Avalanche photo diodes (APDs) internally multiply the primary signal photocurrent before it enters the input circuitry of the following amplifier which increases receiver sensitivity.
For carrier multiplication to take place, the photo carriers generated must traverse a region where a very high electric field is present. This carrier multiplication mechanism is known as impact ionization.
This impact ionization leads to avalanche breakdown in ordinary reverse bias. It requires very high reverse bias voltage in order that the new carriers created by impact ionization can themselves produce additional carriers by same mechanism.
The newly generated carriers are again accelerated by, high electric field and they gain enough energy to cause further impact ionization. This phenomenon is avalanche effect.
Below avalanche breakdown voltage, a finite number of carriers are created whereas above breakdown infinite number of carriers is created for carrier multiplication.
Another basic performance characteristic of APD is the electron hole ionization rate, as it determines gain bandwidth product and noise performance characteristics.
Multiplication factor: Multiplication factor M is a measure of internal gain provided by APD. It is defined as the ratio of total multiplied output current to the primary multiplied current.
$ M = \dfrac I{I_p}$
Where I the Total multiplied output current
$I_p$ is the primary unmultiplied current
- Multiplication depends on physical and operational characteristics of photo detector device. Operational characteristics include the width of avalanche region, the strength of electric field and type of semiconductor material employed.
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