Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL

5.3 LOGIC SWITCHES

As described earlier, the logic 1 and logic 0 states represent closed and open switches, respectively. We know that the operation of a transistor is similar to that of a simple switch, so logic circuits can be built using transistors. Generally, metal—oxide semiconductor field-effect transistors (MOSFETs) are used to implement the switches. There are two different types of MOSFETs: n-channel and p-channel. An n-channel MOSFET has four electrical terminals, known as gate (G), drain (D), source (S), and body/substrate (B). The operation of an n-channel MOSFET is as follows. When the gate voltage (V_G) is greater than the threshold voltage (V_TH), a connection is established between the source and the drain and the transistor is said to be turned on and acts as a closed switch. If the gate voltage (V_G) is low, there is no connection between the source and the drain and it acts as an open switch. The symbol and the equivalent circuit for a simple n-channel MOSFET are shown in Figure 5.2.

Figure 5.2 NMOS Transistor as a Switch

Figure 5.3 PMOS Transistor as a Switch

PMOS transistors behave in exactly the opposite way to NMOS transistors. In this case the transistor acts as an open switch for logic 1 and as a closed switch for logic 0. The PMOS transistor also has four ...