High gain-bandwidth-product avalanche photodiodes for multigigabit data rate

For long-wavelength (1.3- and 1.5-μm) high-bit-rate (>400-Mbit/s) lightwave systems the highest receiver sensitivities have been achieved with III–V compound avalanche photodiodes with separate absorption and multiplication regions (SAM-APDs). Initial APDs of this type exhibited poor frequency response owing to charge accumulation at the heterojunction interfaces. A significant improvement in the bandwidth was achieved by introducing a transition region between the multiplication and absorption layers (SAGM-APD). Early SAGM-APDs exhibited bandwidths in the 1–3-GHz range and gain-bandwidth products as high as 18 GHz. Recently, the progression of new lightwave systems to higher and higher bit rates has stimulated efforts to further increase the bandwidth of these SAGM-APDs.