1

A

Report

On

WIDE BAND GAP SEMICONDUCTORS

BY

Puja Agrawal

07DDEC854

Under the guidance of

Prof Pawan Mishra

Faculty of Science and Technology

ICFAI University, Dehradun

2

A

Report

On

WIDE BAND GAP SEMICONDUCTORS

Prepared in Partial fulfillment of Study oriented project (OC-302)

BY

Puja Agrawal 07DDEC854

Under the guidance of

Prof Pawan Mishra

Faculty of Science and Technology ICFAI University, Dehradun

3

Faculty of Science and Technology ICFAI University, Dehradun

Title of Project: WIDE BAND GAP SEMICONDUCTORS Name of the Student: PUJA AGRAWAL Name of Faculty: PROF. PAWAN MISHRA Date of submission: 30/04/2010 Key words: BAND GAP Project area: NANO ELECTRONICS, WIDE BAND GAP

Abstract: The developing list of wide gap substrates for device production is remarkable and

continues to provide new device design possibilities. Wide band gap semiconductors have expanded the

scope of device applications beyond those of silicon and gallium arsenide. Exploitation of wide band

gap semiconductors holds promise for revolutionary improvements in the cost, size, weight and

performance of a broad range of military and commercial microelectronic devices. The inherent

material properties of silicon carbide, gallium nitride and zinc oxide make them ideal candidates for

high-power, high-temperature electronics, power amplifiers, switches, and short wavelength light

sources.

Puja Agrawal Prof. Pawan Mishra 07DDEC854

Signature of student Signature of Guide

4

ACKNOWLEDGEMENT

First of all I would like to express my sincere gratitude to the dean of my college, Dr R.C. Ramola who

allowed me to take this project. Secondly I would like to thank the Instructor In charge Dr J. K Gupta.

My project would have been incomplete without the help of my faculty, Prof Pawan Mishra. I would

also like to thank Prof. Sanjay Sahu Kumar who guided me throughout the course of this project. Lastly

I would like to thank my parents and friends without whom my project would not have been successful.

Prof. Pawan Mishra Puja Agrawal

Faculty in charge Student

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TABLE OF COTENTS

Acknowledgement 4

1. Introduction 5 2. Wide Band gap semiconductors 6 3. Properties of wide band gap semiconductors 7

3.1 High electric breakdown field 9 3.2 High saturated drift velocity 10 3.3 High thermal stability 14 3.4 Figure of merit comparison 14

4. Silicon Carbide 16 4.1 Comparison of commercial SiC Schottky pn diodes 16 4.1.1 Conduction Losses 16

4.1.2 Switching Losses 29

4.2 System Level Benefits 22 5. Gallium Nitride 23 6. Application of SiC and GaN in electric vehicles 25

6.1 Existing Si IGBTs 26

6.2 Automobile Traction 26

6.3 SiC Switch Boost 27

6.4 Hybrid Cats in India 31 7. Military Interest 33 8. Zinc Oxide 35

8.1 Electronic Properties 35 8.2 Optical Properties 35 8.3 Application of ZnO in LED 40 8.3.1 How it works 40 8.3.2 Use of ZnO in LED 40 8.3.3 Benefits 41

9. Conclusion 42 9.1 Advantages of Wide Band gap semiconductors 42 9.2 Disadvantages of wide band gap semiconductors 43

10. References 44 11. Annexure 45

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1. INTRODUCTION

Semiconductor substrates provide the foundation for a multi $100B’s electronics industry. Silicon is

currently and will remain the material of choice for the foreseeable future due to the low cost, readily

availability, and established device technology and infrastructure. If a device can be made with silicon

it will. In spite of the phenomenal progress being made with silicon technology it does have its

limitations with respect to temperature, frequency operation and voltage blocking capabilities. As

gallium arsenide (GaAs) and indium phosphide (InP) technologies provided the basis for the

phenomenal growth in the wireless and telecommunications industries during the late 1980s – 1990s, a

new class of semiconductors commonly referred to as “wide band gap semiconductors” holds promise

for continued revolutionary improvements in the size, cost, weight and performance of a broad range of

military and commercial microelectronic and optoelectronic applications. Silicon carbide (SiC),

gallium nitride (GaN) and zinc oxide (ZnO) have emerged as candidate substrate materials that may

overcome the performance limitations of silicon, GaAs and InP.

7

2. WIDE BAND GAP SEMICONDUCTERS

The electrons of a single isolated atom occupy atomic orbitals, which form a discrete set of

energy levels. If several atoms are brought together into a molecule, their atomic orbitals split, as in a

coupled oscillation. This produces a number of molecular orbitals proportional to the number of atoms.

When a large number of atoms (of order × 1020 or more) are brought together to form a solid, the

number of orbitals becomes exceedingly large, and the difference in energy between them becomes

very small, so the levels may be considered to form continuous bands of energy rather than the discrete

energy levels of the atoms in isolation. However, some intervals of energy contain no orbitals, no

matter how many atoms are aggregated, forming band gaps. Within an energy band, energy levels are

so numerous as to be a near continuum. First, the separation between energy levels in a solid is

comparable with the energy that electrons constantly exchange with phonons (atomic vibrations).

Second, it is comparable with the energy uncertainty due to the Heisenberg uncertainty principle, for

reasonably long intervals of time. As a result, the separation between energy levels is of no

consequence. Any solid has a large number of bands. In theory, it can be said to have infinitely many

bands (just as an atom has infinitely many energy levels). However, all but a few lie at energies so high

that any electron that reaches those energies escapes from the solid. These bands are usually

disregarded. Bands have different widths, based upon the properties of the atomic orbitals from which

they arise. Also, allowed bands may overlap, producing (for practical purposes) a single large band. A

simplified energy band diagram is shown below [fig 2.1]. The top band is called the conduction band

and the next lower one is called the valence band. The region between the valence band and the

conduction band is called the forbidden band, where, ideally, no electrons exist.

8

Every solid has its own characteristic band structure. The variation in band structure is

responsible for the wide range of electrical characteristics observed in various materials. Wide band

gap semiconductors are semiconductor materials with electronic band gaps larger than one or two

electron volts (eV). The exact threshold of "wideness" often depends on the application, such as

optoelectronic and power devices. Wide band gap materials are often utilized in applications in which

high-temperature operation is important.

