In the following we will describe the construction, working principle and characteristics of a MOSFET transistor. The JFET transistor is similar in many aspects to the MOSFET transistor.
A MOSFET transistor is usually made on a silicon P substrate by making two separated N regions, thus obtaining two PN junctions. The P region that separates the two N regions is called the channel, and has a layer of SiO2 insulator deposited on it. Metallic contacts are made on the two N regions (the drain and source) and over the insulator deposited over the channel (the gate). The working principle is the following: normally when a voltage is applied between drain and source, there is one (of a total of two) pn junction reverse biased. The transistor does not conduct. When a positive voltage is applied to the region gate-source, it will attract electrons just under the insulator layer creating a conducting layer on the surface of the p semiconductor between the n regions. Thus the transistor will open and begin to conduct. The resistance of the channel can be varied by varying the width of the channel (the gate-source voltage) until a saturation regime is attained.
The I-V characteristic of the MOSFET is made up of a cut-off region (1), when the channel is closed and the transistor does not conduct, a linear region when the transistor is just opened and the conductivity of the channel is proportional to the voltage applied between gate and source (2) and a saturation region, when the channel is widely open (3). The saturation current and voltage increases with increasing the applied voltage between gate and source.
describe construction ,working principle and transfer characteristics of FET transistor and JFET transistor ?