Due to the insulation between gate and source terminals, the input resistance of MOSFET may be very high such (usually in the order of 1014 ohms. In these transistors, the gate terminal is electrically insulated from the current carrying channel so that it is also called as Insulated Gate FET (IG-FET). The Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is one type of FET transistor.

Metal Oxide Semiconductor Field Effect Transistor or MOSFET.

Junction Field Effect Transistor or JFET.

In case of a FET, the voltage at the Gate (a terminal in FET equivalent to Base in BJT) determines the current flow between the other two terminals. The other and very important difference is that a BJT is essentially a current controlled device meaning the current at the base of the transistor determines the amount of current flowing between collector and emitter. The first difference is that in BJT, both the majority and minority charge carriers are responsible for current conduction whereas in FETs, only the majority charge carriers are involved. There are two main differences between BJT and FET. To learn more about a basics of transistor and its history, read the Introduction to Transistors tutorial. Transistors come a variety of shapes, sizes and designs but essentially, all transistors fall under two major families. They are semiconductor devices that act as either an electrically controlled switch or a signal amplifier. O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.Transistors, the invention that changed the World. Get Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL now with the O’Reilly learning platform. The resulting truth table for this circuit is also shown in Figure 5.5, which is the same.

On the other hand, if any one of the inputs or both inputs is low (i.e., logic 0), one or both transistors will be turned off and act as an open switch, and thus the output will remain high (i.e., logic 1). Consider the case when both inputs are high (i.e., logic 1) and NMOS transistors T 1 and T 2 are both turned, pulling the output node down to ground, resulting in logic 0 as output. Now observe the circuit diagram shown in Figure 5.5. The truth table is also shown in Figure 5.4.įigure 5.4 NMOS Inverter Gate and Its Truth Table 5.4.2 NMOS NAND Gate It will be replaced with a PMOS transistor in later circuit design. In other words, this resistor acts as a current source load. The purpose of resistor R is to limit the current when the NMOS transistor is turned on. The circuit in Figure 5.4 acts as an inverter gate.

However, if V G = V DD (logic 1), the NMOS switch is closed and the NMOS transistor T 1 starts conducting, thereby pulling down the output node to ground. The output voltage V out is equal to V DD (logic 1). When V G = 0V (logic 0), the NMOS transistor T 1 is off and no current flows through resistor R. The operation of the circuit can be explained as follows. 5.4 NMOS AND PMOS LOGIC GATES 5.4.1 NMOS InverterĬonsider the circuit shown in Figure 5.4.

Nmos transistor diagram