# Microelectronics Questions and Answers – Physics of Bipolar Transistors

This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “Physics of Bipolar Transistors”.

1. A circuit contains 28 transistors named Qi. All the even numbered transistors and all the odd numbered transistors are made parallel. The area of each even numbered B.J.T. is made 36/84 times the odd numbered transistor. The early effect is neglected. Choose the least number of transistors required to reach such a configuration if each series is connected in as a C.E. stage.
a) 2
b) 10
c) 14
d) 1

Explanation: We have a cascade of two stages formed by the even and odd numbered Q’s. If the total area of odd numbered transistors is 14p, where p is the area of each odd numbered transistor, then the total area of even numbered ones is 6p. The even stage and the odd stage can be replaced by one transistor for each stage which have the area 14p and 6p respectively. Hence, the minimum number of transistors required will be 2.

2. A circuit contains 28 transistors named Qi. All the even numbered transistors and all the odd numbered transistors are made parallel. The area of each even numbered B.J.T. is made 36/84 times the odd numbered transistor. The early effect is neglected. If we replace the circuit by one transistor whose area is equal to area of 28 transistors, we will get the same gain.
a) True
b) False

Explanation: Cascading two stages imply multiplying the gain provided by each stage. The total area of all the transistors turn out to be 20*A where A is the area of all the transistors. Even if we make a transistor with the same area, the net gain will only be 20 times whereas the cascaded stage gives a total gain of about 84 times (in the ideal case) of one transistor.

3. Which of the following event leads to maximum current flow from base to emitter within a npn B.J.T.?
a) Drift
b) Diffusion
c) Hole drift
d) Electron drift

Explanation: It is often assumed that the depletion layer sustains the entire electric field generated within the transistor and the amount of drift current is very less. Hence diffusion is the remaining mechanism for the flow of charge carriers, in this case electrons, from the emitter to the base.

4. How does the emitter area affect the reverse saturation current?
a) Directly proportional
b) Inversely proportional
c) Directly proportional to twice the emitter current
d) Directly proportional to thrice the emitter current?

Explanation: The reverse saturation current is directly proportional to the area of the emitter. An increase in the emitter area hence leads to more gain in the transistor.

5. An increase in the doping level of the base would _____ the reverse saturation current.
a) Decrease
b) Increase
c) Double

Explanation: The doping level in the base is kept small so that the concentration gradient can increase. An increase in doping level of the base will decrease the reverse saturation current.

6. If the width of the base increases by factor of 10, what happens to the collector current?
a) It increases 10 times
b) It decreases 10times
c) It decreases 5 times
d) It doesn’t get affected

Explanation: The base width is kept very small to maintain a strong concentration gradient. Hence the collector would decrease to about 10 times the original value.

7. Which co-efficient, present in the mathematical expression relating the collector current to the bias voltage, plays a dominant role in differentiating a npn form a pnp transistor?
a) Diffusivity
b) Mobility
c) Both diffusivity and mobility
d) Emitter width

Explanation: Diffusion is responsible for the majority current flow through the base emitter junction. The value of diffusivity plays the dominant role.

8. A npn transistor allows faster operation than a pnp transistor.
a) True
b) False

Explanation: The current flow within npn transistors is majorly due to electrons since the emitter doping is n-type. Hence, the above statement is true since electrons have a higher mobility than holes.

9. The ratio of collector to emitter current is often approximated as α. If α is required to be 1, which current within the base emitter junction is required to be reduced in the case of a npn transistor?
a) The current due to diffusion of holes from base to emitter
b) The current due to diffusion of holes from collector to emitter
c) The current due to diffusion of holes from collector to base
d) The current due to diffusion of holes from base to collector

Explanation: The current due to diffusion of holes from emitter to base is a part of the emitter current. This current doesn’t contribute to the collector current while the emitter current due to diffusion of electrons from emitter to base contributes to the collector current in an npn transistor.

10. What happens due to the base width modulation?
a) Drift current increases
b) Diffusion current decreases
c) Diffusion current increases
d) Base width increases

Explanation: Base width modulation occurs due to a decrease in the width of the base as the depletion region widens if VCB is reverse biased. A reduction in the width of the base would in further increase the collector current since the concentration gradient increases and the diffusion current increases.

11. The Early effect affects the transconductance of the device. How?
a) It increases the transconductance
b) It decreases the transconductance
c) It doubles the transconductance
d) It increases the transconductance by a factor of 5

Explanation: The early effect increases the transconductance since more current flows from the base to the collector as a result of reduced base width. Specifically, it increases it by a factor of (1 + VCE/vA) where VA is the early voltage.

12. If the B.J.T. is made to operate in the active mode, it will amplify the input signal. Is this statement always true?
a) Yes
b) No
c) No, it depends on the output terminal
d) Yes, it is independent of the output terminal

Explanation: The output terminal of a B.J.T. is either the collector or the emitter terminal. The output from the collector terminal is always an amplified version of the input terminal while that from the emitter terminal is an attenuated version of the input terminal, the input being the base terminal. Hence, the change in the output is dependent on the output terminal.

13. The early effect leads to a change in the output voltage which is measured at the collector terminal. What kind of change is found in absence of the early effect?
a) The gain increases
b) The gain decreases
c) The gain is independent on the early effect
d) Cannot be concluded

Explanation: Comparing the gain in absence of early effect to when early effect is considered, we find that the gain is higher in absence of early effect.

14. The threshold voltage of the diode and the transistor is 0.6V and 0.7V. If VCB=0.2V, VCC=5V and R1=1KΩ, calculate VCE in the active mode?

a) 4.9V
b) 0.9V
c) 5V
d) Incomplete data

Explanation: VBE is 4.4V since the diode takes a drop of 0.6V. It must be noted that the diode is connected to a voltage supply without a resistor and this is harmful as the answer becomes 0 if the diode gets damage. The data is incomplete since for active mode, the assumption that VCB=0.2V doesn’t stay since and the value of Is is required to calculate the value of VCE.

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