Tunnel Diode (Esaki Diode )

Tunnel Diodes

(Esaki Diode)

Tunnel diode is the p-n junction

device that exhibits negative

resistance. That means when the

voltage is increased the current through it decreases.

1

Part I Tunnel Diode principles

Concept of Electron Tunneling

Before contact After contact

E CSiO

2

E CSiO

2

E CSi E CSi E CSi E CSi

E VSi

E VSiO

E VSi

2

E VSi

E VSiO

E VSi

2

Si SiO2 Si Si SiO2 Si

2

…continued…Concept of Electron Tunneling

•

•

For thick barrier, both Newtonian and Quantum mechanics say that the

electrons cannot cross the barrier. It can only pass the barrier if it has more energy than the barrier height.

Electron with energy greater than

EB can pass over the barrier

E=EB

Electron with energy less than

EB cannot pass the barrier

E=0

Si SiO2 Si 3

…continued…Concept of Electron Tunneling

•

•

For thin barrier, Newtonian mechanics still says that the electrons cannot cross the barrier. However, Quantum mechanics says that the electron wave nature will allow

it to tunnel through the barrier.

Tunneling is caused by

the wave nature of electron

E=EB E=EB

E=0

Si SiO2 Si Si SiO2 Si

4

Newtonian Mechanics Quantum Mechanics

Electron Tunneling in p-n junction

•

•

•

When the p and n region are highly doped, the depletion region becomes very thin

(~10nm). In such case, there is a finite probability that electrons can tunnel from the conduction

band of n-region to the valence band of p-region During the tunneling the particle ENERGY DOES NOT CHANGE

High doping

Thick depletion layer Thin depletion layer

EC EC

EV EV

Electrons tunnel through the thin barrier

p n p n 5

Tunnel Diode Operation

• When the semiconductor is very highly doped (the doping is greater than No) the

Fermi level goes above the conduction band for n-type and below valence band for p- type material. These are called degenerate materials.

Under Forward Bias

Step 1: At zero bias there is no current flow

EC

EF

EV

6

…continued…Operation of a Tunnel Diode

Step 2: A small forward bias is applied. Potential barrier is still very high –

no noticeable injection and forward current through the junction.

However, electrons in the conduction band of the n region will tunnel to the

empty states of the valence band in p region. This will create a forward bias

tunnel current

EC

EV

Direct tunneling current starts growing 7

…continued…Tunnel Diode Operation

Step 3: With a larger voltage the energy of the majority of electrons in the

n-region is equal to that of the empty states (holes) in the valence band of p-region; this will produce maximum tunneling current

EC

EV

Maximum Direct tunneling current 8


Step 4: As the forward bias continues to increase, the number of electrons

in the n side that are directly opposite to the empty states in the valence

band (in terms of their energy) decrease. Therefore decrease in the

tunneling current will start.

EC

EV

Direct tunneling current decreases 9


Step 5: As more forward voltage is applied, the tunneling current drops to

zero. But the regular diode forward current due to electron – hole injection

increases due to lower potential barrier.

EC

EV

No tunneling current; diffusion current starts growing 10


Step 6: With further voltage increase, the tunnel diode I-V characteristic is similar to that of a regular p-n diode.

EC

EV

11


Under Reverse Bias

In this case the, electrons in the valence band of the p side tunnel directly towards the empty states present in the conduction band of the

n side creating large tunneling current which increases with the

application of reverse voltage. The TD reverse I-V is similar to the Zener diode with nearly zero

breakdown voltage.

EC

EV

12

Part II Circuits with the Tunnel Diodes

I R

TD

NDR region V

Typical Tunnel Diode (TD) I-V characteristic has two distinct features:

(1) it is STRONGLY non-linear (compare to the resistor I-V).

Current - Voltage relationships for TDs cannot be described using the Ohm’s law

(2) it has a negative differential resistance (NDR) region

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Tunnel Diode I-V

• The total current I in a tunnel diode is given by peak

valley

I = I tun + I diode + I excess

• The p-n junction current,

Ip

I diode

⎡⎛ V

≈ I s exp ⎢⎜

⎜ ηV

⎣⎝ th

⎞ ⎤ ⎟ − 1⎥ ⎟

⎠ ⎦

Iv

Vv

Vp

Is saturation current, η is the

ideality factor and Vth = kT/q

14

Tunnel Diode I-V

• The tunnel current,

I tun

⎡ ⎛ V ⎞m ⎤V

=exp ⎢− ⎜ ⎟ ⎥ ⎜V ⎟

R0

⎢ ⎝ 0⎠ ⎥ ⎣⎦

Ip

Iv

peak

valley

Typically, m = 1….3; V0 = 0.1….0.5 V

R0 is the TD resistance in the ohmic region

The maximum |NDR| can be found as

| Rd max

⎛1+ m ⎞

exp ⎜⎟

⎝ m ⎠

| = R0

m

Vv

Vp

The peak voltage Vp:

