In this chapter, a brief description of the basic concepts governing the flow of current in a pn junction are discussed. Both intrinsic and extrinsic semiconductors are discussed. The characteristics of depletion and diffusion capacitance are explored through the use of example problems solved with MATLAB. The effect of doping concentration on the breakdown voltage of pn junctions is examined. 10.1 10.1.1 Energy bands INTRINSIC SEMICONDUCTORS According to the planetary model of an isolated atom,