Zener, Schottky, Varactor and PIN Diodes

The standard silicon rectifier diode is just the starting point. A whole family of specialized diodes has been developed, each exploiting a different aspect of semiconductor physics to do something useful that a plain diode cannot. In ham radio you will encounter Zener diodes holding voltages steady in power supplies, Schottky diodes detecting faint RF signals, varactor diodes acting as electronically-controlled capacitors for VFO tuning, and PIN diodes switching RF signals without any mechanical relay.

Zener Diodes: Voltage Regulation

You learned in the previous lesson that applying excessive reverse voltage to a diode causes breakdown — normally a destructive event. A Zener diode is designed to operate in this reverse breakdown region reliably and repeatedly. The breakdown voltage is precisely set during manufacture (from about 2.4 V up to hundreds of volts) and the diode holds that voltage almost exactly regardless of how much current flows through it.

This makes the Zener diode a simple voltage regulator. Connect it in reverse across a supply rail, with a resistor in series to limit current, and the Zener clamps the voltage at its specified value V_Z. If the supply rises, the Zener conducts more, dropping more voltage across the series resistor, keeping the output steady.

Zener diodes are used in ham radio for reference voltages in AGC circuits, protection of sensitive inputs, and simple regulated rails for low-current circuits such as VFO oscillators. They appear in back-to-back pairs (two Zeners facing each other) across receiver inputs to clamp any voltage spike from transmit breakthrough to a safe level.

Schottky Diodes: Fast Switching and Low Forward Drop

A Schottky diode is formed by joining a metal to an N-type semiconductor rather than P-type to N-type. This metal-semiconductor junction has two important properties:

• Very low forward voltage drop: Typically 0.15–0.4 V, compared to 0.6–0.7 V for silicon. This makes Schottky diodes ideal where low voltage drop matters — particularly in low-voltage switching power supplies and RF detector circuits.
• Extremely fast switching: The Schottky junction has no stored charge (no reverse recovery time), allowing it to switch at radio frequencies where a standard silicon diode would still be conducting when it should be off.

In ham radio, Schottky diodes appear in RF power detectors, SWR bridge circuits, and as the rectifying elements in switching power supplies. The BAT43 and 1N5711 are common low-power Schottky diodes used in RF work.

Varactor Diodes: Voltage-Controlled Capacitors

When a diode is reverse biased, the depletion layer at the PN junction acts like a capacitor — charge is stored on each side of the insulating depletion region. As the reverse voltage increases, the depletion layer widens and the capacitance decreases. A varactor diode (also called a varicap) is designed and optimized to exploit this effect, providing a useful range of capacitance that is precisely controlled by the reverse bias voltage.

By varying the control voltage, you can electronically tune a resonant LC circuit without any moving parts. This is exactly how VFOs and PLLs in modern transceivers are tuned — voltage-controlled oscillators (VCOs) use varactors as the tuning element, replacing the old variable capacitor with a simple voltage from a DAC or synthesiser circuit.

A varactor diode in a parallel LC tank circuit. Increasing the tuning voltage reduces the varactor capacitance, raising the resonant frequency. The RF choke isolates the DC tuning voltage from the RF circuit.

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Typical varactor capacitance ranges from about 3 pF to 50 pF depending on the device, with the capacitance varying by a 3:1 to 10:1 ratio across the bias voltage range. The BB105 and MV2109 are examples widely used in HF/VHF synthesiser circuits.

PIN Diodes: RF Switching and Attenuation

A PIN diode has an extra layer of intrinsic (undoped) semiconductor material sandwiched between the P and N regions — hence PIN. At DC or low frequencies it behaves like a standard diode. But at RF, the PIN diode's intrinsic region stores charge in a way that makes its RF resistance inversely proportional to the DC bias current through it.

With no forward bias current, the PIN diode presents a high RF impedance (it looks like a capacitor). With forward bias current, it presents a low RF resistance proportional to that current. This makes PIN diodes useful as:

• RF switches: A PIN diode driven into conduction connects an RF path; removing the bias opens it. Used in solid-state T/R switches in modern transceivers — faster and more reliable than electromechanical relays.
• Variable attenuators: Varying the forward current continuously varies the RF attenuation, used in AGC attenuator pads and stepped attenuators.