TABLE OF CONTENTS Capacitors are frequently used in linear circuits. The need to understand integrated-circuit capacitors, however, by far exceeds the application in linear circuits. The capacitors are inherently present, as parasitic devices, in the structure of any diode or transistor, and it is these capacitors that typically determine high-frequency behavior of the circuit. Also, the principle of operation of field-effect transistors (the most frequently used semiconductor devices) is essentially based on the capacitance effect.
This chapter describes the two most commonly used types of capacitors, the P-N junction capacitor and MOS (metal-oxide-semiconductor) capacitor. Therefore, it introduces the basic concepts of two fundamental semiconductor structures, namely the P-N junction and the MOS structures. It also introduces an additional fundamental equation, the Poisson equation, which models the relationship between the electric potential and the charge density.
The capacitance effect is illustrated in Fig. 2.1. This effect is achieved when two conductive "plates" are placed close to each other, but are still physically separated by a dielectric. The dielectric suppresses any direct current (DC) flow through the capacitor. The electrons arriving from the negative terminal of the battery accumulate at the capacitor plate as they cannot cross over the dielectric to continue the flow toward the positive terminal of the battery. The electric field of these electrons, however, can penetrate through the dielectric repelling the electrons from the other capacitor plate inducing, in effect, positive charge accumulation at that plate. Therefore, if the voltage applied across the capacitor...
