Special diodes: Zener, LED, photodiode and solar cell
Changing doping and design gives special diodes. A Zener holds a steady reverse voltage, an LED gives light, and photodiodes and solar cells turn light into current.
The four special diodes at a glance
| Diode | Bias used | How it works | Main use |
|---|---|---|---|
| Zener | Reverse, in breakdown | Heavily doped p and n sides; very thin depletion layer; Zener breakdown at a set voltage Vz | Voltage stabiliser (regulator) |
| LED | Forward | Electrons and holes recombine and give out light (electroluminescence) | Displays, indicator lamps, remote controls, calculators |
| Photodiode | Reverse | Light makes electron-hole pairs near the junction; the reverse current grows with light | Light sensors, remote-control receivers in TVs |
| Solar cell | None: it supplies current | Photovoltaic effect: light makes pairs, and the junction field separates them | Power for satellites, charging batteries, calculators |
The Zener diode and its characteristic
A Zener diode has both sides heavily doped, so its depletion layer is extremely thin, less than 10⁻⁶ m (1 μm). Even a few volts of reverse bias give a huge field, about 5 × 10⁶ V m⁻¹ at 5 V. In forward bias it behaves like an ordinary diode. In reverse bias, only a tiny current flows until the Zener voltage Vz. At Vz the current rises sharply while the voltage stays almost constant. Unlike an ordinary diode, it is built to work in breakdown without damage.
LED, photodiode and solar cell
Memory aid: the LED sends light out in forward bias; the photodiode and solar cell take light in.
How to draw the symbols and characteristics
- Zener symbol: a diode symbol whose bar has small bent ends like a Z. Curve: a normal forward curve; in reverse, almost no current until Vz, then an almost vertical drop.
- LED symbol: a diode symbol with two arrows pointing away (light out). Curve: like a forward diode curve, but starting at a higher threshold voltage.
- Photodiode symbol: a diode symbol with two arrows pointing in (light in). Curve: third quadrant, several nearly flat lines, lower for brighter light; label the intensities I₁ > I₂ > I₃.
- Solar cell curve: fourth quadrant, V to the right and current downward. Mark the open-circuit voltage on the V-axis and the short-circuit current on the I-axis.
Worked example: light and the band gap
(a) An LED is made of a material with a band gap of 1.9 eV. What colour of light does it give? (b) Can light of wavelength 1500 nm make a silicon (Eg = 1.1 eV) photodiode or solar cell work?
- hc/e = (6.63 × 10⁻³⁴ × 3 × 10⁸) ÷ 1.6 × 10⁻¹⁹ ≈ 1.24 × 10⁻⁶ V m, so hc ≈ 1240 eV nm.
- (a) The light from recombination has energy about Eg, so λ = hc/Eg = 1240 eV nm ÷ 1.9 eV ≈ 653 nm: red light.
- (b) Photon energy E = hc/λ = 1240 eV nm ÷ 1500 nm ≈ 0.83 eV.
- 0.83 eV is less than 1.1 eV, so the photon cannot break bonds in silicon. The device will not work.
(a) about 650 nm, red; (b) no, because 0.83 eV is less than 1.1 eV
The TV remote has an infrared LED, and the TV has a photodiode to catch its flashes. Solar panels on homes and farms are big sets of solar cells, and Zener diodes keep voltages steady in chargers.
2 marks: “A Zener diode, an LED, a photodiode and a solar cell are all p-n junctions. Give one practical use of each.” Zener diode: voltage regulator. LED: displays and indicator lamps. Photodiode: light detector, such as a TV remote receiver. Solar cell: power for satellites or charging batteries.
3 marks: “Sketch the V-I curve of an LED and compare it with that of an ordinary silicon diode. How does the light from an LED change as its forward current is raised?” Forward biased; recombination gives light. Curve like a Si diode but with a higher threshold voltage, different for each colour. Brightness rises with current to a maximum, then falls. Reverse breakdown is low, about 5 V. Sketch the curve.
2 marks: “How does a solar cell produce a voltage when light falls on it?” Generation: light with hν > Eg makes electron-hole pairs near the junction. Separation: the junction field sends electrons to n and holes to p. Collection: contacts gather them, so p becomes positive and n negative.
Z holds, L shines, P sees, S feeds: the Zener holds a voltage, the LED shines, the photodiode sees light and the solar cell feeds power.