Simulation study of high injection effects and parasitic barrier formation in SiGe HBTs operating at high current densities

The effects of high carrier densities near the base–collector (B–C) heterojunction in npn SiGe heterojunction bipolar transistors during device operation at high current densities has been investigated using a commercial numerical device simulator. Due to electron velocity saturation in the B–C space charge region and the presence of the valence band discontinuity at the B–C junction, hole accumulation occurs at the collector end of the base at the onset of base pushout at high current densities. Formation of a parasitic barrier to electron flow in the conduction band occurs at the collector junction which increases recombination in the base and the base current and produces saturation in the collector current. Together, these effects produce an abrupt degradation in the transistor’s current gain and cutoff frequency with increasing emitter junction bias. In this paper, we investigate the onset and relationship of parasitic barrier formation and base pushout, and their dependence on device structure and biasing.     

SiGe HBT异质结势垒高度物理模型研究

基于SiGe HBT异质结势垒效应(HBE)产生的物理机制,综合考虑Ge引入集电区和大电流下电流感应基区中少子浓度对空穴浓度的影响,建立了异质结势垒高度解析模型。结果表明,将Ge引入集电区可有效地推迟HBE发生;同时,考虑少子浓度的影响,势垒高度具有明显的饱和趋势,峰值约为0.07 eV。     

THE EFFECT OF DOPANT OUTDIFFUSION ON THE NEUTRAL BASE RECOMBINATION CURRENT IN Si/SiGe/Si HETEROJUNCTION BIPOLAR TRANSISTORS

A new analytical model for the base current of Si/SiGe/Si heterojunction bipolar transistors(HBTs) has been developed. This model includes the hole injection current from the base to the emitter, and the recombination components in the space charge region(SCR) and the neutral base. Distinctly different from other models, this model includes the following effects on each base current component by using the boundary condition of the excess minority carrier concentration at SCR boundaries: the first is the effect of the parasitic potential barrier which is formed at the Si/SiGe collector-base heterojunction due to the dopant outdlffusion from the SiGe base to the adjacent Si collector, and the second is the Ge composition grading effect. The effectiveness of this model is confirmed by comparing the calculated result with the measured plot of the base current vs. the collector-base bias voltage for the ungraded HBT. The decreasing base current with the increasing the collector-base reverse bias voltage is successfully explained by this model without assuming the short-lifetime region close to the SiGe/Si collector-base junction, where a complete absence of dislocations is confirmed by transmission electron microscopy (TEM)[1]. The recombination component in the neutral base region is shown to dominate other components even for HBTs with a thin base, due to the increased carrier storage in the vicinity of the parasitic potential barrier at collector-base heterojunction.     

微波功率SiGe HBT研究进展

Si的热导率比大部分化合物半导体(如GaAs)的热导率高,SiGe HBT在一个较宽的温度范围内稳定,SiGe HBT的发射结电压VBE的温度系数dVBE/dT比Si的小,双异质结SiGe HBT本身具有热-电耦合自调能力,所加镇流电阻可以较小,所有这些使SiGe HBT比GaAs HBT和SiBJT在功率处理能力上占一定优势。文章对微波功率SiGe HBT一些重要方面的国内外研究进展进行评述,希望对从事微波功率SiGe HBT的研究者有所帮助。     

Measure the barrier height of manganite p–n heterojunction by activation energy measurement

The barrier height of the manganite based p–n heterojunction is identified from the activation energy. The La_0.35Pr_0.32Ca_0.33MnO₃/Nb-doped SrTiO₃ p–n heterojunction is fabricated by the pulse laser deposition technology. The junction shows good rectifying behavior which can be well described by the Shockley equation. A satisfactorily logarithmic linear dependence of resistance on temperature is observed, and also the relation between bias and activation energy (E_A) deduced from the R−1/T curves is linear. As a result, the interfacial barrier of the heterojunction is obtained by extrapolating the Bias –E_A plot to Y axis, which is 0.95 eV.     

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.     

