14.6 SEMICONDUCTOR DIODE

. SEMICONDUCTOR DIODE 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 V0.

As a result, the depletion layer width decreases and the barrier height is reduced [Fig. . (b)]. The effective barrier height under forward bias is (V0 – 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.

Due to the applied voltage, electrons from n-side cross the depletion region and reach p-side (where they are minority carries). Similarly, holes from p-side cross the junction and reach the n-side (where they are minority carries). This process under forward bias is known as minority carrier injection. At the junction boundary, on each side, the minority carrier concentration increases significantly compared to the locations far from the junction.

Due to this concentration gradient, the injected electrons on p-side diffuse from the junction edge of p-side to the other end of p-side. Likewise, the injected holes on n-side diffuse from the junction edge of n-side to the other end of n-side (Fig. . ).

This motion of charged carriers on either side gives rise to current. The total diode forward current is sum of hole diffusion current and conventional current due to electron diffusion. The magnitude of this current is usually in mA. .

. p-n junction diode under reverse bias When an external voltage (V ) is applied across the diode such that n-side is positive and p-side is negative, it is said to be reverse biased [Fig. . (a)].

The applied voltage mostly drops across the depletion region. The direction of applied voltage is same as the direction of barrier potential. As a result, the barrier height increases and the depletion region widens due to the change in the electric field. The effective barrier height under reverse bias is (V0 + V ), [Fig.

. (b)]. This suppresses the flow of electrons from n p and holes from p n. Thus, diffusion current, decreases enormously compared to the diode under forward bias.

The electric field direction of the junction is such that if electrons on p-side or holes on n-side in their random motion come close to the junction, they will be swept to its majority zone. This drift of carriers gives rise to current. The drift current is of the order of a few )A. This is quite low because it is due to the motion of carriers from their minority side to their majority side across the junction.

The drift current is also there under forward bias but it is negligible ()A) when compared with current due to injected carriers which is usually in mA. The diode reverse current is not very much dependent on the applied voltage. Even a small voltage is sufficient to sweep the minority carriers from one side of the junction to the other side of the junction. The current FIGURE .

(a) p-n junction diode under forward bias, (b) Barrier potential ( ) without battery, ( ) Low battery voltage, and ( ) High voltage battery. FIGURE . Forward bias minority carrier injection.