The band gap of pure Si semiconductor is 1.1 eV. Doping increases the band gap. When Si is

doped with carbon (C), it forms a compound called silicon carbide (SiC). The band gap of SiC (a wide

band gap semiconductor) is 3.03 eV (6H-SiC) and 3.26 eV (4H-SiC). Similarly, the band gaps of other

wide band gap semiconductors covered under this report are GaN: 3.45 eV and ZnO: 3.3 eV.

9

3 PROPERTIES OF WBG SEMICONDUCTORS [1]

Wide-band gap semiconductor materials have superior electrical properties compared with Si. Some of

these properties for the most popular WBG semiconductors and Si are shown in Table 3.1

Table 3.1 Physical properties of Si and the major WBG semiconductors [2, 3, and 4]

Property Si GaAs 6H-SiC 4H-SiC GaN

Band gap, gE (eV) 1.12 1.43 3.03 3.26 3.45

Dielectric Constant ,( )arε 11.9 13.1 9.66 10.1 9

Electric Breakdown Field,cE (kV/cm) 300 400 2500 2200 2000

Electron Mobility, nµ ( 2cm /V-s) 1500 8500 500 1000 1250

Hole Mobility, pµ ( 2cm /V-s) 600 400 101 115 850

Thermal Conductivity,λ (W/cm-K) 1.5 0.46 4.9 4.9 1.3

Saturated Electron Drift

Velocity, satv ( )710× scm/

1 1 2 2 2.2

zaε 0εε r= , where mF /1085.8 14

0−×=ε

Presently, two SiC polytypes are popular in SiC research: 6H-SiC and 4H-SiC. Before the

introduction of 4H-SiC wafers in 1994, 6H-SiC was the dominant polytype. Since then, both of these

polytypes have been used in research, but recently 4H-SiC has become the more dominant polytype.

Although both of these polytypes have similar properties, 4H-SiC is preferred over 6H-SiC because

the mobilities in 4H-SiC are identical along the two planes of the semiconductor, whereas 6H-SiC

exhibits anisotropy, which means the mobilities of the material in the two planes are not the same. In a

solid, electrons exist at energy levels that combine to form energy bands. A simplified energy band

diagram is shown in Fig. 2.1. The top band is called the conduction band, and the next lower one is

called the valence band. The region between the valence band and the conduction band is called the

forbidden band, where, ideally, no electrons exist. If the electrons in the valence band are excited

externally, they can move to the conduction band. In the valence band, they have energy of Ev. In order

to move to the conduction band, they need Eg= Ec – Ev amount of energy, where Eg is the band

10

gap.For a conductor like copper, the forbidden band does not exist, and the energy bands overlap. For

an insulator, on the other hand, this band is so wide that the electrons need a lot of energy to move from

the valence band to the conduction band. For semiconductors, the gap of the forbidden band is smaller

than for an insulator. Some semiconductors are classified as “wide-band gap” semiconductors because

of their wider band gap. Silicon has a band gap of 1.12 eV and is not considered as a wide-band gap

semiconductor. The band gaps of WBG semiconductors are about three times or more that of Si as can

be seen in Table 3.1. SiC polytypes and GaN have similar band gap and electric field values, which are

significantly higher than those of Si and GaAs.

WBG semiconductors have the advantage of high-temperature operation and more radiation

hardening. As the temperature increases, the thermal energy of the electrons in the valence band

increases. At a certain temperature, they have sufficient energy to move to the conduction band. This is

an uncontrolled conduction which must be avoided. The temperature at which this happens is around

150°C for Si. For WBG semiconductors, the band gap energy is higher; therefore, electrons in the

valence band need more thermal energy to move to the conduction band. This intrinsic temperature for

SiC is around 900°C, and this value is much higher for diamond. The wider the band gap is the higher

the temperatures at which power devices can operate. The above reasoning is also true for radiation

hardening. Radiation energy can also excite an electron like the thermal energy and make it move to the

conduction band.

As a result of the wide band gap, devices built with WBG semiconductors can withstand more

heat and radiation without losing their electrical characteristics. They can be used in extreme conditions

where Si-based devices cannot be used.

3.1 HIGH ELECTRIC BREAKDOWN FIELD

Wider band gap means a larger electric breakdown field (Ec). A higher electric breakdown field results

in power devices with higher breakdown voltages. With a high electric breakdown field, much higher

doping levels can be achieved; thus, device layers can be made thinner at the same breakdown voltage

levels. The resulting WBG-semiconductor-based power devices are thinner than their Si-based

counterparts and have smaller drift region resistances.

For example, the breakdown voltage (BV ) of a pn diode is expressed as follows:

BV ( )dcr qNE 22 ÷≈ ε (3.1)

11

Where q is the charge of an electron and dN is the doping density.

Using the semiconductor parameters in Table 3.1, this expression can be simplified as follows:

dSi

B NV ÷×≈ 171096.2 (3.2)

dSiCH

B NV ÷×≈− 174 10135 (3.3)

dSiCH

B NV ÷×≈− 176 107.166 (3.4)

dGaN

B NV ÷×≈ 17104.99 (3.5)

Using the above equations, the breakdown voltages of diodes made of the materials listed in Table 3.1

were calculated assuming the same doping density, and the results are plotted in Fig.3.1 with the

breakdown voltages normalized to that of a Si diode. As seen in this figure, the theoretical breakdown

voltage of a diamond diode is 514 times more than that of a Si diode. For 6H-SiC, 4H-SiC, and GaN,

this number is 56, 46, and 34 times, respectively, that of a Si diode. With a higher electric breakdown

field, more doping can be applied to the material, further increasing the gap between the upper

breakdown voltage limits of WBG semiconductors compared to Si.

Fig.3.1 Maximum breakdown voltage of a power device at the same doping

density normalized to Si.

12

Another consequence of the higher electric breakdown field and the higher doping density is that the

width of the drift region decreases. The required width of the drift region can be expressed as:

Using the electric breakdown field values for Si and 4H-SiC from Table 3.1, the drift thickness

of the drift region for these two semiconductors are found as

BSi

d VW 61067.6 −×= (3.7)

B

SiCHd VW 64 1091.0 −− ×= (3.8)

BSiCH

d VW 66 1081.0 −− ×= (3.9)

BGaN

d VW 6101 −×= (3.10)

Where BV is the breakdown voltage &

dW is the width of drift

The width of the drift region was calculated for all the semiconductors in Table 3.1, and the

results are plotted in fig. 3.2 for a breakdown voltage range of 100 to 10,000 V. Diamond requires the

minimum width, while 6H-SiC, 4H-SiC, and GaN follow diamond in the order of increasing widths.

