As the temperature increases, the current gains

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Examoogle · Accepted Answer

Can be any of the above

Explanation: The current gain ($\beta$ or $h_{FE}$) of a Bipolar Junction Transistor (BJT) is highly sensitive to temperature variations. As temperature increases, the intrinsic carrier concentration and base-emitter voltage characteristics shift, causing the current gain to change in a manner that is often non-linear and dependent on the device's specific operating point and construction.

Working Principle: The current gain $\beta = \frac{I_C}{I_B}$ is influenced by temperature due to two main factors: the increase in minority carrier density in the base region and the temperature dependence of the base-emitter voltage $V_{BE}$. While $\beta$ generally increases with temperature in many silicon BJTs due to improved emitter injection efficiency, it can exhibit complex behavior depending on whether the device is operating at low or high collector current levels.

Key Formulas: $\beta = \frac{I_C}{I_B}$ — Definition of Common Emitter current gain; $\Delta \beta \approx \frac{\partial \beta}{\partial T} \Delta T$ — Linear approximation of thermal gain dependence

Key Points: The current gain $\beta$ of a BJT is a temperature-dependent parameter.; Thermal runaway is a risk in power BJTs as increasing temperature can lead to higher collector currents.; Operating temperature range influences the stability of the bias point (Q-point).; Designers often use heat sinks to mitigate the thermal sensitivity of the BJT gain.

Advantages: Understanding gain variation helps in designing temperature-compensated bias circuits.; Allows for thermal stability analysis in high-power amplifier stages.

Disadvantages: Thermal dependency leads to DC bias point instability.; Makes the design of high-precision analog circuits more complex.

Applications: Power amplifier biasing circuits.; Thermal sensors based on BJT characteristics.; Temperature-compensated current mirrors.

Additional Info: For most commercial silicon BJTs, $\beta$ increases with temperature at a rate of approximately 0.5% to 1% per degree Celsius.. Option D is correct because the gain does not follow a universal monotonic trend across all currents and device geometries; it is characterized by specific dependencies that can be classified based on operating conditions.

Answer: D is correct — The current gain is a temperature-sensitive parameter that can increase or behave non-linearly depending on the specific operating conditions and device characteristics.

Exam Tip: In competitive exams, always look for the most comprehensive answer regarding BJT parameters like $\beta$ and $V_{BE}$, as they are inherently non-linear and environment-dependent.

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