Introduction
Photo detectors are used primarily as an optical receiver to convert light into electricity. The principle that applies to photo detectors is the photoelectric effect, which is the effect on a circuit due to light. Max Planck In 1900 discovered that energy is radiated in small discrete units called quanta; he also discovered a universal constant of nature which is known as the Planck’s constant. Planck’s discoveries lead to a new form
of physics known as quantum mechanics and the photoelectric effect
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which is
Planck constant multiplied by the frequency of radiation. The photo electric effect is the effect of light on a surface of metal in a vacuum, the result is electrons being ejected from the surface this explains the principle theory of light energy that allows photo detectors to operate. Photo detectors are commonly used as safety devices in homes in the form of a smoke detector, also in conjunction with other optical devices to form security systems.
Photo detector:
A photo detector operates by converting light signals that hit the junction to a voltage or current. The junction uses an illumination window with an anti-reflect coating to absorb the light photons. The result of the absorption of photons is the creation of electron-hole pairs in the depletion region. Examples of photo detectors are photodiodes and phototransistors. Other optical devices similar to photo detectors are solar cells which also absorb light and turn it into energy. A similar but different optical device is the LED which is basically the inverse of a photodiode, instead of converting light to a voltage or current, it converts a voltage or current to light.
Photodiodes:
A commonly used photo detector is the photodiode. A photodiode is based on a junction of oppositely doped regions (pn junction) in a sample of semiconductor. This creates a region depleted of charge carriers that results in high impedance. The high impedance allows the construction of detectors using silicon and germanium to operate with high sensitivity at low temperatures. The photodiode functions using an illumination window (Figure 1), which allows the use of light as an external input. Since light is used as an input, the diode is operated under reverse bias conditions. Under the reverse bias condition the current through the junction is zero when no light is present, this allows the diode to be used as a switch or relay when sufficient light is present.
Figure 1 photo diode with illumination window as shown.
Photodiodes are mainly made from gallium arsenide instead of silicon because silicon creates crystal lattice vibrations called phonons when photons are absorbed in order to create electron-hole pairs. Gallium arsenide can produce electron-hole pairs without the slowly moving phonons; this allows faster switching between on and off states and GaAs also is more sensitive to the light intensity. Once charge carriers are produced in the diode material, the carriers reach the junction by diffusion. Important parameters for the photodiode include quantum efficiency, current and capacitance which will be covered in the equations section.
Phototransistor
Phototransistor is similar to the photodiode except an additional n-type region is added to the photodiode configuration. The phototransistor includes a photodiode with an internal gain. A phototransistor can be represented as a bipolar transistor that is enclosed in a transparent case so that photons can reach the base-collector junction. The electrons that are generated by photons in the base-collector junction are injected into the base, and the current is then amplified. Since phototransistor detection is on the order of the photodiode they cannot detect light any better than a photodiode. The draw back of a phototransistor is the slower response time in comparison to a photodiode.
