Tunnel Diode

A tunnel diode is a pn junction that exhibits negative resistance between two values of
forward voltage (i.g., between peak-point voltage and valley-point voltage).
A conventional diode exhibits *positive resistance when it is forward biase or
reverse biased. However, if a semiconductor junction diode is heavily dope with
impurities, it exhibits negative resistance (i.e. current decreases as the voltage is
increased) in certain regions in the forward direction. Such a diode is call tunnel
diode.

Theory

The tunnel diode is basically a pn junction with heavy doping of p-type
and p-type semiconductor materials. In fact, it is dope approximately
1000 times as heavily as a conventional diode. This heavy doping results in a large
number of majority carriers.

Because of the large number of carriers, most are not use
during the initial recombination that produces the depletion layer. As a result, the
depletion layer is very narrow. In comparison with conventional diode, the depletion
layer of a tunnel diode is 100 times narrower. The operation of a tunnel diode depends
upon the tunneling effect and hence the name.

Tunneling effect

The heavy doping provides a large number of majority
carriers. Because of the large number of carriers, there is much drift activity in p and n
section. This causes many valence electrons to have their energy levels raised closer to
the conduction region. Therefore, it takes only a very small applied forward voltage to
cause conduction.

The movement of valence electrons from the valence energy band to the
conduction band with little or no applied forward voltage is call tunneling. Valence
electrons seem to tunnel through the forbidden energy band.

As the forward voltage is first increased, The diode current rises rapidly due to
tunneling effect. Soon the tunneling effect is reduce and current flow starts to decrease
as the forward voltage across the diode is increase. The tunnel diode is say to have
entered the negative resistance region. As the voltage is further increase, the tunneling
effect plays less and less part until a valley-point is reach. From now onwards, the
tunnel diode behaves as ordinary diode i.e, diode current increases with the increase in
forward voltage.

V-I Characteristic.

Fig.1(i) shows the V-I characteristic of a typical tunnel
diode.

(i) As the forward voltage across the tunnel diode is increased from zero,
electrons from the n-region “tunnel” through the potential barrier to the p-
region. As the forward voltage increases, the diode current also increases
until the peak-point P is reached. The diode current has now reached
peak current I P (= 2.2 mA) at about peak-point V p (=0.07 V). Until now the
diode has exhibited positive resistance.

(ii) As the voltage is increased beyond Vp the tunneling action starts
decreasing and the diode current decreases as the forward voltage is
increased until valley-point V is reached at valley-point voltage V V (=
0.7V). In the region between peak-point and valley-point (i.e, between
points P and V), the diode exhibits negative resistance i.e, as the forward
bias is increased, the current decreases. This suggests that tunnel diode,

when operated in the negative resistance region. can be used as an
oscillator or a switch.
 If current flowing through a circuit or device increases as the applied voltage is
increased, we say that the circuit or device has positive resistance.

(iii) When forward bias is increased beyond valley-point voltage V V (=0.7 V),
the tunnel diode behaves as a normal diode. In other words, from point V
onwards, the diode current increases with the increase in forward voltage
i.e the diode exhibits positive resistance once again. Fig.1(ii) shows
the symbol of tunnel diode. It may be note that a tunnel diode has a high
reverse current but operation under this condition is not generally used.

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