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16.4 TUNNEL DIODES

In a pn diode, the width of the depletion layer varies inversely as the square root of impurity concentration. Thus, the smaller the impurity concentration (doping), the larger the depletion region and vice-versa. The impurity concentration, in a pn junction diode is of the order of 1 in 108. With this impurity concentration, the width of the depletion layer is typically 5 microns (5 × 10−6 m). If on the other hand, if this impurity concentration is increased to 1 in 103, the width of the depletion region may be as small as 0.01 microns (0.01 × 10−6 m) i.e., the depletion layer becomes very thin. Thus, if a pn junction diode is formed with such high impurity concentrations, the barrier becomes very thin. When the depletion layer becomes thin, a large number of electrons may penetrate through this thin barrier. This phenomenon is called ‘tunneling’ and such pn diodes are called ‘tunnel diodes’ or ‘Esaki diodes’. A tunnel diode can be used as a high speed switch as tunneling takes place at the speed of light. A tunnel diode is schematically represented as shown in Fig. 16.18(a).


  

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