Compared to these, Si requires a drift region approximately 10 times thicker.

(3.6)

Fig 3.2 Width of the drift region for each material at different

13

The last device parameter to be calculated from the properties in Table 3.1 is the on-resistance of the

drift region for unipolar devices, which is given by the Eq.:

The calculation results for on-resistance are plotted in Fig. 3.3 with respect to the breakdown

voltage of the device. Again, diamond shows the best performance, with 4H-SiC, GaN, and 6H-SiC

following in increasing order of resistance. The on-resistance of the drift region for the Si device is

approximately 10 times more than for the SiC polytypes and GaN devices. As the breakdown voltage

increases, more doping can be applied to WBG semiconductors than to Si, so the specific on-resistance

ratio between Si and WBG semiconductors increases further. Note that contact resistance and/or

channel resistance must also be considered when on-resistance for the devices is calculated. These two

resistances are dominant at low breakdown voltages (<1 kV) but can be neglected at high breakdown

voltages.

Fig 3.3 Resistance of the drift region for each material at different breakdown voltages

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3.2 HIGH SATURATED DRIFT VELOCITY

The high-frequency switching capability of a semiconductor material is directly proportional to its drift

velocity. The drift velocities of WBG materials are more than twice the drift velocity of Si (1×107);

Therefore, it is expected that WBG semiconductor-based power devices could be switched at higher

frequencies than their Si counterparts. Moreover, higher drift velocity allows charge in the depletion

region of a diode to be removed faster; therefore, the reverse recovery current of WBG semiconductor-

based diodes are smaller, and the reverse recovery time is shorter.

3.3 HIGH THERMAL STABILITY

As explained earlier, because of the wide band gap, WBG semiconductor-based devices can operate at

high temperatures. In addition to this, SiC has another thermal advantage not mentioned previously —

its high thermal conductivity. As seen in Eq. (3.11), junction-to-case thermal resistance, jcthR − is

inversely proportional to the thermal conductivity.

jcthR − ( )Ad λ÷= (3.11)

Where λ is the thermal conductivity, d is the length, and A is the cross-sectional area. Higher thermal

conductivity means lower jcthR − , which means that heat generated in a SiC-based device can more

easily be transmitted to the case, heat sink, and then to the ambient; thus, the material conducts heat to

its surroundings easily, and the device temperature increases more slowly. For higher-temperature

operation, this is a critical property of the material. As seen in Table 3.1, diamond still leads the other

materials by at least a factor of 5, with the SiC polytypes as the next best materials. GaN has the worst

thermal conductivity — even lower than that of Si.

3.4 FIGURE OF MERIT COMPARISON

15

For a comparison of the possible power electronics performances of these materials, some commonly

known figures of merit are listed in Table 2.2. In this table, the numbers have been normalized with

respect to Si; a larger number represents a material’s better performance in the corresponding

category. SiC polytypes and GaN have similar figures of merit, which implies similar performances.

Silicon and GaAs have the poorest performance among the semiconductor materials listed in Tables 3.1

and 3.2. Much of the present power device research is focused on SiC.

Table 3.2: Main figures of merit for WBG semiconductors compared with Si [3]

Si GaAs 6H-SiC 4H-SiC GaN

JFM 1 1.8 277.8 215.1 215.1

BFM 1 14.8 125.3 223.1 186.7

FSFM 1 11.4 30.5 61.2 65

BSFM 1 1.6 13.1 12.9 52.5

FPFM 1 3.6 48.3 56 30.4

FTFM 1 40.7 1470.5 3424.8 1973.6

BPFM 1 0.9 57.3 35.4 10.7

BTFM 1 1.4 748.9 458.1 560.5

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4 SILICON CARBIDE [1]

Silicon carbide technology is the most mature among WBG semiconductor technologies. It has

advanced greatly since 1987 with the foundation of CREE, Inc., which is the major supplier of SiC

wafers. Pending material processing problems like micro pipes and screw dislocations limit the die

size, but these problem have not stopped the commercialization of the first SiC Schottky diodes(power

devices) with twice the blocking voltage (600 V) of Si Schottky diodes (300 V).

Apart from the commercial devices, many other SiC power devices in the kilovolt range with

reduced on-resistances are being investigated; these include 4H-SiC and 6H-SiC pn diodes, Schottky

diodes, IGBTs, Thyristors, BJTs, various MOSFETs, GTOs, MCTs, and MTOs. Except for some of the

diodes, these devices are all experimental devices with very low current ratings.

4.1 COMPARISON OF COMMERCIAL SiC SCHOTTKY DIODES WI TH Si pn DIODES

Silicon carbide Schottky diodes used in this study are rated at 300 V and 10 A and have been obtained

directly from Infineon AG in Germany. The next two subsections describe testing, characterization, and

loss modeling of Si pn and SiC Schottky diodes and compares the two. The main reason for comparing

pn diodes with Schottky diodes is because SiC Schottky diodes are projected to replace Si pn diodes in

the 300- to 1200-V range.

4.1.1 Conduction Losses

The circuit shown in Fig. 4.1 is set up with test diodes in a temperature-controlled oven to obtain the I-

V characteristics of the diodes at different operating temperatures. The dc voltage supply is varied, and

the diode forward voltage and current are measured at different load currents and several temperature

values of up to 250°C (the temperature limit of the oven). The I-V curves obtained as a result of this

test for both Si pn and SiC Schottky diodes are shown in Fig. 4.2; it can be seen that the forward

voltage of the SiC diode is higher than that of the Si diode. This is expected because of the wider band

gap of SiC. Another difference between these two diodes is their high-temperature behavior. As the

temperature increases, the forward characteristics of the Si diode change severely, while those of the

SiC diode stay confined to a narrow region. Note that the pn diode (negative) and the Schottky diode

17

(positive) have different polarity temperature coefficients for on-state resistance; that is why the slope

of the curve at higher currents is increasing in the Si diode case and decreasing in the SiC diode case

with the increase in temperature.

