This set of Optical Communications online test focuses on “Phototransistors and Metal – Semiconductor – Metal Photodetectors”.
1. The _____________ is photosensitive to act as light gathering element.
a) Base-emitter junction
b) Base-collector junction
c) Collector-emitter junction
d) Base-collector junction and Base-emitter junction
Explanation: Base-collector junction is photosensitive in n-p-n phototransistor and act as light gathering element. This light absorbed affects the base current and gives multiplication of primary photocurrent in device.
2. A large secondary current _________________ in n-p-n InGaAs phototransistor is achieved.
a) Between base and collector
b) Between emitter and collector
c) Between base and emitter
Explanation: The photo-generated holes are swept to the base. This increases the forward bias device. This generates secondary current between emitter and collector.
3. _______ emitter-base and collector-base junction capacitances is achieved by use of hetero-structure along with _________ base resistance.
a) Low, high
b) High, low
c) Low, low
d) High, negligible
Explanation: In hetero-structure, there is low doping level in emitter and collector which is coupled with heavy doping base. This is due low emitter-base and collector-base junction capacitance and low base resistance. This allows large current gain.
4. A ________ is created by hetero-junction at collector-base junction.
a) Potential barrier
b) Depletion region
c) Parasitic capacitance
Explanation: Potential barrier is created at emitter-base junction by hetero-junction. This eliminates hole junction from base. This is achieved when junction is forward-biased and provides good emitter-base efficiency.
5. Phototransistors based on hetero-junction using _________ material are known as waveguide phototransistors.
c) InGaAsP/ InAlAs
Explanation: Phototransistor using InGaAsP/ InAlAs are known as waveguide phototransistors. They function as waveguide phototransistors. They function as high performance photo-detectors at 1.3 micro-meter wavelength. They utilize a passive waveguide layer under active transistor region.
6. A phototransistor has collector current of 18 mA, incident optical power of 128 μW with a wavelength of 1.24 μm. Determine an optical gain.
a) 1.407 *102
b) 19.407 *102
c) 2.407 *102
d) 3.407 *102
Explanation: The optical gain is given by-
G0=hcIc/λeP0, where h=Planck’s constant, Ic=collector current, λ=wavelength, P0=incident optical power.
7. For a phototransistor having gain of 116.5, wavelength of 1.28 μm, optical power 123μW. Determine collector current.
a) 0.123 mA
b) 0.0149 mA
c) 1.23 mA
d) 0.54 mA
Explanation: The collector current is given by-
Ic= G0λeP0/ hc, where h=Planck’s constant, Ic=collector current, λ=wavelength, P0=incident optical power.
8. The detection mechanism in the ____________ photo-detector includes inter sub-band transitions.
Explanation: The inter sub-band transitions are also known as type-2 transitions. It comprises of mini-bands within a single energy band, The detection mechanism in DWELL photo-detector includes inter sub-band transitions.
9. Which of the following is the difference between the n-p-n and conventional bipolar transistor?
a) Electric property
b) Magnetic property
c) Unconnected base
d) Emitter base efficiency
Explanation: The n-p-n bipolar transistor differs in the following ways: base is unconnected, base-collector junction is photosensitive as a light gathering element.
10. The n-p-n hetero-junction phototransistor is grown using ______________
a) Liquid-phase tranquilizers
b) Liquid-phase epistaxis
c) Solid substrate
d) Hetero poleax
Explanation: The technique LPE consists of a thin layer of n-type collector based on a p-type base layer. Liquid phase epistaxis is used in hetero-junction technology.
11. The _____________ at emitter-base junction gives good emitter base injection efficiency.
b) Depletion layer
Explanation: The hetero-junction at the emitter-base junction effectively eliminates hole injection from the base when the junction is forward biased. This gives good emitter-base injection efficiency.
12. Waveguide phototransistors utilize a ___________ waveguide layer under the _________ transistor region.
a) Active, passive
b) Passive, active
c) Homo, hetero
d) Hetero, homo
Explanation: Waveguide phototransistors are based on hetero-junction structure. They function as high-performance photo-detectors and thus utilize a passive waveguide layer under the active transistor region.
13. What is the main benefit of the waveguide structure over conventional hetero-junction phototransistor?
a) High depletion region
b) Depletion width
c) Increased photocurrent, responsivity
d) Low gain
Explanation: Waveguide structure offers increased photocurrent. Photocurrent is directly proportional to the responsivity; thus in turn increases responsivity.
14. Waveguide structure provides high quantum efficiency.
Explanation: Responsivity and quantum efficiency follow a different path. They are indirectly proportional to each other. Thus, in waveguide structure, as the responsivity increases, quantum efficiency remains low.
15. Metal-semiconductor-metal (MSM) photo-detectors are photoconductive detectors.
Explanation: MSM photo-detectors are the simplest of photo-detectors. It provides the simplest form of photo-detection within optical fiber communications and are photoconductive.
Sanfoundry Global Education & Learning Series – Optical Communications.
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