Schottky Diodes
Introduction
Schottky diodes are formed by a metal–semiconductor junction rather than the traditional p–n junction.
This unique construction gives them superior performance in high-speed switching and low-power applications.
They are widely used in modern electronics where fast switching speeds and low power dissipation are critical.
Historical Development
Schottky diodes were first described by Walter H. Schottky in 1938, but commercial production began in the 1960s.
Since then, they have become essential in:
- Microprocessor design
- High-speed digital logic
- RF and microwave circuits
- Power supplies
- Solar cells
Basic Construction and Operating Principle
Device Structure
Key features:
- Metal layer: aluminum, platinum, molybdenum, or tungsten
- Semiconductor layer: lightly doped n-type silicon or gallium arsenide (GaAs)
- Ohmic contact: forms the cathode
The junction creates a Schottky barrier, which defines the diode’s electrical behavior.
Comparison with P–N Junction Diodes
| Property | P–N Diode | Schottky Diode |
|---|---|---|
| Junction Type | Semiconductor–Semiconductor | Metal–Semiconductor |
| Forward Voltage Drop | 0.6–0.7V (Si), 0.3–0.4V (Ge) | 0.3–0.4V (Si), 0.15–0.2V (GaAs) |
| Switching Speed | Moderate (ns range) | Very fast (ps range) |
| Reverse Recovery Time | Moderate (μs) | Very fast (ps) |
| Leakage Current | Low | Moderate to high |
| Noise | Low | Moderate |
| Temperature Sensitivity | Moderate | Higher |
Schottky diodes trade speed and efficiency for higher leakage current compared to p–n diodes.
Key Electrical Characteristics
Forward Voltage Drop (Vf)
- Silicon Schottky: 0.3–0.4V
- GaAs Schottky: 0.15–0.2V
Advantages of low Vf:
- Lower heat generation
- Better efficiency in power supplies
- Reduced power loss in high-current circuits
- Longer battery life in portable devices
Reverse Recovery Time (trr)
Schottky diodes have virtually zero reverse recovery time (<1s).
This makes them ideal for high-frequency and fast-switching circuits.
- P–N diodes: 10–200 ns
- Schottky diodes: <1 ns
Reason:
- P–N diodes store charge in the depletion region → must be removed
- Schottky diodes → no significant stored charge
Reverse Leakage Current (Ir)
Schottky diodes have much higher leakage current than p–n diodes (100–1000×).
This increases with temperature and must be considered in precision circuits.
Causes:
- Thermionic emission over the barrier
- Quantum tunneling
- Junction defects
Dynamic Resistance
- Lower than p–n diodes
- Improves voltage regulation
- Reduces on-state voltage drop
- Enhances efficiency in switching applications
Capacitance
- Lower junction capacitance than p–n diodes
- Better high-frequency performance
- Reduced switching losses
- Operation possible at GHz frequencies
Construction Details
Silicon Schottky Diodes
- Material: lightly doped n-type silicon (~1×10^15 cm^-3)
- Barrier height: 0.6–0.8V
Advantages:
- Widely available
- Mature manufacturing process
- Cost-effective
- Wide voltage ratings
Disadvantages:
- Higher leakage current
- Forward voltage not as low as GaAs
Gallium Arsenide (GaAs) Schottky Diodes
- Material: GaAs substrate with metal contact
- Barrier height: 0.6–0.9V
Advantages:
- Lower forward voltage
- Better high-frequency performance
- Useful in microwave and RF circuits
Conclusion
Schottky diodes are indispensable in fast-switching, high-frequency, and low-power applications.
Their low forward voltage and near-instant recovery make them efficient, though designers must account for higher leakage currents and temperature sensitivity.