Fig 4.1 I-V characterization circuit

Fig 4.2 Experimental I-V characteristics of the Si and SiC diodes in an operating temperature range of 27°C to 250°C

If a line is drawn along the linear high-current portion of the I-V curves extending to the x-axis, the

intercept on the x-axis is VD, and the slope of this line is 1/RD. The parameters VD and RD thus obtained

are plotted in Fig. 4.2. As mentioned previously, because of different temperature coefficients, RD of

the Si diode is decreasing and that of the SiC diode is increasing. For low temperatures, the SiC on-

resistance is lower than that of Si’s. In addition to the on-resistance, Si also has a lower voltage drop,

18

which also decreases with temperature. Lower on-resistance and lower voltage drop imply lower

conduction losses for the Si diode.

The changes in RD and VD are modeled using a curve-fitting method, as also plotted in Fig. 4.3. The

equations describing the curves are

7042.02785.0 0046.0 += − TSiCB eV (4.1)

2023.01108.0 0072.0 +−= − TSiCD eR (4.2)

5724.03306.0 0103.0 += TSiB eV (4.3)

0529.02136.0 2193.0 +−= − TSiD eR (4.4)

where T is in °C.

Fig 4.3 Variation of (a) RD and (b) VD with temperature in Si and SiC diodes

Equations (4.1)–(4.4) can be used to derive the diode loss equations in a power converter system. For a

three-phase, sinusoidal PWM inverter, the conduction loss for a diode can be simply expressed as:

(4.5)

where M is the modulation index and φ is the power factor angle.

( ) ( )8/cos2/13/cos8/12, φππφ MIVMRIP DDDcond −+−=

19

4.1.2 Switching Losses The most important part of the diode switching loss is the reverse recovery loss. The rest of the losses

are negligible. In this paper, the energy lost during reverse recovery is calculated experimentally so that

the switching losses can be calculated for any switching frequency. Schottky diodes, unlike pn diodes,

do not have reverse recovery behavior because they do not have minority carriers; however, they still

show some reverse recovery effects. The main reason for these effects is parasitic oscillation due to

parasitic device capacitance and inductances in the circuit. The second reason is the parasitic pn diode

formed by the p-rings inserted to decrease the reverse leakage currents and n-type drift region. For this

test, the chopper circuit shown in Fig. 4.4 was set up with test diodes in a temperature-controlled oven.

The main switch, Q, is turned on and off at 1 kHz with a duty ratio of 75%. The typical Si and SiC

diode turn-off waveforms are given in Fig. 4.5 for three different forward currents. These experimental

waveforms show that the Si diode switching losses are almost three times more than those of the SiC

diode.

Fig. 4.4 Reverse recovery loss measurement circuit

20

The peak reverse recovery current, IR , and the reverse recovery current-time integral of the diodes are

measured at different operating temperatures with varying load currents. The peak reverse recovery

current at different temperatures is plotted in Fig. 4.6 with respect to the forward current. The IR of the

Si diode is higher than that of the SiC diode at any operating temperature. As the temperature increases, the

difference increases because the IR of the Si diode increases with temperature but that of the SiC diode stays

constant.

Fig 4.5 Typical reverse recovery waveforms of the Si pn and SiC Schottky diode (2 A/div.)

Fig 4.6 Peak reverse recovery values with respect to the forward current at different operating temperatures

21

The reverse recovery current-time integral can be used to calculate reverse recovery losses, and thus

diode switching losses. Assuming that the diode “sees” a constant reverse voltage when it is off and

that it is switched at constant frequency, then

(4.6)

Figure 4.7 shows reverse recovery losses for a 20-kHz operation with a 300-V reverse voltage.

The reverse recovery time-integral current can be approximated linearly as a function of the forward

current:

(4.7)

(4.8)

(4.9)

Fig 4.7 Diode switching loss of Si and SiC diodes at different operating temperatures

22

(4.10)

Equations (4.8)-(4.10) can be used to calculate the switching losses of Si and SiC diodes in system

level models to show the system level benefits of SiC devices.

4.2 SYSTEM LEVEL BENEFITS The use of SiC power electronics instead of Si devices will result in system level benefits like reduced

losses, increased efficiency, and reduced size and volume. When SiC power devices replace Si power

devices, the traction drive efficiency in a hybrid electric vehicle (HEV) increases by 10 percentage

points, and the heat sink required for the drive can be reduced to one-third of the original size. In a dc

power supply; the effects of increasing the switching frequency by using SiC devices show that the

sizes of the passive components, which include the transformer and the filter components, decrease

proportionally.

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5 GALLIUM NITRIDE

Gallium nitride (GaN) is a binary III/V direct band gap semiconductor commonly used in bright light-

emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal

structure. Its wide band gap of 3.4 eV affords it special properties for applications in optoelectronic,

high-power and high-frequency devices. Its sensitivity to ionizing radiation is low (like other group III

nitrides), making it a suitable material for solar cell arrays for satellites. Because GaN transistors can

operate at much hotter temperatures and work at much higher voltages than gallium arsenide (GaAs)

transistors, they make ideal power amplifiers at microwave frequencies. Applications of GaN devices

have mainly focused on optoelectronics and radio frequency uses because of the material’s direct band

gap and high-frequency performance, respectively. GaN also has a potential for use in high-power

electronics applications.

In the last few years some papers have been published in the literature on high voltage GaN

Schottky diodes. The comparison of GaN Schottky diodes with SiC Schottky diodes and Si pn diodes

at similar blocking voltages show a performance advantage for the GaN Schottky diode. The main

advantage of GaN schottky diode is the negligible reverse recovery current and consequently lower

switching loss and it is also independent of the operating temperature. Figure 5.1 compares the

switching performances of GaN, Si, and SiC diodes. As can be seen in this figure, GaN and SiC diodes

have similar switching properties, but as the temperature increases, the switching performance of the

GaN diode is better than that of the SiC diode. The switching speed and losses of GaN Schottky diodes

have been shown to be slightly better than similarly rated SiC diodes as seen in Table 5.1. On the other

hand, because of its wider band gap, the GaN Schottky diode has a much higher forward voltage drop

than the Si pn and SiC Schottky diodes.

