The impact of a large bandgap drift region in long-wavelength metamorphic photodiodes

A comparative study of two types of metamorphic double heterojunction long- wavelength photodiodes on GaAs substrates is performed in terms of their ba ndwidths and responsivities. A P-i-I-N heterostructure with a large bandgap drift layer (I-InAlAs) at the cathode end of the photoabsorption region (i -InGaAs) is compared experimentally and theoretically to a P-i-N structure without a drift layer. Both types of photodiodes were fabricated using an I nGaAs-InGaAlAs-InAlAs double heterostructure design to simultaneously achie ve high bandwidths and high responsivities. The inclusion of an I-InAlAs dr ift region resulted in P-i-I-N photodiodes with larger bandwidths than P-i- N photodiodes with the same areas, or conversely a P-i-I-N photodiode can b e made larger than a comparable P-i-N photodiode, but achieve the same band width. Therefore, P-i-I-N photodiodes provide larger optical fiber alignmen t tolerances and better coupling efficiency than P-i-N photodiodes with the same bandwidths. P-i-I-N photodiodes with 10-mum-diameter optical windows typically exhibited low dark currents of 500 pA at 5-V bias, responsivities of 0.6 A/W, and a -3-dB bandwidth of 38 GHz for 1.55-mum operation.