Introduction of MOSFET :

The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is a semiconductor device which is widely used for switching and amplifying electronic signals in the electronic equipments.  The MOSFET is a core of integrated circuit and it can be designed and fabricated in a single chip just because of these very small sizes.  The mosfet transistor is a four terminal device with gate (G), source(S), drain (D) and body (B) terminals. The body structure of the MOSFET is frequently connected to the source terminal so making it a three terminal device like field effect transistor. The MOSFET is very far the most common transistor and can be used in both digital and analog circuits.


A MOSFET may be intuition of as a variable resistor, where the Gate-Source voltage difference can control the Drain-Source Resistance. When there is no applying voltage between the Gate-Source , the Drain-Source resistance is too high, which is almost like a open circuit, so no current may flow through the Drain-Source. So, When Gate-Source potential difference is applied, the Drain-Source resistance is reduced, and there will be current flowing through Drain-Source, which is now a closed circuit.


How to use MOSFET as a Switch

To use a MOSFET as a switch, you have to need its gate voltage (Vgs) higher than the source. If you connect the gate to the source (Vgs=0)then it is turned off.

Suppose,we have a IRFZ44N which is a “standard” MOSFET and only turns on when Vgs=10V – 20V. But basically we try not to push it too hard so 10V-15V is common for Vgs for this type of MOSFET.

Types of MOSFET :

There are three main category of MOSFET which is given below :

N- Channel : - For an N-Channel MOSFET, In this category of MOSFET the source is connected to ground. To turn on the MOSFET, we need to raise the voltage on the gate. To turn off the MOSFET, we need to connect the gate to ground.

P-Channel :-  For an P-Channel MOSFET, The source is connected to the power rail (Vcc). In sequence to allow current to flow the Gate needs to be pulled to ground. To turn off the MOSFET, the gate needs to be pulled to Vcc.

Depletion Mode – It needs the Gate-Source voltage ( Vgs ) applied to switch the device “OFF”.

Enhancement Mode – The transistor needs a Gate-Source voltage ( Vgs ) applied to switch the device “ON”.

There are also Logic-Level and Normal MOSFET, but the only difference is the Gate-Source potential level need to drive the MOSFET.


How does it work in theory?

MOSFET is a voltage controlled field effect transistor that is different from a JFET. The Gate electrode is electrically insulated from the main semiconductor by a thin layer of insulating material like glass etc. This insulated metal gate is like a plate of a capacitor which has an very high input resistance (as high as almost infinite!). Because of the isolation of the Gate there is no current flow into the mosfet transistor from Gate.

When voltage is applied at the gate,then it changes the width of the Drain-Source channel along which charge carriers flow (electron or h***). The wider the channel is directly proportional to the betterment of the device conducts.

The MOSFET are used differently comparing to the conventional junction FET.

The infinite high input impedance makes MOSFETs very useful for power amplifiers. The devices are also well suited to high-speed switching applications and some integrated circuits contain tiny MOSFETs and are used in computers.

Because the oxide layer is so thin, the MOSFET can be destroyed by built up electrostatic charges. In weak-signal radio-frequency work, MOSFET devices do not generally perform as well as other types of FET (Field Effect Transistor).


Where to put the load to a MOSFET? Drain or Source?

Because load has resistance, which is usually a resitor. For N-channel MOSFET the reason we basically put the load at the Drain side is because of the Source is usually connected to GND.


If load is connected at the source side, the Vgs will require to be higher in order to switch the MOSFET, or there will be insufficient current flow between source and drain than expected.


Heat Sink connected to the Drain?

Typically the heat sink on the back of a mosfet transistor is connected to the Drain! If you mount multiple MOSFET transistors on a heat sink, they must be electrically isolated from the heat sink! This is good practice to isolate regardless in case the heat sink is bolted to a grounding frame.


What is the Body Diode For?

MOSFET transistor also have an internal diode which may allow current to flow unintentionally. The body diode will also restrict switching speed. User don’t have to worry about it if you are operating under 1Mhz.

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