Friday, October 12, 2012

What is MOSFET

MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a semiconductor device used to control and amplify current. MOSFET is four electronically active regions that are marked: gate (G), source (S), and drain (D), and the bulk terminal (B) to which the gate, drain, and source voltages are typically referenced. The rectangular gate region lies on top of the bulk separated by a thin silicon oxide dielectric with thickness. Two other important dimensions are the transistor gate length and width. The drain and source regions are embedded in the substrate but have an opposite doping to the substrate.

There are two types of MOSFET transistors, the nMOS transistor and the pMOS transistor, that differ in the polarity of carriers responsible for transistor current. The charged carriers are holes in pMOS transistors and electrons in nMOS transistors.

MOSFET Operate: A MOSFET has a gate that is insulated from the channel. A voltage that is applied to the gate will attract charge from the channel toward the gate. This charge cannot move through the gate because of the insulation. But, this “moved” charge does change the conductivity of the channel.

A simple description treats the MOSFET operate. When the gate has a high voltage, the transistor closes like a wall switch, and the drain and source terminals are electrically connected. Just as a light switch requires a certain force to activate, the transistor gate terminal needs a certain voltage level to switch and connect the drain and source terminals. This voltage is called the transistor threshold voltage Vt and is a fixed voltage for nMOS and for pMOS devices in a given fabrication process.

The most common MOSFET is the enhancement mode MOSFET. In an n-channel enhancement mode MOSFET the resistance from the source to drain is relatively high; therefore, very little current can flow. However, a positive voltage at the gate causes the channel to induce negative charges which allows more electrons to flow from the source to the drain. This means that the resistance of the channel is easily controlled by the gate.

Practical form of the MOSFET

CMOS: Complementary MOSFET

Complementary Metal Oxide Semiconductor Field Effect Transistor is a technology for constructing integrated circuits. In this circuit, two MOSFETs P-channel and N-channel are connected in series so that source of P-channel device is connected to a positive voltage supply + VDD and the source of N-channel device is connected to the ground. Gates of both the devices are connected as a common input and drain terminals of both the devices are connected together as a common output.

When the input is low on the gate, the P-channel MOSFET will be ON, the drain and source terminals are electrically connected and the N-channel MOSFET is OFF.
In other case when the input is high on the gate, the N-channel MOSFET will be ON, the drain and source terminals are electrically connected and the P-channel MOSFET is OFF.