71. The ‘buried layer’ reduces collector series resistance by providing,
A. A low resistivity current path from n-type layer to n+ contact layer
B. A low resistivity current path from p-type layer to n+ contact layer
C. A high resistivity current path from n-type layer to n+ contact layer
D. A high resistivity current path from p-type layer to n+ contact layer
Answer: A
A heavily doped n+ region is sandwiched between the n-type epitaxial collector and p-type substrate. This buried n+ region provides a low resistivity current path from the active collector region (n-type layer) to the collector contact (n+ contact layer). In effect, the n+ layer shunt n-layer of the collector region with respect to the flow of current, thus effectively reducing the collector resistance.
72. At what potential, the substrate of a vertical PNP transistor should be kept to attain good isolation?
A. Same potential
B. Positive potential
C. Different potential
D. Negative potential
Answer: D
The limitation of vertical PNP transistor is that collector has to be held at a fixed negative potential, as the substrate is to be held at the most negative potential in the circuit for providing good isolation.
73. Which method is used in the fabrication of a PNP transistor?
A. Vertical substrate PNP
B. Triple diffused PNP
C. Lateral PNP
D. All of the mentioned
Answer: D
PNP transistors in Integrated Circuits are fabricated in one of the following three ways.
74. State the correct reason for neglecting the PNP transistor.
A. Increase in the series collector resistance of PNP transistor
B. Parasitic capacitance appears between collector and substrate
C. Current gain of PNP transistor is as low as 1.5 to 30
D. None of the mentioned
Answer: C
The lateral PNP transistor has inferior characteristics as the base width is usually larger controlled by lateral diffusion of p-type impurities and photographic limitations during mask marking and alignment.
Therefore, the PNP transistor normally gives a current gain as low as 1.5 to 30 compared to 50-300 for the NPN transistor.
75. The diffusion of collector impurities in NPN transistor should be small because,
A. No additional diffusion or masking steps required
B. Bandwidth is controlled by lateral diffusion of p-type impurity
C. Collector need not be kept at a negative potential
D. None of the mentioned
Answer: D
Generally, n-type impurities have smaller diffusion constant than p-type impurities, the n-type collector moves very little while p-type moves appreciably. Therefore, the diffusion coefficient of the collector impurities should be as small as possible to avoid the movement of the collector junction.
76. The advantage of a Multi-emitter transistor is
A. To reduce fabrication steps
B. To save chip area
C. To lower design consideration
D. To provide a linear output
Answer: B
In a multi-emitter transistor n+ emitter is diffused at three places in the p-type base. Thus, it is possible to save chip area and enhance the component density of an IC.
77. Which transistor is best suitable to achieve very fast switching in digital circuits?
A. Lateral PNP transistor
B. Schottky transistor
C. multi-emitter transistor
D. NPN transistor
Answer: B
Fast switching can be achieved if the transistor is prevented from entering into saturation. In the Schottky transistor, a Schottky diode is used to clamp between base and collector. Whenever the base current increases to saturation, the diode conducts. Thus, the base to collector voltage drops to 0.4v (less than VBE(cut-in)=0.5) and the transistor does not enter into saturation.
78. Choose the appropriate value of diode to get a speedy diode from the given values of storage time (n) in a sec and forward voltage (V γ).
A. n = 56 , V γ = 0.96
B. n = 100 , V γ = 0.92
C. n = 9 , V γ = 0.85
D. n = 53 , V γ = 0.95
Answer: C
The diode with the lowest storage time and the lowest forward voltage drop is useful for getting a high-speed diode to be used in the digital integrated circuit.
79. The number of leads in the Schottky barrier diode is
A. Four
B. Three
C. Two
D. Six
Answer: C
Schottky barrier diode has two contact leads namely,
1. Schottky Barrier contact
2. Ohmic contact.
80. In the Schottky barrier diode, which contact has similar characteristics to that of an ordinary PN diode?
A. Ohmic contact
B. Schottky barrier contact
C. Both ohmic and Schottky barrier contact
D. None of the mentioned
Answer: B
A metal semiconductor is formed when aluminum is deposited directly upon n-type silicon. Its characteristics are found to be the same as in an ordinary PN junction diode (Although the physical mechanism is different).
