IMPORTANCE OF MOSFET

FETs of all types are widely used electronics components today. Of all the types of FET, the MOSFET is possibly the most widely used.

Even though MOSFETs have been in use for many years, these electronics components are still a very important element in today's electronics scene. Not only are MOSFETs found in many circuits as discrete components, but they also form the basis of most of today's integrated circuits.

MOSFETs provide many advantages. In particular they offer a very high input impedance and they are able to be used in very low current circuits. This is particularly important for integrated circuit technology where power limitations are a major consideration.

There a several MOSFET circuit symbols that are used. Some MOSFET symbols are equivalents of each other, while others indicate more detail about the MOSFET itself.

As there are several varieties of MOSFET, the symbols used to indicate them need to be different.

MOSFET symbols for N-channel and P-channel types (enhancement mode)

MOSFET symbol used above generally indicates that the device has a bulk substrate - this is indicated by the arrow on the central area of the substrate.The MOSFET provides some key features for circuit designers in terms of their overall performance.

KEY MOSFET FEATURES
FEATURE	DETAILS
Gate construction	gate is physically insulated from the channel by an oxide layer. Voltages applied to the gate control the conductivity of the channel as a result of the electric field induced capacitively across the insulating dielectric layer.
N / P channel	Both N-channel and P-channel variants are available
Enhancement / depletion	Both enhancement and depletion types are available. As the name suggests the depletion mode MOSFET acts by depleting or removing the current carriers from the channel, whereas the enhancement type increases the number of carriers according to the gate voltage.

The two main types of MOSFET are N-channel and P-channel. Each has different features:

COMPARISON OF THE KEY FEATURES OF N-CHANNEL AND P-CHANNEL MOSFETS
PARAMETER	N-CHANNEL	P-CHANNEL
Source / drain material	N-Type	P-Type
Channel material	P-Type	N-Type
Threshold voltage Vth	negative	doping dependent
Substrate material	P-Type	P-Type
Inversion layer carriers	Electrons	Holes

There are basically three regions in which MOSFETs can operate:

• Cut-off region:   In this region of the MOSFET is in a non-conducting state, i.e. turned OFF - channel current IDS = 0. The gate voltage VGS is less than the threshold voltage required for conduction.
• Linear region:   In this linear region the channel is conducting and controlled by the gate voltage. For the MOSFET to be in this state the VGS must be greater than the threshold voltage and also the voltage across the channel, VDS must be greater than VGS.
• Saturation region:   In this region the MOSFET is turned hard on. The voltage drop for a MOSFET is typically lower than that of a bipolar transistor and as a result power MOSFETs are widely used for switching large currents. 
  
  

  SWITCHING FOR DIFFERENT TYPES OF MOSFET
  MOSFET TYPE	VGS +VE	VGS 0	VGS -VE
  N-Channel Enhancement	ON	OFF	OFF
  N-Channel Depletion	ON	ON	OFF
  P-Channel Enhancement	OFF	OFF	ON
  P-Channel Depletion	OFF	ON	ON