Three mechanisms are limiting the speed of response of an avalanche photodiode (APD): The RC-time constant, the transit time of photo-generated carriers and the avalanche build-up time. Each factor contributes to the total device bandwidth depending on the operation condition.
The -3dB bandwidth of the RC-limitation is a function of the device capacitance, the load resistance and all the parasitics. The RC-limitation becomes negible as soon the APD becomes part of a receiver circuit.
Photo-generated carriers are created in the depletion region and are accelerated by the electric field and transported to their corresponding side of the pn-junction. As long as carriers are traveling through the depletion region a current can be measured at the contact of the APD. The actual transit time limitation depends on the carrier saturation velocity in the semiconductor material and the distance this carriers have to travel.
Avalanche build-up time limitation
Avalanche multiplication is a series of subsequent impact ionization events. In between impact ionization events, the charge carriers are accelerated to achieve sufficient energy for the next impact ionization event. Due to the subsequent nature of this process and the finite time required to accelerate the carriers, this process is time consuming. The avalanche build-up time limitation gives rise to the so called gain-bandwidth product.
The gain-bandwidth product is a figure of merit for avalanche photodiodes (APDs). However, its meaning should not be overestimated. Most applications are targeting optimal sensisitivty. For most state-of-the-art APDs like ours the optimal gain is around M=8 to M=15. At higher gain, the avalanche excess noise is starting to degrade the receiver sensitivity. This means that the bandwidth of the APD needs to be sufficient for 10G operation up to a multiplication gain of around M=15 to avoid intersymbol interference. However, the gain-bandwidth product is the assymptotic value of bandwidth vs. gain curve for M>20. Of course a higher gain-bandwidth product will lead to higher bandwidth at M=15.
For an actual 10G application the bandwidth simply needs to be sufficient around M=10 to M=15 to avoid intersymbol interference at the given bit rate. More important than the gain-bandwidth product is a low avalanche excess noise. I will come back to receiver sensitivity in a later post.