Avalanche photodiodes with low noise, high speed and PIN photodetectors with high output power
This dissertation describes three research projects on high-performance photodetectors. Two projects involved research on avalanche photodiodes (APDs). The third was a study of high speed photodiodes that operate with high input signal levels. APDs are important components in many optical receivers due to the sensitivity margin provided by their internal gain. The SACM structure avalanche photodiode (APD), which consists of an absorbing region and a multiplication region separated by a charge layer, has the advantage that the photon absorption process and the carrier multiplication process are independent and can be optimized individually to improve both the noise and speed performance. We have investigated the detrimental effects of impact ionization in the absorption region on the speed and noise performance of InGaAs/InAlAs SACM APDs. Improvements have been made by implementing p − doping in the absorption region to compensate the n − background doping. viii It is found that impact ionization in the absorption region is significantly suppressed by comparing the gain-bandwidth products, gain curves and excess noise factors of APDs with and without the p-type compensation. Impact ionization engineering (I 2E) is a novel approach that incorporates materials with different impact ionization threshold energy (Eth) into the multiplication region of APDs to achieve low noise and high speed. We demonstrate for the first time the design and successful implementation of an SACM APD with an In0.52Al0.48As/In0.53Ga0.17Al0.3As I2E multiplication region. By implementing an electric field gradient in the multiplication region, better control of impact-ionization has been achieved. Lower noise (k=0.1) and higher speed performance (gain-bandwidth product of 160GHz) compared to SACM APDs with bulk InAlAs multiplication layer (gain-bandwidth product of 120GHz) are demonstrated. For some novel microwave optoelectronic systems, it is necessary to have high-speed PIN PDs with very high RF output power. In this dissertation, we demonstrate an InGaAs/InP charge-compensated UTC photodiode with thick intrinsic region (500nm) that can be biased to relatively high voltages and deliver record high RF power. By using a heatsink, a 40-µm-diameter photodiode has achieved a bandwidth of 10GHz and RF output power of 24.5dBm. For a 100-µm-diameter device, a record high RF output power of 29dBm has been achieved. We have also studied high power detectors with Partially-DepletedAbsorber (PDA) and hybrid structures. A very high responsivity of 1 A/W has been achieved.