FIELD EFFECT TRANSISTORS MADE BY : GROUP (13)/PM

FIELD EFFECT TRANSISTORS MADE BY : GROUP (13)/PM

THE FIELD EFFECT TRANSISTOR (FET) In 1945, Shockley had an idea for making a solid state device out of semiconductors. He reasoned that a strong electrical field could cause the flow of electricity within a nearby semiconductor. He tried to build one, but it didn't work. Three years later, Brattain & Bardeen built the first working transistor, the germanium point contact transistor, which was designed as the junction (sandwich) transistor. In 1960, Bell scientist John Atalla developed a new design based on Shockley's original field-effect theories. By the late 1960s, manufacturers converted from junction type integrated circuits to field effect devices.

FIELD EFFECT TRANSISTORS The current flow in a junction transistor is due to flow of both the electrons and holes. This junction transistors is also called a bipolar juncton transistors (BJT). There are two drawbacks in a junction transistors : 1. The input impedence in CE mode is very small due to which base current is not negligible. 2. The noise level is very high. Both drawbacks are removed in FET. Field effect transistor is a three terminal semiconducting device in which output current is controlled by the electric field. The current flow in FET is only due to the flow of one type of charge carriers (electron or holes). Hence it is also called the UNIPOLAR TRANSISTOR.

KINDS OF FIELD EFFECT TRANSISTORS Field effect transistors can be classified into two categories : 1. Junction Field effect transistor (JFET) 2. Metal Oxide Semiconductor (MOSFET) 1. JFET : It means junction field effect transistor. This can be classified in two categories : (i) p-channel FET. (ii) n-channel FET. 2. MOSFET : It means metal oxide semiconductor field effect transistor. This can also be divided into two categories : (i) Depletion Type. (ii) Enhancement Type.

Important terminals FET 1.Source- It supplies the majority carriers to the channel. 2.Drain- It takes out these majority carriers from the channel. 3.Gate- It controls the motion of majority carriers from Source to Drain. 1. N-channel FET : It consist of a thin bar of N-type semiconductor forming two junctions with the P-type semiconductor near the centre, at opposite side of bar. Thus we get two PN junctions and both P-layers are internally connected i.e. there is one common terminal. This terminal is called Gate. When a potential difference is applied between the drain and source, the majority carriers of N- channel (electron) move in accordance with the applied pot. As a result currentflow through the channel.

2. P-channel JFET: It consist of athin bar of P- type semiconductor forming two junctions with the N- channel semiconductor near the centre, at opposite faces of bar. Thus we get two P-N junctions on eitherside of bar. Both N layers are connected internally. When a potential difference is applied between the drain and source, the majority carriers of P-channel (holes) move in accordance with the applied potential. As a result, the current flow through the channel. Working: when Vgs is zero and Vds is increased the number of electron reaching the drain will also be increase & hence current will also be increased. At particular value of Vds the current becomes constant (saturated) because max. num of electrons reached the drain. After that when Vgs is increased, the width of depletion layer increase and channel width decreases because gate is reverse biased. In this case the current get saturated at lower value.

Pinch of voltage (Vp): Value of Vds at which Id becomes saturated is called pinch of voltage. Charecterstic of JFET : Formula for Drain current - At pinch off, the sum of gate volgate and drain voltage is called Vp & Idss is max. drain current.

MOSFET (METAL OXIDE SEMICONDUCTOR FET) The insulated gate FET (IGFET) or metal oxide semiconductor FET (MOSFET) is also a Field effect tran. but it has a much greater commercial importance than JFET. A MOSFET uses a metal gate electrode (instead of PN-junction as in JFET), separated from the semiconductor by an insulating thin layer of silicon dioxide to modulate the resistance of conducting channel in the semiconductor. Types of MOSFET (i) Depletion MOSFET (ii) Enhancement MOSFET MOSFET is modified form of JFET in which input impedence is larger than JFET so it widly use than JFET.

(i)depletion MOSFET: In N- channel MOSFET two highly doped N-type regions are diffused in a lightly dopped P-type substrate, to serve as the source and drain. Channel is differed between the source and drain, with the same type of impurity as used for source and drain diffusion. The insulating thin layer (1000 to 2000 A) of silicon dioxide is grown over the surface of structure. The metal gate electrode (say of AL) is deposited on top the oxide layer (which is its name) on the surface. A P-channel MOSFET has a similar structure. It has a lightly dopped N-type substrate in which two heavily doped P-regions are formed at the two ends by diffusion to serve as the source and drain respectively.

(ii) Enhancement MOSFET: It consist of a lightly doped substrate of P-type silicon. Two highly doped N-region are diffused in the substrate which serve as the source and drain. As insulating very thin layer of silicon dioxide is deposited over the substrate. Then a thin film of metal aluminium is deposited over SiO². Structurally there exists no channel between the source and drain but still term used as N-channel enhan. FET because the thin layer of P-substrate touching the metal oxide film provides channel for electrons and hence acts like N-type material.

APPLICATIONS OF FIELD EFFECT TRANSISTORS FET can be used as amplifiers to amplify small signals, low noise, High gain. FETs can be used as switches. FETs can be used as protections diodes that is have low leakage currents. FETs can be used as current limiter and also as voltage controlled resistors. FETs can also be used as mixers and oscillators.