FET Characteristics

Objective:

To study Drain Characteristics and Transfer Characteristics of a Field Effect Transistor (FET).

Components:

S.No.	Name	Quantity
1	JFET (BFW11/ BFW10)	1(One) No.
2	Resistor (1K ,100K)	1(One) No. Each
3	Bread board	1(One) No.

Equipment:

Specifications:

For FET BFW11:

• Gate Source Voltage V_GS = -30V
• Forward Gain Current I_GF = 10mA
• Maximum Power Dissipation P_D = 300mW

Circuit Diagram:

Figure 1.

Pin assignment of FET:

Top View

Bottom View

Operation:

The circuit diagram for studying drain and transfer characteristics is shown in the figure1.

1. Drain characteristics are obtained between the drain to source voltage (V_DS)  and drain current (I_D) taking gate to source voltage (V_GS) as the constant parameter.
2. Transfer characteristics are obtained between the gate to source voltage (V_GS)  and drain current (I_D) taking drain to source voltage (V_DS) as the constant parameter.

Procedure:

Drain Characteristics:

1. Connect the circuit as shown in the figure1.
2. Keep V_GS = 0V by varying V_GG.
3. Varying V_DD gradually in steps of 1V up to 10V note down drain current I_D and drain to source voltage (V_DS).
4. Repeat above procedure for V_GS = -1V.

Transfer Characteristics:

1. Connect the circuit as shown in the figure1.
2. Set voltage V_DS = 2V/5V (BFW10/ BFW11).
3. Varying V_DD in steps of 0.5V until the current I_D reduces to minimum value.
4. Varying V_GG gradually, note down both drain current I_D and gate-source voltage(V_GS).
5. Repeat above procedure (step 3) for V_DS = 4V/ 8V (BFW10/ BFW11).

Observations:

Graph:

1. Plot the drain characteristics by taking V_DS on X-axis and I_D on Y-axis at a constant V_GS.
2. Plot the transfer characteristics by taking V_GS on X-axis and taking I_D on Y-axis at constant V_DS.

Calculations from Graph:

1. Drain Resistance (r_d): It is given by the relation of small change in drain to source voltage(V_DS) to the corresponding change in Drain Current(I_D) for a constant gate to source voltage (V_GS), when the JFET is operating in pinch-off region.

   at a constant V_GS (from drain characteristics)

2. Trans Conductance (g_m): Ratio of small change in drain current(I_D) to the corresponding change in gate to source voltage (V_GS) for a constant V_DS.

g_m     at constant V_DS (from transfer characteristics).

The value of g_m is expressed in mho’s () or Siemens (s).

3. Amplification factor (µ): It is given by the ratio of small change in drain to source voltage (V_DS) to the corresponding change in gate to source voltage (V_GS) for a constant drain current (I_D).

Inference:

1. As the gate to source voltage (V_GS) is increased above zero, pinch off voltage is increased at a smaller value of drain current as compared to that when V_GS = 0V.
2. The value of drain to source voltage (V_DS) is decreased as compared to that when V_GS = 0V. 

Precautions:

1. While performing the experiment do not exceed the ratings of the FET. This may lead to damage of FET.
2. Connect voltmeter and ammeter with correct polarities as shown in the circuit diagram.
3. Do not switch ON the power supply unless the circuit connections are checked as per the circuit diagram.
4. Properly identify the Source, Drain and Gate terminals of the transistor.

Result:

Drain and Transfer characteristics of a FET are studied.

Outcomes:  Students are able to

1. analyze the Drain and transfer characteristics of FET in Common Source configuration.
2. calculate the parameters transconductance (g_m), drain resistance (r_d) and amplification factor(µ).

Viva Questions:

1. Why FET is called a Unipolar device?

Ans: FETs are unipolar transistors as they involve single-carrier-type operation.

2. What are the advantages of FET?

Ans: The main advantage of the FET is its high input resistance, on the order of 100 MΩ or more. Thus, it is a voltage-controlled device, and shows a high degree of isolation between input and output. It is a unipolar device, depending only upon majority current flow. It is less noisy. and is thus found in FM tuners and in low-noise amplifiers for VHF and satellite receivers. It is relatively immune to radiation. It exhibits no offset voltage at zero drain current and hence makes an excellent signal chopper. It typically has better thermal stability than a bipolar junction transistor (BJT)

3. What is transconductance?

Ans: Trasconductance is an expression of the performance of a bipolar transistor or field-effect transistor (FET). In general, the larger the transconductance figure for a device, the greater the gain(amplification) it is capable of delivering, when all other factors are held constant. The symbol for transconductance is gm. The unit is thesiemens, the same unit that is used for direct-current (DC) conductance.

4. What are the disadvantages of FET?

Ans: It has a relatively low gain-bandwidth product compared to a BJT. The MOSFET has a drawback of being very susceptible to overload voltages, thus requiring special handling during installation.The fragile insulating layer of the MOSFET between the gate and channel makes it vulnerable to electrostatic damage during handling. This is not usually a problem after the device has been installed in a properly designed circuit.

5. Relation between µ, g_m and r_d?

Ans: µ = g_m * r_d

6. Why an input characteristic of FET is not drawn?

Ans:In FET always input is reverse biased (V_GS ), I_G=0, there exists minimum I_GSS  with high input impedance.It is in the range of Mohms.So, any value of V_GS , I_G=0.

7. What is the importance of high input impedance?

Ans: In FET the input impedance is very high compared to BJT.This very high input impedance makes them very sensitive to input voltage signals.

8. Why wedge shaped depletion region is formed in FET under reverse bias gate condition?

Ans:FET under reverse bias gate condition the gate is more “negative” with respect to Drain voltage than source voltage. Hence, depletion layers penetrate more deeply into the channel at points lying closer to Drain than to Source. Thus wedge-shaped depletion regions are formed.

9. Why FET is less noisy compared to BJT?

Ans:Generally FET is less noisy compared BJT because FET current depends on majority carriers only where as BJT current depends on both majority and minority carriers, BJT has 2-PN junctions when current passing through the junctions more thermal noise will be added where as in FET no junctions exists so, it is less noisy cpmpared to BJT.

10. What is the difference between n- channel FET and p-channel FET?

Ans:

11.What are the applications of FET.

Ans:FET is used as a buffer in measuring instruments, receivers since it has high input impedance and low output impedance, used in RF amplifiers in FM tuners and used digital circuits in computers.

12.Classify different FET devices.

Ans:Based on the construction FETs can be classified into 2-types as Junction FET and Metal oxide semiconductor FET or Insulated gate FET or Metal oxide silicon transistor. Depending upon the majority carriers, JFET has been classified into two types namely, 1. N-channel JFET and 2. P-channel JFET. In the same way MOSFET classified as 1.N-channel MOSFET and 2.P-channel MOSFET.

13.Give the expression for saturation Drain current.

Ans:    Where  I_DS is the saturation drain current, I_DSS is the value of I_DS when V_GS=0, and V_P is the pinch -off voltage.

14.Define Pinch-off voltage.

Ans:In FET the voltage V_DS at which the current I_D reaches to its constant saturation level is called Pinch-off Voltage, V_P.