Chapter 1 - Intro to Diodes.pdf

7/26/2019 Chapter 1 - Intro to Diodes.pdf

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CHAPTER 1INTRODUCTION

TO DIODES

BY AZRUL GHAZALI

TYPES OF DIODES


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CONTENT1. SEMICONDUCTOR MATERIALS AND

PROPERTIES

ELEMENT & COMPOUNDSEMICONDUCTOR

INTRINSIC & EXTRINSICSEMICONDUCTORS

P-TYPE AND N-TYPESEMICONDUCTORS

DRIFT & DIFFUSION CURRENTS

2. THE PN JUNCTION

EQUILIBRIUM PN JUNCTION

REVERSED-BIASED & FORWARD-BIASED PN JUNCTION

IV RELATIONSHIP

3. DC MODEL AND ANALYSIS

IDEAL MODEL

PIECEWISE LINEAR MODEL

CONSTANT VOLTAGE DROP MODEL

4. AC MODEL

5. OTHER TYPES OF DIODES

SOLAR CELL

PHOTODIODE

LIGHT-EMITTED DIODE (LED)

SCHOTTKY BARRIER DIODE

ZENER DIODE

ELEMENT & COMPOUND SEMICONDUCTORS

Compound semiconductor is

composed of elements from two

or more different groups of

periodic table.


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INTRINSTIC SEMICONDUCTORS Intrinsic semiconductor is a pure, single-crystal semiconductor with no impurities or lattice

defects.

In an intrinsic semiconductor, the no. of holes equals to the no. of electrons. Theconcentrations of electrons and holes are represented as ni, measured in cm

-3.

The valence electrons of semiconductor are shared among its atoms. This sharing ofelectrons is known as covalent bonding.

Crystal lattice structure of SiliconValence electrons in Siliconare shared in covalent bond

Valence electronsare electrons locatedat the most outer

shell of an atom.Silicon has 4 valenceelectrons.

ENERGY BAND DIAGRAM At T = 0K, all valence electrons occupy the valence band. Semiconductor behaves like an

insulator.

When T increases, the valence electrons gain thermal energy. When the energy issufficient enough, the covalent bond can be broken. An electron-hole pair is generated.

The valence electrons are now known as free electrons and exists in conduction band.

The minimum energy needed by an electron to become a free electron from a valenceelectron, is known as bandgap energy (Eg).

Crystal lattice structure of SiliconEnergy band diagram of semiconductor


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EXTRINSTIC SEMICONDUCTORS Extrinsic semiconductor is a semiconductor having impurity in its crystal.

An intrinsic semiconductor can be turned into extrinsic semiconductor when it is doped withcontrolled amount of dopants (impurities).

Doping semiconductor with donor atoms (Group V elements P, As, Sb) creates n-typesemiconductor. Doping semiconductor with acceptor atoms (Group III elementsB, Al, Ga)creates p-type semiconductor.

Doping concentration for donor atoms (ND) and acceptor atoms (NA) is measured in cm-3.

Negative-charged electrons in n-type silicon and positive-chargedholes in p-type silicon

Group V elements has 5valence electrons. Whenthis impurity atomdisplaced a Si atom, the

4 valence electronsmade covalence bondswith neighboring Siatoms, leaving somefree electrons.

Group III elements has 3valence electrons. Whenthis impurity atomdisplaced a Si atom, the3 valence electronsmade covalence bondswith neighboring Siatoms, creating someholes.

DIFFUSION AND DRIFT CURRENTS Diffusion current is the current in semiconductor caused by variations in the dopant

concentration. Carriers flow from region of higher concentration to a region of lowerconcentration.

Drift current is the electric current, or movement of charged carriers, which is due to theapplied electric field. The direction of applied electric field will determine the directionof carrier.

Current in semiconductor material is normally measured as current density (current perunit area of cross section, with unit in A/cm2]

Drift current in n-type and p-typesemiconductor

Diffusion current in n-type and p-type semiconductor


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BIASED PN JUNCTION

The pn junction is in forward-bias when +ve terminal of applied voltage is connected to p-

region whileve terminal is connected to n-region. If the polarity is reversed, the pn junctionis in reverse-bias.

In forward-biased pn junction, holes in p-region and electrons in n-region are pushedtowards the depletion layer. The width of the layer becomes narrower. When the appliedvoltage is larger than cut-in voltage (V

), minority carriers in the space-charge region will

diffuse into the respective region, thus creating a current in the pn junction.

