Output Saturation and Linearity of Waveguide Unitraveling-Carrier Photodiodes

Waveguide unitraveling-carrier photodiodes (UTC-PDs) with different absorber and collector layer doping levels have been fabricated and characterized. These photodiode (PD) structures are fabricated on a platform that allows for the monolithic integration of multiquantum-well optical phase modulators and couplers for realizing novel coherent receivers. 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 larger field is induced in the absorber layer at high photocurrent levels. Also the higher n-doping in the collector layer is adequate for providing charge compensation. For PD B, there is an enhancement in the RF response as the photocurrent level is increased. At a frequency of 1 GHz, the saturation current for PD A is around 65 mA and that for PD B is around 63 mA. For PD B, the third-order output intercept point at photocurrent levels of 30 and 40 mA is 37.2 and 34.9 dBm, respectively. That for PD A is 35.8 and 30.4 dBm. PD B is, therefore, favorable for linear operation at high current levels.