Antennas And Microwave Engineering Set 5

Electrical Engineering

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This set of Antennas and Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on Antennas And Microwave Engineering Set 5

Q1 | In a GaAs n-type specimen, the current generated is constant irrespective of the electric filed applied to the specimen.
  • true
  • false
Answer: false
Explaination:
in a gaas n-type specimen, when the electric field applied reaches a threshold value of eth, the current in the specimen becomes suddenly oscillatory and with respect to time and these oscillations are in the microwave frequency range. this effect is called gunn effect.
Q2 | When the electric field applied to GaAs specimen is less than the threshold electric field, the current in the material:
  • increases linearly
  • decreases linearly
  • increases exponentially
  • decreases exponentially
Answer: increases linearly
Explaination:
when the electric field applied is less than the threshold value of electric field, the electrons jump from the valence band to the primary valley of the conduction band and current increases linearly with electric field.
Q3 | GaAs is used in fabricating Gunn diode. Gunn diode is:
  • bulk device
  • sliced device
  • made of different type of semiconductor layers
  • none of the mentioned
Answer: bulk device
Explaination:
a gunn diode is a bulk device, that is, it does not contain any junction but it is a slice of n-type gaas. p- type gaas does not exhibit gunn effect. hence it is a reversible and can be operated in both directions.
Q4 | The electrodes of a Gunn diode are made of:
  • molybdenum
  • gaas
  • gold
  • copper
Answer: molybdenum
Explaination:
gunn diode is grown epitaxially onto a gold or copper plated molybdenum electrode, out of gallium arsenide doped with silicon, tellurium or selenium to make it n-type.
Q5 | When either a voltage or current is applied to the terminals of bulk solid state compound GaAs, a differential              is developed in that bulk device.
  • negative resistance
  • positive resistance
  • negative voltage
  • none of the mentioned
Answer: negative resistance
Explaination:
when either a voltage or current is applied to the terminals of a sample of bulk solid state compound formed by group 5 and 3 elements of periodic table, a differential resistance is developed in the bulk device. this fundamental concept is called rwh theory.
Q6 | The number of modes of operation for n type GaAs is:
  • two
  • three
  • four
  • five
Answer: four
Explaination:
n-type gaas used for fabricating gunn diode has four modes of operation. they are gunn oscillation mode, limited space charge accumulation mode, and stable amplification mode bias circuit oscillation mode.
Q7 | The free electron concentration in N-type GaAs is controlled by:
  • effective doping
  • bias voltage
  • drive current
  • none of the mentioned
Answer: effective doping
Explaination:
the free electron concentration in n-type gaas is controlled through effective doping so that they range from 1014 to 1017 per cc at room temperature. the typical specimen of n-type gaas has the dimensions 150 µm by 150 µm.
Q8 | The modes of operation of a Gunn diode are illustrated in a plot of voltage applied to the Gunn diode v/s frequency of operation of Gunn diode.
  • true
  • false
Answer: false
Explaination:
a graph of plot of product of frequency and the length of the device plotted along y-axis versus the product of doping concentration and length along x- axis. these are the parameters on which the four modes of operation of gunn diode are explained.
Q9 | The mode of operation in which the Gunn diode is not stable is:
  • gunn oscillation mode
  • limited space charge accumulation mode
  • stable amplification mode
  • bias circuit oscillation mode
Answer: gunn oscillation mode
Explaination:
in gunn oscillation mode, the device is unstable due to the formation of accumulation layer and field domain. this high field domain moves from cathode to anode.
Q10 | The frequency of oscillation in Gunn diode is given by:
  • vdom/ leff
  • leff/ vdom
  • leff/ wvdom
  • none of the mentioned
Answer: vdom/ leff
Explaination:
in gunn oscillation mode, the frequency of oscillation is given by vdom/ leff, where vdom is the domain velocity, leff is effective length that the domain moves from the time it is formed until the time a new domain is formed.
Q11 | In Gunn diode oscillator, the Gunn diode is inserted into a waveguide cavity formed by a short circuit termination at one end
  • true
  • false
Answer: true
Explaination:
the gunn diode is mounted at the centre of the broad wall of a shorted waveguide since for the dominant te10 mode; the electric field is maximum at the centre.
Q12 | In a Gunn diode oscillator, the electron drift velocity was found to be 107 cm/second and the effective length is 20 microns, then the intrinsic frequency is:
  • 5 ghz
  • 6 ghz
  • 4 ghz
  • 2 ghz
Answer: 5 ghz
Explaination:
the intrinsic frequency for a gunn oscillator is given by vd/l. here vd is the drift velocity and l is the effective length. substituting the given values in the above equation, intrinsic frequency is 5 ghz.
Q13 | The material used to fabricate IMPATT diodes is GaAs since they have the highest efficiency in all aspects.
