A little later in this book, you’ll learn about semiconductors. These materials, in their many different forms, have revolutionized electrical and electronic circuit design in the past several decades.

These materials can be employed to make capacitors. A semiconductor diode conducts current in one direction, and refuses to conduct in the other direction. When a voltage source is connected across a diode so that it does not conduct, the diode acts as a capacitor. The capacitance varies depending on how much of this reverse voltage is applied to the diode. The greater the reverse voltage, the smaller the capacitance. This makes the diode a variable capacitor. Some diodes are especially manufactured to serve this function. Their capacitances fluctuate rapidly along with pulsating dc. These components are called varactor diodes or simply varactors.

Capacitors can be formed in the semiconductor materials of an integrated circuit (IC) in much the same way. Sometimes, IC diodes are fabricated to serve as varactors. Another way to make a capacitor in an IC is to sandwich an oxide layer into the semiconductor material, between two layers that conduct well. You have probably seen ICs in electronic equipment; almost any personal computer has dozens of them. They look like little boxes with many prongs (Fig. 11-7).

Semiconductor capacitors usually have small values of capacitance. They are physically tiny, and can handle only low voltages. The advantages are miniaturization, and an ability, in the case of the varactor, to change in value at a rapid rate.