Graded Heterojunction of AlGaInP High-brightness Light Emitting Diodes

A simple model of the graded heterojunction in AlGaInP compound semiconductors was introduced to analyze the band profile. The band profiles are analyzed with the different grading ways but the same grading length and under the different doping densities. The effect of the different grading lengths on the surplus of the potential of the spike to the potential of N region are also analyzed under the different doping densities.Through the experiments,it proves that the performances of high brightness light emitting diodes can be improved by the effects of the graded heterojunction.     

An Ebers-Moll model for the heterostructure bipolar transistor

An Ebers-Moll model for the heterostructure bipolar transistor (HBT) is developed. The model describes both single and double heterojunction transistors with or without band spikes and applies to uniform or graded base HBTs. Model parameters are directly related to device parameters such as doping densities, dimensions and band spikes. Junction velocities are introduced to describe the transport of carriers across the junctions. Results demonstrate that even for compositionally graded junctions, transport across the junctions may limit HBT performance if the base is graded. Use of the model is illustrated by examining a recently proposed technique for extracting conduction band spikes by comparing forward and inverted I-V characteristics.     

Analysis of the operation of GaAlAs/GaAs HBTs

Drift-diffusion modeling in two dimensions has been used to characterize and analyze storage, transport and recombination effects in GaAlAs/GaAs heterostructure bipolar transistors. Both intrinsic and parasitic effects have been studied, and their relationship to the design of the device is discussed. For conventional dopings and high current densities, the heterojunction grading potential causes a barrier in the base-emitter junction, which results in a large increase in the dynamic resistance. In heterojunction collectors, a similar barrier leads to a large increase in base charge storage and to spreading of the collector current. It is shown that increased doping levels can successfully suppress these barrier effects. The capacitance and transport phenomena at the base-emitter junction are also analyzed under conditions of large forward bias, where the junction space-charge region is shorter than the alloy grading length. Recombination is analyzed in the limit of high surface recombination velocities using Shockley-Read-Hall theory in the presence of Fermi-level pinning due to surface states. The pinning results in a potential energy saddle point at the edge of the base-emitter junction, which largely determines the surface recombination behavior of the transistor when the recombination velocity is high     

低电阻P-DBR结构的模拟分析

设计了低电阻的渐变型分布布拉格反射器(DBR)结构,模拟分析了采用渐变p-DBR结构对价带能带的影响,并详细讨论了渐变形状、渐变区宽度、掺杂浓度等参数对低电阻渐变DBR结构的价带势垒的影响.利用MOCVD技术制备了实验样品,模拟结果与实验数据基本吻合.     

Design of flat-band AlGaAs heterojunction Bragg reflectors

Bragg reflectors using periodic AlGaAs heterojunction layers can be built into a light emitting diode to reduce the absorption in the substrate, or into a laser diode to form a cavity. In this paper, a numerical model for the design of low resistance flat-band p-type or n-type graded heterojunction reflectors is presented. It computes the required doping profile to achieve a flat conduction band or valence band across a graded junction, such that the reflector will add minimum series resistance to the devices     

Base transit time for SiGe-base heterojunction bipolar transistors

The base transit time expressions for SiGe base heterojunction bipolar transistors are presented including the accelerating field effects due to the base bandgap grading and doping grading, and the retarding field (opposing drift field) effect from the graded boron profile down towards the emitter. It is found that the retarding field exhibits 40-80% contribution to the base transit time, depending on the boron concentration near the emitter. The results of base transit time from these analytic expressions are unambiguously supported by the published simulation data.<>     

Improved current gain and fT through dopingprofile selection in linearly graded heterojunction bipolar transistors

Analytical and experimental results are used to show that extension of a thin p-doped layer of base doping into the graded-gap region, close to the base, of an n-p-n AlGaAs/GaAs heterojunction bipolar transistor and removing n-type dopant from the rest of the linearly graded AlGaAs region improves current gain β and unity gain cutoff frequency f_T. Current gain is significantly improved by reducing recombination near the metallurgical interface and using the effective electric field from the grading to accelerate electrons as they are injected into the p-base. The doping profile also inhibits the formation of a potential minimum in which electrons can be stored in close proximity to the base. This greatly improves f_T, and does not hamper the current injection or increase the turn-on voltage. Space-charge recombination current is also reduced, due to the carrier density reduction associated with the effective electric field due to the graded gap     

Heterojunction bipolar transistor using a (Ga,In)P emitter on a GaAs base, grown by molecular beam epitaxy

We report the first N-p-n heterojunction bipolar transistor (HBT) using a (Ga,In)P/GaAs heterojunction emitter on a GaAs base. This combination is of interest as a potential alternate to (Al,Ga)As/GaAs, because of theoretical predictions of a larger valence band discontinuity and a smaller conduction band discontinuity, thus eliminating the need for grading of the emitter/base junction. The structure was grown by molecular beam epitaxy, with the base doping (∼10¹⁹cm^-3) far exceeding the n-type doping ∼5¹⁷cm^-3) of the (Ga,In)P wide gap emitter (Eg= 1.88 eV). Common-emitter current gains of 30 were attained at a current density of 3000 A/cm², the highest current density achieved without burnout.     

