FIELD EFFECTTRANSISTORS (FET)
The main difference between a Bi‐polar transistor and a FET is that, Bi‐polar transistor is a current controlled device In simple terms, it means that the main current in a bipolar transistor (collector current) is controlled by the a base current.
FET is a voltage controlled device
This means that the voltage at the gate(similar to base of a bi‐polar transistor) controlles the main current. In addition to the above, in a bi‐polar trasistor (NPN or PNP), the main current always flows through N‐doped and P‐ doped semiconductor materials. Whereas, in a FET the main current flows either only through the Ndoped semiconductor or only through the P‐doped semiconductor as shown in Fig 1
If the main current flow is only through the N‐doped material, then such a FET is referred as a N‐channel or N‐type FET. The current through the N‐doped material in the N‐type FET is only by electrons.
If the main current flow is only through the P‐doped material, then such a FET is referred as a P‐channel or P‐type FET. The current through the P‐doped material in the P‐type FET is only by Holes.
in bipolar transistors in which the main current is both by electrons and holes, in constrast in FETs depending on the type(P or N type) the main current in
Either by electrons or by holes and never both. For this reason FETs are also known as Unipolar transistors or Unipolar device.
There are a vide variety of FETs. In this lesson MOSFET (Metal Oxide Semiconductor FET) and one of the fundamental type called as Junction Field Effect Transistor (JFET) are discussed.
Junction Field effect Transistor(JFET)
It is a three terminal device and looks similar to a bi‐polar transistor. The standard circuit symbols of N‐channel and P‐channel type FETs are shown in Fig 2.
Construction
As shown in Fig 3a, a n‐Channel JFET has a narrow bar of n‐type. To this, two p‐type junctions are diffused on opposite sides of its middle part Fig 3a. These diffused junctions form two P‐N diodes or gates. The n‐type semiconductor area between these junctions/gates is called channel. The diffused P regions on opposite sides of the channel are internally connected and a single lead is brought out which is called gate lead or terminal. Direct electrical connections are made at the two ends of the bar. One of which is called source terminal, S and the other drain terminal, D.
A p‐channel FET will be very similar to the n‐channel FET in construction except that it uses P‐type bar and two N type junctions as shown in Fig 3b. FET notation listed below are essential and worth memorizing,
- Source terminal: It is the terminal through which majority carriers enter the bar(N or P bar depending upon the type of FET).
- Drain terminal: It is the terminal through which majority carriers come out of the bar.
- Gate terminal: These are two internally connected heavily doped regions which form two P‐N junctions.
- Channel: It is the space between the two gates through which majority carriers pass from source to drain when FET is working(on).
