What is a Power MOSFET?

The term MOSFET stands for "Metallic oxide semiconductor field effect transistor". A power MOSFET is a type of majority carrier device. It is also a voltage-controlled device with a higher input impedance due to the presence of the SiO2 layer. The conduction losses are few here; also, it has a higher ON stage voltage drop. The switching power loss is very low for this device. It has a positive temperature coefficient, so a secondary breakdown does not occur. Parallel operation of power MOSFET is possible. It has a higher switching speed. There are two types of structures; vertical diffused metal oxide semiconductor (VDMOS) and Double-diffused metal oxide semiconductor (DMOS) structure. There are also logic-level MOSFET's which is turned ON completely from the microprocessor's logic level. Another kind of MOSFET is super-junction MOSFET. In this type of MOSFET, the p-layer and n-layer are aligned alternatively.

Construction of Power MOSFET

The power MOSFET transistor structures are enhancement types. The voltage rating is enhanced in the enhancement-mode MOSFETs by the use of a drift layer. The MOSFET generally contains four layers. The middle layer is the p-type layer also known as the body, whereas the n-type layer is called the drift layer or region. The drift region decides the breakdown voltage, and hence it is the lightly doped region in power MOSFETs. The first and last layers are the n+ layers. The first layer and the last layers are the source and drain layers. The structure of N-channel MOSFET (e-MOSFET) is n+ p n- n+, but the shape of p-channel is the opposite doping shape.

Power MOSFET circuit

The main terminals of this type of circuit are drain-source, and the current direction is from the drain-to-source terminal. It is controlled through the gate voltage terminal to the source by applying zero voltage.

Working of Power MOSFET

Power MOSFET structure
CC-BY-SA-3.0 | Image credit: https://en.m.wikipedia.org | Cyril BUTTAY

When the gate signal is not given, the drain voltage reverse biases the MOSFET. There is no path for drain current, and the MOSFET is in the cut-off region. Only the leakage current flows at this time. When a gate signal is given, positive charges are induced near the metallic surface; and on the other side, negative charges are induced, that is, it acts as a capacitor. These charges are called gate charges. To turn on the MOSFET, the gate charge's must be induced into the gate to turn the MOSFET ON. If the gate charge is large, it takes time to charge the capacitor but switching losses increase. This N-channel provides a path for drain current. Conduction is only due to electrons, so it is a majority carrier device, and also a high-speed device. When a positive gate signal is given, the N-channel is induced in the p-type layer. This provides a path for drain current. Due to the absence of minority carriers in the MOSFET, the reverse recovery time is reduced to a significant extent. Therefore, it operates with a very high switching frequency. This device has a very higher input impedance. Here, the controlling parameter is the gate-source voltage and the gate-to-source voltage is denoted as 'Vgs'. When Vgs< Vgt, the MOSFET is in the ON state, and only leakage current flows. Power MOSFET also acts as a switch in most modern-day applications due to its low conduction losses, zero gate current, and low switching losses. When a switch is used, one can drive it to turn ON faster or turn ON slower. That is why, it is used as a very efficient switch, as it allows to switch faster ON and OFF than the bipolar junction transistors. 

ON-state resistance

Between the drain and source terminals, a power MOSFET exhibits resistance (ON-resistance) behavior when it is in an ON state. This resistance RDSon as shown in the figure below, is the sum of many elementary contributions.

  • RS- Source resistance
  • Rch - Channel resistance
  • Ra - Access resistance
  • RJFET - Transistor resistance
  • Rn - Epitaxial layer resistance
On - state resistance diagram showing all the resistances
CC BY-SA 3.0 | Image Credit: https://commons.wikimedia.org | CyrilB

Breakdown voltage/on-state resistance trade-off

MOSFET acts as a PIN diode when it is in the OFF state. When it is reversed biased, the space charge region extends over the n-layer. Therefore, most of the drain to source voltage is withstanded by this layer in OFF state. The n- layer has no function in the ON state because it is the lightly doped region. A resistance RDSon is created due to this. The breakdown voltage and the RDSon are governed by two main parameters, the doping concentration, and thickness of the n-layer. The breakdown voltage is be higher as much as the layer is thicker and the doping level is lower. Also, the RDSon is lower, as the layer is thinner and the doping level is higher. Therefore, in the design of MOSFET, there is a trade-off between its voltage rating and its ON-resistance. If the device has a high voltage then the channel resistance and threshold voltage will increase. So, we need to do careful optimization.

