Components Expert

Overview and working principle of power MOSFET


The original meaning of MOSFET is a metal oxide semiconductor field effect transistor. MOS refers to a field effect transistor in which the gate of the metal layer (M) is separated by the oxide layer (O) to control the semiconductor (S) through the electric field effect.

Power field effect transistors are divided into junction type and insulated gate type, but the insulated gate type usually refers to MOS type (metal oxide semiconductor FET), that is, power MOSFET. Junction power field effect transistors are commonly referred to as static induction transistors (SIT). Its characteristics are the use of gate voltage to control the drain current, simple drive circuit, low drive power, fast switching speed, high operating frequency, and better thermal stability than GTR. However, its current capacity is very small and can only withstand low voltages. Generally speaking, it is only suitable for power electronic equipment with a power not exceeding 10kW.

Types of power MOSFET:

  • According to the conductive channel, it can be divided into P channel and N channel.
  • According to the magnitude of the gate voltage, it can be divided into: depletion type and enhanced type.
  • The power MOSFET is mainly N-channel enhancement type.

The working principle of power MOSFET

Cut off: Add a positive power supply between the drain and the source, and the voltage between the gate and the source is zero. The J1 of the PN junction formed between the P base region P and the drift region N is reverse biased, and no current flows between the drain and the source.

Conduction: By adding a positive voltage UGS between the gate source, the gate is insulated, so no gate current flows. However, the positive voltage of the gate pushes away the holes in the region P below it, and attracts the electrons and minority carriers in the region P to the surface of the region P below the gate.

When UGS is greater than UT (turn-on voltage or threshold voltage), the electron concentration on the surface of the P area under the gate will exceed the hole concentration, so that the P-type semiconductor is inverted to N-type and becomes an inversion layer. The inversion layer forms an N channel, which makes the PN junction J1 disappear. The drain pipe and the water source are conductive.