GaN Schottky diodes up to 2 kV and GaN pn diodes up to 6 kV have already been

demonstrated; however, 4.9-kV SiC Schottky diodes and 19.2-kV pn diodes have also been

demonstrated. These figures show how advanced SiC technology is at this point compared with GaN

technology. GaN has some disadvantages compared to SiC. The first one is that GaN does not have a

native oxide, this is required for MOS devices. SiC uses the same oxide as Si, SiO2. For GaN, more

studies are under way to find a suitable oxide; without it, GaN MOS devices are not possible. The

second important problem is that with present technology, GaN boules are difficult to grow. Therefore,

pure GaN wafers are not available); instead, GaN wafers are grown on sapphire or SiC. Even then,

24

thick GaN substrates are not commercially available. As a consequence, GaN wafers are more

expensive than SiC wafers.

An additional disadvantage of GaN compared with SiC is that its thermal conductivity is almost

one fourth that of SiC. This property is especially important in high-power, high-temperature operation

because the heat generated inside the device needs to be dissipated as quickly as possible. The higher

the thermal conductivity, the more quickly the heat is dissipated. Growing GaN on SiC wafers

increases the overall thermal conductivity, but the material still does not equal the performance of SiC.

Fig 5.1 Comparison of switching performances of Si, SiC , and GaN diodes at room temperature and at 623K [5]

25

Table 4.1. Reverse recovery performance of 6000-V Si, SiC, and GaN diodes[5]

26

6. Application of SiC and GaN in Electric Vehicles [6]

Environmental concerns have fallen off the agenda somewhat in recent months with the focus

on economic recovery. However, the basic long term concerns remain in building efficient systems

capable of harnessing existing and future energy creation and conversion technologies. For

transportation, hybrid electric vehicles (HV or HEV), pure electric (EV), and fuel cell hybrids (FCHV)

are being considered to reduce the reliance on gasoline and to reduce exhaust and global warming gas

pollution. These systems require greater control over high power electricity compared with traditional

fossil fuel vehicles. Power inverters are used to convert from the direct current battery or fuel cell to the

kilohertz currents needed to drive high power motors ( . Hybrids also want to conserve excess

energy generated during braking and convert the power back to storage. Such demands require

switches that can handle high power, current and voltage. One also wants to develop intelligent power

systems for response and efficiency- that means producing power semiconductor systems that have

extreme capabilities.

6.1 Existing Si IGBTs

Commercial HEVs/EVs presently use Si based insulated gate bipolar transistors (IGBTs),

combined with suitable diodes, as the main driving switch. For hybrids particularly, where the electric

power is running in tandem with a high temperature fossil fuel engine, Si is not the ideal semiconductor

material(see table 6.1). These devices require a secondary cooling system to maintain suitable junction

temperatures. Also materials with higher breakdown voltages are desired to enable the use of higher

power motors and to protect against surge voltages.

The wide band gap semiconductor SiC has the potential to operate up to 600, so interfacing

with standard engine coolant systems aiming at 105 should be easy. Jettisoning special cooling would

cut cost. Also, SiC’s breakdown voltage is 10 times that of Si. Since the devices are smaller, the

capacitance is lower and switching can be faster. Losses can be lower during both turn-on and turn-off.

SiC has much better thermal conductivity, allowing fast heat transfer out of the device. However, SiC is

a difficult material to grow and work- developments are still needed in terms of wafer quality and size,

gate isolation and packaging to handle the higher temperature environment. Once mature, the

technology could enable significant reductions in weight and size and increases in efficiency. GaN is

another wide band gap semiconductor with potential that is being explored. This option also has a high

breakdown field and higher carrier saturation velocity, along with better carrier mobility.

27

Table 6.1 Sample of material properties for application to high power electronics [7]

6.2 Automobile Traction

The first successful hybrid vehicles for the general automotive market came out of Japan in the

1990s with the Toyota Prius and the Honda Insight. While Prius is a parallel series hybrid, meaning that

the electric motor can operate the car on its own, the Insights electric power is used to assist the main

engine. The defense is seen in the amount of power that the electric motor needs to deliver: 60kW for a

recent model of the Prius; 10kW for the Insight. Both vehicles have gasoline engine capable of around

70kW. These vehicles, along with the ranges coming from US and European competitors, are based on

silicon IGBT technology. However, these companies are not blind to the advantages that would accrue

to successful implementation of wide band gap alternatives. Toyota recently presented some of its GaN

power device research.

Toyota has produced a range of HVs since the first Prius in 1997. While the first Prius used

277V battery directly connected to the inverter stage, later versions have used a voltage booster from

202V to 500V before linking into inverter and motor [fig 6.1]. Some newer HV systems have increased

the boost beyond 600V [Fig 6.2].

28

Fig 6.1 Typical set-up for powering electric motor from battery supply, as used in Toyota prius

Fig 6.2 Toyota has been increasing the voltage level to enable higher power motors in its EVs

29

The present Si based component used in HV inverters has a breakdown voltage of about 1kV.

For Hybrid applications, Toyota is attracted to GaN due to its wide band gap, high breakdown field and

high saturation velocity. AlGaN/GaN field effect transistors have been developed mostly in the lateral

direction, aiming at relatively low power(of the order of watts) and high frequency (of the order of

GHz) for applications such as RF power amplification. For high power and lower frequency of electric

motors, a vertical structure is most suitable for small chip sizes, simple wiring and high breakdown.

‘Current collapse’ which is a feature of lateral devices, is not expected to be a problem in the new

format. The current collapse phenomenon is associated with the effect of surface states on current flow.

The vertical structure current flow is through the bulk of the material, not just a thin layer near the

surface, and hence surface states are much less significant.

However running current through the bulk, one may come into play from the relatively high

dislocation density in such materials, favoring the use of high quality free standing material. Recently

2-inch substrates of such material have become available from several suppliers. Toyota has explored

two GaN MOSFET types: an AlGaN/GaN heterostructure FET similar to the double diffused Si

MOSFET (fig6.3) and a device with a U-shaped trench (fig6.4).

