Schottky Diode

A Schottky diode switches off in picoseconds because it has no minority carrier storage, and that speed makes it the clamping diode in every fast TTL logic gate and switching power supply rectifier. The 1N5817 found on virtually every DC-DC converter board is a Schottky device.

Core Concept

A Schottky diode is formed by a metal-semiconductor junction, not a p-n junction. The metal (typically platinum, titanium, or aluminum) is deposited directly on lightly doped n-type silicon. This creates a Schottky barrier at the interface, which allows rectification without involving minority carriers at all.

Because conduction is entirely by majority carriers (electrons in n-type silicon), there is no minority carrier injection and no reverse recovery charge stored in the junction. When the diode switches from forward to reverse bias, it turns off in 100 ps to 1 ns, compared to 50 ns to 200 ns for a standard 1N4007. This is why switching power supplies use devices like the 1N5817 (40V, 1A Schottky) instead of regular rectifier diodes.

The forward voltage drop of a Schottky diode is 0.2V to 0.45V, compared to 0.6V to 0.7V for silicon p-n. In a 5V switching supply rectifying at 1A, replacing a 1N4007 with a 1N5817 saves 0.4W of conduction loss, which is significant in battery-powered designs. The trade-off is higher reverse leakage current, typically 100 µA to 1 mA at rated voltage, compared to under 1 µA for a silicon p-n diode.

Key Equations

Schottky diode current: I = A* × T^2 × exp(-q×φ_B / kT) × (exp(qV/kT) - 1) where A* is Richardson constant (110 A/cm^2·K^2 for silicon), T is temperature in K, φ_B is barrier height in V (0.7V for Pt-Si), k = 1.38×10^-23 J/K.

Simplified form: I = I_s × (exp(V/V_T) - 1) with V_T = 26 mV at 300K. For a Schottky diode, I_s is much larger than for a p-n diode (10^-6 A vs 10^-12 A), explaining the lower turn-on voltage.

Conduction loss power: P_cond = I_F × V_F where I_F is average forward current and V_F is forward voltage drop at that current. This is the main loss advantage of Schottky over p-n in power circuits.

Given:
  Schottky diode: V_F = 0.35 V at I_F = 2 A (e.g. MBR2045)
  Silicon p-n diode: V_F = 0.7 V at I_F = 2 A (e.g. 1N4007)
  Switching frequency = 100 kHz (rectifier in SMPS)
  Duty cycle = 0.5 (diode conducts 50% of cycle)

Why this formula:
  Average conduction loss = average current × V_F.

Formula:
  P_cond = I_avg × V_F
  I_avg = I_F × duty cycle

Substitution:
  I_avg = 2 × 0.5 = 1 A
  P_Schottky = 1 × 0.35
  P_silicon  = 1 × 0.70

Calculation:
  P_Schottky = 0.35 W
  P_silicon  = 0.70 W
  Power saved = 0.70 - 0.35 = 0.35 W

Final Answer:
  Schottky conduction loss = 0.35 W
  Silicon p-n conduction loss = 0.70 W
  Using the Schottky diode saves 0.35 W (50% reduction in rectifier loss).

Exam Tip: GATE questions on Schottky diodes often ask why it has no reverse recovery time. The correct answer is that it is a majority carrier device with no minority carrier injection, so there is no stored minority charge to sweep out during turn-off. Do not say it is because of low V_F. Also, remember that Schottky diodes have higher reverse leakage than p-n diodes, which limits their use in high-voltage circuits above 150V.

Key Properties

• Metal-semiconductor junction, not p-n. No minority carriers involved in conduction.
• Forward voltage V_F = 0.2V to 0.45V at low currents. The 1N5817 drops 0.45V at 1A.
• Reverse recovery time t_rr = 100 ps to 1 ns. A 1N4007 has t_rr of 30 µs. The difference is three to four orders of magnitude.
• Reverse leakage current is 100 µA to 1 mA at rated voltage, much higher than p-n diodes. This limits use to voltages below 150V in most designs.
• Common devices: 1N5817 (20V, 1A), MBR2045 (45V, 20A), BAT85 (30V, 200 mA for signal applications).
• Schottky barrier height φ_B depends on the metal: Pt on Si gives φ_B = 0.85V, Al on Si gives φ_B = 0.72V. Lower φ_B means lower V_F.
• Operating temperature is limited to 125°C to 150°C. Above this, leakage current rises to the point where the diode fails to block reverse voltage.

Quick Revision

• Schottky = metal-semiconductor junction. Majority carrier device only.
• V_F = 0.3V to 0.45V (versus 0.7V for silicon p-n).
• No minority carrier storage, so t_rr is 100 ps to 1 ns.
• Higher reverse leakage than p-n: limits use above 150V.
• Used in SMPS rectifiers, TTL clamping, RF detectors, mixer diodes.
• 1N5817: 20V, 1A. MBR2045: 45V, 20A. BAT85: 30V signal Schottky.
• Barrier height φ_B determines V_F: lower φ_B gives lower turn-on voltage.
• Exam trap: Students say Schottky has no recovery time because V_F is low. The real reason is majority-carrier-only conduction with no stored minority charge. V_F has nothing to do with recovery speed.