As pointed out in section 1.6, when the impurity concentrations on both the P and N sides of a P—N junction are sufficiently large (> 1020 cm−3), the Fermi levels lie within the appropriate energy bands and the materials are said to be degenerate. This is the situation in the tunnel diode, sometimes called the Esaki diode after its Japanese inventor. In the absence of an applied voltage, the energy-level diagram is as shown in figure 14.1a. When a small forward bias is applied, besides the usual current that arises from carriers climbing the energy barrier at the junction, a current flows owing to a process graphically known as tunnelling. In general, if an energy barrier exists between states of the same energy at different positions, there is always a finite probability of particles traversing the barrier by tunnelling through it rather than climbing over it. The tunnelling process can be understood in terms of quantum theory and the tunnelling probability calculated using wave mechanics. Because of the heavy doping, the depletion region and hence the energy barrier in a tunnel diode is extremely narrow and the tunnelling probability for current carriers is high. Consequently appreciable numbers of electrons can tunnel from the conduction band states on the N side to any vacant states (holes) of the same energy on the P side. The electron tunnel current clearly reaches a maximum when the forward bias matches the levels of the conduction electrons on the N side with those of the holes on the P side as depicted in figure 14.1 b.
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