Ultra-fast Photodiodes under Zero- and Forward-Bias Operations
This chapter reviews several kinds of high-speed photodiodes (PDs), which have the capability in zero-bias or forward-bias operations for the application of Optical interconnect, coherent detection, and millimeter wave (MMW) photonics. It introduces their working principles and explains why they can have high-speed performance within an extremely small electric (E-) field inside their active layers. Regarding the receiving end, there are PDs, transimpedance amplifiers, and limiting amplifiers (LAs) inside. To see the energy consumption breakdown of the whole channel, the major consumed power is in the receiver side. It mainly consists of two parts: one is from LAs, which need to have high peak-to-peak driving voltage output to drive the next-stage digital circuit; the other is from the DC component of photocurrent generated from the high-speed optical data stream. In order to overcome the two aforementioned speed-limiting factors, UTC-PD structures, which can eliminate the hole transport, have successfully demonstrated excellent speed and power performance under zero- or forward-bias operations.