When light hits the metal, it causes a change in the electrical properties of the substance. This phenomenon of photoelectric power is collectively known as the photoelectric effect. The photoelectric effect is divided into photoelectron emission, photoconductive effect and barrier layer photoelectric effect, also known as photogenerated volt effect. The former phenomenon occurs on the surface of an object and is also known as the photoelectric emission. The latter two phenomena occur inside the object and are known as the internal photoelectric effect.
Many types of photodetectors – such as photodiodes, phototransistors, photocells, and CCD or CMOS image sensors – work on the basis of the photoelectric effect. This actually comes in two completely different forms: external and internal photoelectric effects.
External photoelectric effect
For example, if light with sufficient high photon energy (higher than the so-called work function) hits a metal (or semiconductor) surface, some of the light may be absorbed and electrons will be emitted from the metal surface. If the metal part (called the photocathode) is kept in a vacuum and an electric field is applied using a second electrode (anode) that attracts these electrons, one can detect the photocurrent, which is proportional to the intensity of the incident light. The external photoelectric effect provides the operating principle for vacuum photocells. Here, external refers to the electrons that are obtained from the release of the surface of the material.
The photoelectric effect only occurs in light with wavelengths below a certain limit, depending on the material; For longer wavelengths, even fairly high light intensities cannot produce this effect. The maximum energy of the photoelectron (measured by the desired stopping potential) is independent of the light intensity, which is the opposite of what is expected. Instead, this maximum energy depends on the wavelength of light: the shorter the wavelength, the higher the energy of the photoelectron, while the intensity of light only affects the speed at which the photoelectron is emitted.
The photonic model explains the above observations well. It is only when the photon has a sufficiently high energy that it can trigger the emission of the photoelectron – a process that requires a certain amount of energy, i.e., binding energy plus the resulting kinetic energy of the electron.
The external photoelectric effect is used in vacuum tube photodetectors, especially photocells and photomultiplier tubes, but also in infrared observers, fringe cameras, image intensifiers (image amplifiers) and image converters. In addition, pulsed photocathodes irradiated with ultrashort laser pulses are used in some particle accelerators.
Internal photoelectric effect
The internal photoelectric effect does not produce photoelectrons that can be observed outside the material, but only excites the electrons to a higher level, i.e., from the valence band of the semiconductor material. The result is that photocurrents are usually detected in a back-biased p-n junction or p-i-n junction.
The internal photoelectric effect is utilized in various types of semiconductor photodetectors, i.e., in photodiodes and phototransistors. The prerequisite for this is that the photon energy is greater than the bandgap energy of the material in the active region. While typical dielectrics, such as fused silica or other optical glasses, have large bandgap energies and carrier lifetimes that are too short, various semiconductors exhibit smaller bandgap energies. For example, silicon-based detectors can operate at wavelengths of about 1.1 microns, although the responsiveness often drops significantly above 1 micron. Materials with lower bandgap energies allow for photoelectric detection at longer wavelengths – for example, gallium arsenide indium (InGaAs) can reach ≈1.7 μm.
There are even materials for detecting mid-infrared light, for example in infrared cameras. Their bandgap energy is so small that a large amount of thermal excitation has already occurred at room temperature. To avoid this, such detectors need to be operated at low temperatures, e.g. with a Stirling cooler.
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