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Ch7Sec5 - atoms are ionized These new free charges go...

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THE FIBER FORUM Fiber Optic Communications JOSEPH C. PALAIS PRESENTED BY
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Joseph C. Palais 7.5 2 Section 7.5 Avalanche Photodiode The avalanche photodiode (APD) is a pin photodiode having internal gain. The circuit is the same as that for a conventional pin photodiode. V B APD R L + -
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Joseph C. Palais 7.5 3 AVALANCHE PHOTODIODE The supply voltage is on the order of a few hundred volts to produce a high electric field in the junction. When a photon is absorbed in the junction, free charges are released as we described for the pin photodiode. These charges are accelerated by the force of the high electric field. The charges gain kinetic energy and collide with neutral atoms.
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Joseph C. Palais 7.5 4 AVALANCHE PHOTODIODE The kinetic energy causes the release of new
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Unformatted text preview: atoms are ionized). These new free charges go through the same process, ionizing even more atoms, creating even more free charges. An avalanche of new charges flow in the junction increasing the total circuit current. In this way, there is an increase (gain) in the current. Joseph C. Palais 7.5 5 AVALANCHE PHOTODIODE Gains up to a few hundred are possible. The APD is the solid state equivalent of the photomultiplier tube (PMT). 1. Unfortunately, it does not have as much gain as the PMT. 2. Fortunately, it is a solid state device and is compatible with fiber-optic system requirements. Joseph C. Palais 7.5 6 AVALANCHE PHOTODIODE 3. The APD improves the receiver sensitivity....
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