TABLE OF CONTENTS p n junctions, also called p n diodes, are important devices as well as important components of all MOSFET and bipolar devices. The characteristics of p n diodes are therefore important in determining the characteristics of VLSI devices and circuits. A p n diode is formed when one region of a semiconductor substrate is doped n-type and an immediately adjacent region is doped p-type. In practice, a silicon p n diode is usually formed by counterdoping a local region of a larger region of doped silicon. For instance, a region of a p-type silicon substrate or well can be counterdoped with n-type impurities to form the n-type region of a p n diode. The n-type region thus formed has a donor concentration higher than its acceptor concentration.
A doped semiconductor region is called compensated if it contains both donor and acceptor impurities such that neither impurity concentration is negligible compared to the other. For a compensated semiconductor region, it is the net doping concentration, i.e., N d ?N a if it is n-type and N a ? N d if it is p-type, that determines its Fermi level and its mobile carrier concentration. However, for simplicity, we shall derive the characteristics and behavior of p n diodes assuming none of the doped regions are compensated, i.e., the n-sides of the diodes have a net donor concentration of N d and the p-sides have a net acceptor concentration of N a. The resultant equations can be extended...
