Zener Diode and Special Diodes: LED and Photodiode

Special diodes exploit other corners of diode physics beyond forward conduction. The Zener diode is designed to operate safely in reverse breakdown, holding a nearly constant voltage — a simple voltage regulator. The LED turns forward current into light, and the photodiode does the reverse, turning light into current.

These three cover three directions of energy conversion — Zener (electricity → stable electricity), LED (electricity → light), photodiode (light → electricity) — found in power supplies, displays and sensors.

Key Concepts

• Zener breakdown — below ~5 V a strong field pulls electrons directly across the junction (tunnelling); above ~5 V, avalanche breakdown (carrier multiplication by collisions) dominates. Both are non-destructive in a Zener by design.
• Zener voltage (Vz) — the rated reverse voltage where the diode clamps, available from ~2.4 V to 200 V. In breakdown the voltage stays nearly constant while current varies widely.
• LED — a forward-biased diode of direct-bandgap material. Recombining electrons and holes release photons; the band gap sets the colour — red ~1.9 eV, blue ~2.7 eV, white via phosphor coating.
• LED forward voltage — typically 1.8–2.2 V for red, 3.0–3.4 V for blue/white, higher than silicon's 0.7 V because of the wider band gap.
• Photodiode — a reverse-biased pn junction where incoming photons generate electron-hole pairs, producing a photocurrent proportional to light. Unbiased it works like a tiny solar cell; reverse-biased it responds faster for sensing.

Formulas

• LED series resistor: R = (Vs − Vf) / If. For a 9 V supply, 2 V LED at 20 mA: R = (9 − 2)/0.02 = 350 Ω — use the nearest standard value, 390 Ω, for safety.
• Resistor power: P = If² × R. With ~18 mA through 390 Ω: P ≈ 0.125 W, so a standard 1/4 W resistor is fine.
• Photodiode current: I = responsivity × optical power. A 0.5 A/W photodiode under 2 mW of light gives 1 mA of photocurrent.

Worked Example

Light a red LED (Vf = 2 V) at 20 mA from a 9 V battery. What series resistor and power rating are needed?

1. The resistor must drop 9 − 2 = 7 V.
2. R = 7 / 0.02 = 350 Ω; the nearest standard value above is 390 Ω, keeping current safely under 20 mA.
3. Actual current: I = 7 / 390 ≈ 17.9 mA — still bright, gentler on the LED.
4. Power: P = (0.0179)² × 390 ≈ 0.125 W, so a common 1/4 W resistor handles it. Never connect an LED directly across a battery — without the resistor the current is uncontrolled and the LED burns out in seconds.

Common Misconceptions

• "A Zener is just a broken regular diode." It is deliberately engineered for stable, non-destructive breakdown — a regular diode in breakdown overheats and dies.
• "LEDs can run straight from a battery." LEDs are current-driven with a steep I–V curve; a small voltage rise causes a huge current spike, so a series resistor or constant-current driver is mandatory.

Real-World Applications

• Zener: simple voltage references and overvoltage protection in power supplies and sensor circuits.
• LED: room lighting, phone flashes, traffic signals, TV backlights, seven-segment displays.
• Photodiode: TV remote receivers, smoke detectors, fibre-optic receivers, pulse oximeters — plus optocouplers pairing an LED with a photodiode for isolated signalling.

Special diodes are the pn junction put to specialised work: the Zener tames reverse breakdown into a voltage clamp, the LED turns current into light, and the photodiode turns light back into current. Size the LED resistor, respect the Zener's current range and bias the photodiode correctly — and these tiny devices do jobs nothing else can.