Basic MOS Transistors : MOSFET-Fundamentals
Our primary focus in this section would be on the most important semiconductor device in current technology and applications, namely the metal oxide semiconductor field effect transistor (MOSFET).
MOSFETs have several unique features which distinguish them from BJTs these are :
1) BJTS are current controlled where as MOSFETs are voltage controlled devices.
2) BJTs are bipolar and MOSFETs are unipolar i.e. the current transport takes place either by electrons or by holes but not by both.
3) Because of lower current drive capability of MOSFETs the charging and discharging of load capacitors is lower hence speed MOSFETs is lower than BJT.
The one way of classifying MOSFETs is depending upon the type of channel.
There are basically two types of MOSFETs namely n-channel and p-channel shortly called as NMOS and PMOS respectively.
In n-channel MOSFETs (NMOS) the type of carriers in the channel are electrons that will flow from source to drain terminals depending upon the bias applied at terminals, whereas in p-channel MOSFETs (PMOS) the type of carriers in the channel are holes.
The another way of classifying the MOSFETs is depending upon the availability of channel between source and drain terminals i.e. Enhancement mode MOSFETs and depletion mode MOSFETs.
Thus, there are four basic MOSFET device types there are :
1) n-channel enhancement mode MOSFET
2) n-channel depletion mode MOSFET
3) p-channel enhancement mode MOSFET
4) p-channel depletion mode MOSFET.