WHAT IS MOSFET?
- MOSFET is metal-oxide semiconductor field-effect transistor.
- MOSFET is an important semiconductor device and widely used in many applications. The input impedance of MOSFET is much more than that of JFET because of very small gate leakage current.
- There are two types of MOSFETs – Depletion MOSFET and Enhancement MOSFET.
The following figures show the construction details and symbol of Depletion MOSFET.
Figure 1.Depletion MOSFET
- It consists of conducting bar of N-type material with an insulated gate on the left and p-type region on the right side. Free electrons can flow from source to drain through N-type bar. The p-region is called substrate.
- A thin layer of SiO2 is deposited on left side of the channel, which insulates the gate from the channel. The basic construction of p-channel MOSFET is similar to n-channel MOSFET except that conducting bar is of p-type material and substrate is of n-type material.
WORKING OF DEPLETION MOSFET:
- The working of MOSFET may be explained easily if we visualize the entire structure of the device as a parallel plate capacitor. The negative voltage on the gate induces a positive charge in the channel. Because of these free electrons in the vicinity of positive charge are repelled away in the channel. As a result of this the channel is depleted of free electrons.
- This reduces the no. of free electrons which reduces the drain current passing through the channel. Therefore if larger the value of VGS then smaller the values ID. When the VGS = – VP then ID = 0. On the other hand conductivity of n-channel increases for positive value of VGS as shown in the figure below.
- The figure shows symbol of n-channel and p-channel Enhancement MOSFETs. It consist of a supporting P region called as substrate, which has two broad n+ material regions forming drain (D) and source (S). The n+ region means a region which is heavily doped.
WORKING OF ENHANCEMENT MOSFET:
- This MOSFET is always operated with the positive gate to source voltage VGS. There is no drain current for VGS = 0.
- Therefore Enhancement MOSFET is also called normally off MOSFET. If some positive voltage is applied at the gate it induces a negative charge in p-type substrate just adjacent to SiO2.
- The induced negative charge is produced by attracting the free electrons from the source. When the gate is positive enough it can attract a no. of free electrons.
- This forms a thin layer of electrons, which stretches from source to drain. This effect is equivalent to producing a thin layer of n-type channel in p-type substrate.
- This layer of free electrons is called n-type inversion layer. The minimum gate to source voltage VGS, which produces inversion layer is called threshold voltage VGS(Th). Figure below shows characteristics of n-channel enhancement MOSFET.
Applications of MOSFET
- It is used as buffer amplifier
- In low noise amplifier.
- Large scale integrate circuits for microprocessors, memory devices etc.
- In cascaded amplifiers
- In sample and hold circuits which are used in analog to digital convertors.
Advantages of MOSFET over JFET
- They are easier to manufacture.
- They are smaller in size.
- Capacitive effects are lower and input resistance is higher.
- They can operate in depletion and enhancement mode, while JFETs operate only in depletion mode.