Editing Content :: Admin Scraper

Chapter

Content type

Position

Value

Physics Initially, diffusion current is large and drift current is small. As the diffusion process continues, the space-charge regions on either side of the junction extend, thus increasing the electric field strength and hence drift current. This process continues until the diffusion current equals the drift current. Thus a p-n junction is formed. In a p-n junction under equilibrium there is no net current. The loss of electrons from the n-region and the gain of electron by the p-region causes a difference of potential across the junction of the two regions. The polarity of this potential is such as to oppose further flow of carriers so that a condition of equilibrium exists. Figure 14.11 shows the p-n junction at equilibrium and the potential across the junction. The n-material has lost electrons, and p material has acquired electrons. The n material is thus positive relative to the p FIGURE 14.11 (a) Diode under material. Since this potential tends to prevent the movement of equilibrium (V = 0), (b) Barrier electron from the n region into the p region, it is often called a potential under no bias. barrier potential. . Example 14.3 Can we take one slab of p-type semiconductor and 4 physically join it to another n-type semiconductor to get p-n junction? 1 Solution No! Any slab, howsoever flat, will have roughness much P M larger than the inter-atomic crystal spacing (~2 to 3 Å) and hence X continuous contact at the atomic level will not be possible. The junction E will behave as a discontinuity for the flowing charge carriers. 14.6 S EMICONDUCTOR D IODE A semiconductor diode [Fig. 14.12(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. 14.12(b). The direction of arrow indicates the conventional direction of current (when the diode is under forward p n bias). The equilibrium barrier potential can be altered by applying an external voltage V across the diode. The FIGURE 14.12 (a) Semiconductor diode, situation of p-n junction diode under equilibrium (b) Symbol for p-n junction diode. (without bias) is shown in Fig. 14.11(a) and (b). 14.6.1 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. 14.13(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 334 and p-side.) The direction of the applied voltage (V ) is opposite to the Reprint 2025-26