渐变异质结在HB-LED器件中的应用以及实现技术

本文首先讨论通过渐变方式使异质结处的尖峰消失或减小，从而改善高亮度发光二极管(HB-LED)的器件性能．讨论在实际中用双层突变拟合缓变异质结遇到的问题．     

Doping effects and compositional grading in AlxGa1-xAs/GaAs heterojunction bipolar transistors

First-order analytical calculations were made for the energy-band diagrams for n-AlxGa1-xAs/p-GaAs heterojunctions for x = 0.15, 0.3, and 0.5 employing different compositional gradings and doping densities specifically for heterojunction-bipolar-transistor (HBT)applications. In the calculations most recently determined, conduction-band discontinuity ΔECof 65 percent of the bandgap difference ΔEgbetween the AlxGa1-xAs and GaAs, and the donor activation energies in n-AlxGa1-xAs of 60 and 160 meV for x = 0.3 and 0.5, respectively, were used. The results show that the position of the heterojunction spike barrier, and the depth and width of the notch in the conduction-band edge for a compositionally abrupt heterointerface depend on the respective doping densities on the p and n sides of the heterojunction. Also, for an abrupt heterointerface the difference in barrier heights for electron and hole injections varies between ΔEgand ΔEV(the valence-band discontinuity), depending on the doping densities and the applied bias, and is not necessarily the generally accepted value of ΔEV. Analytical expressions and curves were obtained to estimate the minimum compositional grading L for eliminating the spike barrier and the notch as a function of the doping densities and the applied bias.     

A simplified model for graded-gap heterojunctions

A simplified method for calculating the energy band profiles of graded-gap heterojunctions, based on the generalized model of Oldham and Milnes, is presented. The profiles are derived by superposing an energy band grading function and the electrostatic potential in the heterojunction. The latter is obtained by using the depletion layer approximation as for conventional p-n homojunctions. The energy band profiles of hypothetical p(GaAs)-n(Al_0·4Ga_0·6As) heterojunctions are calculated using the simplified method. For small grading layer widths, the results are in good agreement with the generalized model. The barrier lowering factor η as a function of the graded layer width l is calculated for such heterojunctions. It is found, for acceptor and donor densities of 10¹⁸ and 10¹⁶ cm^?3 respectively, that the barrier height is reduced from 0·47 eV to zero as l increases from zero (abrupt case) to ≈300 Å. The applications of these analyses to practical heterojunctions are discussed.     

Energy transport simulation for graded HBT's: Importance of settingadequate values for transport parameters

An energy transport simulation method for graded AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is presented in which Al composition-, doping density-, and energy-dependences of transport parameters are considered. For several representative Al composition and doping densities, parameters such as electron mobility, energy relaxation time, and upper-valley fraction are evaluated as a function of electron energy by a Monte Carlo method. For the other Al composition, these are determined by a linear interpolation method. Calculated cutoff frequency characteristics and electron velocity profiles are compared with those obtained by using more simplified approaches, demonstrating the importance of giving adequate transport parameters, particularly in analyzing graded band-gap base HBTs     

Numerical investigation of the effects of graded layer on the performance of AlGaAs/GaAs heterojunction bipolar transistors

The effects of grading on the performance of heterojunction bipolar transistors (HBTs) are studied by numerically solving the basic semiconductor equations. The numerical modelling is based on the extended drift–diffusion formulation with inclusion of thermionic emission current at the heterointerface of abrupt emitter HBTs. The results of the simulation show that the graded layer improves significantly the current driving capability of the HBT and lowers its offset voltage. However, owing to the increase in the recombination in the graded layer, the current gain of the graded HBT is lower and depends strongly on the bias at low and medium bias range. The simulation also reveals the presence of a potential minimum in the conduction band of the graded emitter HBT, which results in an electron accumulation in the region. This accumulation increases the emitter-base capacitance and its charging time, leading to a smaller unity-gain frequency f _T.     

Resolving degradation mechanisms in ultra-high performance N-p-nheterojunction bipolar transistors

The conduction band barrier caused by base dopant outdiffusion is investigated for MBE-grown Al_xGa_1-xAs/GaAs N-p-n linearly graded heterojunction bipolar transistors (HBTs). The change of the B-E heterojunction conduction band barrier can be directly revealed by a novel technique based on the diffusion-thermionic emission current model. This is accomplished by measuring the inverted collector current ratio at two different heterojunction reverse biases. In addition, this ratio is found to correlate with an anomalous base current component measured at low temperature. The heterojunction potential barrier and the anomalous base current component are attributed to beryllium redistribution during MBE growth and forward current stress. This suggests that the diffusion and incorporation of beryllium dictate V _BE uniformity and long-term reliability of HBTs     

