A semiconductor diode, discovered in 1957 by Leo Esaki (1925– ), based on the tunnel effect. It consists of a highly doped p−n semiconductor junction, which short circuits with negative bias and has negative resistance over part of its range when forward biased. Its fast speed of operation makes it a useful device in many electronic fields. In a tunnel diode the shape of the characteristic for current and voltage is affected sharply by the tunnel effect, with a large tunnelling current associated with tunnelling from the valence band to the conduction band (see energy bands). As the forward bias on the junction is increased the tunnel effect becomes less and less pronounced.
A p-n junction diode that has extremely high doping levels on each side of the junction, i.e. it is doped into degeneracy. The energy bands are shown in Fig. a for zero applied bias. At such high doping levels tunnelling of electrons (see tunnel effect) across the junction can occur in the forward direction (positive applied voltage on the p-region). As the positive bias is increased the height of the potential barrier at the junction is decreased and the width increased; the diode therefore exhibits a negative resistance portion of the characteristic as the tunnel effect contributes progressively less towards the conductance. A minimum current is reached at the valley point when the tunnel effect ceases and for voltages above this point the diode behaves as a normal p-n junction diode. Tunnelling also occurs in the reverse direction in a similar manner to that in the Zener diode but the effective Zener breakdown voltage can be considered to occur at a small positive value of voltage, termed the peak point. A typical tunnel diode characteristic is shown in Fig. b.
(a) Energy bands of tunnel diode (b) Characteristic of tunnel diode
The backward diode is similar to the tunnel diode but the doping levels are slightly lower so that the semiconductor regions are not quite degenerate (Fig. c). The tunnel effect occurs readily for small values of reverse bias but the negative-resistance portion of the characteristic disappears. The current flowing in the reverse direction is larger than in the forward direction (Fig. d).
(c) Energy bands of backward diode (d) Characteristic of backward diode
The backward diode can be used for rectification of small signals, when the conventional forward-direction positive bias on the p-region becomes the reverse direction for this device. Carrier storage at the junction does not occur and the backward diode therefore has a high speed of response and may be used at microwave frequencies. Very little variation of the current-voltage characteristics occurs with temperature or incident radiation. Unfortunately the high doping levels required for both tunnel diodes and backward diodes make reliable manufacture difficult.
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