Characteristics of Polyaniline/Si Heterojunction Solar Cell By Electrochemical Dye Sensitization

Using the electrochemical polymerization dye sensitization （ECDS） method, polyaniline （PAn）, which is used as top region material in solar cells, is sensitized with direct blue dye（DS）, and sensitized Al grid/DS-PAn/n-Si/Al heterojunction solar cells is prepared by ECDS. Influences of the ECDS on the absorption spectrum and the junction characteristics of the solar cell were discussed, and the output characteristics were measured. The results show that the absorption spectrum of the sensitized PAn films is much wider and stronger in Vis-range; the diode quality factor is about 6.3 and the height of latent barrier potential of p-n junction is 0.89 eV; the short-circuit current and the conversion efficiency of sensitized DS- PAn/Si heterojunction solar cells are greatly improved, which the short-circuit current can increase 6 times, the fill factor is 57% and the efficiency can reach 1.42 % under the illumination of 37.2 W/m^2 , respectively.     

Determination of bandgap narrowing and parasitic energy barriers inSiGe HBT's integrated in a bipolar technology

This paper describes a method for characterizing the bandgap narrowing and parasitic energy barrier in SiGe heterojunction bipolar transistors (HBTs), fabricated using a single-polysilicon self-aligned bipolar process. From a comprehensive study of the temperature dependence of the collector current, the bandgap narrowing in the base due to germanium has been dissociated from that due to the heavy dopant concentration. The same approach has been used to characterize the height and width of parasitic energy barriers which appear when boron out-diffusion from the SiGe base is present. The method has been applied to SiGe heterojunction bipolar transistors fabricated using a single polysilicon, self-aligned, bipolar process, as well as mesa transistors. The experimental results show that small geometry transistors have degraded collector currents due to boron out-diffusion around the perimeter of the emitter. This behavior has been explained by accelerated boron diffusion due to point defects generated during the extrinsic base implant. The values of undoped SiGe spacer thickness needed to suppress the parasitic energy barrier are described. Finally, high-frequency results are reported, which correlate the frequency transition to these parasitic energy barriers     

White‐Light Emitting Diode Array of p+‐Si/Aligned n‐SnO2 Nanowires Heterojunctions

We report on the fabrication and optoelectronic properties of p‐n heterojunction arrays of p⁺‐type Si and aligned n‐type SnO₂ nanowires with high rectification ratios of >10⁴ at ±15 V. The electrical stability of the p‐n heterojunction devices was improved by coating the junction with poly(methylmethacrylate) to minimize the degradation of the interface layer at the junction. As a photodiode an enhanced UV photosensitivity higher than 10² was recorded under reverse bias. Using a large forward bias in the light‐emitting diode mode white light was emitted from the large‐scale heterojunction devices with at least three broad peaks in the visible range, which can be attributed to the interband transitions of the injected electrons or holes mediated by an interfacial SiO₂ layer with a contribution of trap‐level energies. These results indicate the high potential of Si/SnO₂ nanowires heterojunctions as optoelectronic devices with proper tuning of the recombination center at the junctions.     

Electrical characteristics of rectifying polycrystalline silicon/silicon carbide heterojunctions

This paper presents a study of the rectifying properties of heavily doped polycrystalline silicon (polysilicon) on 4H silicon carbide (4H-SiC). Current properties and barrier heights were found using analysis of the heterojunction. This revealed that Schottky analysis would be valid for the large barrier height devices. Isotype and an-isotype devices were fabricated on both p-type and n-type SiC and the electrical characteristics were investigated using capacitance vs voltage measurements, current vs voltage measurements (I-V), and temperature I-V measurements. Extraction of the barrier height, built-in potential, and Richardson constant were made and then compared to theoretical values for the heterojunction. Temperature I-V measurements demonstrated that the current transport mechanism is thermionic emission, confirming the validity of the Schottky diode model. The I-V characteristics show near ideal diode rectifying behavior and the capacitance-voltage characteristics show ideal junction space charge modulation for all polysilicon/SiC combinations. These experimental results match well with heterojunction band-offset estimated barrier heights and demonstrate that the barrier height of the polysilicon/4H SiC interface may be controlled by varying the polysilicon doping type.     

Modeling the temperature-dependent Early voltage of a silicongermanium heterojunction bipolar transistor

This paper compares the temperature-dependent Early voltage of a silicon germanium heterojunction bipolar transistor (HBT) to that of a silicon bipolar junction transistor (BJT) fabricated with identical geometry. Derived is an isothermal expression for the forward Early voltage specifically suited to the base composition of a SiGe HBT. The expression includes two fit factors, one for the Si_1-xGe_x alloy and the other for the nonuniform doping density. The fit factors are functions of the device temperature and are determined through pulsed bias measurements     

InGaN/GaN MQW p–n junction photodetectors

InGaN/GaN multiquantum well (MQW) p–n junction photodetectors with semi-transparent Ni/Au electrodes were fabricated and characterized. It was found that the fabricated InGaN/GaN MQW p–n junction photodetectors exhibit a 20 V breakdown voltage and a 3.5 V forward 20 mA turn on voltage. It was also found that the photocurrent to dark current contrast ratio is higher than 10⁵ when a 0.4 V reverse bias was applied to the InGaN/GaN MQW p–n junction photodetectors. Furthermore, it was found that the maximum responsivity was 1.28 and 1.76 A/W with a 0.1 and 3 V applied reverse bias, respectively.     

