Zener, Schottky, TVS & Other Special-Purpose Diodes

A diode for every job. For each type learn the symbol idea, construction, operation, I–V characteristic, key parameters, advantages, limitations, applications and selection criteria: PN signal and power diodes, fast/ultrafast recovery diodes, Schottky barrier diodes, Zener and avalanche diodes (shunt regulators, references), TVS diodes (surge/ESD clamps), varactors (voltage-tuned capacitance), tunnel diodes (negative resistance), and SiC Schottky — with Python designing a Zener shunt regulator and checking TVS pulse power.

Beyond basic rectification, diodes regulate voltage (Zener), protect circuits from surges and ESD (TVS), switch fast with low loss (Schottky), tune radios (varactor) and even oscillate (tunnel). Choosing the right type prevents failures and wasted power.

Each type is optimised by its doping and construction to emphasise one part of the junction's behaviour.

An analogy

Think of diodes as specialised valves: a Zener is a pressure-relief valve that opens at a set pressure, a TVS is a burst valve for sudden surges, a Schottky is a low-friction check valve, and a varactor is a valve whose chamber size changes with pressure.

Rectifier and signal diodes

General-purpose families:

Schottky and Zener diodes

Schottky: metal–semiconductor junction, majority-carrier device — low V_F (0.2–0.5 V) and no stored charge (fast), but higher leakage (rises sharply with temperature) and lower reverse ratings. Used for low-voltage rectification, OR-ing, reverse-polarity protection and clamps. Zener: heavily doped junction designed to break down at a precise voltage (2.4–200 V); ~5–6 V Zeners have the lowest temperature coefficient (tunnelling and avalanche effects cancel) and lowest r_z.

TVS and protection diodes

Transient voltage suppressors are large-area avalanche diodes rated for pulse power (e.g. 400 W, 600 W, 1.5 kW for 10/1000 µs pulses). Choose V_RWM (stand-off) above the normal maximum voltage, check V_C at the expected surge current is below what the protected IC can withstand, and select uni- or bidirectional. Low-capacitance TVS arrays protect USB and Ethernet lines from ESD (IEC 61000-4-2).

Varactor, tunnel and other diodes

Varactors exploit C_j(V_R) for voltage-controlled oscillators, tuning and FM modulators (hyperabrupt profiles give large tuning ratios). Tunnel diodes (very heavy doping) show negative differential resistance, historically used for microwave oscillators. PIN diodes act as RF switches and attenuators; step-recovery diodes generate harmonics; current-regulator diodes (JFETs) provide constant currents; LEDs, photodiodes and laser diodes are covered in the optoelectronics lesson.

Formulas

Worked example: Schottky vs PN loss

Given: 3 A average, V_F 0.45 V vs 0.95 V

Find: Conduction loss.

Answer: Schottky saves 1.5 W

Worked example: Varactor tuning

Given: L 100 nH, C from 10 pF to 40 pF

Find: Tuning range.

Answer: 159 MHz down to 79.6 MHz (2:1)

Worked example: TVS stand-off

Given: 24 V industrial input (max 30 V)

Find: Choose V_RWM.

Answer: e.g. SMBJ33A (V_RWM 33 V)

Zener shunt regulator

  V_in (10–14 V) ──[R_S 180 Ω]──┬────── V_out ≈ 5.1 V ──► load (0–20 mA)
                                 │
                               ─┴─  Zener 5.1 V (cathode up)
                                ▲
  GND ──────────────────────────┴──────────────────────────

R_S drops the difference between input and Zener voltage. The Zener absorbs whatever current the load doesn't take, holding the output near V_Z. Worst case for the Zener is maximum input with no load.

Zener regulator design and TVS pulse power

# Zener shunt regulator design and TVS clamp check
Vin_min, Vin_max, Vz, Iz_min, IL_max = 10.0, 14.0, 5.1, 5e-3, 20e-3
Rs = (Vin_min - Vz) / (Iz_min + IL_max)
print(f"series resistor Rs <= {Rs:.0f} ohm -> choose 180 ohm")
Rs = 180.0
Iz_worst = (Vin_max - Vz) / Rs              # no load, max input
print(f"worst-case zener current {Iz_worst*1e3:.1f} mA, zener power {Iz_worst*Vz*1e3:.0f} mW, resistor power {(Vin_max-Vz)**2/Rs*1e3:.0f} mW")
rz = 10.0
print(f"line regulation: dVout/dVin ~ rz/(Rs+rz) = {rz/(Rs+rz):.3f} -> {4*rz/(Rs+rz)*1e3:.0f} mV for a 4 V input change")
# TVS: 5 V line, SMBJ5.0A (VBR ~6.4 V, Vclamp 9.2 V at 43.5 A)
print(f"TVS peak pulse power at 43.5 A: {9.2*43.5:.0f} W")
# series resistor Rs <= 196 ohm -> choose 180 ohm
# worst-case zener current 49.4 mA, zener power 252 mW, resistor power 440 mW
# line regulation: dVout/dVin ~ rz/(Rs+rz) = 0.053 -> 211 mV for a 4 V input change
# TVS peak pulse power at 43.5 A: 400 W

The Zener must survive 252 mW at no load (choose ≥ 500 mW) and the resistor 440 mW (≥ 1 W). Line regulation is mediocre (≈ 5%), which is why Zener regulators are used only for low-current references or pre-regulators; linear regulator ICs do far better. The TVS briefly handles 400 W — but only for microseconds.

Common mistakes

Applications

Key takeaways

Going further