Tunnel Diode

Leo EsakiThe Nobel Prize in Physics 1973

A NEGATIVE RESISTANCE DEVICE

NEGATIVE RESISTANCE NEGATIVE RESISTANCE DEVICEDEVICE

• It is a device which exhibits a negative incremental resistance over a limited range of V-I characteristic.

• It is of two types :- 1. Current controllable type : V-I curve is a

multi valued function of voltage and single valued function of current .eg:- UJT, p-n-p-n diode

2. Voltage controllable type : V-I curve is a multi valued function of current and single valued function of voltage. eg:- SCS, Tunnel diode -

TUNNEL DIODE (Esaki Diode)• It was introduced by Leo Esaki in 1958.• Heavily-doped p-n junction

– Impurity concentration is 1 part in 10^3 as compared to 1 part in 10^8 in p-n junction diode

• Width of the depletion layer is very small(about 100 A).

• It is generally made up of Ge and GaAs.• It shows tunneling phenomenon.• Circuit symbol of tunnel diode is :

EV

WHAT IS TUNNELINGWHAT IS TUNNELING• Classically, carrier must have energy at least

equal to potential-barrier height to cross the junction .

• But according to Quantum mechanics there is finite probability that it can penetrate through the barrier for a thin width.

• This phenomenon is

called tunneling and

hence the Esaki Diode

is know as

Tunnel Diode.

- Ve Resistance Region

VfVp

Ip

Vv

Forward VoltageReverse voltage

Iv

Re

ve

rse

Cu

rre

nt

Fo

rwa

rd C

urr

en

t

Ip:- Peak Current

Iv :- Valley Current

Vp:- Peak Voltage

Vv:- Valley Voltage

Vf:- Peak Forward

Voltage

CHARACTERISTIC OF TUNNEL DIODE

Reference:-

Dan Wheeler ,”Advanced Semiconductor Devices” . Chapter –”Tunneling Devices”.( Microsoft ppt.)

ENERGY BAND DIAGRAM

Energy-band diagram of pn junction in thermal equilibrium in which both the n and p region are degenerately doped.

Reference:-D.A.Neamen,”Semiconductor Physics and Devices,”TataMcGraw-Hill,3rd edition,2002 ( Microsoft ppt.).

-Zero current on the I-V diagram;

-All energy states are filled below EF on both sides of the junction;

AT ZERO BIAS

Simplified energy-band diagram and I-V characteristics of the tunnel diode at zero bias.

Reference:- D.A.Neamen,”Semiconductor Physics and Devices,”TataMcGraw-Hill,3rd edition,2002 ( Microsoft ppt.).

-Electrons in the conduction band of the n region are directly opposite to the empty states in the valence band of the p region.

-So a finite probability that some electrons tunnel directly into the empty states resulting in forward-bias tunneling current.

AT SMALL FORWARD VOLTAGE

Simplified energy-band diagram and I-V characteristics of the tunnel diode at a slight forward bias.

Reference:-D.A.Neamen,”Semiconductor Physics and Devices,”TataMcGraw-Hill,3rd edition,2002 ( Microsoft ppt.).

-The maximum number of electrons in the n region are opposite to the maximum number of empty states in the p region.

- Hence tunneling current is maximum.

AT MAXIMUM TUNNELING CURENT

Simplified energy-band diagraam and I-V characteristics of the tunnel diode at a forward bias producing maximum tunneling current.

Reference:-D.A.Neamen,”Semiconductor Physics and Devices,”TataMcGraw-Hill,3rd edition,2002 ( Microsoft ppt.).

-The forward-bias voltage increases so the number of electrons on the n side, directly opposite empty states on the p side decreases.

- Hence the tunneling current decreases.

AT DECREASING CURRENT REGION

Simplified energy-band diagram and I-V characteristics of the tunnel diode at a higher forward bias producing less tunneling current.

Reference:- D.A.Neamen,”Semiconductor Physics and Devices,”TataMcGraw-Hill,3rd edition,2002 ( Microsoft ppt.).

-No electrons on the n side are directly opposite to the empty states on the p side.

- The tunneling current is zero.

-The normal ideal diffusion current exists in the device.

AT HIGHER FORWARD VOLTAGE

Simplified energy-band diagram and I-V characteristics of the tunnel diode at a forward bias for which the diffusion current dominates.

Reference:-D.A.neamen,”Semiconductor Physics and Devices,”TataMcGraw-Hill,3rd edition,2002( Microsoft ppt.).

- Electrons in the valence band on the p side are directly opposite to empty states in the conduction band on the n side.

-Electrons tunnel directly from the p region into the n region.

- The reverse-bias current increases monotonically and rapidly with reverse-bias voltage.

