Microwave Engineering Multiple Choice Questions on “Field Effect Transistors”.
1. Field effect transistors are different from BJTs in that they are _________
A. monopolar devices
B. bipolar devices
C. bidirectional device
D. none of the mentioned
Answer: A
Clarification: FETs are called monopolar devices, with only one carrier type, either electrons or holes providing current flow through the device. N-channel FETs employ electrons while p-channel FETs employ holes as source of current.
2. GaAs MESFET –metal semiconductor field effect transistor are one of the widely used categories of FETs.
A. true
B. false
Answer: A
Clarification: One of the most important developments in microwave technology has been the GaAs metal semiconductor field effect transistor, as this device permitted the first practical solid-state implementation of amplifiers and oscillators.
3. At frequencies above 10GHz, MESFET are not suitable for microwave applications due to parasitic effects.
A. true
B. false
Answer: B
Clarification: GaAs MESFETs can be used at frequencies well into the millimeter wave range, with high gain and low noise figure often making them the device of choice in monolithic microwave integrated circuits above frequencies of 10 GHz.
4. Advantage of using GaAs in MESFET as compared to use of silicon is:
A. GaAs are cost effective
B. they have higher mobility
C. they have high resistance for flow of current in the reverse direction
D. none of the mentioned
Answer: B
Clarification: The desired high gain and noise features of this transistor are a result of high electron mobility of GaAs compared to silicon and the absence of shot noise in them.
5. In MESFET, an applied signal at the gate modulates the electron carriers; this produces _______ in the FET.
A. voltage amplification
B. voltage attenuation
C. electron multiplication
D. electron recombination
Answer: A
Clarification: In operation, the electrons are drawn from the source to drain by a positive voltage applied to the source and drain. These carriers are modulated by the voltage applied to the gate hence resulting in voltage amplification.
6. The frequency of operation of an FET is limited by:
A. drain to source voltage
B. gate to source voltage
C. gate length
D. effective area of an FET
Answer: C
Clarification: The frequency of operation of an FET is given by the gate length. Present FETs have a gate length of the order of 0.2-0.6 µm, with corresponding upper frequency limits of 100-50 GHz.
7. The S21 parameter for a MESFET is lesser than 1.
A. true
B. false
Answer: B
Clarification: From the analysis of the small signal equivalent circuit of MESFE, S21 parameter of the transistor was found to be greater than one under normal operating conditions. Here port 1 is at the gate and port 2 is at the drain.
8. The expression for short circuit current gain of an FET is given by:
A. gm/ ωCgs
B. Ig/gmVc
C. ωCgs/ gm
D. none of the mentioned
Answer: A
Clarification: Short circuit current gain of an FET is defined as the ratio of drain current to gate current when the output is short circuited. This is expressed as ID/IG. This ratio in simplified form is given as gm/ ωCgs.
9. The upper threshold frequency of an FET, where short circuit gain is unity is given by:
A. gm/2πCgs
B. gm/Cgs
C. gm/ 2π
D. none of the mentioned
Answer: A
Clarification: The upper threshold frequency is dependent on the factor gm, associated with the current generator of the small signal equivalent circuit. Cgs is the capacitance measured between the gate and source terminals.
10. The scattering parameter S11 for an FET __________ with increase in the frequency of operation of the transistor.
A. increases
B. decreases
C. remains constant
D. none of the mentioned
Answer: B
Clarification: S11 parameter of an FET decreases with the increase in the frequency of operation of an FET. The measured values are 0.97 at 1 GHz, and 0.49 at 12 GHz.
11. The curve of IDS v/s VDS of an FET does not vary with the gate to source voltage applied.
A. true
B. false
Answer: B
Clarification: Curve of IDS v/s VDS of an FET varies with the gate to source voltage applied. As the gate to source voltage applied becomes more positive, the drain to source current goes on increasing for an applied constant gate to source voltage.
12. High-power circuits generally use higher values of:
A. gate to source current
B. drain to source current
C. drain current
D. gate to source voltage
Answer: C
Clarification: In order to achieve high drain current for high power applications, DC bias voltage must be applied to both gate and the drain, without disturbing the RF signal paths.
13. High drain current at RF levels is achieved with the biasing and decoupling circuitry for a dual polarity supply.
A. true
B. false
Answer: A
Clarification: High drain current at RF levels is achieved with the biasing and decoupling circuitry for a dual polarity supply. The RF chokes provide a very low DC resistance for biasing, and a very high impedance at RF frequencies to isolate the signal from the bias supply.
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