What is MOSFET??

MOSFET

MOSFET(metal-oxide semiconductor field-effect transistor) is a family member of semiconductor devices mainly used for signal amplification and as digital switches.it is a type of transistor but works totally different from ancient BJT(bipolar junction transistor).

Types of MOSFET

There are two types of MOSFET, Enhancement type and Depletion type.

these are subdivided into n-channel and p-channel MOSFET.

for now, we only discuss n-channel because the working of p-channel MOSFET is the same.

Working of MOSFET

Enhancement type

now starting with enhancement type suppose we have a p-type material and we sandwiched two n-type material on p-type as shown in the figure

D is drained and S is source.we also have a third pin G (gate) which is a conductor in between the gate and p-type material body we have dielectric(SiO2 insulator denoted with green slab).source is always internally connected to the p-type material and they both are grounded. so source always remains at 0 volts. the depletion region is also created at the junction of n and p-type material as created in diodes. if we want to flow electron from drain to source we can't directly apply a voltage across drain and source.

if we apply positive voltage at the drain and negative at the source so the junction near drain becomes reverse bias and junction near-source become forward what is why electron can't flow from drain to source.even if we change the polarity the other junction will become reverse biased.

Now, what to do?? 

now to make an electron flow we have to create a channel from drain to source. for doing this we have to apply a voltage across gate and source.gate is at positive voltage as both body and source are grounded so they both are at 0 volts. Now comes the trick as there is dielectric placed between gate and p-type material it behaves like a parallel plate capacitor as the gate is at higher voltage it pulls the electron near the dielectric. electron density increases near the junctions of drain and source.n-type material has electrons as a majority charge carrier as there are a lot of electrons they behave as n-type material and creates a channel from drain to source.now you can increase the voltage across gate and source so the channel becomes wider.to flow current from drain to source we add more voltage source across drain and source which causes current to flow from drain to source.

Increasing voltage led to a problem!!

now you want to increase the flow of current from drain to source so you increase the voltage across (drain and source) now more current can flow but if you keep on increasing the voltage reverse bias takes place near drain junction because the drain is n-type and you are increasing the positive voltage.

now channel become narrow near the drain and current also flow at low rate.but if you want to increase the current flow rate there are two methods .first is to decrease the drain-source voltage or second is to increase the gate-source voltage because after that more electron come near the junction and widens the junction.

Depletion Type MOSFET

In depletion type MOSFET channel is permanently present for the flow of current.but you can widen the channel why applying a voltage to gate and source.