Hi all
I have always had difficulty in understanding how FETs work. N channel and P channel also all the variants such as enhancement and depletion types. Also the operation modes ie. Linear mode triode mode etc
I would like to learn this practically.
Are there any good resources where i could learn practically and with easy examples.
Thanks in advance.
You have a right to be confused. There is a lot of confusing information about transistors everywhere.
Let's start with the enhancement type MOSFET, also known as the insulated gate FET or IGFET. Below is a picture of the structure of a N-channel enhancement MOSFET. A P-channel enhancement MOSFET acts the same, but the voltages, charge carriers and semiconductors are reversed.
Notice it has a thin layer of silicon oxide insulating material under the gate lead. When no voltage is applied to the gate, the FET acts like two back-to-back diodes, and a high resistance exists between the source and drain. Applying a positive voltage with respect to the source causes the holes to move further into the p-type substrate leaving behind a depletion region beneath the silicon oxide layer consisting of bound negative charges created by the uncovering of the acceptor atoms. Increasing the gate voltage attracts electrons from both the source and drain N-regions so that a electron rich or N-channel is formed. This is shown below.
So the enhancement type MOSFET creates a channel by electrical induction. No current will exist between the source and drain until the gate voltage is at a sufficient level and a voltage difference between the source and drain is present.
The deplection-type MOSFET operates the same way except the N-channel is physically implanted between the source and gate. This means that current will exist between the source and drain even if no gate voltage is present, provided there is a voltage difference between the source and drain. A forward gate voltage will increase the source to drain current and a reverse gate voltage is need to deplete the N-channel and stop the current. So this FET type can be operated in either the enhanced mode by a forward gate voltage or depletion mode by a reverse voltage.
The JFET is shown below.
This simple structure will allow current between the source and drain even if no voltage is applied to the gate. To reduce current, a reverse voltage has to be applied to the gate which causes the p-depletion region to expand and make the N-channel narrower and restrict current.
To summarize: FET,s, TUBE,s, AND BJT,s are all transconductance devices. The TUBE,s and FET,s control current by electric fields and BJT,s control current by diffusion. FET,s manage current by sending a single polarity charge flow along depletion regions in channels, and BJT's do it by sending charges of both polarities through depletion regions by diffusion. The charge flow is a 90 degree difference in direction between FET's and BJT's.
Ratch