Special Diodes (Varactor / Tunnel / PIN / Gunn)
Diode
The 'special devices' chapter of EDC in one place: diodes engineered for jobs other than rectifying — voltage-variable capacitance (varactor), negative resistance (tunnel, Gunn) and RF switching/attenuation (PIN).
Schematic symbol
Diode
How special diodes (varactor / tunnel / pin / gunn) appears in a circuit diagram.
Types & variants
Key specs
Capacitance ratio (varactor)
Cmax/CminJunction capacitance falls as reverse bias rises (C ∝ 1/√V) — e.g. 30 pF at 1 V down to 3 pF at 28 V tunes an LC circuit electronically.
Peak/valley current (tunnel)
mAThe N-shaped I-V curve: current peaks at ~0.1 V, dips at the valley, rises again — the negative-resistance region between them enables GHz oscillation and the classic exam sketch.
Carrier lifetime (PIN)
ns–µsThe intrinsic layer stores charge, so at RF the diode behaves as a current-controlled RESISTOR (not a rectifier) — the property that makes clean switches and attenuators.
Frequency (Gunn)
GHzTransferred-electron oscillation in bulk GaAs: 8–100 GHz. The 10.525/24 GHz Gunn oscillator in door-opener radars and the microwave lab bench.
Reverse voltage / dissipation
V / mWThese are small-signal parts: varactors ~30 V, tunnel diodes millivolt-scale operation, PIN switches rated by RF watts handled.
Markings
Small-signal packages (DO-35 glass, SOT-23, ceramic pills for microwave). Part numbers do the talking: BB109/BB910/MV2109 (varactor), 1N3712 (tunnel, now collectible), BA479/HSMP-381x (PIN), MA49104 (Gunn). Gunn diodes sit in waveguide cavity mounts.
Standard values
Varactors: BB910 (VHF tuning), MV209. PIN: BAP64, HSMP series; also the humble 1N4007's PIN-like structure abused as an RF switch by hams. Gunn: X-band 10 GHz modules in every microwave engineering lab.
How to choose
1) Electronic tuning (VCOs, AFC, varactor-tuned filters): varactor with the right C range and Q. 2) RF signal routing/attenuation without relays: PIN diodes, biased with a few mA. 3) Microwave lab source: Gunn oscillator module. 4) Tunnel diodes: essentially historical/educational — modern circuits use transistor oscillators. Know them for exams, not BOMs.
Pinout & package
Two-terminal diodes; polarity marked by band. Varactors work reverse-biased through a large isolation resistor/RF choke; PIN switches need DC bias tees separating control current from RF path; Gunn diodes screw into resonant cavities with a tuning screw.
Example circuits
- Varactor-tuned VCO: reverse-bias voltage sweeping an LC oscillator's frequency (every PLL's heart)
- Remote antenna selection: PIN diodes switching RF paths on a bias voltage
- PIN attenuator: bias current setting RF resistance for AGC
- Microwave lab: Gunn oscillator + isolator + frequency meter + slotted line measuring guide wavelength
Common failures
ESD punch-through (tiny junctions die invisibly — device reads shorted/leaky), varactor drift with temperature detuning circuits, PIN diodes cooked by RF overdrive, and Gunn diodes degraded by bias overvoltage or cavity mis-tuning.
How to test
Varactor: diode test forward ~0.7 V, then verify capacitance change on an LCR meter at two reverse voltages. Tunnel: curve tracer shows the N-curve (a DMM alone can't). PIN: forward drop normal; function proven in an attenuator jig. Gunn: bias to spec in its cavity and detect output with a crystal detector.
Substitutes
Varicap duty in a pinch: an ordinary LED or rectifier junction gives a few pF of swing (ham trick). PIN switching: RF relays or MMIC switches. Gunn sources: modern PLL synthesizer + multiplier chains. Tunnel diode roles: any transistor negative-resistance/oscillator circuit.