Transistor Base Emitter Voltage Behavior with Temperature Change

This explanation details how the base-emitter voltage ($V_{BE}$) of a transistor changes when the operating temperature increases. Understanding this behavior is crucial for designing stable electronic circuits.

Understanding Base Emitter Voltage ($V_{BE}$)

• The base-emitter voltage, denoted as $V_{BE}$, is the voltage potential difference applied across the base and emitter terminals of a bipolar junction transistor (BJT).
• This voltage is essential for turning the transistor "ON". For a silicon transistor, $V_{BE}$ is typically around 0.6V to 0.7V at standard room temperature (25$^\circ$C or 298K) for small collector currents.
• The relationship between the collector current ($I_C$) and $V_{BE}$ is exponential, often described by the Ebers-Moll model.

Temperature Effects on Semiconductor Junctions

• Transistors are manufactured using semiconductor materials, most commonly silicon. The electrical characteristics of these materials are significantly influenced by temperature.
• The base-emitter junction of a BJT acts like a forward-biased diode. The behavior of semiconductor diodes is temperature-dependent.
• As the temperature of a semiconductor device increases, the thermal energy of the charge carriers (electrons and holes) also increases.
• This increased thermal energy affects the semiconductor's properties, including the intrinsic carrier concentration and the energy required to move charge carriers across the P-N junction.

Impact of Increased Temperature on $V_{BE}$

• For a constant collector current ($I_C$), the base-emitter voltage ($V_{BE}$) required to maintain that current decreases as the temperature rises.
• This is primarily because the reverse saturation current ($I_S$) of the base-emitter junction diode increases significantly with temperature. To maintain the same forward current level ($I_C$), the forward bias voltage ($V_{BE}$) needs to be reduced.
• Quantitatively, for silicon bipolar junction transistors, the base-emitter voltage ($V_{BE}$) typically decreases by approximately 2 millivolts for every 1 degree Celsius increase in temperature. This can be expressed as:

  $$ \frac{\Delta V_{BE}}{\Delta T} \approx -2 \text{ mV/}^\circ C $$

• Therefore, as the temperature goes up, the base-emitter voltage ($V_{BE}$) drops.

Conclusion on Temperature Dependence

In summary, an increase in temperature causes the base-emitter voltage ($V_{BE}$) of a transistor to decrease, assuming the collector current remains constant. This characteristic is important for thermal compensation in circuit design.

Based on this analysis, the correct option describing the effect of increased temperature on the base-emitter voltage is that it decreases.