Body diode

In the first figure, we can see that the metal contact connects both the N+ and P+ layers. However, the operation of MOSFET only requires the connection between the source and the N+ layer. So, it results in a floating P zone between the drain and the N-doped source, which is the same as the NPN transistor circuit. Therefore, a body diode is created that has two terminals, that is, the cathode (drain) and the anode (source) which can block current in one direction.

Power MOSFET transistor testing

A power MOSFET is usually tested using a multi-meter through the following methods:

Testing N-channel MOSFET

An N-channel power MOSFET is tested using the following method:

The digital multi-meter is first fixed to the range of the diode and then the MOSFET is placed on a wooden table. After this, the drain and gate terminals are shorted using a screwdriver or a meter probe. The internal capacitance of the N-channel MOSFET will discharge completely. The red color probe is placed toward the drain side and the black color probe is placed on the source side. The digital multimeter will show an open circuit symbol. Then, the black color probe is kept connected toward the source only, but the red color probe is removed from the drain terminal of the N-channel MOSFET and is connected to the gate terminal for a while. It is again disconnected from there and is placed back to the drain terminal. The digital multi-meter will then display a short circuit. This helps us to know that the MOSFET is in a good condition and is normal. This method is repeated again and again for confirmation.

Testing P-channel MOSFET

A P-channel MOSFET is tested using the following method:

For testing the P-channel MOSFET, the above method will remain the same, except that the polarities of the meter will vary. The red color probe will remain as it is from the source, while the black color probe is detached from the drain terminal and is connected to the gate terminal for a while, and then again it is disconnected and placed back toward the drain of the MOSFET. We will see that the multi-meter will display continuity or a less value. This means that the MOSFET is in a good condition and any other value in the multi-meter will mean that it is a faulty MOSFET.

Context and Applications

This topic is significant in the following courses.

  • Bachelors in Technology (Electrical Engineering/Electronics Engineering)
  • Masters in Technology (Electrical Engineering/Electronics Engineering)

Practice Problems

1. What is the full form of MOSFET?

  1. Metal oxide semiconductor field-effect transistor
  2. Modern oxidized silicone-based field-effect transistor
  3. Modern oxidized silicone-based force effect transistor
  4. Metal oxide silicone field equivalent transistor

Answer: Option a

Explanation: The full form of MOSFET is a metal oxide semiconductor field-effect transistor.

2. What type of device is MOSFET?

  1. Current-controlled
  2. Voltage-controlled
  3. Voltage-controlled current source
  4. Voltage-controlled voltage source

Answer: Option b

Explanation: Power MOSFET is a voltage-controlled device. If we increase the voltage magnitude of the drain current increases.

3. What type of carrier device is a power MOSFET?

  1. Majority
  2. Minority
  3. Majority and minority
  4. Either majority or minority

Answer: Option a

Explanation: A power MOSFET is a majority carrier device. Here, conduction is only due to electrons.

4. How many terminals are there in power MOSFET?

  1. One
  2. Two
  3. Three
  4. Four

Answer: Option c

Explanation: A power MOSFET is a three-terminal device. The terminals are drain, source, and gate terminals.

5. What are the main terminals that carry current?

  1. Source
  2. Drain
  3. Drain-source
  4. Gate

Answer: Option c

Explanation: The main terminals that carry current are drain-source. The n-channel provides the path for the drain current to flow.

  • Switch-mode power supply
  • DC-DC converters
  • Motor control

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