7.2 Basic Structure of the pn Junction

Fig.7.1（a）schematically shows the pn junction. It is important to realize that the entire semiconductor is a single-crystal material in which one region is doped with acceptor impurity atoms to form the p region and the adjacent region is doped with donor atoms to form the n region. The interface separating the n and p regions is referred to as the metallurgical junction.

The impurity doping concentrations in the p and n regions are shown in Fig.7.1（b）. For simplicity，we will consider a step junction in which the doping concentration is uniform in each region and there is an abrupt change in doping at the metallurgical junction. Initially，at the metallurgical junction，there is a very large density gradient in both the electron and hole concentrations. Majority carrier electrons in the n region will begin diffusing into the p region and majority carrier holes in the p region will begin diffusing into the n region. If we assume there are no external connections to the semiconductor，then this diffusion process cannot continue indefinitely. As electrons diffuse from the n region，positively charged donor atoms are left behind. Similarly，as holes diffuse from the p region，they uncover negatively charged acceptor atoms. The net positive and negative charges in the n and p regions induce an electric field in the region near the metallurgical junction，in the direction from the positive to the negative charge，or from the n to the p region.

The net positively and negatively charged regions are shown in Fig.7.2. These two regions are referred to as the space charge region . Essentially all electrons and holes are swept out of the space charge region by the electric field. Since the space charge region is depleted of any mobile charge，this region is also referred to as the depletion region：these two terms will be used interchangeably. Density gradients still exist in the majority carrier concentrations at each edge of the space charge region. We can think of a density gradient as producing a ‘diffusion force’ that acts on the majority carriers. These diffusion forces，acting on the electrons and holes at the edges of the space charge region，are shown in the figure. The electric field in the space charge region produces another force on the electrons and holes which is in the opposite direction to the diffusion force for each type of particle. In thermal equilibrium，the diffusion force and the E-field force exactly balance each other.