Photodetector
Photodetectors are electromagnetic radiation sensors converting the electromagnetic radiation into readable output through various mechanisms.
Types of photodetectors
Light incident on a material can cause various changes in them, depending on the phenomenon of conversion of light to any measurable signal, photodetectors can be classified as
| Phenomenon | Mechanism |
|---|---|
| photoelectric effect | Photons cause electrons to shift from the conduction band of a material to valence band producing free electrons in a vacuum or gas. |
| Thermal | Photons cause electrons to transition to mid-gap states then decay back to lower bands, making phonon which is heat. |
| Polarization | Photons induce changes in polarization states of suitable materials, that can change the index of refraction or other polarization effects. |
| Photochemical | Photons induce a chemical change in a material |
| Weak interaction effects | photons induce secondary effects such as in photon drag. detectors or gas pressure changes in Golay cells. |
Configurations of Photodetectors
Arrangement of the sensors in a photodector, there are two types of configuration.
-
1-D array - to measure the distribution of light along a line – Eg: spectrophotometer
-
2-D array- form images from the pattern of light – Eg: image sensor
Properties of Photodetectors
The figure of merit for the selection of photodetectors are as follows
| Parameters | Description |
|---|---|
| Spectral response | The response of a photodetector as a function of photon frequency |
| Quantum efficiency | The number of carriers (electrons or holes) generated per photon |
| Responsivity | The output current divided by total light power falling on the photodetector. |
| Noise-equivalent power | The amount of light power needed to generate a signal comparable in size to the noise of the device. |
| Detectivity | The square root of the detector area divided by the noise equivalent power. |
| Gain | The output current of a photodetector divided by the current directly produced by the photons incident on the detectors, i.e., the built-in current gain. |
| Dark current | The current flowing through a photodetector even in the absence of light |
| Response time | The time needed for a photodetector to go from 10% to 90% of final output. |
| Noise spectrum | The intrinsic noise voltage or current as a function of frequency. This can be represented in the form of a noise spectral density. |
| Nonlinearity | The RF-output is limited by the nonlinearity of the photodetector |
Photodetector devices
Based on each phenomenon, there are various devices to choose from to be used in different application
Photoemission or photoelectric
| Device | Description |
|---|---|
| Gaseous ionization detectors | Detect photons and particles with sufficient energy to ionize gas atoms or molecules. Electrons and ions generated by ionization cause a current flow which can be measured. Experimental particle physics |
| Photomultiplier | A setup with multiple tubes that encloses a photocathode which emits electrons when illuminated, the electrons are then amplified by a chain of dynodes. A dynode is a vacuum tube that serves as an electron multiplier through secondary emission. |
| Phototubes | A tube with photocathode which emits electrons when illuminated, such that the tube conducts a current proportional to the light intensity. |
| Microchannel plate detectors | Silicon-based photomultipliers used for detection of single particles (electrons, ions and neutrons) and low intensity impinging radiation (ultraviolet radiation and X-rays). |
Semiconductor
| Device | Description |
|---|---|
| Active-pixel sensors | Image sensors usually made using cmos fabrication process, commonly used in cell phone cameras, web cameras, and some dslrs. |
| Cadmium zinc telluride radiation detectors | Operating in direct-conversion (or photoconductive) mode at room temperature, unlike some other materials (particularly germanium) which require liquid nitrogen cooling. High sensitivity for x-rays and gamma-rays, due to the high atomic numbers of Cd and Te, and better energy resolution than scintillator detectors. |
| Charge-coupled devices | Used to record images in astronomy, digital photography, and digital cinematography. Used instead of photographic plates a predecessor to cryogenic detectors. |
| HgCdTe infrared detectors. | Detection occurs when an infrared photon of sufficient energy kicks an electron from the valence band to the conduction band. Such an electron is collected by a suitable external readout integrated circuits (ROIC) and transformed into an electric signal. |
| LEDs | Reverse-biased to act as photodiodes. |
| Photoresistors/ Light Dependent Resistors (LDR) | Change resistance according to light intensity. Normally the resistance of LDRs decreases with increasing intensity of incident light |
| Photodiodes | Operate in photovoltaic mode or photoconductive mode,they are often combined with low-noise analog electronics to convert the photocurrent into a voltage that can be digitized. |
| Phototransistors | Amplifying photodiodes. |
| Quantum dot photoconductors or photodiodes | Detecting wavelengths in the visible and infrared spectral regions. |
| Semiconductor detectors | Used in gamma and X-ray spectrometry and as particle detectors |
| Silicon drift detectors | X-ray radiation detectors used in x-ray spectrometry (EDS) and electron microscopy |
Photovoltaic
| Device | Description |
|---|---|
| Photovoltaic cells or solar cells | A combination of layers of semiconducting material that can produce a voltage across it and supply an electric current when light is incident on it |
Thermal
| Device | Description |
|---|---|
| Bolometers | Measure the power of incident electromagnetic radiation by heating of a material that has temperature-dependent electrical resistance. A microbolometer is a specific type of bolometer used as a detector in a thermal camera. |
| Cryogenic detectors | Sensitive to measure the energy of single x-ray, visible and infrared photons. Used in space applications |
| Pyroelectric detectors | Detect photons through the heat they generate and the subsequent voltage generated in pyroelectric materials. |
| Thermopiles | Detect electromagnetic radiation through heat, then generating a voltage in thermocouples. |
| Golay cells | Detect photons by the heat they generate in a gas-filled chamber, causing the gas to expand and deform a flexible membrane whose deflection is measured. |
Photochemical
| Device | Description |
|---|---|
| Photoreceptor cells | Used in bionics for the retina that detect light through biological function. |
| Chemical detectors - photographic plates | A chemical reaction that results in splitting silver halide molecule into an atom of metallic silver and a halogen atom. The photographic developer causes adjacent molecules to split similarly. |
Polarization
| Device | Description |
|---|---|
| photorefractive effect | Used in holographic data storage. |
| Polarization-sensitive photodetectors | Use optically anisotropic materials to detect photons of a desired linear polarization |
Application of photodetectors
Photodetectors have application in various fields from industries to medical and residential to commercial uses.
- Sensing applications
- Communication couplers for electrical isolation between circuits
- As recording element.