3.3 MOS Transistors

Presently, the most popular technology for realizing microcircuits makes use of MOS transistors. Unlike most bipolar-junction transistor (BJT) technologies, which make dominant use of only one type of transistor ( npn transistors in the case of BJT processes ^[4]), MOS circuits normally use two complementary types of transistors n-channel and p-channel. While n-channel devices conduct with a positive gate voltage, p-channel devices conduct with a negative gate voltage. Moreover, electrons are used to conduct current in n-channel transistors, whereas holes are used in p-channel transistors. Microcircuits containing both n-channel and p-channel transistors are called CMOS circuits, for complementary MOS. The acronym MOS stands for metal-oxide semiconductor, which historically denoted the gate, insulator, and channel region materials, respectively. However, most present CMOS technologies now utilize polysilicon gates rather than metal gates.

Before CMOS technology became widely available, most MOS processes made use of only n-channel transistors (NMOS). However, often two different types of n-channel transistors could be realized. One type, enhancement n-channel transistors, is similar to the n-channel transistors realized in CMOS technologies. Enhancement transistors require a positive gate-to-source voltage to conduct current. The other type, depletion transistors, conduct current with a gate-source voltage of 0 V. Depletion transistors were used to create high-impedance loads in NMOS logic gates.

A typical cross section of an n-channel enhancement-type MOS transistor is shown in Fig. 3.12. With no...