Field effect transistor and its characteristics:

Field effect transistor and its characteristics:

FET is the three terminal unipolar semiconductor devices in which output current is connected by applied electric field, it is called as FET. The current flow through FET is only due to only one type of charge carrier i.e. either electron or hole so called unipolar devices.

The input impedance of FET is very high and the flow of signal is due to only one type of majority charge carrier so commonly used as switching device. These are of following two type:

• Junction field transistor (JFET )
• Metal oxide semiconductor field effect transistor (MOSFET)

Junction field effect transistor:

The JFET is most commonly used than BJT (Bipolar junction transistor e.g. NPN, PNP). Also the size of JFET is smaller than BJT. These are following two types:

1. N-channel JFET
2. P-channel JFET

The JFET can be fabricated with N-channel or P-channel where as N-channel more preferred. The two p-type junction are diffused as the middle of the N-type junction narrow semiconductor bar at its opposite side to construct N-channel JFET.

This diffusion produces the two depletion layer across the bar. In between these two depletion layer, there is a space called as channel. The two p-type junction are internally connected and single load is taken out from its called as Gate(G). The upper and lower end of the N-channel JFET are called as Drain (D) and source (s) respectively charge carrier into the bar and the Drain has to leave the majority carrier out of bar.

Working principle:

The properly biased (reversed biasing gate source terminal and forward biasing of drain source terminal). The properly biased N-channel JFET is shown in figure with supply voltage \(V_{GG}\) and \(V_{DD}\) as in figure. The potential across the gate source terminals \(V_{GS}\) and that across drain source terminal \(V_{DS}\) can controls drain current (\(I_D\)).

When \(V_{DS}\) is given with no \(V_{GS}\) (i.e.\(V_{GS}=0\)) the current starts to flow from source to drain terminal. There is depletion layer between two p-n junction which determines the width of channel between them and further determine drain current.

When \(V_{GS}\) is given the depletion layer increases with increase in reverse potential. The width of depletion decreases channel width and hence current flow from source to drain also decreases when the value \(V_{GS}\) decreases the depletion layer also decrease increasing channel width. This finally causes the drain current.

In this way, current flow from source to drain is controlled by application of electronic field across gate. Hence it is called as FET.

Characteristics of JFET:

The JFET characteristics can be studied under following two headings

1. Drain characteristics

The variation of drain current (\(I_D\)) with drain source voltage (\(V_{DS}\)) for constant value of gate source voltage (\(V_{GS}\) is called as drain characteristics of JFET. It is shown in figure and also called as output characteristics of JFET.

The plot of \(I_D\) versus \(V_{DS}\) for different constant values of \(V_{GS}\) is shown. From this plot it is seen that the drain current increases rapidly up to certain limiting value of \(V_{DS}\) in ohmic region. After, this value \(V_{DS}\), \(I_D\) almost remains constant in active region of it. This is because, in this region due to applied \(V_{DS}\) the depletion layer increases decreasing the channel width so that the small current through narrow channel flows (almost constant) as in figure. The value of applied drain source voltage at which maximum drain current at which maximum drain current flow, when the gate source voltage is reduced to zero, is called pinch off voltage. The value of drain current at shorted gate source voltage(\(V_{GS}\)) at which maximum drain current flows is called as \(I_{DSS}\) i.e. drain current at shorted gate. When the reverse voltage (\(V_{GS}\) is strong enough so that the depletion layer almost touch as each either which blocks the drain current due to blocking of channel is called as \(V_{GS}\) (off) i.e. shorted drain current due to \(V_{GS}\). After this value it enters to breakdown region.

B) Transfer characteristics:

The variation of drain current with variation of gate-source reverse voltage at constant value drain source voltage is called as transfer characteristics is shown in figure . From the figure it is seen that, the maximum drain current in flow at shorted \(I_D=0\) at \(V_{GS}=V_{GS}(off)\) due to blocking of current.

The variation of drain current is given by the relation,

$$I_D=I_DSS\biggl(1-\frac{V_{GS}}{V_{GS}(off)}\biggr)$$

$$I_D=I_{DSS}$$When \(V_{GS}=0\)

This maximum value of drain current .

Reference:

(1)Theraja, B.L. Basic Electronics. N.p.: S.Chand, n.d. Print.

(2)C.L.Arora. Refresher Course in Physics. Vol. II and III. N.p.: S.Chand, 2006. Print.

(3)Malvino. Electronic Principles. N.p.: Tata McGraw-Hill, n.d. Print.

(4)N.Nelkon and P.Parker. Advanced Level Physics. 5th ed. N.p.: Arnold Heinemann, n.d. Print.

(5)Priti Bhakta Adhikari,Diya Nidhi Chaatkuli, Ishowr Prasad Koirala. A Textbook of Physics (2nd Year). N.p.: Sukunda Pustak Bhawan, 2070. Print.