Biased pn Junction and Its Types | Semiconductors | Electricity

A biased pn junction is formed by connecting a source of dc potential across the p and n regions. When the positive terminal of a battery is connected to the p-side and negative terminal to the n side, the junction is said to be forward biased. If the terminals of the battery are reversed, i.e., the positive terminal is connected to the n region and the negative terminal to the p region, the junction is said to be reverse-biased.

Forward Biased pn Junction:

Where a pn junction is forward biased with a DC potential V which is less than V_B, the characteristic potential barrier at the junction decreases to the value V_B – V. Also since the applied field opposes the built-in field, the electric field within the transition region decreases by the forward bias. This forces the majority carriers to move towards the junction. Thus the width of the uncovered charges and the width of the height of the potential barrier are reduced as shown in Fig. 7.15.

This also disturbs the balance between the diffusion and drift currents. Some additional holes in the p-region now acquire sufficient thermal energy to overcome the potential barrier and diffuse into the n-region. Similarly, some additional electrons diffuse from the n side to the p side. Such movement of majority carriers constitutes an additional diffusion current.

The holes diffusing into the n-region behave as minority carriers and constitute an injected minority hole current which decays exponentially with distance from the junction owing to recombination of holes with electrons of the n region.

Similarly, the electrons diffusing into the p-region constitute the injected minority electron current which also decays exponentially with distance. The resultant current at any distance from the junction is equal to the electron current and hole current and remains constant.

The drift current caused by the movement of minority carriers is, however, not much affected by the change in the height of the potential barrier. This is because the drift current is limited by the number of minority carriers present in the two regions and not by the size of the barrier existing at the junction.

Therefore, as long as the temperature remains constant, the electron and hole drift currents at junction are independent of the applied voltage. It is thus apparent that under forward bias, the diffusion current exceeds the drift current and a current flow through the junction in the direction of the former.

The change in the electrostatic potential at the junction also has a direct effect on the separation of energy bands as the energy is given by the product of electronic charge and the height of the potential barrier. Thus the band separation decreases to e(V_B – V) under forward bias.

The Fermi levels on either side of the junction are, therefore, shifted with respect to each other by an energy in electron volts which is equal to eV. The effects of forward bias on the transition region width, electrostatic potential and energy diagram of a pn junction are shown in Fig. 7.15 (a).

It is thus apparent that the forward bias acts to reduce the effective barrier potential. As V exceeds V_B, the effect of barrier potential is completely eliminated and the pn junction, like a normal conductor, offers little resistance to the flow of current.

Reverse Biased pn Junction:

A reverse biased pn junction is shown in Fig. 7.15 (b). In this case, the applied voltage is in the same direction as the intrinsic electrostatic potential barrier and hence increases the height of the barrier potential. This increases the built-in field which forces the majority carriers to move further away from the junction, thereby widening the depletion layer.

The balance between the diffusion and drift currents is disturbed again. The diffusion current falls drastically and becomes almost negligible due to the inability of the majority carriers to surmount the large potential barrier. The drift current, as in the previous case, is almost unaffected for the same reasons. Therefore, the drift current predominates over the diffusion current and small reverse current flows in the direction of the applied voltage.

Apparently, this current arises due to the movement of minority carriers which are generated within and near the depletion region by the thermal effects. This current is known as the reverse saturation current. It is almost independent of the applied reverse bias, but increases with temperature due to increase in the minority carrier concentration.

The reverse bias also increases the band separation by an account equal to eV. The changes in the depletion width, the barrier potential and the energy band diagram brought about by reverse biasing a pn junction are depicted in Fig. 7.15 (b). The symbolic representation of a forward biased and reverse-biased pn junctions are given in Fig. 7.17.

Since a pn junction allows a large current to flow through it on forward biasing and a negligibly small reverse current on reverse biasing, it behaves like a vacuum diode and thus can be used as a rectifier. Hence pn junction is also referred to as semiconductor diode.