AlGaAs/GaAs/AlGaAs渐变异质结光致发光仿真研究

分析了AlGaAs/GaAs/AlGaAs渐变异质结的光致发光特性。根据理论及仿真结果,确定了GaAs发光的最优能带结构为双异质结P-AlGaAs/P-GaAs/P-AlGaAs或者N-AlGaAs/N-GaAs/ N-AlGaAs,并且异质结两边能带渐变。基于所选结构,研究了能带渐变及层宽对发光效率的影响。研究结果表明,外体区吸收层的能带渐变,且外体区激发层的能带不变,发光区域的载流子最多,发光能量值最大。激励光源的波长不同,各层有不同的最优宽度,为器件的整体优化提供了依据。AlGaAs/GaAs/AlGaAs渐变异质结的光致发光研究为高效率器件如太阳电池、发光二极管等的实用化设计、研制提供了有价值的参考。     

The interface of metalorganic chemical vapor deposition-CdTe/HgCdTe

The metalorganic chemical vapor deposition (MOCVD) growth of CdTe on bulk n-type HgCdTe is reported and the resulting interfaces are investigated. Metalinsulator-semiconductor test structures are processed and their electrical properties are measured by capacitance-voltage and current-voltage characteristics. The MOCVD CdTe which was developed in this study, exhibits excellent dielectric, insulating, and mechano-chemical properties as well as interface properties, as exhibited by MIS devices where the MOCVD CdTe is the single insulator. Interfaces characterized by slight accumulation and a small or negligible hysteresis, are demonstrated. The passivation properties of CdTe/ HgCdTe heterostructures are predicted by modeling the band diagram of abrupt and graded P-CdTe/n-HgCdTe heterostructures. The analysis includes the effect of valence band offset and interface charges on the surface potentials at abrupt hetero-interface, for typical doping levels of the n-type layers and the MOCVD grown CdTe. In the case of graded heterojunctions, the effect of grading on the band diagram for various doping levels is studied, while taking into consideration a generally accepted valence band offset. The MOCVD CdTe with additional pre and post treatments and anneal form the basis of a photodiode with a new design. The new device architecture is based on a combination of a p-on-n homojunction in a single layer of n-type HgCdTe and the CdTe/HgCdTe heterostructure for passivation.     

Emitter—Base bandgap grading effects on GaAlAs/GaAs heterojunction bipolar transistor characteristics

Emitter-base bandgap grading effects on GaAlAs/GaAs heterojunction bipolar transistor characteristics are theoretically investigated using an accurate one-dimensional numerical model including aluminum mole-fraction-dependent velocity versus field characteristics and donor energy level. The bandgap grading is shown to influence not only the electron injection but also the carrier recombination and the hole injection, resulting in a significant common-emitter current gain dependence on the graded layer thickness. The cutoff frequency dependence on the graded layer thickness is also described. Detailed discussion is given for the underlying physical mechanism that determines the device performance.     

Ag掺杂ZnO/GaN异质结可见光吸收特性的第一性原理

ZnO/GaN异质结带隙宽度较宽,制约了对可见光的吸收。为研究Ag对ZnO/GaN异质结可见光吸收的影响,在(1-100)非极性面上构建GaN/ZnO异质结,并用Ag分别取代不同位置的Zn和Ga原子,采用第一性原理计算Ag掺杂对ZnO/GaN异质结稳定性、电子结构、光学性质和带边位置的影响。研究结果表明：Ag掺杂ZnO/GaN异质结形成能为负值,结构稳定;Ag置换Zn和Ga使带隙宽度由2.93 eV分别减小至2.7 eV和2.3 eV,吸收系数和光电导产生红移,有利于可见光的吸收,Ag掺杂ZnO/GaN异质结具有良好的光催化活性。     

异质结位置对缓变集电结SiGe HBT性能的影响

研究了npn型SiGe HBT集电结附近的异质结位置对器件性能的影响.采用Taurus-Medici 2D器件模拟软件,在渐变集电结SiGe HBT的杂质分布不变的情况下,模拟了各种异质结位置时的器件直流增益特性和频率特性.同时比较了处于不同集电结偏压下的直流增益和截止频率.分析发现即使没有出现导带势垒,器件的直流和高频特性仍受SiGe层中性基区边界位置的影响.模拟结果对SiGe HBT的设计和分析都具有实际意义.     

异质结位置对缓变集电结SiGe HBT性能的影响

研究了npn型SiGe HBT集电结附近的异质结位置对器件性能的影响.采用Taurus-Medici 2D器件模拟软件,在渐变集电结SiGe HBT的杂质分布不变的情况下,模拟了各种异质结位置时的器件直流增益特性和频率特性.同时比较了处于不同集电结偏压下的直流增益和截止频率.分析发现即使没有出现导带势垒,器件的直流和高频特性仍受SiGe层中性基区边界位置的影响.模拟结果对SiGe HBT的设计和分析都具有实际意义.