Diode Families: Rectifier, Fast-Recovery, PIN, Varactor, Tunnel & TVS

The diode zoo and how to choose. For each family — signal/switching, standard rectifier, fast, ultrafast and soft-recovery, high-voltage and high-current diodes, PIN diodes (RF switches, attenuators, photodiodes, power), varactors (abrupt and hyperabrupt, VCOs, PLLs, tuned filters, matching), tunnel diodes (heavy doping, quantum tunnelling, negative differential resistance, oscillators), and TVS/ESD protection diodes (stand-off, breakdown, clamping, peak pulse power, uni/bidirectional, automotive and data lines) — learn structure, operation, I–V, key parameters, applications and selection — with Python computing varactor tuning ranges, PIN RF resistance, tunnel-diode NDR and TVS pulse energy.

By tailoring doping profiles, intrinsic layers and lifetimes, the humble PN junction becomes a rectifier, an RF switch, a tunable capacitor, an oscillator or a surge protector.

Selecting the right family means matching the dominant physics — storage, capacitance, tunnelling or controlled breakdown — to the application.

An analogy

Diode families are like specialised vehicles built on the same chassis: a truck (rectifier) for heavy loads, a sports car (fast/Schottky) for speed, a camper van with adjustable space (varactor), an off-road buggy (tunnel diode) with unusual handling, and an ambulance (TVS) that springs into action only during emergencies.

Rectifier and switching diodes

Choose by speed and voltage:

PIN and varactor diodes

PIN diodes: at RF, carriers can't follow the signal, so the forward-biased diode behaves as a resistor controlled by DC current (switches, attenuators, limiters, phase shifters); reverse-biased, the intrinsic layer gives low, voltage-independent capacitance and high isolation. The same structure makes fast photodiodes and high-voltage power diodes. Varactors: reverse-biased junctions with controlled C(V) for VCOs in PLLs, electronically tuned filters, matching networks and parametric amplifiers; Q and linearity matter.

Tunnel diodes

With both sides degenerate, filled states on one side face empty states on the other at small forward bias: electrons tunnel, current peaks, then falls as the bands misalign (negative differential resistance), before normal diffusion current takes over. NDR enables microwave oscillators and extremely fast switching; low voltages and small signal swings limit use today (resonant-tunnelling diodes continue the idea for THz).

TVS and ESD protection

TVS diodes stay off below the stand-off voltage V_WM, begin conducting at V_BR, and clamp at V_C during the rated pulse current I_PP. Choose V_WM above the highest normal voltage (including tolerance and load-dump where relevant), V_C below the protected device's absolute maximum, P_PP for the surge standard, and low capacitance for high-speed data lines (ESD arrays, < 1 pF). Bidirectional parts protect AC or bipolar lines. Place them at the connector with minimal inductance.

Formulas

Worked example: Varactor tuning

Given: C 20 → 5 pF, L 1 µH

Find: Frequency range.

Answer: 35.6 → 71.2 MHz

Worked example: PIN attenuation current

Given: W 50 µm, τ 2 µs, target 5 Ω

Find: I_F.

Answer: ≈ 1.4 mA

Worked example: TVS choice

Given: 12 V line (max 14 V), protected IC max 30 V

Find: Requirements.

Answer: e.g. 15 V stand-off, ~24 V clamp part

Structures

  PIN:      p⁺ │ intrinsic (i) W = 10–200 µm │ n⁺   → forward: conductivity-modulated resistor;
                                                     reverse: tiny, flat capacitance
  Varactor: p⁺ │ n (hyperabrupt: doping falls away from junction) │ n⁺ → large C(V) swing
  Tunnel:   p⁺⁺│n⁺⁺ (degenerate both sides, W ≈ 10 nm) → band-to-band tunnelling, NDR
  TVS:      large-area avalanche junction, low thermal impedance, fast (ps) response

Each family is a profile engineering exercise. The PIN's wide intrinsic layer stores charge for RF switching and gives high breakdown; the hyperabrupt varactor's decreasing doping makes W (and C) vary strongly with voltage; the tunnel diode's degenerate doping makes the junction thin enough for tunnelling; the TVS is a rugged avalanche diode designed to absorb energy.

Varactor, PIN, tunnel and TVS numbers

import math
# Special diodes: varactor tuning, PIN RF resistance, tunnel-diode negative resistance, TVS clamp energy
C0, Vbi = 30e-12, 0.7
L = 1e-6
for m, name in ((0.5, "abrupt"), (1.5, "hyperabrupt")):
    Cmax, Cmin = C0 / (1 + 1 / Vbi) ** m, C0 / (1 + 10 / Vbi) ** m
    f1, f2 = 1 / (2 * math.pi * math.sqrt(L * Cmax)), 1 / (2 * math.pi * math.sqrt(L * Cmin))
    print(f"{name:11s} varactor 1-10 V: {Cmax*1e12:5.1f} -> {Cmin*1e12:5.2f} pF, LC tank {f1/1e6:.1f} -> {f2/1e6:.1f} MHz")
W, mu_sum, tau = 100e-4, 1350 + 480, 1e-6
for IF in (1e-3, 10e-3, 100e-3):
    Rrf = W**2 / (mu_sum * tau * IF)
    print(f"PIN diode (W 100 um) forward {IF*1e3:5.1f} mA: RF resistance {Rrf:.2f} ohm")
Ip, Vp, Iv, Vv = 10e-3, 0.065, 1e-3, 0.35
print(f"tunnel diode: peak {Ip*1e3:.0f} mA at {Vp*1e3:.0f} mV, valley {Iv*1e3:.0f} mA at {Vv*1e3:.0f} mV -> average NDR {-(Vv-Vp)/(Ip-Iv):.1f} ohm")
Vc, Ipp = 9.2, 65.2                         # SMBJ5.0A-class TVS at its rated 10/1000 us pulse
print(f"TVS: {Vc*Ipp:.0f} W peak; 10/1000 us pulse (half value at 1 ms) energy ~ P*1ms/ln2 = {Vc*Ipp*1e-3/math.log(2):.2f} J")
# abrupt      varactor 1-10 V:  19.3 ->  7.67 pF, LC tank 36.3 -> 57.5 MHz
# hyperabrupt varactor 1-10 V:   7.9 ->  0.50 pF, LC tank 56.5 -> 224.6 MHz
# PIN diode (W 100 um) forward   1.0 mA: RF resistance 54.64 ohm
# PIN diode (W 100 um) forward  10.0 mA: RF resistance 5.46 ohm
# PIN diode (W 100 um) forward 100.0 mA: RF resistance 0.55 ohm
# tunnel diode: peak 10 mA at 65 mV, valley 1 mA at 350 mV -> average NDR -31.7 ohm
# TVS: 600 W peak; 10/1000 us pulse (half value at 1 ms) energy ~ P*1ms/ln2 = 0.87 J

A hyperabrupt varactor tunes an LC tank ~4× in frequency over 1–10 V versus ~1.6× for an abrupt one. A PIN diode's RF resistance falls inversely with bias current — from 55 Ω at 1 mA to 0.55 Ω at 100 mA — making it an excellent current-controlled RF switch/attenuator. The tunnel diode's −32 Ω region can sustain oscillation. A 600 W-class TVS absorbs ~0.9 J during its rated 10/1000 µs surge.

Common mistakes

Applications

Key takeaways

Going further