Research On High-speed And High-power Photodetector And Its AM-PM Conversion Effect

The unique ability of microwave photonic technology in processing ultra-broadband and high-frequency microwave signals makes it a promising option for broadband microwave signal processing,which has developed rapidly in recent years.Whether increasing link gain,reducing noise figure,or increasing spurious-free dynamic range requires high-power,high-response photodetectors.In this dissertation,a high-speed and high-power photodetector based on In Ga As/In P is designed,fabricated and characterized according to the requirements of microwave photonic systems for photodetectors.The bandwidth,microwave saturation power,and AM-PM（amplitude modulation to phase modulation）conversion effect of photodetectors are studied from both theoretical and experimental levels.The main object of the experimental study is the Charge-Compensated Modified Uni-Travelling-Carrier（CC-MUTC）photodetector,and the PIN photodetector is also studied for comparison.In addition,the low temperature characteristics of photodetectors are also a research hotspot in recent years.This paper also studies the low temperature characteristics of photodetectors.The main contents and achievements of this paper include:（1）The epitaxial structure of the CC-MUTC photodetector is optimized,and the effects of various epitaxial structures on the photodetector bandwidth and saturation characteristics are analyzed.The coplanar waveguide electrode of the high-speed photodetector is optimized to ensure that the transmission characteristics and characteristic impedance of the electrode meet the transmission requirements of high-speed signals.（2）Using the fabrication process of high-speed and high-power photodetectors,experimental samples of CC-MUTC photodetectors of different sizes were actually fabricated,and tests and theoretical analyses were carried out.The experimental samples of the PIN photodetector were simultaneously fabricated for comparison.For the CC-MUTC photodetector and PIN photodetector with a diameter of 40μm,when the reverse bias voltage is greater than 1 V,the responsivity is 0.41 A/W and 0.4 A/W,respectively.Under-5V bias,the 3d B bandwidths of CC-MUTC photodetector and PIN photodetector with a diameter of 20μm reach 20.364 GHz and 21.608 GHz,respectively.The saturated photocurrents at 20 GHz are 23.079 m A and 20.879 m A,respectively,and the corresponding output microwave powers are 7.313 d Bm and 6.297 d Bm,respectively.The saturated photocurrents of the 40μm CC-MUTC photodetector and the PIN photodetector at 5 GHz are 48.989 m A and 34.692 m A,respectively,and the corresponding output microwave powers are 15.688 d Bm and 12.493d Bm,respectively.The tests show that the CC-MUTC photodetector with the same size is higher than the PIN photodetector in terms of saturation current and output microwave power.The measured bandwidth is lower than the simulation results,mainly due to the larger lateral resistance in the p-doped layer of the normal incidence device.（3）For the normal incidence CC-MUTC photodetector,a nonlinear equivalent circuit model corresponding to the device structure is proposed,and the model parameters vary with the bias voltage and photocurrent.Using this model,the effects of different photocurrent carrier transit processes and RLC responses on the bandwidth were investigated.The results show that for a device with a diameter of 20μm,the 3d B bandwidth is mainly limited by the carrier transit process;while for a device with a diameter of 40μm,the 3d B bandwidth is determined by the carrier transit process and the RLC response.In addition,using this model,the effects of different photocurrent carrier transit processes and RLC responses on the phase change of the microwave signal were studied for the first time,and the contributions of these two factors to the AM-PM conversion effect were clarified.The research results show that for a device with a diameter of 40μm,the phase change caused by the RLC response dominates the change of the total phase of the device,and the AM-PM conversion coefficient?_z,modelcorresponding to the RLC response dominates the total AM-PM conversion coefficient?_measure;while for a device with a diameter of 20μm,Both the carrier transition process and the RLC response have significant contributions to the phase change of the device,and it is necessary to consider the contribution of the carrier transition process and the RLC response to the AM-PM conversion effect at the same time.（4）The low temperature characteristics of CC-MUTC photodetector and normal incidence PIN photodetector were tested and analyzed.In the temperature range of 190K-300K,the dark current activation energies of the PIN photodetector and the CC-MUTC photodetector were measured respectively.The activation energy E_a of the dark current of the PIN photodetector is equal to 0.43 e V,which is about 58%of the In_0.53Ga_0.47As band gap of0.74 e V,indicating that the dark current is dominated by the generation-recombination current and the diffusion current.While the activation energy E_a of the dark current of the CC-MUTC photodetector is equal to 0.35 e V,which is about 47%of the In_0.53Ga_0.47As band gap of 0.74e V,indicating that the dark current is dominated by the generation-recombination current.The experiment is consistent with the theory.In addition,the bandwidth of the CC-MUTC photodetector was measured at low temperature and the corresponding physical mechanism was analyzed.When the temperature drops from 300 K to 150 K,the heterojunction barrier increases with the decrease of temperature,and the electron transport time is mainly increased by the blocking effect of the heterojunction barrier,resulting in a decrease in the bandwidth.When the temperature drops from 150 K to 77 K,the electron mobility and saturation velocity of In Ga As and In P materials increase significantly with the decrease of temperature,which is the main factor affecting the bandwidth.In this temperature range,the bandwidth increases with the decrease of temperature.