Advanced: Semiconductor Diode Physics
The diode equation
Over an important operating range, the current of an idealized PN junction can be approximated by the Shockley diode equation:
- IS is reverse saturation current.
- VD is junction voltage.
- n is an ideality factor.
- VT is thermal voltage.
At about 300 K (27°C), thermal voltage is approximately 25.9 mV. This small number helps explain why diode current rises so sharply with forward voltage.
Small-signal resistance
Around a chosen bias point, the nonlinear diode can be replaced by an incremental resistance for small signals:
The bulk semiconductor, contacts, leads, and package add series resistance, so the measured slope at high current is not determined by junction physics alone.
Depletion width
Reverse bias increases depletion-region width. Greater doping generally narrows the depletion region, while lighter doping permits a wider region and can support higher reverse voltage.
Power rectifier design therefore involves tradeoffs among reverse-voltage rating, forward resistance, chip area, stored charge, and switching speed.
Junction capacitance
The depletion region behaves as a dielectric between charged semiconductor regions. Reverse voltage changes depletion width and therefore changes capacitance. Varactor diodes deliberately optimize this voltage-dependent junction capacitance.
Diffusion capacitance
During forward conduction, excess carriers stored near the junction create another effective capacitance associated with carrier storage. This is important when a PN diode switches rapidly from forward conduction to reverse blocking.
Reverse recovery
A forward-conducting PN diode contains stored minority carriers. When voltage is suddenly reversed, current can continue briefly while those carriers are removed. The interval is called reverse recovery.
In switch-mode power supplies, reverse recovery can increase switching loss, device stress, ringing, and electromagnetic interference. Fast-recovery, ultrafast, Schottky, and silicon-carbide diodes are selected partly to control this problem.
Schottky behavior
Because a Schottky diode is a majority-carrier device, it avoids the same minority-carrier storage mechanism as a PN diode. That permits very fast switching. Tradeoffs can include leakage current and reverse-voltage behavior.
Zener and avalanche breakdown
Reverse breakdown can occur through different physical mechanisms. Heavily doped, lower-voltage junctions can exhibit strong Zener tunneling, while higher-voltage junctions are increasingly dominated by avalanche multiplication. Practical devices may involve both mechanisms over some voltage range.
Temperature coefficients
Low-voltage Zener-dominated breakdown tends to have a negative temperature coefficient, while avalanche-dominated breakdown tends to have a positive coefficient. Near a few volts, selected devices can combine the mechanisms to produce relatively small net temperature coefficients.
Power dissipation
Junction temperature is determined by power, ambient temperature, thermal resistance, pulse duration, mounting, airflow, and nearby heat sources. Datasheet current ratings are meaningful only with their stated thermal conditions.
Rectifier conduction loss
For a rough constant-drop model:
More accurate estimates integrate instantaneous voltage and current over the actual waveform.
Breakdown and surge energy
TVS and avalanche-rated diodes may safely absorb specified transient energy. Ordinary small-signal and rectifier diodes are not automatically safe in breakdown. The allowed pulse depends on junction area, waveform, repetition, temperature, and package thermal impedance.
Semiconductor materials beyond silicon
| Material/system | Why it is useful |
|---|---|
| Germanium | Low forward voltage; historically important; relatively high leakage. |
| Silicon | Dominant general-purpose semiconductor with mature processing and useful temperature performance. |
| Gallium arsenide / III-V materials | Important for RF, LEDs, laser diodes, and optoelectronics. |
| Gallium nitride systems | Blue/UV light emission and high-field electronic devices. |
| Silicon carbide | High-voltage Schottky rectifiers with very low reverse-recovery charge and high-temperature capability. |
Engineering selection
- Forward current and surge current.
- Reverse voltage with transient margin.
- Forward drop at operating current and temperature.
- Reverse leakage.
- Reverse-recovery behavior or junction capacitance.
- Power dissipation and thermal resistance.
- Package inductance and layout for fast switching.
- Failure mode and required protection.