BJT Vbe vs Temperature

Formula

Description

The base-emitter voltage of a BJT decreases approximately 2 mV per degree Celsius increase in temperature. This well-known temperature coefficient is remarkably consistent across different transistor types and is exploited in bandgap voltage references and temperature sensors. At room temperature, Vbe is typically 0.6-0.7V for silicon transistors, dropping to about 0.5V at 100°C above ambient. This temperature sensitivity must be accounted for in bias circuit design to prevent thermal runaway in power transistors.

Variables

• Vbe — Base-emitter voltage at operating temperature (V)
• Vbe_ref — Vbe at reference temperature (V)
• ΔT — Temperature change from reference (°C)

Practical Notes

The -2 mV/°C coefficient is an approximation that works well from -40°C to +125°C. The actual relationship is: Vbe = (kT/q) × ln(Ic/Is), where Is doubles approximately every 10°C. Bandgap references combine this negative tempco with the positive tempco of ΔVbe between two transistors to produce a temperature-independent 1.2V reference.