How to use MOSFET. Design note 2

MOSFET is Metal Oxide Semi conductor Field Effect Transistor, a very efficient type of switching transistor used in power devices like Inverters, UPS, Power amplifier etc. Its high current capability and fast switching makes it as an ideal choice for these applications. MOSFET is a four terminal device. The terminals are Drain, Source, Gate and the Substrate or Body terminal. The Drain and Source terminals are connected to the heavily doped regions. The Gate terminal is connected top on the oxide layer while the Substrate terminal is connected to the intrinsic semi conductor.

There are two types of MOSFETs. In N-Channel MOSFET, the current is due to the flow of electrons in the inversion layer while in P-Chanel MOSFET, it is due to the flow of Holes.

How MOSFET works?

The working of MOSFET depends on its MOS capacitor which is the main part. The Semi conductor surface present below the oxide layer and between the drain and source terminal can be inverted from P-type to N-type by applying a positive or negative voltages. When a positive voltage is applied at the gate, the holes present beneath the oxide layer experience a repulsive force and they pushed downward with the substrate. The positive voltage also attracts the electrons from the N+ source and drain regions in to the channel. Thus if a voltage applied between the source and the drain, current flows freely between the source and drain .Gate voltage controls the electrons concentration in the channel. If a Negative voltage is applied, the hole channel will be formed beneath the oxide layer. Controlling the Source to Gate voltage causes the conduction of current between the source and drain.

Difference between P-Chanel MOSFET and N-Chanel MOSFET

MOSFET having a P-Chanel region between the Source and Drain is called P-Chanel MOSFET. It is a four terminal device, and the terminals are Gate, Drain, Source and Substrate or body. The drain and source are heavily doped P+ region and the Substrate is N-type. Current flow depends on the flow of positively charged holes. In N-channel MOSFET, the region between Source and Drain is an N-Chanel. It is also a four terminal device and the terminals are Gate, Drain, Source and Substrate or body. The Drain and Source are heavily doped N+ region and the substrate is P-type. Current flow is due to the flow of Negatively charged Electrons.
MOSFETs are sensitive devices and static electricity can damage them easily. So do not short its pins with hand.


Proximity Sensor using MOSFET

Here is a simple circuit to see the working of MOSFET. IRF 511 N-Chanel MOSFET is used here to pick up the static field from the hand. When hand is close to the pickup plate, buzzer sounds and LED glows. Pick up plate is a 4×4 inch copper or aluminum sheet. VR adjust the sensitivity of the circuit. Since the gate of MOSFET is very sensitive, a high value resistor chain R1 through R3 is used. You can add 7 more 10 M resistors in series to make the resistance as 100M. This makes the circuit very sensitive.

How to test MOSFET?
You need a Multimeter with minimum 3.3 volts over the Diode Under Test (D.U.T) . If the multimeter is less than this, MOSEFET cannot be tested.
1. First identify the Pins (Drain, Source and Gate) of the MOSFET.
2. Hold the MOSFET in its case or Tab and connect the Source ( S) pin with the Negative ( Black) lead of Multimeter.
3. First touch the Positive (Red) lead to Gate (G) and then quickly touch Drain ( D ). The reading should be a low value. During this test, MOSFET turns on due to the charging of its internal capacitance on the gate
4. Keeping the Positive lead on Drain, touch the Source and Gate together with finger. Meter reading should go high due to the discharge of Gate.


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