MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

 A power MOSFET is a voltage managed device and requires simplest small enter current due to the fact output (drain Current) may be controlled through gate-supply voltage (VGS).The power MOSFET has 3 terminals referred to as Drain (D), Source (S) and Gate (G). The switching velocity could be very excessive and the switching times are the order of nano seconds. Low power excessive frequency applications. MOSFETs do now no longer have the issues of secondary breakdown phenomenon as BJTs. Power MOSFET is a unipolar device. Current conduction happens thru delivery of majority carriers with inside the drift place with out presence of minority carrier injection required for bipolar transistor operation. It have very excessive enter impedance so as of 109 ohm.


  • Linear power supplies,
  • Speed control of dc and ac motors,
  • Stepper motor controllers,
  • Relays,
  • Lighting controls,
  • Induction heating etc.,

Types of Power MOSFET

The  primary styles of strength MOSFETs are
1. Depletion kind MOSFET
    a. N Channel MOSFET
    b. P Channel MOSFET
2. Enhancement type MOSFET
    a. N Channel MOSFET
    b. P Channel MOSFET
Out of those  types, n-channel enhancement MOSFET is greater common due to better mobility of electrons.

Depletion Type Power MOSFET

The Depletion MOSFET operated in  modes:
The depletion mode and enhancement mode. The gate voltage is poor for depletion mode and
nice gate voltage for enhancement mode.

Enhancement type Power MOSFET

mosfet(metal oxide semiconductor field effect transistor)
 This MOSFET only works in enhancement mode and does not have a depletion mode. Its structure differs from that of a Depletion type MOSFET in that it lacks a physical channel.

Power MOSFET Symbols

mosfet(metal oxide semiconductor field effect transistor)

Power MOSFET Characteristics

The static characteristics 

(a) Transfer Characteristics  The source terminal S is taken as common terminal, as usual, between the input and output of a MOSFET.  This characteristic shows the variation of drain current I D as a function of gate- source voltage VGS.The VGST (threshold voltage) of a MOSFET is an important metric. The minimal positive voltage between gate and source required to induce an n-channel is known as VGST. Thus, for threshold voltage below VGST device is in the off-state . The VGST has a magnitude of 2 to 3 volts. Transconductance, which is the ratio of drain current to gate voltage, defines the transfer characteristics and is a very important parameters.
  • Transconductance (gm)= (βˆ†πΌπ·/ βˆ†π‘‰πΊπ‘†)*( VDS=Constant)
  • Output Resistance (𝒓𝒐 ) = 𝑅𝐷𝑆 = βˆ†π‘‰π·π‘† /βˆ†πΌ.

The static characteristics – (b) Output Characteristics

Output Characteristics contd..

The graph between ID – VDS. 
  •  On State resistance π‘Ÿπ‘œ = 𝑅𝐷𝑆= 𝑉𝐷𝑆/𝐼𝐷 .
In the power electronics applications, the MOSFET is used as a switch, these device must be operated in the cut off and ohmic region when turned OFF and ON respectively.  A denotes a fully ON state, while B denotes a fully OFF state. As a result, MOSFETs serve as switches.

Three Regions of Operation

 1. Cut off region ( VGS≀ VT ) .

When the gate source voltages VGS is less than the threshold voltage VT.  The device must withstand the applied voltage and to avoid the breakdown, the drain source breakdown voltage should be greater than the applied voltage. When the breakdown occurs, it is due to the avalanche breakdown of the drain body junction. 
  • Cut off region ( VGS≀ VT ) 
  • Active region (or) Pinch off Region (or) Saturation Region (VDS β‰₯ VGS – VT ) 
  • Ohmic Region (or) Linear Region. (VDS ≀ VGS – VT ).
2. Ohmic Region (or) Linear Region. (VDS ≀ VGS – VT )

When large gate to source voltage is applied the device driven into the ohmic region where the VDS(on) is small. In this operating region, the power dissipation can be kept reasonably low, by minimizing the on state voltage.
3. Active region (or) Pinch off Region (or) Saturation Region (VDS β‰₯ VGS – VT ) 

ID is almost independent of the VDS . It is only dependent on the VGS . The power dissipation of MOSFET is high in the active region.

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