Research On Low-noice High-speed And High Saturation Uni-Traveling-carrier Photodetectors For Optical Fiber Communications

With the explosive development of smart terminals, social networks,cloud computing and online communication services, the demand for network bandwidth and transmission capacity is also growing fast. In order to meet this demand, the optical fiber communication system is developing towards the new generation optical network with high speed and large transmission capacity. The photodetector as the key components of optical fiber communication system, plays an important role in the development of optical fiber communications.In optical fiber communication system, because uni-traveling-carrier photodetectors (UTC-PD) utilize only electrons as active carriers, it reduces the transmission time and the space charge effect. As a result,UTC-PD can realize high speed and high saturation performance at the same time. Besides, the dark current of UTC-PD is very low and pA order of magnitude. In a word, UTC-PD has excellent low noise characteristics.The main content of this work is to study low noise, high speed and high saturation UTC-PD in optical communication systems.The main contents and innovations of this work are as follows:1. Basic thories, performance parameters and newest research advance of UTC-PD are analyzed. The carrier mobility, electric field distribution and energy band of UTC-PD are studied in detail. It provides a guideline for the design of low-noise, high speed and high saturation current UTC-PD.2. The physical mechanism of dark current is deeply studied. It is found that dark current includes diffusion current, surface recombination current, generation-recombination current and tunneling current. The dark current is dominated by generation-recombination mechanism at lower bias, whereas dominated by tunneling effects at higher bias. Based on the carrier density rate equation and the theory of dark current, the equivalent noise circuit model of PIN-PD and UTC-PD are developed and the characteristic of dark current of PIN-PD and UTC-PD are simulated.3. A comparative study with different Gaussian doping profiles in absorber is made on the high speed performance of UTC-PDs through simulation analysis, which is based on electric field and energy band diagram. Under the condition of same doping concerntration in absorber,the results show that the bandwidth has a positive relationship with the location of the peak doping concentration and the negative correlation between bandwidth and characteristic length is also found. As a result, the thickness of absorption layer of UTC-PD is 200nm, the peak concerntration of Gaussian doping is 3E19cm-3, the location of peak doping is the top of the absorber and the characteristic length is 0.037.For the device with 18 ?m2 active area, the bandwidth of UTC-PD is up to 81 GHz at a reverse bias of 3 V.4. A novel InP-based UTC-PD with low-noise, high speed and high saturation current is designed and discussed. Instead of the In0.53Gao.47As and uniform doping profile in absorber, the linear compositionally-graded In0.62Ga0.38As0.82P0.18?In0.53Ga0.47As and Gaussian doping profile are introduced, which enhance the potential gradient and build-in electric field to improve the bandwidth. Meanwhile, the step doping profile is adopted in collection region, which relaxes the space charge effect to improve the DC saturation current. For the device with 18?m2 active area,the 3-dB bandwidth is up to 85GHz, the DC saturation current of 152mA is reached. Besides, the dark current of zero-bias UTC-PD is only 27.4pA and the dark current is as low as about 1nA at a reverse bias of 3 V.5. The fabrication and performance test of UTC-PD are made in corporation with seniors. For the device with 12?m active diameter, the 3-dB bandwidth is 5.2GHz, the dark current is 7.87nA and the quantum efficiency is 29.5% at 3V bias and 1550nm wavelength.