Microelectronics Questions and Answers – Characteristics of a BJT

This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “Characteristics of a BJT”.

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. What will be the total area of odd numbered 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. Choose the ratio of current contribution if the two series are made parallel even further.
a) 2/7
b) 1/5
c) 3/9
d) 4/901

Explanation: The current flowing from the transistor is directly proportional to the area of the transistor. Thus, the ratio of total area of even and odd numbered series of transistors gives us the ratio of current distribution under the given condition. The answer becomes 2/7.

3. The following figure shows the small signal model of a transistor. Determine the output resistance of the transistor.

a) Rout
b) 0
c) Infinite
d) Undefined

Explanation: The output resistance of the transistor is simply Rout. This resistance is the total resistance connected to the collector junction. To calculate the output resistance, we perform the Thevenin equivalent at the output or the collector node.

4. Each transistor results in a current Ic with an IS of 10-14A. Calculate the change in the current through R1 if the area of Q2 is doubled but the base width of Q1 is halved.

a) No change
b) It increases by a factor of 4
c) It decreases by a factor of 2
d) It increases by a factor of 1.5

Explanation: The total current through the resistor is the current generated by each Q. We find that the base voltage is fixed so the collector current is directly proportional to IS. IS gets doubled for Q2 but is halved for Q1. Thus the total current increases by factor of 1.5.

5. The width of the base region, of a transistor, is kept a bit larger than the minority carrier diffusion length in the base region.
a) True
b) False

Explanation: The width of the base region is kept sincerely lower than the minority carrier diffusion length in the base region. This is to maintain a high concentration gradient so that all the minority carriers travel into the collector in a very short time.

6. The base transport factor, for a npn transistor, is a ratio of electron current density at the BC junction to that that of the _____
a) BE junction
b) CE junciton
c) Hole current density at the BC junction
d) No such factor exists

Explanation: The base transport factor represents the amount of recombination happening in the base due to excess minority carriers, in this case electrons, in the base.

7. Which carriers are considered while calculating the bae transport factor of a pnp transistor?
a) Electrons
b) Holes
c) Electrons & Holes
d) No such factor is present

Explanation: Holes are the majority carriers which contribute to the maximum current in a pnp transistor. The base transport factor, for a pnp transistor, is thus a ratio of hole currents.

8. What happens to the recombination current if the bias voltage of the transistor decreases?
a) It increases
b) It decreases
c) It remains constant
d) It cannot be determined

Explanation: As the bias voltage decreases, the minority carriers take more time to diffuse through the base region. This leads to more recombination in the base region and the collector current reduces as should be the case.

9. If the recombination factor becomes 1, what does it imply?
a) Recombination current is negligible
b) Recombination current is very high
c) Recombination current is moderately high
d) There is no such factor

Explanation: If the recombination factor becomes 1, then the recombination current should become negligible. The ratio of total emitter current without recombination to that with recombination is the recombination factor. It becomes less dominant in the active region.

10. The Ic vs VCE curve becomes in a straight line in a certain region. What does the slope of the straight line represent?
a) The resistance at the base
b) The resistance at the base-collector region
c) The early effect
d) The resistance at the base-emitter region

Explanation: The early effect leads to degradation of the B.J.T. when it’s in the active region. This keeps the B.J.T. from behaving as an ideal current source.

Sanfoundry Global Education & Learning Series – Microelectronics.

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