Leo Esaki
The Nobel Prize in Physics 1973
   A NEGATIVE RESISTANCE DEVICE
THE TUNNEL DIODE
NEGATIVE RESISTANCE DEVICE
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 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.
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.).



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.)

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=gs
P 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.
Kenndy,G. and B. Davis,”Electronics Communication system,” Tata McGraw-Hill,4th edition,pp.440-447,1999.
Donald A.neamen,”Semiconductor Physics and Devices,” Tata McGraw-Hill,3rd edition,pp.313-316,2002.
Millman,J. and H.Taub,”Pulse ,Digital and Switching Waveforms,”Tata McGraw-Hill,1991.
Millman,J. and C.C.Halkias,”Integrated Electronics :Analog and Digital Circuits and System,” Tata McGraw-Hill ,1991.
Ananda Kumar,A.,”’Pulse and Digital Circuits,”PHI,2005.
Leo Esaki , ”Tunnel Diode”, U.S. Patent, 4,198,644, Apr 15,1980.
S.Paull,C.A.Cancro and N.M.Garraban,”Low Power Nanosecond Pulse & Logic Circuit Using TD,”NASA.Washington,August 1966.
REFERENCES
THANK-YOU
Queries?

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