Low bias, low noise single-avalanche-stage APDs

Avalanche photodiodes (APDs) are attractive for use in large capacity long distance optical communication systems. They provide an advantage over p-i-n detectors because of their internal gains. A gain-bandwidth product of ∼300 GHz has been achievable using an APD with a <100 nm thick multiplication layer. Avalanche multiplication based on impact ionization is a noisy process intrinsically. In a conventional APD, the multiplication region is long. Carriers can be ionized essentially anywhere within the avalanche region. This causes a large gain fluctuation; therefore, it becomes another source of noise in an APD in addition to the shot noise. The ultra low noise APDs device has two large band discontinuities (and high E-field regions too), where the deterministic impact ionizations will take place. Such a device will have a very high operational speed limited by the drifting time in the absorption region. Competing with the impact ionization processes, the tunneling effect increases with the reverse bias voltage and the tunneling leakage current can dominate the total current. Although the leakage current has limited us to do more meaningful system measurement at this moment, we have demonstrated avalanche gains at quite low bias voltage. We also expect that by using a combination of the InAIAs and InP materials. We will be able to obtain better results and achieve our proposed device performance.