III.8. THE TUNNEL DIODE
1. Theory The Japanese physicist Leo Esaki invented the tunnel diode in 1958. It consists of a p-n junction with highly doped regions. Because of the thinness of the junction, the electrons can pass through the potential barrier of the dam layer at a suitable polarization, reaching the energy states on the other sides of the junction. The current-voltage characteristic of the diode is represented in Figure 1. In this sketch i p and U p are the peak, and iv and
U v are the valley values for the current and voltage respectively. The form of this dependence can be qualitatively explained by considering the tunneling processes that take place in a thin p-n junction.
For the degenerated semiconductors, the energy band diagram at thermal equilibrium is presented in Figure 2. In Figure 3 the tunneling processes in different points of the currentvoltage characteristic for the tunnel diode are presented.
c) Figure 3.
In Fig. 3a, the thermal equilibrium situation corresponding to point 1 from the Fig. 1 diagram presented; in this case the electrons will uniformly tunnel in both directions, so the current will be null. At a direct polarization, a non-zero electron flow will tunnel from the occupied states of the conduction band of the n region to the empty states of the valence band from the p region. The current attains a maximum when the overlap of the empty and occupied states reaches the maximum value; a minimum value is reached when there are no states for tunneling on the sides of the barrier. In this case, the tunnel current should drop to zero, but thanks to the tunneling through the local levels from the semiconductor forbidden band, a finite 175
current, called valley current, will exist. For U > U v , a thermal current passes through the diode, which is the normal p-n junction current. 2. Experimental determination of the I –U characteristic Theoretical determination of...
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