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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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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:
Hole mobility increases in the collector region
The collector resistance decreases due to depletion region expansion
The emitter injection efficiency decreases
The effective base width decreases with collector–base voltage
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 (IC).
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 (IC).
Think of a sponge (the base) that absorbs water (holes); if you squeeze the sponge to make it thinner (Early effect), more water passes through to the other side without being trapped in the sponge.
Early Effect = Effective Base Width reduction (EBW).
IC=αIE+ICBO — relationship between collector, emitter, and leakage currents
Weff=WB−Wdep — effective base width where Wdep is the depletion width
As the collector-base reverse bias voltage (VCB) 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 (Weff=WB−Wdep) 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 IC to rise even if the emitter current (IE) is kept constant.
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.
Used to model the output conductance of a transistor
Limits the maximum output impedance of the amplifier
BJT circuit design
Small-signal analysis
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.
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).
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.