9.1 Junctions and Transistors

The p-n junction, located at the juncture between p-type and n-type material, is the basic building block for semiconductor devices. At the transition between n- and p-type materials, a potential gradient is formed to separate the regions with high and low electron and hole concentrations.

A piece of Si with joined n-type and p-type regions, each with 10 ¹⁶ donors or acceptors, is shown in Fig. 9.1. In thermal equilibrium, the Fermi level, E _F, is constant across the transition from n- to p-type materials. The magnitude of the difference between conduction band edges (i.e., the barrier height eV ₀) is given by

(9.1)

Figure 9.1: Schematic of a p n junction with an abrupt change in doping from N _D = 10 ¹⁶ donors cm ^-3 to N _A = 10 ¹⁶ acceptors cm ^-3 for the symmetrical step junction. The energy level and carrier concentration versus distance are shown for the transition from n- to p-type. The shaded area shows the extent of the depletion region for a junction potential of V ₀ volts

For semiconductors at room temperature ( kT ~ 0.025 eV) with 10 ¹⁶ carriers cm ^-3 and densities of states N _c and N _v equal to 10 ¹⁹ cm ^-3, the values of ( E _c - E _f) _n and ( E _f - E _v) _p are...