A metal-oxide-semiconductor field-effect transistor (MOSFET) has three terminals, source, gate, and drain. In an n-MOSFET (or p-MOSFET), both the source S and drain D are N-type (or P-type) and the substrate between them is P-type (or N-type). The gate and the P-type substrate is insulated by a thin layer of silicon dioxide (). Due to this insulation, there is no gate current to either the source or drain.
Typically the polarities of the voltages applied to the MOS transistor are such that
The MOS transistor can be used in either analog circuits or as a switch in binary logic circuit:
More acturately, the drain current and the gate voltage can be modeled by
The current is affected by voltage as well as . It can therefore considered as a function of both and plotted below (similar to a bipolar transistor ):
This function can be divided into three different regions:
In the plot of vs , the triode region and the saturation region are separated by the curve of .
Example: Assume .
Example: Assume and , and both MOSFETs in the following circuit are in the saturation region. Find output voltage .
Since both MOSFETs are in saturation region with the same which is determined only by but independent of , their must be the same. The upper MOSFET must have the same as the lower one , i.e., the output voltage has to be .
Comparison between BJT and FET
The BJT and FET can be compared with the old technology of vacuum tube. Although the specific physics of each of these devices is quite different from others, the working principles of these devices are essentially the same. In all three devices, a small AC input voltage (signal) is applied to the input terminal of the device (base, gate, or grid) to control the current that flows through the device (from collector, drain, or plate to emitter, source, or cathode, respectively), causing a much amplified voltage to appear at the output terminal (collector, drain, or plate) of the device.