What is a Photodiode?
A photodiode is a semiconductor device with a P-N junction that converts photons (or light) into electrical current. The Player has an abundance of holes (positive), and the N layer has an abundance of electrons (negative). Photodiodes can be manufactured from a variety of materials including, but not limited to, Silicon, Germanium, and Indium Gallium Arsenide. Each material uses different properties for cost benefits, increased sensitivity, wavelength range, low noise levels, or even response speed.
A photon can strike an atom within the device and release an electron if the photon has enough energy. This creates an electron-hole pair (e- and h+) where a hole is simply an “empty space” for an electron. If photons are absorbed in either the P or N layers, the electron hole pairs will be recombined in the materials as heat if they are far enough away (at least one diffusion length) from the depletion region. Photons absorbed in the depletion region (or close to it) will create electron hole pairs that will move to opposite ends due to the electric field. Electrons will move toward the positive potential on the Cathode, and the holes will move toward the negative potential on the Anode. These moving charge carriers form the current (photocurrent) in the photodiode.
Photodiode sensors have a high degree of linearity over a large range of light power levels: from fractions of a nanowatt to about 2 mW. Above that light level, corresponding to a current of about 1 mA, the sensor saturates and reads erroneously low.
Types of Photodiodes
This is the most basic photodiode. The physics of how the P-N junction photodiode operates was reviewed earlier. The PIN and APD photodiode are variations from the P-N junction. The depletion region contains few free charge carriers, and the width of the depletion region can be manipulated by adding voltage bias. Current passing through the photodiode can only flow in one direction based on the P and N doped materials. If reverse biased, current will not flow through a photodiode without incident light creating photocurrent.
The PIN photodiode is similar to the P-N Junction with one major difference. Instead of placing the P and N layers together to create a depletion region, an intrinsic layer is placed between the two doped layers. This intrinsic layer is highly resistive and increases the electric field strength in the photodiode. There are many benefits to the added intrinsic layer because the depletion region is greatly increased. The capacitance of the junction is decreased, and so the speed of the photodiode increased. The increased layer also allows for a larger volume of photon to electron-hole conversion and higher Quantum Efficiency.
Application of Photodiodes
Photosensors of all types may be used to respond to incident light or to a source of light which is part of the same circuit or system. A photodiode is often combined into a single component with an emitter of light, usually a light-emitting diode (LED), either to detect the presence of a mechanical obstruction to the beam (slotted optical switch) or to couple two digital or analog circuits while maintaining extremely high electrical isolation between them, often for safety (optocoupler). The combination of LED and photodiode is also used in many sensor systems to characterize different types of products based on their optical absorbance.
Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors.