• FET :
FET stands for Field-Effect Transistor. FET devices do not have the problem of storage and removal of minority charge carriers, when these devices are switched from off to on and on to off. Therefore, the problem of distribution of charges do no arise in these device, which contributes to delay in bipolar devices.
FET devices have significant capacitance contributes to delay in these switches, because of the charging and discharging of these capacitors through the register in the drain circuit in switching operation. In general, the turn-on and turn-off delay times of unipolar devices are significantly higher than those of bipolar devices but developments in the most devices have made it possible for these devices to have speed comparable to those of bipolar devices.

• MOSFET :
MOSFET stands for Metal-Oxide Semiconductor Filed-Effect Transistor. A field-effect transistor is made by growing a very thin layer of SiO2 (0.1µm thick) over a semiconductor material. A metal such as aluminium is deposited over a dielectric layer of SiO2. This structure is known as 'Metal-Oxide Semiconductor Field-Effect Transistor' (MOSFET).
The metal gate is insulated from the channel and therefore it is also referred to as an 'Insulated Gate FET (IGFET)'. Similar to junction field-effect transistor we can have n-channel as well as p-channel MOSFET's. Since n-channel devices are more popular than p-channel devices because of their higher speeds, only n-channel devices are discussed here. However, the physical principles of operations of p-channel MOSFET's are the same as n-channel devices.