1⎞ m⎛

V p = ⎜ ⎟ V0

⎝m⎠

1

15

Tunnel Diode I-V

• The excess current, peak

I excess

⎡⎛ V − Vv ⎞⎤ V

⎟⎥=exp ⎢⎜

⎜ V ⎟

Rvex

⎠⎦⎣⎝

valley

Ip

Iv

Vv

Vp

Iexcess is an additional tunneling

current related to parasitic

tunneling via impurities.

This current usually determines the

minimum (valley) current, Iv

Rv and Vex are the empirical parameters; in high-quality diodes, Rv >> R0. Vex = 1…..5 V

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NDR of the Tunnel Diode

Tunnel Diode differential resistance is

NEGATIVE in the voltage range 100 mV – 200 mV

17

Energy dissipation in resistors and Energy

generation in Negative Resistors

R

+

- VS

Power = Voltage x Current = I2 R

If current direction is from “-” toward “+”, then R =V/I is negative;

For R<0, P <0,

Positive power means energy dissipation (e.g. conversion into the

Joule heat);

Negative power corresponds to the power GENERATION (Energy

supply);

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Differential resistance and negative differential resistance

Static resistance: R = V/I

I R

I

∂ V ΔV

Differential resistance: Rd =≈

∂ I ΔI

I R ΔI

Rd = cot (α )

α

ΔV V V

V

For linear (“Ohmic”) components, R = Rd.

For many semiconductor devices, R ≠ Rd:

I α

I

Rd < R

V

I α

I Rd << R

I

α2

α1

Rd2 < 0

TD

α3

V

Diode (forward bias)

V

Zener Diode

(reverse bias)

V V

19

Transients in Negative Differential Resistance

Circuits

R

VS

After turning the switch ON:

C

VS -t/(RC) i (t ) = ×e R

i

R>0

R<0

t t

20

i

Tunnel Diode as a microwave oscillator

Tunnel diode Cd Microwave cavity

(LC- resonance circuit)

~ Rd

us

RL

R

Load resistance is chosen so that RL < |Rd | in the NDR region

At the TD operating point, the total circuit differential resistance is negative

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Tunnel Diode as a microwave oscillator

Transient in resonant cavity after turning the bias voltage ON

1

Cd 0.8

0.6

0.4

0.2

~ Rd

us

RL

R

LC 0

-0.2

-0.4

-0.6

0 1 2 3 4 5 6 7 8 9

The resonant circuit with NDR can oscillate. Maximum frequency of the TD-oscillator is

limited by the

characteristic tunneling

time:

-0.8

-1

Rd >0 or Rd<0 and RL > |Rd|

5

4

3

2

1

0

-1 0

-2

-3

-4

-5

-6

1 2 3 4 5 6 7 8 9 fMAX ≤ (1/2π) (1/τtun)

Tunneling time in TDs is

extremely small: << 1 ps FMAX > 100 GHz Rd<0 and RL < |Rd|

22

Tunnel Diode microwave oscillators

After: M. Reddy et.al,

IEEE ELECTRON DEVICE LETTERS, VOL. 18, NO. 5, MAY 1997

~ 600 GHz oscillation frequencies has

been achieved.

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Nonlinear Circuit Analysis: Load Line technique

Vs = Vd + IR

Vd Vs

⇒I=−+

R R

Vd

⎛ Vs ⎞⎛ 1⎞

I = ⎜ − ⎟Vd + ⎜ ⎟

⎝ R⎠⎝R⎠

Vs R

y = mx + c

X − axis intercept,Vs

y ⎛Vs

⎜

⎝

⎞

R⎟

⎠

slope = − 1

R

Y − axis intercept, c =

Slope, m = − 1

Vs

R Vs

x

24 R

Nonlinear Circuit Analysis: Load Line technique

Vs = Vd + I × R

⎛ Vs ⎞⎛ 1⎞

I = ⎜ − ⎟Vd + ⎜ ⎟

⎝ R⎠⎝R⎠

In the load line equation, I is the resistor current when the

voltage across the diode is Vd

On the other hand, when the voltage

across the diode is Vd, the diode

current is given by the diode I-V curve

For example, when the diode voltage

is Vd1 the diode current is Id1

However, in this circuit, Id must be equal IR. Hence the actual operating point is given

by the load line – I-V intercept.

