Research Of High Speed And High Saturation Uni-Traveling-Carrier Photodiodes Under Low Bias

With the continuous development of network,we have entered a new era of rapid development of information technology.Our life is full of new technologies such as face recognition,mobile payment and so on.It can be said that our lives are interacting with huge amounts of information every day.In this context,the optical fiber communication system also has new development requirements.It is required to achieve larger capacity and longer distance transmission.Photodetector,as the one of the most important part of optical fiber communication system,also need to have faster response speed,greater responsivity and better high saturation performance.Uni-traveling-carrier photodiode(UTC-PD)uses electrons as single-traveling carrier,which effectively reduces the space charge effect of devices.Compared with conventional photodetectors,UTC-PD has advantages in high speed and high saturation characteristics.In addition,the optimum electric field required for UTC-PD to obtain overshoot velocity of electron is relatively small,which enables the device to provide high speed output response even under relatively low bias.Meanwhile,Low bias means low power consumption,simplifies packaging and maintenance,and prolongs the lifetime of devices.It conforms to the trend of integration of optical communication technology and has broad application prospects.This paper focuses on the theoretical and experimental research of uni-traveling-carrier photodiode under low bias(0-1V).The main research contents and innovations are as follows:1.The performance and function layer optimization of UTC-PD structure under low bias are completed.The results show that a classical UTC-PD structure proposed by Ishibashi et al.has better high speed performance near 0.1 V bias than other low bias values.Compared with performance under high bias and zero bias,the high speed performance under low bias is between them.Under 0.1V bias,the 3dB bandwidth of 7um diameter device reaches 112GHz,which is significantly higher than 105GHz under zero bias,slightly less than 122GHz under 6V bias.On the other hand,it is found that the step doping of absorption layer and collector layer,and the parameters optimization of the other functional layers can significantly improve the high speed performance of devices.2.A new structure of UTC-PD is proposed.A novel doping collector layer and a gradient doping space layer are used to significantly improve high speed response under low bias.Under 0.1V bias,the 3dB bandwidth of the new UTC-PD structure with a diameter of 7μm is up to 150GHz,which is 33.9%higher than that of a classic UTC-PD structure proposed by Ishibashi et al.The device bandwidth can be further improved to 181GHz by using dipole doping in the absorption layer and step doping near the cliff layer.This bandwidth value is higher than the current reported 3dB bandwidth record under zero bias,and can completely match the bandwidth under high bias.3.The effects of different vertical light illuminating ways on device performance under low bias are analyzed in detail.The results show that for the Ishibashi classical UTC-PD structure with 50nm thick P-contact layer,when the incident wavelength is 1550nm,the back illuminated device has the highest responsivity and bandwidth,followed by both sides illuminated device,and the worst is the front illuminated device.The reasons are analyzed theoretically.First,for front illuminated device,Ino.53Gao.47As in the P-contact layer absorbs the incident light,which reduces the effective light power to reach the absorption layer.Second,when the detector is not saturated,the electric field intensity of UTC-PD absorption layer is small for front illuminated device,so the electron transit time in the absorption layer is longer and the bandwidth of 3dB is smaller.4.The performance of UTC-PD arrays under low bias voltage is systematically simulated and analyzed.The analysis shows that,due to the effect of capacitance,the 3dB bandwidth of UTC-PD arrays decrease with the increase of cascade number when the incident light intensity is the same.However,the UTC-PD arrays significantly improve the high saturation characteristics of the devices,and the optical power responsive range of which increase linearly with the number of cascades.In addition,when the area of active layer is the same for 1×4 array structure and UTC-PD discrete large-size structure,the high speed of array structure is better because the transverse transmission distance of electrons in array structure is smaller and the electric field at the center of device is larger.5.The UTC-PD device was fabricated and its performance was tested.The experimental results show that under 0.5V bias,the 3dB bandwidth of UTC-PD structure with diameter of 32um can reach 9.8GHz,which is significantly higher than the corresponding 2.6GHz under zero bias.