EC-6.5 Optoelectronic Devices: LED, Photodiode and Solar Cell

What this is and why it exists

Three devices, one structure.

A junction under forward bias can give up its recombination energy as light. The same junction under reverse bias produces a current when light creates carriers inside it. With no bias at all it produces a voltage from the same effect.

Emitter, detector, generator. One p-n junction, three jobs, decided by how it is biased and how it is built.

That symmetry is worth seeing plainly. Otherwise you meet these as three unrelated components in three different courses. It also explains why the material matters here in a way it did not for the plain diode. Only a direct gap emits light usefully, and the size of the gap sets the colour.

The vocabulary

• Photon — a single quantum of light, carrying one particular energy.
• Radiative recombination — an electron and hole recombining and releasing a photon.
• Direct gap — a gap crossable without a change of momentum, so light can be emitted.
• Indirect gap — one that also needs a lattice vibration, so the energy leaves as heat.
• Photodiode — a junction used to detect light, operated in reverse bias.
• Responsivity — the current a detector gives per unit of incident optical power.
• Photovoltaic mode — operation with no applied bias, generating a voltage.
• Fill factor — how close a solar cell's output comes to the product of its best voltage and best current.

The mental model

Start with the symmetry, because it makes the rest short.

Forward biased, carriers are injected across the junction and recombine. Each recombination releases the gap energy. If the material allows, that energy leaves as a photon: this is a light-emitting diode.

Reverse biased, light absorbed inside the depletion region creates electron and hole pairs, and the field there immediately sweeps them apart before they can recombine. That gives a current proportional to the light: a photodiode.

Unbiased, the same light-generated pairs are separated by the built-in field alone. That produces a voltage across the terminals, and a current when a load is connected: a solar cell.

Now the material question, which decides whether the first of these works at all.

An electron falling across the gap must shed energy and conserve momentum. In a direct gap material, the top of the valence band and the bottom of the conduction band line up in momentum. The electron can then drop straight down and give everything up as a photon. In an indirect gap material they do not line up. A lattice vibration has to take up the difference, and the energy comes out as heat instead.

Silicon is indirect. That is the entire reason there is no silicon light-emitting diode, and why emitters are made from compound materials instead. It is a materials decision rather than a circuit one, and no amount of clever biasing gets around it.

The gap size sets the photon energy, which the eye reads as colour. A wider gap gives bluer light. This is why colours arrived historically in the order they did. Red was straightforward. Blue took decades and a different material system, because a wide direct gap that also dopes well is hard.

For the photodiode, reverse bias does two things. It widens the depletion region, so more of the incoming light is absorbed where the field can act on it. And it reduces the junction capacitance, so the device responds faster. Detectors are biased that way for both reasons.

For the solar cell, the priorities are completely different: area, cost and efficiency rather than speed. It is optimised to absorb as much of the spectrum as possible and to lose as little as possible to internal resistance.

One practical rule to take away. An emitter is driven with a current, never a voltage. The relation between them is exponential, so a small voltage error becomes a large current error, and connecting one directly across a supply destroys it. Use a series resistor at minimum, and a current source when the brightness has to be stable.

What you should now be able to explain or do

• Explain how one junction structure gives an emitter, a detector and a generator.
• Say why silicon cannot emit light and what is used instead.
• Explain why photodiodes are reverse biased, giving both reasons.
• Say what sets the colour of an emitted photon.
• Explain why an emitting device is driven with a current rather than a voltage.

Check yourself