The APD is a semiconductor device that, when reverse-biased, creates strong fields in the junction region. When a photon causes an electron-hole pair, the pair flows through the junction. Because of the strong fields in the junction, the electron gains enough energy to cause secondary electron-hole pairs, which in turn cause more.
Thus a multiplication (or avalanche) process takes place (hence the name), and a
substantial current is generated from few initial photons.
- Asymmetric doped p-n junction - Reversebiased PIN diode
- Incoming light signal initiate an avalanche breakdown: photogenerated carriers are accelerated due to applied large electric field and generate more carriers via impact ionization, providing internal gain => Built-in amplifiers
- response to varying light is extremely short
Gain of APD:
The gain M of an APD is expressed by
M = IAPD/IPrimary
where IAPD is the APD output current and Iprimary is the current due to photon-electrons conversion. However, during this multiplication process, shot noise is multiplied as well, and is estimated as
where P is the APD noise factor.
If is the effective transit time through the avalanche region, the APD bandwidth is approximated as:
- Designed to have low dark current and single carrier type reducing noise since gain factor and frequency response are better than any other common photodetectors.
- Separation of Absorption and Multiplication regions for reducing tunneling current.
- Separation of Charge and Grading regions decoupling the requirement of high doping.
- Multicell PIN APD cell: I-(dP-doping)-I-(dn-doping)-I one type multiplication of carrier occurs.