Design And Theoretical Analysis Of Zero-bias High-speed High-power Photodiodes

In recent years,the new services of the telecommunication and the Internet industry have been emerging and maturing,including 4G/5G network,large data,cloud computing and so on.Because the emerging multimedia services are gradually being widely applied,the demand for high-speed and large-capacity mobile data transmission will be higher,which will bring great challenges to the optical fiber communication network technology.As one of the most important devices of the optical receivers,the photodiode is bound to have high-speed and high-photocurrent performance,and the ability to withstand the high optical injection level(high-power)to deal with the above challenges.With the concept of "Radio-over-Fiber" being put forward,the photodiodes used in digital optical communication systems will not be able to meet the requirements of high-speed and high-power transmission in analog signal communication systems.Uni-traveling-carrier photodiode(UTC-PD)has many advantages,such as high-speed response,high-linearity and high-photocurrent,which satisfies the demand of optical communication system.UTC-PD is the most suitable choice for the receiver of high-power analog optical communication system.In addition,the explosive growth of data transmission makes the number,energy consumption and the size of chips increased.Therefore,the development trend of optical fiber communication technology is:integration,low-energy consumption and maintainability.UTC-PD without a power supply will save the energy,simplify the packaging,reduce the dimensions,and ease the maintenance,so that more attention is paid to studying bias-free photodiodes in recent years.The work of this paper is mainly focused on the research and principle analysis of high-speed and high-power UTC-PD under zero bias.The innovations and main works are as follows:1.The advantages and disadvantages of several types of common photodiodes used in optical communication system are studied.Among which,UTC-PDs can widely apply in high-speed and large-capacity optical communication systems.The high-speed and high-power performance of zero-bias UTC-PD is analyzed theoretically,and the important performance parameters of UTC-PD are researched.2.The f3dB and high-power performance of UTC-PD are discussed when the absorption layer is under different doping profiles,and different thicknesses and doping concentrations of the cliff layer and the collection layer.By comparing the results of simulations,we find the best structure of zero-bias UTC-PD is when the absorption layer is uniformly doped,the thickness of cliff layer is 10nm and doping concentration is 1×1018cm-3,and the thickness of collection layer is 200nm and doping concentration is 1 × 1016cm-3.With this structure and a 14?m2 active area,the 3dB bandwidth(f3dB)of the designed zero-bias UTC-PD can be up to 88.6GHz when incident light intensity is 6×105W/cm2(84mW),and the f3dB can still achieve 71.7GHz when incident light intensity is 9×105W/cm2(126mW).The responsivity of the PD is 0.16A/W.The optimized UTC-PD not only has high-speed response,but also has better ability to receive high-power incident light intensity.3.We analyzed the influence on the f3dB and high-power performance when the absorption layer of zero-bias UTC-PD is doped with different profiles.Zero-bias UTC-PD with gaussian doping can enhance the electric field of absorption layer,which can accelerate the movement of electrons and thus increase f3dB.However,it will also reduce the electric field at the InGaAs/InP heterojunction.Thus make it more difficult for the electrons in absorption layer to pass through the heterojunction barrier,resulting in accumulation electrons in absorption layer and reduction of high-power performance.4.The simulation results show that the f3dB of zero-bias UTC-PD increases at first but then decreases with the incident light intensity.We find that the bandwidth of UTC-PD increases because of the self-induced field and decreases because the electrons are accumulated in the absorption layer with the gradually increasing incident optical intensity.The 1dB compression points of zero-bias UTC-PD operating at different frequencies can form a curve that determines the incident optical intensity range of UTC-PD with linear performance.By combining the above simulation result and conclusion,we can obtain the incident optical intensity range in which the zero-bias UTC-PD have high-speed and high-linearity performance simultaneously.The method can be used to evaluate the incident light intensity range of different photodiodes,in which the photodiodes have best state of performance.5.Zero-bias UTC-PD integrated with high-reflectivity distributed Bragg reflector(DBR)mirrors,referred to as DBR-UTC-PD,is presented.The responsivity of zero-bias DBR-UTC-PD is 0.198A/W,which is nearly twice as high as ordinary UTC-PD's.After the fabrication of DBR-UTC-PD was finished,the responsivities of zero-bias DBR-UTC-PD at different incident optical intensity were tested,which increased 0.05-0.09A/W compared with ordinary UTC-PD.The f3dB of zero-bias DBR-UTC-PD is similar to the ordinary UTC-PD,which can be up to 11.4GHz.6.As the basis of the research on DBR-UTC-PD,the fabrication and performance test of UTC-PD are accomplished.The ohmic contact resistance of the p and n poles is 27.8Q and 7.7?,respectively.The dark current of UTC-PD(Diameter=44?m)is 3.2nA at 3V reverse bias.The responsivity of UTC-PD(Diameter=24?m)is 0.22A/W and the f3dB,is 23GHz at 0.5V bias and 11.4GHz at 0V.