In reversed-biased pn junction, holes in p-region and electrons in n-region are attractedtowards the supply terminals. The width of the layer becomes wider and now function as aninsulation layer, preventing diffusion from taking place. Ideally, no current flow in the pnjunction.

The cut-in voltage (orturn-on voltage) is theminimum voltageneeded to turn on thediode, i.e., overcomethe barrier and causecurrent to flow.

BREAKDOWN VOLTAGE IN REVERSE-BIASEDPN JUNCTION

The maximum reverse bias voltage that can beapplied to a pn junction is limited bybreakdown.

When the junction is reverse-biased, the electricfield in the space charge region increases. Ifthe electric field is large enough, covalentbonds will be broken and electron-hole pairswill be generated.

Electrons are then swept into n-region whileholes are swept into p-region by the electricfield, generating large reverse-biased current.The corresponding applied voltage is referredto as breakdown voltage.

There are two mechanisms that can causebreakdownavalanche multiplication(avalanche breakdown) and tunneling ofcarriers (Zener breakdown).


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APPLICATIONS OF PN JUNCTION

A range of devices can be created using the

principles of pn junction.

The first device to be explored is the pn

junction diode, which symbol is shown inFigure below.

ANALOGY OF PN JUNCTION DIODE

A diode can be thought as a directional valve (check valve).

In the forward direction, the diode (check valve) will exhibit a small resistance,

which will be a function of V.

In the reverse direction, the diode resistance is very large and is treated as infinite

(i.e., diode is replaced by an open circuit.


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IV CHARACTERISTICS OF PN-JUNCTION DIODE

In forward bias operation, diode will

not conduct significant current untilthe bias reaches about 0.7V, whichis the diode internal barrier voltage.After that point, forward currentincreases rapidly for a very smallincrease in voltage.

In reverse bias operation, diodeblocks current except for anextremely small leakage current. Thecurrent blocking continues untilsome breakdown voltage isreached, resulting a sudden

increase in reverse current. IS = reverse-bias saturation current (in the range of 10-18 to 10-12).(Actual value depends on doping concentrations and crosssectional of pn junction)

n = ideality factor (in the range between 1 and 2)vD = voltage across diodeVT = 26mV (thermal voltage at room temperature)

DC ANALYSIS & AC ANALYSIS

DC ANALYSIS

The analysis determines the behavior orresponse of a circuit with only DC supply(voltage or current) and no AC supply.

The results of this analysis is generallyreferred to as bias operating points orquiescent point (Q-point).

In DC analysis,

All AC voltage sources are shorted-circuited

All AC current sources are opened-circuited

All large capacitors are opened-circuited

AC ANALYSIS (SMALL-SIGNAL ANALYSIS)

The analysis determines the small-signalresponse of a circuit with only AC supply(voltage or current) and no DC supply.

In AC analysis, non-linear components (diodesand transistors) have to be linearized at the DCoperating point.

In AC analysis,

All DC voltages sources are shorted-circuited

All DC current sources are opened-circuited

All large capacitors are short-circuited

The results from DC and AC analysis need to be summed

together to produce total instantaneous value.


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DIODE DC ANALYSISIDEAL MODEL

An ideal diode will conduct or

turned on when the voltage acrossdiode is greater than zero (forwardbias). Current then flows through thediode. Under forward bias, the idealdiode is modelled as a closed-circuitor short-circuit.

An ideal diode will turned off whenthe voltage across diode is less thanzero (reverse bias). No current flow.Under reverse bias, the ideal diode ismodelled as an opened-circuit.

DIODE DC ANALYSISPWL MODEL

In piecewise linear (PWL) model, the current-voltage characteristics of a real diode isapproximated using two linear segments.

The diode will conduct or turned onwhen the voltage across diode is greaterthan cut-in voltage (V

). Current then flows

through the diode.


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DIODE DC ANALYSISCVD MODEL

Constant voltage drop (CVD) model is similarto PWL model, except that the forward

diode resistance is considered to be 0.Hence, a vertical slope at cut-in voltage (V

).

The diode will conduct or turned onwhen the voltage across diode is greaterthan cut-in voltage (V

). Current then flows

through the diode.

DIODE DC ANALYSISSUMMARY

(CVD

model)


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EXAMPLE 2

Find the values of I and V.

Assume ideal diode model.

EXAMPLE 3

Solve i, using PWL diode model. Let V= 0.7V and rf = 80.


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EXAMPLE 4


Assume CVD diode model, with V= 0.7V.

EXAMPLE 5


Assume CVD diode model, with V= 0.7V.


