Uni-traveling-carrier photodiodes with increased output response and low intermodulation distortion

Waveguide uni-traveling-carrier photodiodes have been fabricated and tested to investigate the influence of the doping profile in several of the device layers on saturation characteristics and linearity. Two particular photodiode (PD) structures are discussed. Compared to PD A, PD B has a lower and more graded p-doping profile in the absorber layer and also a higher n-doping level in the collector layer. For PD B a higher field is induced in the absorber layer and the higher doping in the collector layer provides charge compensation. An enhancement in the response for PD B is observed with increasing photocurrent. At a frequency of 1 GHz the saturation current for PD A is 65 mA and that for PD B is 63 mA. The third-order intermodulation distortion was also measured at various photocurrent levels. For PD A, the third-order output intercept point (OIP3) for photocurrent levels of 10 mA, 20 mA, and 30 mA is 34.7 dBm, 38.6 dBm, and 35.8 dBm respectively. Those for PD B are 28.1 dBm, 41.0 dBm, and 37.2 dBm. These are all for a bias of -5 V. While PD A has a higher OIP3 at lower photocurrent levels, PD B is favorable for high current operation.