Advantages and Limitations
The name field effect transistor itself hints the attributes of this device. Like BJT it is also capable of transferring a signal from high resistance to low resistance (Transfer resistor-transistor). The term field effect stands for the fact that the operation of the device mainly depends on the electric field applied between its terminals called Gate and source analogous to the base and collector in transistor to control the current through it. That is the electrical field applied between two terminals (Gate and source Vgs) controls the flow of current in other terminal (Drain).
History of Field Effect Transistor
The field-effect transistor (FET) was first patented by Julius Edgar Lilienfeld in 1926 and by Oskar Heil in 1934. First Practical FET was JFET which is the simplest transistor available developed only much later after the transistor effect was observed and explained.
Types of Field Effect Transistors
Field Effect transistor’s are of mainly of two types based on construction features of the device
- Junction Field Effect Transistor
- Metal oxide semiconductor Field Effect Transistor.
JFET’s are again subdivided into two types based on the type of channel namely p channel JFET and n- channel JFET. Similarly MOSFET’s are divided into two types a) Enhancement mode MOSFET 2) Depletion mode MOSFET. In Enhancement mode MOSFET channel is induced by applying gate voltage contrary to the depletion mode MOSFET in which already existing channel is modulated by applying gate voltage like JFET.
Types of JFET
There are two types of JFET based on the type of channel i.e. based on the majority carrries flow.
- N- channel JFET
- P- channel JFET
The circuit symbol of P channel JFET and N channel JFET is as shown in the figure (a), (b) with the arrow on gate terminal shows the device polarity. Since JFET is a symmetric device either ends of N-channel can be used as source or drain. It is useful for the circuit design to show either of two terminals as source and other as drain. It is achieved here by placing gate terminal close to source.
N-channel JFET consists of a p+ type semiconductors grown by doping acceptor impurities on either side of N-type semiconductor as shown in the figure.
Current is allowed to flow through the length of the N-channel between the two p+ semiconductors. FET is a three terminal devicewith the terminals being source, gate and drain.
It is there terminal where the majority carriers enter in to JFET bar. As for the FET bar concern this terminal is the source of majority carriers so it is called as source terminal and the current flow in this terminal is Is.
This terminal is the end terminal which collects the majority carriers sourced by the source. I.e. it is draining the majority carriers from FET bar and giving to the output terminal so it is called as drain terminal and the current in drain is Id.
This important control element in FET as it acts like gate to the majority carriers flow. I.e. by operating the gate terminal voltage opening or closing of majority carries can be obtained, so this terminal called Gate terminal and the current in gate is Ig.
Practically FET is manufactured by doping donor impurities in p-type substrate. So along with these three terminals there exists substrate terminal which is often shorted to source. Both terminals of p+ type semiconductor are joined to form gate terminal. Metal contacts are provided for all the three terminals. Electrons flow from source to drain hence the conventional current of positive charge carriers flows from drain to source.