Introduction / Context:
Current gain (β = I_C / I_B) in a BJT is not a fixed constant; it depends on operating conditions such as collector current I_C, collector–emitter voltage V_CE, device temperature, and the transistor's fabrication. A practical understanding of how β trends with I_C is important for bias stability, saturation calculations, and robust amplifier or switch design.
Given Data / Assumptions:
- BJT operating in normal active region unless otherwise noted.
- We consider typical silicon small-signal or power BJTs.
- Focus is on the qualitative trend of β as I_C increases.
Concept / Approach:
- At very low I_C, recombination effects can make β small; as I_C rises to a moderate range, β often increases.
- Beyond a certain I_C (device-dependent), high-level injection and series resistances cause β to roll off (decrease) with further increase in I_C.
- In many textbook and exam contexts, the emphasized trend is that β decreases as I_C becomes large, which is the design-relevant caution for saturation and thermal effects.
Step-by-Step Reasoning:
Recognize that β vs I_C typically exhibits a peak: it rises from low I_C to a maximum and then decreases at higher I_C.For questions emphasizing robustness, designers assume β falls at higher currents to ensure worst-case base drive for switches (forced β).Thus, with increasing collector current in the practical higher-current region, β generally decreases.
Verification / Alternative check:
Datasheets often include β (or h_FE) vs I_C curves showing a hump: β improves up to a point and then declines as I_C grows further due to high-level injection and ohmic drops.
Why Other Options Are Wrong:
- stays the same: Unrealistic; β varies with operating point.
- increases: True only up to a moderate I_C; not a general statement for increasing I_C.
- any of the above: Too vague; the dominant high-current trend is a decrease.
- None of the above: Incorrect because 'decreases' captures the standard high-current behavior.
Common Pitfalls:
- Assuming a single β value for all currents and conditions.
- Failing to provide base overdrive in switching applications where β can drop at high I_C.
Final Answer:
decreases
Discussion & Comments