What is MOSFET?
MOSFET stands for metal oxide semiconductor FET may be a semiconductor unit which is widely used for switching and amplifying electronic signals within the electronic devices.it also can be named as Insulated gate FET (IGFET), because the gate terminal is separated by an insulating layer of the silicon die oxide (SiO2) from the substrate. MOSFETS are broken further into depletion type and enhancement type. The term depletion and enhancement define their basic mode of operation.
From the graphic symbols, the lack of a direct connection between the gate and channel is represented by a space between the gate and other terminals of the symbol. The vertical line representing the channel is connected between the drain and source and is supported by a substrate.
Here a slab of p-type/n-type material (p-type in case of n channel and vice-versa) is formed from a silicon base and is referred to as the substrate. It is the foundation on which the device will be constructed. The source and drain terminals are connected through metallic contact to n doped regions linked by an n channel. The gate is also connected to a metal contact surface but remains insulated from n channel by a very thin silicon die oxide layer.
There is no direct electrical connection between the gate terminal and therefore the channel of a MOSFET.
There’s no possibility of current injection from the gate terminal either since the gate oxide may be an excellent insulator. However, the appliance of a positive voltage at the gate terminal with reference to the source will covert the silicon surface beneath the gate oxide into an n-type layer or “channel”, thus connecting the Source to the Drain. The gate region of a MOSFET consists of gate metallization, the gate (silicon) oxide layer, and therefore the p-body silicon forms a high-quality capacitor. When a little voltage is an application to the present capacitor structure with gate terminal positive with reference to the source (note that body and source are shorted) a depletion region forms at the interface between the SiO2 and therefore the silicon.
The charge induced on the gate metallization repels the bulk hole carriers from the interface region between the gate oxide and therefore the p-type body. This exposes the charged acceptors and a depletion region is made. Further increase in VGS causes the depletion layer to grow in thickness. At the same time, the electric field at the oxide-silicon interface gets larger and begins to attract free electrons as shown in Fig (b). The immediate source of the electron is the electron-hole generation by thermal ionization. The holes are repelled into the semiconductor bulk before the depletion region. the additional holes are neutralized by electrons from the source.
As VGS increases further the density of free electrons at the interface becomes adequate to the free hole density within the bulk of the body region beyond the depletion layer. The layer of free electrons at the interface is named the inversion layer and is shown in Fig (c). The inversion layer has all the properties of a semiconductor device and maybe a conductive path or “channel” between the drain and therefore, therefore, the source which allows the flow of current between the drain and the source. Since current conduction during this device takes place through an n-type “channel” created by the electrical field because of the gate-source voltage it’s called “Enhancement type n-channel MOSFET”.
The worth of VGS at which the inversion layer is taken into account to possess formed is named the “Gate to Source threshold voltage VGS (th)”. As VGS is increased beyond VGS(th) the inversion layer gets somewhat thicker and more conductive, since the density of free electrons increases further with an increase in VGS. The inversion layer screens the depletion layer adjacent to it from increasing VGS. The depletion layer thickness now remains constant.
The three-figure shows.
(a) Depletion layer formation;
(b) Free electron accumulation;
(c) Formation of the inversion layer.
Characteristics of n channel depletion type MOSFET
(image source google search)