What happens inside a transistor when it releases photons

A transistor emits light when electrons moving through its semiconductor material lose energy and drop to a lower energy state. That energy release takes the form of a photon — a particle of light. This happens most visibly in light-emitting diodes (LEDs) and laser diodes, where the semiconductor is designed specifically to produce photons rather than just conduct electricity. In standard transistors used for computing, photon emission is usually a side effect rather than the main purpose, but the mechanism is the same.

The key to understanding photon emission is recognizing that electrons in a semiconductor exist at different energy levels. When an electron is excited — knocked to a higher energy level by an electrical current — it becomes unstable. It naturally wants to fall back down to a lower energy level. When it makes that jump, it releases the extra energy as a photon. The color or wavelength of that photon depends on how far the electron falls: a larger energy drop produces a higher-energy photon, which appears as blue or ultraviolet light, while a smaller drop produces lower-energy photons that appear as red or infrared light.

Key Takeaways

  • Photon emission occurs when electrons in a semiconductor drop from a higher energy level to a lower one, releasing the difference as light.
  • The wavelength of the emitted photon is determined by the size of the energy gap in the semiconductor material — larger gaps produce shorter wavelengths (blue light), smaller gaps produce longer wavelengths (red or infrared).
  • LEDs and laser diodes are engineered to maximize photon emission, while standard transistors emit photons as an unintended byproduct of normal operation.
  • The process requires both an electron and a "hole" (the absence of an electron) to meet in the semiconductor's junction, where recombination releases the photon.

The role of the semiconductor junction

Photon emission happens at the junction where two different types of semiconductor material meet — typically where a p-type layer (which has an excess of holes, or missing electrons) meets an n-type layer (which has an excess of free electrons). When you explore voltage across this junction, electrons from the n-type side are pushed toward the p-type side, and holes from the p-type side are pushed toward the n-type side. They meet in the middle at what is called the depletion region.

When an electron and a hole meet in the depletion region, they recombine — the electron fills the hole. This recombination releases energy. In a regular transistor, most of that energy is released as heat. But in a semiconductor designed for light emission, the energy is released as a photon instead. The difference comes down to the material itself: semiconductors like gallium arsenide (GaAs) or indium phosphide (InP) are much better at converting electrical energy into photons than silicon is, which is why LEDs and laser diodes use these materials rather than the silicon used in most computer chips.

How the energy gap determines the color of light

Every semiconductor material has a characteristic bandgap — the minimum amount of energy needed to knock an electron from the valence band (where it is bound to an atom) to the conduction band (where it is free to move). When an electron recombines with a hole, it releases energy equal to the bandgap. That energy becomes a photon, and the photon's wavelength is directly determined by the bandgap size.

A large bandgap means a big energy drop, which produces a high-energy photon with a short wavelength — these appear as blue or ultraviolet light. A small bandgap means a smaller energy drop, which produces a lower-energy photon with a longer wavelength — these appear as red or infrared light. This is why different LED colors require different semiconductor materials: a red LED uses a material with a smaller bandgap (around 1.8 to 2.0 electron volts), while a blue LED uses a material with a larger bandgap (around 2.7 to 3.4 electron volts). Infrared LEDs, used in remote controls and fiber-optic communications, have even smaller bandgaps and emit light invisible to the human eye.

Spontaneous emission versus stimulated emission

There are two ways a photon can be released from a semiconductor. Spontaneous emission is what happens in an LED: an electron randomly recombines with a hole, and a photon is emitted in a random direction with a random phase (timing). Many photons are emitted this way, but they do not reinforce each other — they scatter in all directions. This is why an LED produces incoherent light.

In a laser diode, something different happens. The semiconductor is placed inside an optical cavity — a structure with mirrors on both ends. When a photon is emitted, it can stimulate another electron to recombine and emit a second photon that is identical to the first: same wavelength, same direction, same phase. This is called stimulated emission. The two photons reinforce each other, and if they trigger more photons, the light amplifies. This is how a laser diode produces coherent, directional light from the same basic recombination process that happens in an LED.

