High bandwidth planar InP/InGaAs avalanche photodiodes

The design, fabrication, and performance of a planar InP/InGaAs APD (avalanche photodiode) with high bandwidth for use in high-bit-rate lightwave systems are discussed. The InP multiplication region is highly doped to reduce the avalanche build-up region and increase the gain bandwidth product. This also lowers the transit time across the InP layer. An InGaAsP layer reduces hold trapping at the heterointerface. The thickness of the absorbing InGaAs layer is a compromise between reasonable quantum efficiency and short transit time. To further reduce transit-time effects, the InP buffer layer between the substrate and the InGaAs layer is highly doped to stop the depletion region at the edge of the InGaAs. This material structure is grown by VPE (vapor-phase epitaxy). Devices with gain-bandwidth products up to 70 GHz and bandwidths greater than 5 GHz at M=10 have been fabricated. Bandwidths at low gain of 7 GHz or greater, which can be explained solely by transit time effects have been measured. It is believed that a single InGaAsP layer in this material reduces hole trapping to negligible amounts at a heterointerface field of 6×10⁶ V/m or greater. A strong dependence of gain bandwidth product on heterointerface field has been observed, even down to fields as low as 7×10⁶ V/m. This can be explained by a small amount (<5%) of electron ionization in the InGaAs layer