In a PNP transistor, an increase in collector–base reverse bias causes a slight increase in collector current

Question

In a PNP transistor, an increase in collector–base reverse bias causes a slight increase in collector current even when the emitter current is held constant. This occurs primarily because:

Examoogle · Accepted Answer

The correct answer is Option C: The emitter injection efficiency decreases. In a PNP transistor, increasing the collector-base reverse bias causes the depletion region at the collector-base junction to widen. This phenomenon, known as the Early effect, results in a decrease in the effective base width, which leads to a higher probability of charge carriers reaching the collector, thereby increasing the collector current ($I_C$).

Step-by-Step Solution & Concepts:
In a PNP transistor, increasing the collector-base reverse bias causes the depletion region at the collector-base junction to widen. This phenomenon, known as the Early effect, results in a decrease in the effective base width, which leads to a higher probability of charge carriers reaching the collector, thereby increasing the collector current ($I_C$).

Working Principle: As the collector-base reverse bias voltage ($V_{CB}$) increases, the depletion layer widens into the base region because the base is usually more lightly doped than the collector. This reduction in the effective base width ($W_{eff} = W_B - W_{dep}$) reduces the rate of recombination of holes within the base. Since fewer holes recombine in the base, more holes are collected at the collector, causing $I_C$ to rise even if the emitter current ($I_E$) is kept constant.

Key Formulas: $I_C = \alpha I_E + I_{CBO}$ — relationship between collector, emitter, and leakage currents; $W_{eff} = W_B - W_{dep}$ — effective base width where $W_{dep}$ is the depletion width

Key Points: The Early Effect is also known as base-width modulation.; Increased reverse bias leads to a narrower neutral base region.; Higher collector current results because the recombination current in the base is reduced.; This effect leads to a finite output resistance in transistors.

Advantages: Used to model the output conductance of a transistor

Disadvantages: Limits the maximum output impedance of the amplifier

Applications: BJT circuit design; Small-signal analysis

Additional Info: The provided official answer key option C (Emitter injection efficiency) is incorrect; the physical mechanism described is the Early Effect (Base-width modulation), which typically corresponds to option D in standard textbook questions.. Option D is the most scientifically accurate description of the Early Effect.

Answer: D is correct — The increase in collector current due to collector-base reverse bias is fundamentally caused by the reduction of the effective base width (Early Effect).

Exam Tip: Always remember that in BJT physics, the 'Early effect' is synonymous with 'Base-width modulation'; it is a common source of confusion in competitive exams where answer keys may occasionally misattribute the phenomenon.

Examoogle · Answer

Option A: Hole mobility increases in the collector region

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Option B: The collector resistance decreases due to depletion region expansion

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Option D: The effective base width decreases with collector–base voltage

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