Search This Blog

Sunday, November 13, 2016

TUNNEL DIODE - DETAILED EXPLANATION



We know that a conventional PN Diode is doped by impurity atoms in the concentration 1 part in 108 . With this order of doping, there exists a potential barrier of order 5 microns which restrains the flow of majority carriers to the side of where they constitute minority carriers.
If the concentration of impurity is greatly increased, (by about 1000 times) in a PN Junction diode then the potential barrier width reduces from order of 5 microns to less than 100 A0  . This thickness is only about one-fiftieth of wavelength of visible light. And thus the devices characteristics are completely changed.

“Esaki” introduced a new device, and it is names as “Tunnel diode” as it uses the phenomenon of “Tunneling”.

“A Tunnel diode is a high conductivity two terminal P-N diode doped heavily about 1000 times higher than a conventional diode”.

The heavy doping produces following effect:
(i)                  Width of Depletion layer is reduced.
(ii)                Reverse breakdown voltage reduces to a very small scale.
(iii)               It produces a negative resistance section on V-I Character





From above fig, Tunnel diode is an excellent conductor in Reverse direction i.e when it is Reverse biased.For as soon as forward bias is applied, significant current is produced. The current quickly reaches its peak value ’Ip’ when the applied forward voltage reaches a value ` Vp’.
At the peak current ‘Ip’ corresponding to voltage ‘Vp’, the slope dI/dV of characteristic is zero.

This current variation in the vicinity of origin is due to quantum mechanical tunneling of electrons through narrow space charge region of the Junction.
When ‘V’ is increased beyond ‘Vp’, the current decreases. As a consequence, the dynamic conductance
g = dI/dV is negative. The current reduces to its minimum value ‘Iv’ (valley current) at ‘Vv” (valley voltage).

Tunnel diode exhibits ‘ Negative resistance characteristic’ between Ip & Iv.

At valley voltage Vv at which I=Iv, The conductance is again zero, and beyond this point the resistance becomes and remains positive.

At the so called peak forward voltage ‘Vf’ the current again reaches the value ‘Ip’ and for larger voltages the current increases beyond this value.

SYMBOL:
Standard circuit symbol for a Tunnel diode is given by
 
 Uses of a Tunnel diode:-
è         For currents whose values are between ‘Iv’ and ‘Ip’, the curve is triple valued, because each current can be obtained at three different applied voltages. It is this multi valued featured which makes Tunnel Diode useful in “Pulse and Digital circuitry”.
       
     It can be used as a very high speed switch. Since Tunneling takes place at speed of light, the transient response is limited only by a shunt capacitance (Junction Plus stray wiring capacitance) and peak driving current switching times of order of  a nano second are reasonable, and times as low as 50 pico sec have been obtained.

       Tunnel diode can be used as a very high frequency (microwave) oscillator.

Manufacturing of Tunnel diodes:
è        The most Common commercially available tunnel diodes are made from “Germanium” or “Gallium Arsenide”. It is difficult to manufacture a silicon tunnel diode with a high ratio of peak to valley current Ip/Iv.
è      
      The below table summarizes important static characteristics of these devices. The voltage values are almost independent of current rating.

Ge
Ga AS
Si
Ip/Iv……
Vp,V….
Vv,V…..
Vf,F…….
8
0.055
0.35
0.50
15
0.15
0.50
1.10
3.5
0.065
0.42
0.70


Note:
  • Gallium Arsenide has highest ratio Ip/Iv and lagest voltage swing Vf-Vp ⩯͌ 1.0 V as against 0.45V for Ge.
  • The peak current ‘Ip’ is determined by impurity concentration (resistivity) and Junction area.
  •  The peak point (Vp, Ip) which is in tunneling region is not a very sensitive function of Temperature.
  • The temperature coefficient of ‘Ip’ may be positive or negative depending upon impurity concentration and operating temperature, but temperature coefficient of ‘Vp’ is always negative.
  • Valley point ‘Vv’ which is affected by injection current is temperature sensitive. ‘Iv’ increases rapidly with temperature.
  •  Tunnel diodes are found to be several orders of magnitudes less sensitive to nuclear radiation than are transistors.

ADVANTAGES OF TUNNEL DIODE
  •   Low cost, Low Noise, simplicity
  •   High speed, environmental immunity, low power
 DISADVANTAGES
  • Low output – voltage swing
  • Fact that is a two terminal device, as there is no isolation between input & output and this leads to serious circuit design difficulty.
  • Transistors are preferred for switching times longer than several nano seconds.