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MOSFET Interview Question Answer

State the full form of MOSFET.

Metal oxide field effect transistor

How many terminals the MOSFET consists of?

Three terminals : Source, Drain and Gate

Describe the main advantage of the Power MOSFET over other semi conductor devices.

Advantages of the MOSFET
  • Low input power
  • High switching speed

Whether the MOSFET is voltage controlled or current controlled device? Why?

Voltage controlled device
  • The MOSFET is a voltage controlled majority charge carrier device in the output current is controlled by the input voltage.
  • The conduction of current in the n channel MOSFET is done only by electrons whereas the current conduction in the p channel is done by holes.

What is reason behind the name MOSFET?

The majority charge carriers in the MOSFET are controlled by gate voltage. The gate is insulated by metal oxide layer therefore it is called as MOSFET.

Describe the different types of MOSFET.

Types of MOSFET
  • Depletion enhancement MOSFET : ( 1 ) N channel and ( 2 ) P channel
  • Enhancement MOSFET : ( 1 ) N channel and ( 2 ) P channel

Which are the operating mode of the Depletion enhancement      ( DE ) MOSFET?

The operating mode of DE MOSFET
  • Depletion mode
  • Enhancement mode

Whether the MOSFET is operated at low voltage?

No, the MOSFET is not operated below 500 Voltage.     

What do you mean by the Depletion mode and Enhancement mode in the MOSFET?

Depletion mode
  • When the negative gate to source voltage is applied to N channel MOSFET, it is called as depletion mode of the MOSFET.       

Enhancement mode
  • When the positive gate to source voltage is applied to N channel MOSFET, it is called as enhancement mode of the MOSFET.

Why the vertical structure of the MOSFET is used?

The vertical structure of the MOSFET is used in order to achieve
  • High current rating and
  • High break down voltage

Whether the doping level in the P type material and N type material is same in the N channel MOSFET?

Doping level
  • The two ends n+ type material is heavily doped whereas the middle P type material is moderately doped in the N channel MOSFET. 
  • The n- drift layer has lowest doping level.

Which parameter greatly affects the breakdown voltage of the n channel MOSFET?

n- drift drain layer

Which parameter affects the ON state resistance of the MOSFET?

Geometric shape of source regions

In which type of MOSFET, there is no physical channel?

Enhancement MOSFET

Explain the term : Cut off region and ohmic region

Cut off region
  • When the gate to source voltage is less than the threshold voltage, the MOSFET is operated in the cut off region. 
  • The applied voltage must be less than the break down voltage of the MOSFET. 
  • No drain current flows.        

Ohmic region
  • The MOSFET is operated in the ohmic region when the gate to source voltage is greater than the gate to source threshold voltage ( VGS  > VGS (TH) ).
  • The drain current is almost directly proportional to drain to source voltage for small value drain to source voltage.

In which region, the power dissipation is very high in the MOSFET?

Active region

Give reason : There is no current flow between drain to source in the MOSFET without gate bias.

The current does not flows between drain to source in the  MOSFET without gate bias due to two back to back PN junction diode
Describe the disadvantage of the planner type MOSFET construction.

The planner type construction has long source to drain channel therefore large on state resistance.

Which are the resistances present in the internal circuit of the MOSFET?

Internal resistance in the MOSFET
  • Source region resistance
  • Drift region resistance
  • Drain resistance and
  • Channel resistance

Describe the effect of gate to source voltage on the on state drain to source resistance.

As the gate to source voltage increases, the on state drain to source resistance decreases and vice versa.

What is main reason for switching delay in the MOSFET?

The main reason for switching delay is due to charging and discharging of input and output capacitors in the MOSFET.

Give reason : The parallel connection of the MOSFET is easily done.

The on state resistance of the MOSFET has positive temperature    co – efficient therefore the parallel connection is easily done.

Why the switching speed of the Power MOSFET is higher than the bipolar transistors?

Switching speed
  • The Power MOSFET is a unipolar device.
  • The current conduction is done through only majority charge carriers. 
  • The turn off delay does not occur due to recombination of minority charge carriers. 
  • Therefore the speed of the Power MOSFET is higher than the bipolar transistors.

State the applications of the Power MOSFET.

Applications of Power MOSFET
  • Speed control of AC and DC motors
  • Switching device in stepper motor controllers
  • Lighting control
  • Medical equipment
  • Relays
  • Induction heating
  • Robotics

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