A semiconductor diode is composed of a p-n junction with metallic electrical terminals (see Figure 3.8). If a positive voltage Vis applied in the direction shown (from p to n), holes from the p-side and electrons from the n-side penetrate the barrier layer, whereupon the space charges, diffusion voltage and potential difference between the n-zone and p-zone disappear. The numerous majority carriers comprising electrons on the n-side and holes on the p-side inundate the barrier layer, thus allowing for a substantial flow of current that is conducted by the diode.
But if, on the other hand, a negative voltage V is applied, even more n-side electrons and p-side holes will flow out of and away from the boundary layer, whereupon the space charge zone is enlarged, the voltage passing through the barrier layer will exceed diffusion voltage VD by the level of the external voltage source, and the potential difference between the n-zone and p-zone will increase. The inverse current is now very low as a result of the thermally generated minority carriers (n-zone holes and p-zone electrons), which can penetrate the potential barrier; thus the diode forms a barrier.
The characteristic diode curve I — f(V) (Figure 3.9) is roughly the following;
V — voltage applied at the diode (from p to n)
I — current conducted by the diode (— ID in the solar cell equivalent circuit shown in Figure 3.12)
Is — saturation current (idealized inverse current) e — elementary charge — 1.602■ 10-19 As
n — diode quality factor (1 < n < 2; this factor is close to 1 in most cases) k — Boltzmann’s constant — 1.38 ■ 10-23 J/K T — absolute temperature (in K)
Equation 3.2 is particularly suitable for describing the characteristic line in the forward direction (V > 0). In the reverse direction (V < 0) the inverse current of diodes rated for higher voltages is not constant, as is meant to be the case according to Equation 3.2, but increases somewhat with increasing voltage. At very high reverse voltages, reverse current may suddenly increase very much creating a breakdown and the diode is destroyed, unless the current is limited to non-hazardous values by external circuitry.