Fig 6.3 Schematic of Toyota GaN-based heterojunction field effect transistor (HFET),

which is targeted at vehicle applications

30

The HFET used a polysilicon gate with high temperature silicon dioxide(HTO) insulation. The

source drain electrodes were Ti/Al stacks. The current is blocked using a Mg doped p-GaN blocking

layer. The gate channel consists of the AlGaN/ GaN interface, which enables flow through an aperture

in the center of the blocking layer. Metal organic chemical vapor deposition (MOCVD) was used,

along with inductively coupled plasma(ICP) etching of the aperture in the blocking layer. Si ion

implants were made to form the source regions. Isolation came from N ion implantation. The gate stack

growth was by low pressure CVD. Ohmic contact is also provided on the p-GaN layer, along with

thicker p-GaN and n GaN (drift) regions. For trench gate structure, smooth side walls are needed

which will enable a suitable gate channel current flow. Tetra methyl-ammonium hydride (TMAH) wet

process is used for this purpose. TMAH is commonly used to develop photo resists and for anisotropic

etching of Si. In the GaN structure developed by Toyota, TMAH created smooth m-plane side walls.

Fig 6.4 Schematic of Toyota GaN-based metal-oxide field effect transistor using U-shape trench structure(UMOSFET)

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Polysilicon and HTO were used for the gate structure for an accumulation mode device.

6.3 SiC Switch Boost

Japanese electronic component firm Rohm has worked with Honda and Nissan on applying SiC

material to the electric vehicle sector. Work with Honda resulted in a full SiC inverter module

containing a one phase converter circuit and a three phase inverter. The device contained both SiC

MOSFETs and Schottky barrier diodes. It was concluded that SiC has 1/7th of the switching loss of Si

IGBTs (fig6.5).

The use of SiC also enables a faster switching speed to be used- 80kHz rather than 20kHz for Si IGBTs

for the same power loss. In 2008, tests of an inverter containing SiC diodes were launched by Nissan.

The tests were carried out on company’s X-TRAIL fuel cell vehicle (fig 6.6).

Fig 6.5 Comparison by Rohm And Honda of losses for various configurations of SiC implementation to replace silicon diodes and IGBTs

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Nissan developed a special heterojunction diode (HJD) structure combining SiC and polysilicon for use

in the application (fig6.7).

The surface of the diode was reduced by 70% while giving 20% better power efficiency.

Simplifications in the inverter design resulting from SiC application reduced the size/weight of the

device by 15-20%. More general use in EVs and HEVs is hoped for.

Fig 6.6 Nissan’s X-TRAIL fuel cell vehicle

Fig 6.7 Schematic for Rohm and Nissan’s heterojunction diode(left), compared with SBD

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6.4 Hybrid Cars in India

1) Toyota has launched its famed hybrid car the Prius at the Auto Expo in India making it the

second Hybrid car to be available in the country. Remarkably, the third generation Prius was

launched in international markets only last year. The new Prius has a more aerodynamic design

compared to its predecessors with the coefficient of drag reduced to 0.25 Cd, The 1.8-liter

petrol engine generates about 100PS and with the added power of the electric motor combined

power is 136PS. With an electric water pump, the Prius engine is the first production engine that

requires no accessory belts improving fuel economy.

2) Chevrolet Volt is an electric car that can create its own electricity. Plug it in, let it charge

overnight, and it's ready to run on a pure electric charge for up to 40 miles - gas and emissions

free. After that, Volt keeps going, even if you can't plug it in. Volt uses a range-extending gas

generator that produces enough energy to power it for hundreds of miles on a single tank of gas.

3) General Motors India has showcased a total of 11 production and concept vehicles at the

Auto Expo. The Chevrolet Spark and Aveo U-VA will give the Beat company in the 'Chevrolet

Hot Hatch' category, just one of the three themes the 11 vehicles have been showcased under.

The Electric Spark will run on an all electric drive train while the Volt will save an estimated

1,892 liters of petrol every year if driven for 64kms every day.

4) The Reva Electric Car Company is leading from the front as it prepares to launch two new

models - Reva NXR and NXG. The three-door, four-seater NXR is more spacious compared to

the previous models of the carmaker. It can accelerate to a maximum speed of 105 km/h with a

range of 160 kilometers on a single charge of its lithium-ion battery pack. Moreover, the car

will come with a 15-minute fast-charge capability, even though the full charge would take eight

hours from a standard household outlet.

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7. Military Interests

Much of the US development of wide band gap semiconductors has emanated from defense research

contracts and the high power sector has had its share in such funding.

SiC device producer Cree has worked with power module developer Powerex to produce an all-

SiC dual switch 1200V, 100A power module demonstrator for the US Air Force Research Laboratory

(ARFL). Aim was to produce components that can operate at higher temperatures, in this case a

junction temperature of 200. The module used both SiC MOSFETs and diodes to achieve its purpose.

The MOSFETs are normally off. The companies hoped to be able to develop drop-in replacements for

existing IGBT based components along with smaller, light weight systems with reduced cooling

requirements, but with increased reliability and overload capacity due to SiC’s high temperature

operation capability. Operating the SiC MOSFET based module at a junction temperature of 150 and

a frequency of 20kHz gave 38% lower conduction losses and 60% lower switching losses, compared

with a Si IGBT module of equal rating. The total power loss reduction came to 54%. Low conduction

and switching losses give high efficiency with applications seen in solar energy power inverters and

electric drives, and power conversion for hybrid and electric vehicles. The SiC module is scalable to

higher currents and the layout can be modified for other switch configurations.

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8. ZINC OXIDE

Zinc Oxide (ZnO) is often called II-VI semiconductor because zinc and oxygen belong to the

2nd and 6th groups of the periodic table. This semiconductor has several favorable properties: good

transparency, high electron mobility, wide band gap, strong room-temperature luminescence, etc. Those

properties are already used in emerging applications for transparent electrodes in liquid crystal displays

and in energy-saving or heat-protecting windows, and electronic applications of ZnO as thin-film

transistors and light-emitting diodes.

8.1 Electronic properties

ZnO has a relatively large direct band gap of ~3.3 eV at room temperature therefore, pure ZnO is

colorless and transparent. Advantages associated with a large band gap include higher breakdown voltages,

ability to sustain large electric fields, lower electronic noise, and high-temperature and high-power operation.

The band gap of ZnO can further be tuned from ~3–4 eV by its alloying with magnesium oxide or cadmium

oxide.

