. S EMICONDUCTOR D IODE A semiconductor diode [Fig. . (a)] is basically a p-n junction with metallic contacts provided at the ends for the application of an external voltage.
It is a two terminal device. A p-n junction diode is symbolically represented as shown in Fig. . (b).
The direction of arrow indicates the conventional direction of current (when the diode is under forward bias). The equilibrium barrier potential can be altered by applying an external voltage V across the diode. The situation of p-n junction diode under equilibrium (without bias) is shown in Fig. .
(a) and (b). . . p-n junction diode under forward bias When an external voltage V is applied across a semiconductor diode such that p-side is connected to the positive terminal of the battery and n-side to the negative terminal [Fig.
. (a)], it is said to be forward biased . The applied voltage mostly drops across the depletion region and the voltage drop across the p-side and n-side of the junction is negligible. (This is because the resistance of the depletion region – a region where there are no charges – is very high compared to the resistance of n-side and p-side.) The direction of the applied voltage ( V ) is opposite to the FIGURE .
(a) Diode under equilibrium (V = ), (b) Barrier potential under no bias. FIGURE . (a) Semiconductor diode, (b) Symbol for p-n junction diode. built-in potential V .
As a result, the depletion layer width decreases and the barrier height is reduced [Fig. . (b)]. The effective barrier height under forward bias is ( V – V ).
If the applied voltage is small, the barrier potential will be reduced only slightly below the equilibrium value, and only a small number of carriers in the material—those that happen to be in the uppermost energy levels—will possess enough energy to cross the junction. So the current will be small. If we increase the applied voltage significantly, the barrier height will be reduced and more number of carriers will have the required energy. Thus the current increases.