Written by Kevin Tavolaro
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An APD is one of several varieties of avalanche diodes, special semiconductor instruments utilized as safety measures within a system. Avalanche diodes safeguard circuits by protecting them from dangerous high voltages. APD stands for "avalanche photodiode," referring to the most precise and sensitive type of these semiconductors.

All avalanche diodes employ reverse bias voltage. In this situation, the cathode-anode relationship is inverted, and the diode is attached to the circuit. These diodes are non-conducting, and their presence does not interfere with the operation of the circuit. The avalanche diode is then designed to breakdown if the voltage exceeds a certain limit. This shields the circuit from hazardously high voltage, which is then diverted to the earth.

APD Usage

The difference between an avalanche diode and an APD is that an APD involves a specific type of semiconductor known as a "photodetector." Photodetectors are transducers that receive an optical signal and convert it into an electrical signal containing identical information When used as a protective measure for circuitry, avalanche photodiodes offer the highest level of sensitivity, and are suited for the most precise of operations. For example, single photon detection is made possible with the use of avalanche photodiodes

Thanks to impact ionization, avalanche photodiodes achieve a higher gain by means of a reverse bias voltage. This process links the gain to the reverse voltage, allowing the APD to regulate the voltage that passes through the circuit. When the maximum voltage is exceeded, the avalanche photodiode breaks down and redirects the current to the ground, sparing the circuits. The precision and fragility of avalanche photodiode's makes them best suited for delicate applications. One such use is in the development of laser range detection.

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