8.2 OPTICAL PROPERTIES [8]

The optical properties of ZnO films were carried out with a double beam spectrophotometer (Perkin

Elmer Lambda 2S) in the UV/VIS/NIR regions. The optical transmittance at normal incidence was recorded in

the wavelength range of 300-1100 nm. Swanepoel’s envelope method was employed to evaluate the optical

constants such as the refractive index n, extinction coefficient k, and absorption coefficient a from transmittance

spectra. The thickness of ZnO films was determined from the interference fringes of transmission data measured

over the visible range. The structure and lattice parameters of ZnO films were analyzed by a Rigaku RadB X-ray

diffractometer (XRD) with Cu radiation with

= 1.54056 (30 kV, 15 mA, scanning speed = 6°/min).

Fig. 8.1 shows the X-ray diffraction pattern of ZnO thin film deposited at 400°C only with one sharp and three

small peaks present. Diffraction pattern was obtained with 2 from 10° to 70° at 6° glancing angle. The XRD

pattern of the film shows that the film is crystallized in the wurtzite phase and presents a preferential orientation

along the c-axis. The result is in agreement with the literature (JCPDF card no 36-1451). The strongest peak

observed at = 34.39° (d = 0.260 nm) can be attributed to the (002) plane of the hexagonal ZnO. The (101),

(102) and (103) peaks were also observed at 2 = 36.17°, 47.47° and 62.78°, respectively but these peaks are of

much lower intensity than the (002) peak. The c-axis lattice constant of the ZnO thin film was calculated from

XRD data as 5.21 . The grain size g can be estimated using the Scherer’s formula:

36

Here is the x-ray wavelength (1.54056 ), q and D (2q) are the Bragg diffraction angle of the XRD

peak in degree and the full width at half maximum (in radian) of (002) diffraction peak respectively.

The crystallite size is estimated about 40 nm.

Fig. 8.2 shows the optical transmittance spectrum of ZnO thin film in the wavelength range from 300 to

1100 nm. The films are highly transparent in the visible range of the electromagnetic spectrum with an

average transmittance values up to 95 %, and present a sharp ultraviolet cut-off at approximately 380

nm.

Fig 8.2 UV/VIS/NIR transmission curve of ZnO film

Fig 8.1 X-Ray diffraction pattern of ZnO film deposited on glass substrate at 400o C

37

The thickness of the film was calculated using the following relation:

Where n( ) and n( ) are the refractive indices at the two adjacent maxima (or minima) at and . The

zinc oxide film thickness was found to be 0.52 mm. The optical constants such as refractive index n and

extinction coefficient k were determined from a transmittance spectrum (Fig. 8.3) using envelope method. The

refractive index can be calculated from the following equations:

Where is the refractive index of the substrate. and are maximum and minimum transmittances

at the same wavelength in the fitted envelope curves on the transmittance spectrum. The extinction coefficient

can be also calculated by the following equations:

Where is the absorption coefficient and t is the film thickness. and are the wavelengths at the two

adjacent maxima or minima. The optical constants such as refractive index n and extinction coefficient k were

determined from a transmittance spectrum by envelope method as explained in the previous section. The

variations of refractive index n and extinction coefficient k with wavelength in the region 400 nm-1100 nm are

shown in Fig. 8.3 and Fig. 8.4.

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Fig 8.3 Plot of refractive index (n) as a function of wavelength

Fig 8.4 Plot of extinction coefficient (k) as a function of wavelength

The absorption coefficient a of ZnO films was determined from transmittance measurements. Since

envelope method is not valid in the strong absorption region, the calculation of the absorption

coefficient of the film in this region was calculated using the following expression:

39

Where T is the normalized transmittance, t is the film thickness. These absorption coefficients values were used

to determine optical energy gap. Fig. 8.5 shows the plot of vs. , where is the optical absorption

coefficient and is the energy of the incident photon. The energy gap (Eg) was estimated by assuming a direct

transition between valence and conduction bands from the expression:

Where K is a constant, Eg is determined by extrapolating the straight line portion of the spectrum to = 0.

From this drawing, the optical energy gap, Eg = 3.27 eV is deduced. This value is slightly smaller than the bulk

value of 3.31 eV and in good agreement with previously reported data of ZnO thin film.

Fig 8.5 Plot of vs. photon energy, for ZnO thin film

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8.3 Application of ZnO in LED

LED technology is already used in exit signs and traffic signals, as well as digital clocks, plasma TV

screens, and remote controls.

8.3.1 HOW IT WORKS:

LEDs are essentially tiny light bulbs that fit into an electrical circuit, but they are lit solely by the

movement of electrons in a semiconducting material. A diode is the simplest semiconductor device. It is

made by bonding a section of a positively-charged material to a section of a negatively-charged material

with electrodes on each end so that it only conducts electricity (in the form of free-moving electrons) in

one direction whenever a voltage is applied to the diode. Electrons move in a series of fixed orbits

around the nuclei of atoms. Whenever an electron absorbs extra energy from the added voltage, it jumps

to a higher orbital, and when it returns to a lower orbital, it emits the extra energy as a photon, a particle

of light. LEDs are specially constructed to emit a large number of photons, unlike ordinary diodes, in

which the semiconductor material absorbs most of the light energy before it can be released. LEDs are

also housed in a plastic bulb to concentrate the light in a particular direction.

8.3.2 Use of ZnO in LED:

Since unintentially doped ZnO tends to have n-type properties, p-type doping is difficult to achieve.

Such an assymetric doping limitation is common in wide band gap materials. The n-type doping in ZnO

can be enhanced using doping with Group III elements such as Al or Ga. For p-type properties , one

looks for acceptor levels, and here Group I or III elements are in the frame. The electron concentration in

ZnO grown on sapphire is typically The electron concentration can be reduced to

when ZnO is deposited on ScAlMg . Nitrogen is seen as the best option for acceptor

doping because its small ion size reduces compensation effects. However, high quality zinc oxide films

require growth at more than 800 , while high concentrations of N are only possible to achieve at

temperatures below 500 . One method- alternating low and high temperature growth (‘reverse

temperature modulation’ growth) has achieved a hole concentration of / with a carrrier mobility

of 8 /Vs. The low temperature phase grows 15 nm of N-doped ZnO, while the high temperature

phase leaves 1nm of high quality N-doped material. The process is repeated to achieve the required

thickness. Some blue LEDs have been produced using this technique.