I

Vd

Vs R

⎛ Vs ⎞

⎜ R⎟

⎝⎠

slope = − 1

R

Diode

I-V

Id1

Vd1

Vs

V

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Load Line : example

Vd

Vs

2V

Id=2.4 mA

Id=2.4 mA

V axis intercept, Vs = 2 V

I axis intercept, (Vs/R) = 4 mA

Vd=0.78V R

500Ω

Vd=0.78V

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Load Line : another example

Vd

Vs

2.5V

Id=1.4 mA

Vd=0.76V R

1250Ω

Id=1.4 mA

V axis intercept, Vs = 2.5 V

I axis intercept, (Vs/R) = 2 mA

Vd=0.76V

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…continued… Load Line (Variation of R)

Vd

Vs

2.0V

R

R= 500Ω R= 750Ω R= 1000Ω

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…continued… Load Line (Variation of Vs)

Vd

Vs R

1000Ω

Vs= 1V Vs=2V Vs=3V

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Circuit with the Tunnel Diode and Resistor

8

6

4

2 4

1

I, mA

3

2

TD

V, V

Vs

Vd

R

0.1 0.2 0.3 0.4 0.5 0.6 0.7

Example 1: Vs = 0.7 V; R = 100 Ω; ⇒ Ιmax = 0.7V/100 Ω = 7 mA

The circuit has three possible operating points. Point 2 is typically unstable

(depending on parasitic L and C components. The circuit will operate at the point 1 or point 3 depending on the history.

Example 2: Vs = 0.3 V; R ≈ 10 Ω; ⇒ Ιmax ≈ 30 mA

The circuit has only one operating point - point 4. The total differential resistance is NEGATIVE (because R < |Rd|). Depending on the L and C components, the circuit can be stable (amplifier) or unstable (oscillator)

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insulator–metal (MIM) diode, but present application

appears restricted to research environments due to in- herent sensitivities.* [7] There is also a metal–insulator–

insulator–metal MIIM diode which has an additional insulator layer. The additional insulator layer allows "step tunneling" for precise diode control.* [8]

I

i1

rdiff < 0

i

v v V

1 Forward bias operation Under normal forward bias operation, as voltage begins

to increase, electrons at first tunnel through the very nar-2

row p–n junction barrier because filled electron states in

the conduction band on the n-side become aligned with

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empty valence band hole states on the p-side of the p- n junction. As voltage increases further these states become more misaligned and the current drops – this is

called negative resistance because current decreases with IV curve similar to a tunnel diode characteristic curve. It has

increasing voltage. As voltage increases yet further, the negative resistance in the shaded voltage region, between v1 and

diode begins to operate as a normal diode, where elec- v2 . trons travel by conduction across the p–n junction, and

no longer by tunneling through the p–n junction barrier. The most important operating region for a tunnel diode is

the negative resistance region.

2 Reverse bias operation

Main article: Backward diode

When used in the reverse direction, tunnel diodes are

called back diodes (or backward diodes) and can act as

fast rectifiers with zero offset voltage and extreme linear- ity for power signals (they have an accurate square law

characteristic in the reverse direction). Under reverse

bias, filled states on the p-side become increasingly

aligned with empty states on the n-side and electrons now

tunnel through the pn junction barrier in reverse direction.

I-V curve of 10mA germanium tunnel diode, taken on a Tek- tronix model 571 curve tracer.

The tunnel diode showed great promise as an oscillator and high-frequency threshold (trigger) device since

it operated at frequencies far greater than the tetrode

could, well into the microwave bands. Applications for tunnel diodes included local oscillators for UHF televi- sion tuners, trigger circuits in oscilloscopes, high-speed

counter circuits, and very fast-rise time pulse generator circuits. The tunnel diode can also be used as low-noise

microwave amplifier.* [9] However, since its discovery, more conventional semiconductor devices have surpassed

its performance using conventional oscillator techniques. For many purposes, a three-terminal device, such as a

field-effect transistor, is more flexible than a device with

only two terminals. Practical tunnel diodes operate at a

few milliamperes and a few tenths of a volt, making them

low-power devices.* [10] The Gunn diode has similar high

Tunnel Diode (Esaki Diode )

Devices & Hardware

Transcript of Tunnel Diode (Esaki Diode )