JFET - Junction Field Effect Transistor

A Junction field effect transistor is a three terminal device. The terminals are source (S) ,drain (D) and gate (G).

Majority charge carriers enter the transistor through gate terminal and leave the transistor through drain terminal.

Gate is the controlling terminal  i.e  voltage at gate with respect to source, controls the amount of current flowing through the JFET.

JFETs are of two types depending on their channel :- n-channel JFET and p-channel JFET . The JFET symbols are shown below.

JFET Construction

A JFET (n-channel JFET is discussed as it is commonly used.The p-channel JFET construction will be almost same) in its simplest form looks like as

shown below.

To make a n-type JFET a pure semiconductor bar is taken. This bar is then doped p-type at two positions as shown in above diagram and the remaining portion is converted to n-type. Thus two pn-junctions are created.

The p-type region is heavily doped and the n-channel is lightly doped so that pn-junction grows much deeper into n-channel and less deeper into p-region.

The so created P-regions are also provided ohmic contacts  and the two terminals from the two p-regions are joined together to make single terminal called gate terminal.

JFET Working

If an n-channel JFET is biased as shown with gate-to-source voltage   (V_GS)  zero. Then, due to positive drain-to-source voltage V_DS , few electrons available for conduction in the n-type material will start moving through the channel  towards the drain. Thus, a current called drain current I_D , starts flowing. 
Due to finite resistance of the channel, a voltage drops across the length of the channel. So now, voltage at  ,n-channel nearer to pn-junction, becomes greater than the voltage at ,p-type material nearer to pn-junction. Therefore, Pn- junction gets reverse biased and starts widening and penetrates more into the n-type material as it is lightly doped. Due to this, cross-section available for electrons to conduct electricity, has reduced.

when we apply a negative V_GS  and  increase  it ,the junction spreads more and more into the n-type bar and the drain current  keeps decreasing. At a certain V_GS  value, the drain current becomes zero as the two junctions have spread into the n-channel so much that they completely touch each other and therefore blocks electron path.

As seen the  gate-to-source voltage  controls current through the device. Hence, JFET is a voltage controlled device. 

The relationship between drain current and gate-to-source voltage is given by shockley 's equation

Where, V_P is the pinch-off voltage. Pinch off voltage is the value  of drain-to-source voltage (V_DS) at which drain current reaches its constant saturation value. Any further increase in V_DS  does not increase drain current (I_D) as all the electrons available for conduction have already been swept.

As current is mainly carried by only one type of carriers, electrons for n-channel JFET and holes for p-channel JFET,  a JFET is a unipolar device.  

JFET Characteristics 

The JFET static characteristics are explained along with JFET applications .

(1) Drain characteristics :- are the curves drawn between drain current I_D and drain-to-source voltage V_DS with gate-to-source voltage  V_GS as the parameter.

It has three regions :-

(1) Ohmic region :- Below pinch-off , the channel essentially behave like a constant resistance as I_D varies linearly with V_DS .Therefore, by varying V_GS for a given V_DS  , a JFET can be used as a variable resistor.

(2) Active or saturation region :- Beyond pinch-off voltages, the curves become essentially flat, this region is called active or saturation region. This is where we want to be to perform linear amplification i.e JFET is used as an amplifier. For a given V_DS , I_D decreases as gate-to-source voltage V_GS becomes more and more negative.

(3) Breakdown region :- As V_DS continue to increase, a point is reached when drain-to-source voltage becomes so large that avalanche breakdown occurs at the drain end of the gate-channel junction. This voltage is called breakdown voltage V_BDS  of the JFET.  V_BDS  is function of V_GS . As V_GS  becomes more negative, for n-channel JFET, the breakdown voltage decreases.

(2) Transfer characteristics :-  The graphical plot of the saturation drain current against the gate-to-source voltage is known as the transfer characteristics of a Junction Field Effect Transistor. It can be obtained from drain characteristics very easily.

JFET as a switch :- This is another JFET application. From characteristics it can be understood easily that the JFET can be used as a switch by operating it in two regions - cut off and saturation region.

When V_GS is zero ,the JFET operates in saturation region acting as a closed switch. when gate-to-source voltage is sufficiently negative making the device operate in cut-off region, thus acting as an open switch.