Diode Behavior: Temperature effects

Diode Behavior: Temperature effects

1. How Temperature Affects Diode

Temperature strongly affects reverse saturation current $I_0$, forward voltage $V_γ$, and breakdown voltage $V_{BR}$.

Parameter	$T$ Effect	Quantitative Rule (Si)
$I_0$ (reverse saturation)	Increases with $T$ (more thermal generation $n_i²∝e^{-E_g/kT}$ ⇒ more minority)	Doubles every ~10°C rise ($I_0(T2)=I_0(T1)·2^{(T2-T1)/10}$). At 25°C $I_0≈10$ nA ⇒ 35°C ~20 nA
Forward voltage $V_{γ}$ (or $V_D$ at fixed $I$)	Decreases with $T$ (since $I_0$ up, need less $V$ for same $I$)	$-2$ mV/°C (e.g., 0.7V at 25°C ⇒ 0.6V at 75°C)
Breakdown voltage	Zener $V_Z$ (<6V) decreases** slightly with $T$ (tunneling needs less field when band gap shrinks); **Avalanche $V_{BR}$ (>6V) increases with $T$ (more phonon scattering needs higher field for ionization)	Zener $TC$ negative (~-2mV/°C), Avalanche positive

2. Why $I_0$ Doubles

dominates exponential ⇒ approximate doubling per 10°C for Si near 300K.

3. Consequences

• Thermal runaway: forward diode without series resistor: $T↑$ ⇒ $I_0↑$ ⇒ $I↑$ (since $V$ fixed) ⇒ more heating $I·V$ ⇒ $T↑$ loop ⇒ burn.
• Reverse leakage: at high $T$, $I_0$ may become µA–mA, power dissipation in reverse may still be small but capacitive effects change.
• Circuit design: need temperature compensation (e.g., diode-connected transistor for $V_{BE}$ tracking) or heat sink.

4. Worked Checks

(a) Si diode I0=5nA at 20°C, at 40°C: ΔT=20 ⇒ doubles twice ⇒ 5·4=20nA ✓

(b) Forward V at 10mA is 0.7V at 25°C, at 75°C: ΔT=50 ⇒ ΔV≈ -2·50= -100mV ⇒ 0.6V ✓

(c) Need to keep I constant despite T rise: reduce V by 2mV/°C ⇒ use temperature-compensating diode.

5. Exam Points

• State $I_0$ doubles per 10°C and $V_{γ}$ drops 2mV/°C — 5 marks, very frequent (almost certain to ask temperature effects).
• Explain why $I_0$ increases (more $n_i$) — 3-5 marks, frequent.
• Mention breakdown $TC$ sign difference (Zener negative, avalanche positive) — 5 marks, frequent.
• Show thermal runaway risk — 3-5 marks, frequent.