Tantalum capacitors have high reliability and excellent efficiency. They are oftenused in military applications because they do not fail often. They can be used in au-dio-frequency and digital circuits in place of aluminum electrolytics.Semiconductor capacitorsA little later in this book, you’ll learn about semiconductors. These materials, in theirmany different forms, have revolutionized electrical and electronic circuit design in thepast several decades.These materials can be employed to make capacitors. A semiconductor diode con-ducts current in one direction, and refuses to conduct in the other direction. When avoltage source is connected across a diode so that it does not conduct, the diode acts asa capacitor. The capacitance varies depending on how much of this reverse voltage isapplied to the diode. The greater the reverse voltage, the smaller the capacitance. Thismakes the diode a variable capacitor. Some diodes are especially manufactured to servethis function. Their capacitances fluctuate rapidly along with pulsating dc. These com-ponents 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 semi-conductor material, between two layers that conduct well.You have probably seen ICs in electronic equipment; almost any personal computerhas dozens of them. They look like little boxes with many prongs (Fig. 11-7).Variable capacitors20711-7An integrated-circuitpackage.Semiconductor capacitors usually have small values of capacitance. They are phys-ically tiny, and can handle only low voltages. The advantages are miniaturization, and anability, in the case of the varactor, to change in value at a rapid rate.Variable capacitorsCapacitors can be varied in value by adjusting the mutual surface area between theplates, or by changing the spacing between the plates. The two most common types ofvariable capacitors (besides varactors) are the air variable and the trimmer. Youmight also encounter coaxial capacitors.