  • true
  • false
Answer: false
Explaination:
impatt diodes can be fabricated using silicon, germanium, gaas or indium phosphide. out of these materials, gaas have highest efficiency, low noise and high operating frequencies. but gaas has a major disadvantage of complex fabrication process and higher cost. so, gaas are not preferred over silicon and germanium.
Q14 | 3 IMPATT DIODES, SCHOTTKY BARRIER DIODES, PIN DIODES
  • avalanche multiplication
  • break down of depletion region
  • high reverse saturation current
  • none of the mentioned
Answer: avalanche multiplication
Explaination:
a reverse bias voltage exceeding the breakdown voltage is applied to an impatt diode, a high electric field appears across the n+ p junction. this high field imparts sufficient energy to the holes and also to valence electrons to raise themselves to the conduction band. this results in avalanche multiplication of electron hole pair.
Q15 | To prevent an IMPATT diode from burning, a constant bias source is used to maintain                at safe limit.
  • average current
  • average voltage
  • average bias voltage
  • average resistance
Answer: average current
Explaination:
avalanche multiplication is a cumulative process resulting in rapid increase of carrier density. to prevent the diode from burning due to this increased carrier density, a constant bias source is used to maintain average current at safe limit.
Q16 | The number of semiconductor layers in IMPATT diode is:
  • two
  • three
  • four
  • none of the mentioned
Answer: four
Explaination:
impatt diode consists of 4 layers according to the construction. it consists of a p+ region and n+ layers at the two ends. in between these layers, a p type layer and an intrinsic region is sandwiched.
Q17 | The resonant frequency of an IMPATT diode is given by:
  • vd/2l
  • vd/l
  • vd/2πl
  • vdd/4πl
Answer: vd/2l
Explaination:
the resonant frequency of an impatt diode is given by the expression vd/2l. here vd is the carrier drift velocity; l is the length of the intrinsic region in the impatt diode.
Q18 | If the length of the intrinsic region in IMPATT diode is 2 µm and the carrier drift velocity are 107 cm/s, then the drift time of the carrier is:
  • 10-11 seconds
  • 2×10-11 seconds
  • 2.5×10-11 seconds
  • none of the mentioned
Answer: 2×10-11 seconds
Explaination:
the drift time of the carrier is defined as the ratio of length of the intrinsic region to the carrier drift velocity.
Q19 | If the length of the intrinsic region in IMPATT diode is 2 µm and the carrier drift velocity are 107 cm/s, then the nominal frequency of the diode is:
  • 12 ghz
  • 25 ghz
  • 30 ghz
  • 24 ghz
Answer: 25 ghz
Explaination:
nominal frequency is defined as the ratio of the carrier drift velocity to twice the length of the intrinsic region.
Q20 | IMPATT diodes employ impact ionization technique which is a noisy mechanism of generating charge carriers.
  • true
  • false
Answer: true
Explaination:
impatt devices employ impact ionization techniques which is too noisy. hence in order to achieve low noise figure, impact ionization is avoided in baritt diodes. the minority injection is provided by punch through of the intermediate region.
Q21 | An essential requirement for the BARITT diode is that the intermediate drift region be completely filled to cause the punch through to occur.
  • true
  • false
Answer: false
Explaination:
an essential requirement for the baritt diode is that the intermediate drift region be completely filled to cause the punch through to the emitter-base junction without causing avalanche breakdown of the base collector junction.
Q22 | If the RMS peak current in an IMPATT diode is 700 mA and if DC input power is 6 watt, with the load resistance being equal to
  • 10.1 %
  • 10.21 %
  • 12 %
  • 15.2 %
Answer: 10.21 %
Explaination:
efficiency of impatt diode is defined as the ratio of output rms power to the input dc power. calculating the rms output power from the given rms current and substituting in the equation of efficiency, the efficiency is 10.21%.
Q23 | If the critical field in a Gunn diode oscillator is 3.2 KV/cm and effective length is 20 microns, then the critical voltage is:
  • 3.2 v
  • 6.4 v
  • 2.4 v
  • 6.5 v
Answer: 6.4 v
Explaination:
critical voltage of a gunn diode oscillator is given by the expression lec where l is the effective length and ec is the critical field. substituting the given values in the above equation, critical voltage is 6.4 volts.
Q24 | The production of power at higher frequencies is much simpler than production of power at low frequencies.
  • true
  • false
Answer: false
Explaination:
as frequency increases to the millimeter and sub millimeter ranges, it becomes increasingly more difficult to produce even moderate power with solid state devices, so microwave tubes become more useful at these higher frequencies.
Q25 | Microwave tubes are power sources themselves at higher frequencies and can be used independently without any other devices.
  • true
  • false
Answer: false
Explaination:
microwave tubes are not actually sources by themselves, but are high power amplifiers. these tubes are in conjunction with low power sources and this
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