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DC ANALYSIS (REAL DIODE)ITERATIVESOLUTION

Applying KVL to the circuit yields VS = IDR + VD

Current that flows through resistance, R and diode can bederived using the following equations:

IR = (VSVD) / R --------- Eqn (1)

ID = ISeVD/VT --------- Eqn (2)

Important to notice that current IR equals to current ID.

The iteration starts with an assumption of VD= 0.7V (cut-involtage) in Eqn (1).

IR can then be solved. Since IR = ID, VD in Eqn (2) can then besolved.

If both VD in the two equations do not converge to a singlevalue, the iteration process continues. This time, the newassumed VD is the midpoint between the two values of VD.

This iteration continues until the two VD closely matches witheach other.

DC ANALYSIS (REAL DIODE)GRAPHICALSOLUTION Load line = a linear relationship between ID

and VD for a given voltage supply andresistance, R.

Load line equation can be obtained byderiving the KVL equation from the circuit.

VDD = IDR + VD

The load line must be plotted on the same

graph as the IV characteristics of thediode.

When VD = 0, ID = VDD/R y-intercept

When ID = 0, VD = VDD x-intercept

The intersection of load line and diode IVcurve is the operating point (Quiescentpoint) of the circuit.

(VDQ, IDQ)


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EXAMPLE 6

Design the circuit to provide an output voltage of 2.4V.

Assume all diodes have 0.7V drop at 1mA.

SMALL-SIGNAL ANALYSIS

Small-signal analysis is

performed after dc

analysis is carried out to

determine its operating Q-

point.

At Q-point, the diodes

small-signal resistance, rdcan be determined.


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SMALL-SIGNAL ANALYSISAC MODEL

In small-signal analysis, replaced

the nonlinear diode with

linearized small-signal resistance,

rd in the ac equivalent circuit.

The circuit can then be solved

using KCL and KVL techniques.

EXAMPLE 7

Find vo. Assume CVD diode model, with V = 0.7V.


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OTHER TYPES OF DIODES

OTHER DIODE: PHOTOVOLTAIC CELL

A solar cell is a pn junction device with no directapplied voltage across the junction.

The pn junction has the ability to convert solar energy(photons) into electrical energy (current).

When light hits the space-charge region of the pnjunction, electron-hole pairs are generated. They arethen quickly swept out of the region by the electricfield, thus creating a photocurrent.


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OTHER DIODE: PHOTODIODE

Photodiodes is similar to solar cells except that the pnjunction is operated with reverse-bias voltage.

When light hits the space-charge region of the pnjunction, electron-hole pairs are generated. They arethen quickly swept out of the region by the electricfield, thus creating a photocurrent.

OTHER DIODE: LIGHT-EMITTING DIODE (LED)

LEDs are made from compound semiconductors. Theyconvert current to light.

When the pn junction is forward-biased, electrons andholes flow across the space-charge region andbecome excess minority carriers.

The electron and holes can recombine and a photonor light wave can be emitted.


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OTHER DIODE: LIGHT-EMITTING DIODE (LED) 2

OTHER DIODE: SCHOTTKY BARRIER DIODE

Unlike pn junction, Schottky diode is composed ofmetal made in contact with n-type semiconductor.

The current-voltage characteristics of Schottky diodeis very similar to pn junction diode, but with two majordifferences:

Current is resulted from the flow of majority carriers overthe potential barrier.

The reverse-saturation current IS for a Schottky diode is

larger than that of a pn junction


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OTHER DIODE: ZENER DIODE

For a pn junction, the applied reverse-bias voltage cannot be increased without limit. Atsome point, breakdown will occur and the current will increase rapidly. The voltage at thispoint is called breakdown voltage.

A Zener diode can be designed to have a specific breakdown voltage, |VZ|.

Zener diodes are normally operated with reverse-bias voltage.

SUMMARY A pnjunction diode is turned on or conducting when a forward

bias is applied to the diode. If a reverse bias is applied, the diode isturned off or non-conducting.

The current that flows through the pn junction is due to themovement of minority carriers.

DC analysis on diode circuits can be simplified by modelling the non-linear diode using diode equivalent circuits. Three models werediscussed; ideal, piecewise linear (PWL) and constant voltage drop(CVD).

Graphical and iterative techniques can be applied to determine theoperating point of the nonlinear diode, when both VD and ID areunknown.

If a circuit has both dc and ac supplies, then ac analysis also needto be performed. In ac analysis, the non-linear diode is replaced witha small-signal resistance, rd.


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COURSE OUTCOME

CO1- Understand the characteristics ofdiode, and its DC and AC models andbehavior in relation to circuit analysis.

Chapter 1 - Intro to Diodes.pdf

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