Avalanche Theory Of Narrow Bandgap Semiconductors

Narrow band-gap semiconductors are semiconductors with band-gap less than 0.5eV.The narrow band-gap brings unique properties,such as high non-parabolic coefficient,easier impact ionization and larger band-to-band tunneling.Especially,Hg Cd Te,as a typical ternary alloy narrow band-gap semiconductor,also has large alloy scattering,single carrier avalanche and other unique properties.Avalanche photodetectors have many applications in many areas that need to detect weak optical signals,such as remote sensing,combined active and passive detection,lidar,quantum communication,and astronomical observation.However,the current avalanche theory is mainly based on Si,Ge,and other semiconductor materials with relatively wider bandgap.It is difficult to explain the avalanche phenomenon and its microscopic mechanisms clearly in the narrow bandgap semiconductors.Within the scope of narrow band-gap semiconductor materials,Hg Cd Te has a high detection rate with a very matured growth technology.As a single carrier avalanche material,Hg Cd Te has become an ideal avalanche photodetector material because of its high internal gain and low excess noise factor.The theoretical simulation of avalanche process is the basis of the research and design of avalanche diode.At present,the theoretical simulation methods of avalanche process could be divided into macroscopic methods and microscopic methods: the macroscopic methods first establishe the phenomenological models with fitting parameters,substitute the experimental data into the established model to get the fitting parameters,and then substitute those parameters back to the established model to get the analytical descriptions of avalanche process;the microscopic method analyzes the intra-band and the inter-band transition mechanism of the corresponding material system,and the dynamic process and transition process of carriers are simulated by the sampling of events by Monte Carlo method or other simliar methods,then,the physical quantities in the sampling results is statistically estimated from the simulation results.In this paper,we use quantum mechanics to analyze the intra-band and the inter-band transition mechanism,and propose an analytical theory,the spatial description theory,to describe the single carrier avalanche in narrow band-gap semiconductors.Different from the classical transport theory based on Boltzmann transport equation,the basic concepts of relaxation time and motion equation of the transport process are different under the spatial description theory established in this paper.These newly proposed concepts are discussed and modified in this paper.In this paper,the history dependent avalanche model is generalized,in which the concept of partial avalanche is proposed.Based on the physical picture of partial avalanche theory,the photogenerated carrier avalanche and dark current avalanche caused by different microscopic mechanisms are discussed quantitatively for the first time.The microscopic mechanism of the change of the excess noise factor with gain in Hg Cd Te avalanche photodiodes is clarified.The primary research contents and main achievements of this work are listed as follows:1.The dynamics of electrons in the narrow band-gap semiconductors is discussed in detail and the modification of scattering process based on quantum mechanics is presented.In this paper,a series of effects of nonparabolic conduction band are analyzed,and the equation of motion of electron in nonparabolic conduction band is obtained.At present,the theory of impact ionization rate is improper.In this paper,the calculation of the impact ionization rate based on the quantum mechanics is further modified.In narrow band-gap semiconductors,the scattering of polar optical phonons in non-parabolic conduction band needs to be considered.Based on previous works on parabolic conduction band,the polarimetric optical phonon scattering rate in nonparabolic conduction band is obtained.2.The spatial description theory of avalanche process is proposed.Based on the abstract model of avalanche signal readout process,the spatial description theory of avalanche process for narrow band-gap semiconductors is developed.The nonparabolic conduction band and large-angle scattering are considered in this theory,which can accurately calculate the motion of electrons in avalanche region.The alloy scattering is isotropic.The alloy scattering rate of Hg Cd Te is very high,which permit us to utilize the spatial description theory proposed in this paper.In the spatial description theory,the orbit of the carrier is divided into backscattering motion and forward motion.In this paper,the impact ionization rate is modified by including the discussion of backscattering,and the energy relaxation of polarized optical phonon scattering with the process of backscattering included is obtained.3.The generalized history dependent model of avalanche is proposed.The partial avalanche theory is used to discuss the influence of hot carriers generated at different positions in the junction region on the dark current gain and excess noise factor.The concept of shadow region is proposed to solve the problem of energy non conservation in the more accurate history dependent avalanche model.Based on the partial avalanche theory and the concept of shadow region,the history dependent avalanche model proposed by McIntyre is extended.The extended history dependent avalanche model is applied to the simulation of specific devices.The simulation results show that the gain and excess noise factor of partial avalanche of electron hole pairs generated in the junction region are obviously different from those of photogenerated electron hole pairs.It is necessary to discuss the process of avalanche in narrow band-gap semiconductor respectively,and the cause of excess noise in Hg Cd Te avalanche photodiodes is clarified The microscopic mechanism of the variation of the quantum with gain.