ScAlMg is insulating, so a mesa structure is used, with both contacts made through the top

surface.

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Fig 8.6 ZnO LED structure on ScAlMg substrate (left) and on pure substrate(right) [9]

Devices with nitride emitter structures in combination with ZnO semiconductors are used to produce

white LEDs. A project with Georgia Tech and the US Air Force involves developing light emission on ZnO p-n

junctions for UV/blue emission. It is hoped that homoepitaxial growth of ZnO will result in devices with far

fewer of the defects that seriously degrade performance of GaN devices. Such devices will even cost less to

produce than GaN based technology. P-type ZnO layers are also produced on its n-type substrates using

MOCVD( metal oxide chemical vapor deposition). While Cu and P doping have been achieved, the company’s

efforts have focussed on N-doping using N and O sources. LEDs made from resulting p-n junction were

driven at 140mA to produce light with a peak wavelenght of 384nm. This is the first clear EL (

electroluminesence) peak measured from a ZnO p-n junction LED. This light is attributed to recombination

between shallow donors and nitrogen luminescent centers on the p-side of the junction. N-doped ZnO has a

photoluminescence peak at aroud 380nm.

8.3.3 BENEFITS:

LED lighting consumes 50 percent less energy than traditional sources. It is four times more energy efficient

than regular light bulbs because more of the energy is converted into light than is lost as heat. There is no glass

or filament, as in a light bulb, so LED tiles would last forever, and the tiles are so rugged, someone can jump on

them without breaking them. LED lighting also covers the entire color spectrum of visible light so lighting can

change from one color, or tone, to another with just one touch of a control panel.

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9. CONCLUSION

I would like to conclude that these wide band gap semiconductors are very beneficial for us especially

in the field of transportation. The reliability on gasoline can be reduced by using electric vehicles. Here

the only problem is charging the battery and if this problem is taken care of then this technology can

prove to be a boon in the this field. Transportation will be much cheaper and even faster compared to

vehicles driven by petrol or gasoline. Electric vehicles will also not cause any pollution. Use of wide

band gap semiconductor is also very beneficial in the field of solid state lighting. LEDs are much

brighter compared to normal bulbs. It also consumes less power. Wide band gap semiconductors have

many other advantages in different fields which are not possible to cover in this report.

9.1 Advantages of WBG compared to Si based power devices:

• WBG semiconductor-based unipolar devices are thinner and have lower on-resistances. Lower R-on

also means lower conduction losses; therefore, higher overall converter efficiency is attainable.

• WBG semiconductor-based power devices have higher breakdown voltages because of their higher

electric breakdown field; thus, while Si Schottky diodes are commercially available typically at

voltages lower than 300 V, the first commercial SiC Schottky diodes are already rated at 600 V.

• WBG devices have a higher thermal conductivity (4.9 W/cm-K for SiC and 22 W/cm-K for diamond,

as opposed to 1.5 W/cm-K for Si). Therefore, WBG-based power devices have a lower junction-to-case

thermal resistance, . This means heat is more easily transferred out of the device, and thus the

device temperature increase is slower. GaN is an exception in this case.

• WBG semiconductor-based power devices can operate at high temperatures. The literature notes

operation of SiC devices up to 600°C. Si devices, on the other hand, can operate at a maximum

junction temperature of only 150°C.

• Forward and reverse characteristics of WBG semiconductor-based power devices vary only slightly

with temperature and time; therefore, they are more reliable.

• WBG semiconductor-based bipolar devices have excellent reverse recovery characteristics. With less

reverse recovery current, switching losses and electromagnetic interference (EMI) are reduced, and

there is less or no need for snubbers. As a result, there is no need to use soft-switching techniques to

Reduce switching losses.

• Because of low switching losses, WBG semiconductor-based devices can operate at higher

frequencies (>20 kHz) which is not possible with Si-based devices in power levels of more than a few

43

tens of Kilowatts.

9.2 Disadvantages of WBG semiconductors:

• Low processing yield because of defects for SiC and processing problems for GaN and diamond

• High cost

• Limited availability, with only SiC Schottky diodes at relatively low power is commercially available

• The need for high-temperature packaging techniques that have not yet been developed.

These drawbacks are expected because WBG semiconductor technology has not been yet matured.

44

10. REFERENCES

1) B. Ozpineci, L. M. Tolbert, “Comparison of Wide Band Gap Semiconductors for power

Electronics Applications”,2003

2) K. Shenai, R. S. Scott, and B. J. Baliga, “Optimum semiconductors for high power electronics,”

IEEE Transactions on Electron Devices, 36(9),1811-1823, 1989.

3) Figures of Merit,” EEEnet: Electronics for Extreme Environments,

http://www.eeenet.org/figs_of_merit.asp.

4) A. K. Agarwal, S. S. Mani, S. Seshadri, J. B. Cassady, P. A. Sanger, C. D. Brandt, and N. Saks,

“SiC power devices,” Naval Research Reviews, 51(1),14–21, 1999

5) J. L. Hudgins, G. S. Simin, M. A. Khan, “A new assessment of the use of wide-band gap

semiconductors and the potential of GaN,” 33rd Annual IEEE Power Electronics Specialists

Conference (PESC’02), Cairns, Australia, 2002, 1747–1752.

6) Mike Cooke Reports, ‘Wide load Potential for electric Vehicles’

7) Tolbert et al Oak Bridge national laboratory report on ‘Power electronics for distributed material

systems and transmission and distribution application, 2005’.

8) C Gumus, O.M.Oakendir, H. Kavak, Y. Ufuktepe,C. ’Journal of Optoelectronics and

Advanced Materials, Vol 8, No. 1, Feb 2006.’

9) Pa et al, Proc of SPIE, vol. 6122, p61220M,2006

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11. ANNEXURE

JFM : Johnson’s figure of merit, a measure of the ultimate high-frequency capability of the

material

BFM : Baliga’s figure of merit, a measure of the specific on-resistance of the drift region of a

vertical field effect transistor (FET)

FSFM : FET switching speed figure of merit

BSFM : Bipolar switching speed figure of merit

FPFM : FET power-handling-capacity figure of merit

FTFM : FET power-switching product

BPFM : Bipolar power handling capacity figure of merit

BTFM : Bipolar power switching product