E6B: Diodes

Diodes are the simplest active semiconductor devices, but they come in many specialized forms — each optimized for a different role. The basic PN junction diode conducts in one direction only, but Zener, Schottky, PIN, varactor, point-contact, and LED diodes all exploit specific physical properties to serve distinct functions in radio circuits.

This lesson covers the characteristics, applications, and failure modes of the diode types that appear on the Extra class exam, including how to identify the Schottky diode schematic symbol from Figure E6-2.

Zener Diodes

A Zener diode is designed to operate in the reverse-breakdown region. Unlike standard diodes that are destroyed by reverse breakdown, Zener diodes are engineered to survive it reliably. Their most useful characteristic is that they maintain a constant voltage drop under conditions of varying current.

This makes Zener diodes ideal for voltage regulation and reference circuits. Even as the current through the Zener changes significantly, the voltage across it stays at its rated Zener voltage. This behavior is the defining property of the Zener diode.

Schottky Diodes

A Schottky diode uses a metal-semiconductor junction rather than a P-N semiconductor junction. This construction eliminates minority carrier storage, which gives Schottky diodes two important advantages: extremely fast switching speed and a lower forward voltage drop than standard silicon junction diodes.

The lower forward voltage drop (typically 0.15–0.45 V vs. 0.6–0.7 V for silicon) makes Schottky diodes a better choice for power supply rectifier applications where reducing voltage loss is important. Their high-speed switching also makes them useful as VHF/UHF mixer and detector diodes, where they outperform slower junction types.

LEDs and Band Gap

A light-emitting diode (LED) emits photons when electrons recombine with holes at a forward-biased PN junction. The energy of each photon — and therefore the color of the light — is determined by the semiconductor's band gap. Materials with a larger band gap emit higher-energy (shorter wavelength, bluer) light; smaller band gaps emit lower-energy (longer wavelength, redder) light.

The forward voltage drop of an LED is also determined by the band gap. Blue LEDs have a higher forward voltage drop than red LEDs because their larger band gap requires more energy per electron.

Varactor Diodes

A varactor diode (also called a varicap) is designed to function as a voltage-controlled capacitor. When reverse biased, the depletion region at the PN junction acts as the dielectric of a capacitor, with the P and N regions as the plates. Increasing the reverse bias voltage widens the depletion region and decreases capacitance; decreasing bias narrows it and increases capacitance.

Varactors are used in voltage-controlled oscillators (VCOs), automatic frequency control (AFC) circuits, and electronically tuned filters where you need to vary capacitance without mechanical components.

PIN Diodes

A PIN diode has an intrinsic (undoped) semiconductor layer sandwiched between P-type and N-type regions. This structure creates a very wide depletion region, resulting in extremely low junction capacitance. At RF frequencies, low junction capacitance is critical for minimizing signal leakage and maintaining high isolation in the off state.

PIN diodes are widely used as RF switches. The amount of forward DC bias current flowing through the PIN diode controls its RF resistance — more forward bias current lowers the RF resistance, allowing more signal through. This makes PIN diodes useful for RF attenuation as well as switching, with the forward DC bias current being the control variable.

Point-Contact Diodes

Point-contact diodes are an older technology where a fine metal wire (called a cat whisker) touches the surface of a semiconductor crystal. They have very low junction capacitance and can rectify RF signals at high frequencies. Their most common application today is as an RF detector, particularly in simple crystal radio receivers and RF power meters.

Diode Failure

Junction diodes fail when excess current flows through them. The root cause of failure is excessive junction temperature — when too much power is dissipated at the junction, the heat destroys the semiconductor material. Managing thermal dissipation through proper current limiting and heat sinking is essential for reliable diode operation.

Schematic Symbols: Figure E6-2

Figure E6-2 shows schematic symbols for several types of diodes. One exam question asks you to identify the Schottky diode symbol from this figure.

The Schottky diode symbol is a standard diode triangle with a distinctive bent cathode bar. Look for symbol 6 in Figure E6-2. Knowing the symbol allows you to recognize Schottky diodes on circuit diagrams — they are commonly found in RF mixer stages, detector circuits, and high-efficiency power supplies.

E6B Practice Questions