In bipolar junction transistors (BJTs), how does the DC current gain (beta, β) generally vary as junction temperature increases, under typical operating conditions?

Introduction / Context:The DC current gain β of a bipolar junction transistor is not a fixed constant. It depends on device geometry, collector current, and temperature. Recognizing the qualitative temperature trend of β is important for bias stability, thermal design, and predicting the shift of operating points in amplifiers and switches.

Given Data / Assumptions:

• The device is a silicon BJT operated in the forward-active region.
• Temperature rises within normal operating limits (no breakdown or thermal runaway).
• We are concerned with the typical trend, not extreme edge cases.

Concept / Approach:As temperature increases, carrier mobility and recombination mechanisms change. For most silicon BJTs in the common operating range, β tends to increase with temperature because the base-emitter voltage required for a given collector current decreases, and minority carrier lifetimes change in a way that effectively boosts the collector current for a fixed base drive. Designers exploit or guard against this by using negative feedback, emitter resistors, or bias-stabilizing networks to maintain a stable Q point.

Step-by-Step Solution:Identify the parameter: β = I_C / I_B in forward-active region.Note temperature effect: V_BE falls roughly 2 mV/°C; recombination dynamics shift.Result: For a fixed base current, I_C typically increases with temperature → β increases.Choose “increases.”

Verification / Alternative check:Examine datasheets: β (h_FE) curves often show higher gain at elevated junction temperatures for a given I_C, up to a region where excessive current or high temperature can reverse the trend or induce breakdown—outside normal design targets.

Why Other Options Are Wrong:“decreases” contradicts the common midrange trend; “constant” ignores real temperature dependence; “can be any of the above” is too vague for typical design assumptions; “None” is incorrect because a clear trend exists in standard usage.

Common Pitfalls:Assuming β is constant; overlooking that β also varies with I_C; forgetting to include temperature compensation in precision analog designs.

Final Answer:increases