Why standard transistors emit photons as a side effect

Every transistor, whether it is designed to emit light or not, has electrons moving through a semiconductor and losing energy. In a standard silicon transistor used in a computer processor, most of that energy is released as heat through phonons — vibrations in the crystal lattice. But some energy is also released as photons, even though the transistor was not designed for it. These photons are usually infrared and very dim, and they escape the device without being captured or amplified.

The reason silicon transistors do not glow visibly is twofold: silicon has a relatively small bandgap (about 1.1 electron volts), which means the photons it emits are mostly infrared and invisible to human eyes. Second, silicon is an indirect bandgap semiconductor, meaning that recombination is less likely to produce a photon and more likely to produce heat instead. Materials like gallium nitride (GaN) and gallium arsenide are direct bandgap semiconductors, where recombination much more readily produces photons. This is why these materials are chosen for LEDs and laser diodes, while silicon is chosen for logic and memory chips where light emission would be wasted energy.

The role of doping and impurities

The purity and composition of the semiconductor material strongly affect how many photons are emitted. Semiconductors are doped — intentionally contaminated with small amounts of other elements — to create the p-type and n-type regions. The type and concentration of dopants determine how many electrons and holes are available to recombine, and therefore how many photons can be produced.

Unintended impurities can also affect photon emission. If a semiconductor contains defects or unwanted atoms, electrons can recombine at those defect sites instead of at the main junction. When this happens, the energy is often released as heat rather than as a photon, or the photon is emitted at a wavelength different from the intended one. This is why high-quality LEDs and laser diodes require very pure semiconductor materials and careful control of the doping process. A small amount of contamination can dramatically reduce the light output or shift the color of the light.

Temperature effects on photon emission

The temperature of a semiconductor affects both the number of photons emitted and their wavelength. As temperature increases, the bandgap of the semiconductor decreases slightly, which means electrons need less energy to jump to the conduction band. This can increase the number of recombination events and therefore the number of photons produced. However, higher temperature also increases the number of non-radiative recombinations — recombinations that release energy as heat instead of photons — which reduces the overall light output.

The wavelength shift with temperature is also important in practical applications. As a semiconductor heats up, the photons it emits shift toward longer wavelengths (toward red). This is why the color of an LED can appear to change slightly as it warms up, and why laser diodes must be temperature-controlled to maintain a stable output wavelength. In high-power applications, managing heat is critical not just for efficiency but for maintaining consistent light output.

Frequently Asked Questions

Why do some transistors glow and others don't?

Transistors designed as LEDs or laser diodes use direct bandgap semiconductors like gallium arsenide, where recombination readily produces photons. Standard silicon transistors have an indirect bandgap, where recombination more often produces heat. Silicon transistors do emit photons, but mostly infrared ones that are invisible and very dim.

Can you see the light coming from a computer chip?

No. Computer chips are made of silicon, which emits mostly infrared photons that human eyes cannot see. Even if the photons were visible, they are so few and so dim compared to the heat generated that you would not notice them. The chip would appear dark to your eye.

What determines whether a photon is red, blue, or infrared?

The bandgap of the semiconductor material determines the photon wavelength. A large bandgap produces high-energy photons (blue or ultraviolet), while a small bandgap produces low-energy photons (red or infrared). Different semiconductor materials are chosen specifically to produce the desired color.

Why do LEDs need to be made from special materials instead of silicon?

Silicon is an indirect bandgap semiconductor, meaning recombination is unlikely to produce a photon — the energy is released as heat instead. Materials like gallium nitride and gallium arsenide are direct bandgap semiconductors, where recombination readily produces photons. This makes them far more efficient for light emission.

Does a laser diode emit photons the same way an LED does?

Both use the same recombination process to produce photons. The difference is that a laser diode is placed in an optical cavity with mirrors, which allows photons to stimulate other recombinations and amplify the light. This produces coherent, directional light instead of the scattered light from an LED.