Editing Content :: Admin Scraper

Chapter

Content type

Position

Value

Semiconductor Electronics: Materials, Devices and Simple Circuits 14.5 p-n J UNCTION A p-n junction is the basic building block of many semiconductor devices like diodes, transistor,etc.Aclearunderstandingofthejunctionbehaviour is important to analyse the working of other semiconductor devices. We will now try to understand how a junction is formed and how the junction behaves under the influence of external applied voltage (also called bias). h o p m 14.5.1 p-n junction formation h a Consider a thin p-type silicon (p-Si) semiconductor wafer. By adding y o e n precisely a small quantity of pentavelent impurity, part of the p-Si wafer h n can be converted into n-Si. There are several processes by which a s w semiconductor can be formed. The wafer now contains p-region and c o p i n-region and a metallurgical junction between p-, and n- region. y g Two important processes occur during the formation of a p-n junction: a f diffusion and drift. We know that in an n-type semiconductor, the r p s n concentration of electrons (number of electrons per unit volume) is more . n compared to the concentration of holes. Similarly, in a p-type d t semiconductor, the concentration of holes is more than the concentration h n b d of electrons. During the formation of p-n junction, and due to the e o concentration gradient across p-, and n- sides, holes diffuse from p-side s e to n-side (p ﬁ n) and electrons diffuse from n-side to p-side (n ﬁ p). This i s motion of charge carries gives rise to diffusion current across the junction. n When an electron diffuses from n ﬁ p, it leaves behind an ionised u donor on n-side. This ionised donor (positive charge) is immobile as it is . m bonded to the surrounding atoms. As the electrons continue to diffuse l from n ﬁ p, a layer of positive charge (or positive space-charge region) on n-side of the junction is developed. Similarly, when a hole diffuses from p ﬁ n due to the concentration gradient, it leaves behind an ionised acceptor (negative charge) 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 depleted the region of its free charges (Fig. 14.10). The thickness of depletion region is of the order of one-tenth of a micrometre. Due to the positive space-charge region on n-side of the junction and negative space charge region on 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 moves to n-side and a hole on n-side of the junction moves to p- FIGURE 14.10 p-n junction side. The motion of charge carriers due to the electric field formation process. is called drift. Thus a drift current, which is opposite in 333 direction to the diffusion current (Fig. 14.10) starts. Reprint 2025-26