AT REVERSE BIAS VOLTAGE

Reference:-D.A.Neamen.”Semiconductor Physics and Devices,”TataMcGraw-Hill,3rd edition,2002 (Microsoft ppt.).

Cj -R

rs

Ls

TUNNEL DIODE EQUIVALENT CIRCUIT

•This is the equivalent circuit of tunnel diode when biased in negative resistance region.

•At higher frequencies the series R and L can be ignored.

•Hence equivalent circuit can be reduced to parallel combination of junction capacitance and negative resistance.

Reference:- Kenndy,G. and B. Davis,”Electronics Communication system,” Tata McGraw-Hill,4th edition,pp.440-447,1999

MONOSTABLE OPERATION OF TUNNEL DIODE

Here the Load line cuts the V-I characteristic at a single point in the positive resistance region. It has one stable operating point.

Reference :- Don Arney ,”Diode and Diode circuits”, Chapter -9(”Negative Differential Resistance”Microsoft ppt.)

ASTABLE OPERATION OF TUNNEL DIODE

Here the Load line cuts the V-I characteristic at a single point in the negative resistance region. This point is unstable.

Reference:- Don Arney ,”Diode and Diode circuits”, Chapter -9(”Negative Differential Resistance”, Microsoft ppt.)

BISTABLE OPERATION OF TUNNEL DIODE

Here the Load line cuts the V-I characteristic at a two points in the positive resistance regions. It has two stable operating points.

Reference:-Don Arney ,”Diode and Diode circuits”, Chapter -9(”Negative Differential Resistance”,Microsoft ppt.)

is gs gl -g

NEGATIVE RESISATANCE AMPLIFIER

•With diode the load voltage VL= is /(gs-g+gl)

•Power delivered to load is PL=glVL^2=gl.is^2/(gs-g+gl)^2

Ap = PL/Pmax = 4gs.gl/(gs-g+gl)^2

•For maximum power transfer gl=gs.

Ap=4.gl^2/(2gl-g)^2

=4.gl^2/(4gl^2 – 4gl.g + g ^2 )

=4.gl^2/(4gl^2 + g(g – 4.gl))

•If Cj is tuned out then – g will represent the tunnel diode.In absence of diode the maximum power will be gl=gsP max=is^2/4gs

Reference:- Kenndy,G. and B. Davis,”Electronics Communication system,” Tata McGraw-Hill,4th edition,pp.440-447,1999.

•A tunnel diode, biased at the center point of the negative-resistance range and coupled to a tuned circuit or cavity, produces a very stable oscillator.

• The oscillation frequency is the same as the tuned circuit or cavity frequency.

•Microwave tunnel-diode oscillators are useful in applications that require microwatts or a few milliwatts of power, such as local oscillators for microwave.

TUNNEL DIODE OSCILLATOR

REFERENCE:-

Kenndy,G. and B. Davis,”Electronics Communication system,” Tata McGraw-Hill,4th edition,pp.440-447,1999.

TUNNEL DIODE AS AMPLIFIER

•The low-noise generation, gain ratios of up to 30 dB, high reliability, and light weight make these amplifiers ideal for use as the first stage of amplification in communications and radar receivers.

•The tunnel diode is biased to the center point of its negative-resistance region, but a CIRCULATOR replaces the tuned cavity

REFERENCE:-

Kenndy,G. and B. Davis,”Electronics Communication system,” Tata McGraw-Hill,4th edition,pp.440-447,1999.

**OTHER APPLICATIONS**

• Used in high speed switching circuit.

• Used as pulse generator.

• Used for storage of binary information.

• Used for the construction of shift register.

• Sensor modulator for telemetry of temperature in human beings and animals.

• Used in electron tunneling microscope.

ADVANTAGES OF TUNNEL DIODE

•Relatively resistant to nuclear radiation.

•Useful for high speed operation.

•Useful for high frequency operation.

•Low power is consumed.

1. Kenndy,G. and B. Davis,”Electronics Communication system,” Tata McGraw-Hill,4th edition,pp.440-447,1999.

2. Donald A.neamen,”Semiconductor Physics and Devices,” Tata McGraw-Hill,3rd edition,pp.313-316,2002.

3. Millman,J. and H.Taub,”Pulse ,Digital and Switching Waveforms,”Tata McGraw-Hill,1991.

4. Millman,J. and C.C.Halkias,”Integrated Electronics :Analog and Digital Circuits and System,” Tata McGraw-Hill ,1991.

5. Ananda Kumar,A.,”’Pulse and Digital Circuits,”PHI,2005.6. Leo Esaki , ”Tunnel Diode”, U.S. Patent, 4,198,644, Apr

15,1980.7. S.Paull,C.A.Cancro and N.M.Garraban,”Low Power

Nanosecond Pulse & Logic Circuit Using TD,”NASA.Washington,August 1966.

REFERENCES