Question

Describe MOSFIT switching Mechanism, types, characteristics, Applications with neat diagrams. You can collect the details from the Internet.

Answer 1

MOSFET stands for Metal-oxide-semiconductor field-effect transistor. It is a field-effect transistor (FET with an insulated gate) where the voltage at gate determines its conductivity. It is a voltage controlled device and requires only a small input current.

TYPES AND SWITCHING MECHANISM

Two types of MOSFETs are (1) depletion MOSFETs and (2) enhancements MOSFETs

An n-channel depletion-type MOSFET is formed on a p-type silicone substrate as shown in the figure, with two heavily doped n⁺ silicon for very low resistance connections. The gate is isolated n channel by a thin layer of of oxide. The substrate is normally connected to the source and so it is three terminal device i.e., source, gate and drain. If gate-to-source voltage is negative, some of the electrons in the n-channel area are repelled and a depletion region is created below the oxide layer. This results in a narrower effective channel and a high resistance from the drain to source. If the gate-to-source voltage is made high enough (called pinch-off voltage),the channel becomes completely depleted and so no current flows from the drain to source.

An n-channel enhancement-type MOSFET has no physical channel, as shown in the diagram. If gate-to-source voltage is positive, an induced voltage attracts the electrons from the p-substrate and accumulate them at the surface beneath the oxide layer. If gate-to source voltage is greater than or equal to a value known as threshold voltage (V_T), a sufficient number of electrons are accumulated to form a virtual n-channel and the current flows from the drain to source.

CHARACTERISTICS: There are two type of characteristics of MOSFETs - transfer characteristics and output characteristics as shown below. The transconductance, which is the ratio of drain current to gate voltage, defines the transfer characteristics. This is a very important parameter.

There are three regions of operation as shown in the output characteristics.

1. cutoff region (V_GS less than or equal to V_T)

2. pinch-off or saturation region ( V_DS is more than or equal to V_GS - V_T). In this region, the drain current remains almost constant for any increase in the value of drain-to-source voltage and the MOSFETs are used in this region for voltage amplification.

3. linear region ( V_DS is less than or equal to V_GS - V_T). In this region, the drain current varies in proportion to the drain-source voltage.

The pinch-off occurs at V_DS = V_GS - V_T

APPLICATIONS

1. AMPLIFIER: It is excellent for amplifier applications because it has very high input impedance and requires almost no input current to control the load current.

2. SWITCHING: It can be used for switching applications because its switching speed is very high and switching timings are of the order of nanoseconds.  

For the depletion-type MOSFETs. the gate voltage can be either positive or negative. However, the enhancement-type MOSFETs respond to a positive gate voltage only. The power MOSFETs are generally of the enhancement type.

Similarly, there are p-channel depletion-type and enhancement-type MOSFETs.