Characteristics of JFET

Characteristics of JFET

• To understand electrical behaviour of a JFET, it is necessary to study the interrelation of the current and voltages in JFET. These relationships can be plotted graphically which are commonly known as the characteristics of JFET.

• The important characteristics of JFET are drain characteristics and transfer characteristics.

1. Drain Characteristics

Drain Characteristics of n‒Channel JFET

• Fig. 5.5.1 shows the drain characteristics of a n‒channel JFET. The curves represent relationship between the drain current I_D and drain to source voltage V_DS for different values of V_GS. Fig. 5.5.2 shows the experimental setup required to plot this characteristics.

• V_GS and V_DS both = 0: When V_GS = 0 the channel is entirely open. But V_DS = 0, so there is no attractive force for the majority carriers (electrons in n‒channel JFET) and hence drain current does not flow.

• Self pinch‒off at no bias (V_GS = 0): At V_GS = 0, in response to a small applied voltage V_DS, the n‒type bar acts as a simple semiconductor resistor, and the current I_D increases linearly with V_DS. As V_DS increases, the voltage drop along the channel also increases. This increase in voltage drop increases the reverse bias on gate‒source junction and causes the depletion regions to penetrate into the channel, reducing channel width. The effect of reduction in channel width provides more opposition to increase in drain current I_D. Thus, rate of increase in I_D with respect to V_DS is now reduced. This is shown by the curved shape in the characteristics.

• At some value of V_DS, drain current I_D cannot be increased further, due to reduction in channel width. Any further increase in V_DS does not increase the drain current I_D. I_D approaches the constant saturation value. The voltage V_DS at which the current I_D reaches to its constant saturation level is called 'Pinch‒Off Voltage', V_P.

• V_GS with negative bias: When an external bias, of say ‒1 V, is applied between the gate and the source, the gate channel junctions are further reverse biased, reducing the effective width of the channel available for the conduction. Because of this, drain current will reduce and pinch off voltage is reached at a lower drain current than when V_GS = 0, as shown in Fig. 5.5.1.

• By applying several values of negative external bias voltage (V_GS), a family of curves are obtained as shown in Fig. 5.5.1. From Fig. 5.5.1 it can be observed that for more negative values of V_GS, the pinch‒off voltage is reached at lesser values of I_D.

• Breakdown region: We can observe from Fig. 5.5.1 that if we increase value of V_DS beyond pinch‒off voltage, Vp, the drain current I_D remains constant, up to certain value of V_DS. If we further exceed V_DS, the voltage will be reached at which the gate‒channel junction breaks down, due to avalanche effect. At this point the drain current increases very rapidly, and the device may be destroyed.

• It can be observed that the values of V_DS for breakdown are reduced as the negative gate bias is increased. This is because the total reverse breakdown voltage is the addition of the reverse voltage due to self pinch‒off and the externally applied voltage V_GS.

• Ohmic and saturation regions:

It is seen that the drain characteristics of JFET is divided into two regions ohmic region and saturation region. In the ohmic region, the drain current I_D varies with V_DS and the JFET is said to behave as voltage variable resistance.

In the saturation region, the drain current I_D remains fairly constant and does not vary with V_DS.

• Cut‒off: As we know, for an n‒channel JFET, the more negative V_GS causes drain current to reduce and pinch‒off voltage to reach at a lower drain current. When V_GS is made sufficiently negative, I_D is reduced to 0, as shown in Fig. 5.5.1. This is caused by the widening of the depletion region to a point where it completely closes the channel. The value of V_GS at the cut‒off point is designated as V_GS(OFF).

• Relation of V_GS(off) and V_P: I_D is 0 when V_GS = ‒ V_P

Drain Characteristics of p‒Channel JFET

• In a p‒channel JFET the source is positive with respect to the drain. Here the source is the source of holes which flow through the channel to the drain. The pinch‒off is achieved by making the source to gate voltage, V_SG negative (i.e. V_GS positive) there by reverse biasing the p‒n junction diode formed by the channel and the gate.

• Fig. 5.5.3 shows the drain characteristics of p‒channel JFET. Note the similarities between these characteristics and those shown for n‒channel JFET in Fig. 5.5.1.

• The curves are identical except that voltage V_GS and V_DS have reversed polarities and current I_D flows in reverse direction.

2. Transfer Characteristics

Transfer Characteristics of n‒Channel JFET

• The relationship between the drain current I_D and gate to source voltage V _GS is non‒linear as shown in Fig. 5.5.4. This relationship is defined by Shockley's equation

I_D = I_DSS ( 1 ‒ V_GS/V_P )²

             ………….. (5.5.1)

• The squared term of the equation will result in a non‒linear relationship between I_D and V_GS producing a curve that grows exponentially with decreasing magnitudes of V_GS. From equation we can also write,

• In the equation values of I_DSS and V_P are constants, value of V_GD controls I_D.

• A point A at the bottom end of the curve on the V_GS‒axis represents V_GS(off), and point B at the top end of the curve on the I_D axis represents I_DSS (maximum drain current at V_GS =0). Thus, this curve shows the operating limits of a JFET. These are :

• I_D = 0 when V_GS = V_{GS (off)}

• I_D = I_DSS when V_GS = 0

Transfer Characteristics of p‒Channel JFET

• Fig. 5.5.5 shows the transfer characteristics of p‒channel JFET. It is identical to transfer characteristics of n‒channel JFET except that the polarities of V_GS and I_D are reversed.

Review Questions

1. Sketch and explain the drain characteristics for an n‒channel FET.

2. Draw a circuit diagram to obtain the drain characteristics for an n‒channel JFET. Thus draw drain characteristics and explain them.

3. Explain the drain characteristics for p‒channel FET.

4. Explain the transfer characteristics for n‒channel JFET.

5. Explain the transfer characteristics for p‒channel JFET.

6. What do you understand by 'pinch off voltages' and 'cut‒off voltages'?

7. Draw a circuit for obtaining drain and transfer characteristics for an n‒channel JFET.

8. Sketch a typical drain characteristic for an n‒channel JFET. Explain the shape of the characteristic and identify the regions.

9. Sketch the basic circuits for both N channel JFET and P‒channel JFET.

10. Sketch and explain the drain‒characteristics and transfer‒characteristics of P‒channel JFET.

11. When a FET acts as a voltage variable resistor?

12. Explain the four distinct regions of the output characteristics of the JFET.

13. Discuss the drain and transfer characteristics of JFETS.