Consider a thin p-types silicone (p- Si) semiconductor water. By adding precisely a small quantity of penravelent impurity, part of the p- SI wafer can be converted into n- Si. There are several processes by which a semiconductor can be formed. The wafer now contains p- region and n- region and a metallurgical junction between the formation of a p- n- junction.
Two important processes occur during formation of p- n junction: diffusion and drift. We know that in an n- type semiconductor, that compared to the concentration of holes is more than the concentration of electrons. During the formation of p- n junction, and due to the concentration gradient across p-, and n- side, holes diffuse from p- side to n- side (p→n) and electrons diffuse from n- sides to p- side (n⟶p).
This motion of charge carries gives rise to diffusion current across the junction. When an electron diffuses from n⟶ p, it leaves behind an ionised donor on n- side. This ionised (positive charge) is immobile as it is bonded to the surrounding atoms. As e electrons controns continue to diffuse from n⟶ p, a layer of positive charge v(or positive space - charge region) on n- side of the junction is developed. Similarly, when a hole diffuses from p⟶ n due to the concentration gradient, it leaves behind an ionised acceptor (negative change) which is immobile. As the holes continue to diffuse, a layer of negative charge (or negative space - charge region) on the p- side of the junction is developed.
This space- charge region on either side of the junction together is known as depletion region as the electrons and holes taking part in the initial movement across the junction deleted the region of its free charge region on either side of the junction together is together is known as depletion region as the electrons and holes taking part in the initial free charges . This thickness of depletion region is of one n- side of the junction and negative space charge region on n- side of the junction and negative space charge region von p- side of the junction, an electric field directed from positive charge towards negative charge develops. Due to this field, an electron on p - side of the junction moved to p- side.The motion of charge carriers due to the electric field is called drift.Thus a drift current, when is opposite I'm direction to the diffusion current starts.
Initially, diffusion current is large and draft current is small. As the diffusion process continues, the space - charge regions on either side of the junction under equilibrium there is no net current.
The loss of electrons from the n- region and gain of electron by the p- region causes a difference of potential across such as to oppose further flow of carriers so that a condition of equilibrium exists. Shown 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 material. Since this potential tends to prevent the movement of electron from the n region into the p region, if often called a borrier potential.
