While ordinary junction diodes are useless at radio frequencies, three specialized diode families do the heavy lifting inside RF chains: PIN, Schottky, and varactor diodes. Because they are inexpensive, fast, and linear over a wide dynamic range, they remain the default choice for switching, detection, and tuning — even in designs built around integrated RFICs. Knowing how each family behaves helps you pick the right part and avoid subtle layout or bias mistakes.
PIN Diodes: The RF Workhorse Switch
A PIN diode sandwiches an intrinsic (undoped) semiconductor layer between the P and N regions. At RF, this intrinsic layer behaves like a current-controlled resistor: forward bias floods it with carriers and the resistance drops to well under an ohm; reverse or zero bias leaves it an open circuit with only a small capacitance. That makes a PIN diode an excellent series or shunt RF switch.
Two properties matter most in practice:
- Carrier lifetime — sets the lowest frequency at which the diode still behaves as a resistor. Below roughly 1/(2π·τ), the diode rectifies instead of switching cleanly.
- Bias current vs. RF resistance — insertion loss and isolation both scale with bias current. A switch rated for 0.5 dB loss may need 10–20 mA of forward current; attenuator applications run the diode deliberately in its resistive region.
PIN diodes also excel as RF limiters (protecting sensitive receivers) and as the variable-resistance element in reflective and bridged-T attenuators. When comparing parts, check junction capacitance (sets off-state isolation at high frequency), series resistance at your operating bias, and power dissipation if the diode must survive large RF swings.
Schottky Diodes: Fast Detection and Mixing
Schottky (metal–semiconductor) diodes conduct with a forward drop of roughly 0.15–0.4 V and, crucially, store almost no minority carriers — so they rectify efficiently into the tens of gigahertz. Their classic RF roles:
- Envelope and power detection — a zero-bias Schottky followed by an RC network converts RF amplitude to DC with superb sensitivity.
- Mixers — the nonlinearity of the diode's I–V curve multiplies two signals, producing sum and difference frequencies. Single, balanced, and double-balanced mixer designs all reduce to how cleverly the diodes are arranged.
- Clamping and protection — low forward drop clamps transients before they reach fragile gate oxides.
For detector work, prioritize low zero-bias video resistance and low junction capacitance; for mixers, the local-oscillator drive level and the diode's noise figure dominate. Datasheets usually quote these directly — the trap is comparing parts measured under different LO drive.
Varactors: Voltage-Controlled Capacitance
A varactor is a junction diode run in reverse bias, where the depletion region width — and therefore the capacitance — varies smoothly with voltage. Place one in the resonant tank of a VCO or a tunable filter, and a 0–20 V control line sweeps the frequency. The two specs that define a varactor are the tuning ratio (C at minimum vs. maximum reverse voltage, typically 2:1 to 12:1) and the Q factor at your band, which sets phase noise and insertion loss. Keep the control node high-impedance and well-decoupled: RF leaking onto the tuning line modulates the oscillator and shows up as spurious output.
Selection Checklist
- Frequency range and the diode's self-resonant frequency (package parasitics matter as much as the die)
- Bias scheme: can your supply provide the switch current or tuning voltage the design needs?
- Linearity and power handling where the diode sees large signals
- Package: wire-bonded parts beat SOT packages above a few GHz
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