Effect on ion implanted germanium profile on the characteristics of Si/sub 1-x/Ge/sub x//Si heterojunction bipolar transistors

An optimum profile for Ge ion implantation in SiGe/Si heterojunction bipolar transistors is determined by using a two-dimensional simulator code for advanced semiconductor devices. The simulation code is based on a two-dimensional drift-diffusion model for heterostructure degenerate semiconductors with nonparabolicity included in the energy band structure. The model allows accurate simulations of carrier transport in short base devices. The simulation results indicate that for high current gain the Ge profile maximum must be close to the base-collector junction, and that the unavoidable tail of the implanted germanium in the collector region does not deteriorate the gain.<>     

Parasitic energy barriers in SiGe HBTs

Parasitic energy barriers can easily be introduced during processing. Measurements and calculations of experimental n-p-n HBTs (heterojunction bipolar transistors) are presented, showing that a parasitic conduction-band barrier at the base-collector junction reduces the collector current and the cutoff frequency. A simple analytical model explains the f_T degradation, caused by the reduction of the collector current and a pileup of minority carriers in the base. With the model the effective height and width of the barrier can also be derived from the measured collector current enhancement factor I_C(SiGe)/I_C(Si)     

Extraction of small-signal equivalent circuit model parameters for Si/SiGe HBT using S-parameters measurements and one geometrical information

A physical and simple method is proposed to extract the hybrid-π small-signal equivalent circuit model of Si/SiGe heterojunction bipolar transistor (HBT). In this method, we use test (dummy) structures to extract by means of fitting techniques the extrinsic bias-independent parameters representing the contact pads plus the transmission line connections to the core of the active device. All intrinsic bias-dependent parameters are calculated analytically from S-parameters only. The ratio of the area of the emitter contact to base area is used to solve the base–collector feedback problem due to the distributed nature of the base. Using this physical (geometry) constraint instead of the measured direct current (DC) information helps to get more reliable parameters and easier calculations. When we applied this methodology, a good agreement is obtained between the modelled S-parameters with the corresponding measured ones over the broad band from 40 MHz to 20.02 GHz. The error for three different bias points was less than 1.2%.     

Reverse recovery transient characteristic of PEDOT:PSS/n-Si hybrid organic-inorganic heterojunction

We study the junction behavior of poly (3,4-ethylenedioxythiophene):polystyrenesulphonate/n-Si hybrid organic/inorganic heterojunction by reverse recovery transient (RRT) characterization. RRT response for PEDOT:PSS/n-Si hybrid junction is reported for various n-Si doping concentration and forward bias current injection level. The presence of settling time of 8.3–23.5 μs in the RRT response in contradiction to Schottky junction model commonly assumed for PEDOT:PSS/n-Si hybrid structure. The decrease in the minority carrier lifetime from 126.8 μs to 39.5 μs with increased n-Si doping concentration, suggests that minority carriers are stored at n-Si side of the junction, which is consistent with a p⁺-n junction model for the hybrid structure. The minority carrier lifetime is found to depend on forward bias current injection level, attributed to trap-saturation effect of the recombination-centers at the PEDOT:PSS/n-Si junction. The DC-IV characteristics of the PEDOT:PSS/n-Si hybrid junction are also consistent with the notion of diffusion and trap assisted recombination dominated dark current. The diffusion dominated transport of PEDOT:PSS/n-Si leads to an ideal p⁺-n junction behavior that leverages on the good transport properties of Si. Our findings are important in the modeling and optimization of the characteristics of electronic devices based on the organic/Si hybrid junction.     

Reduction of p+-n+ junction tunneling currentfor base current improvement in Si/SiGe/Si heterojunction bipolartransistors

The authors report a three-order-of-magnitude reduction in parasitic tunneling current at heavily doped p⁺-n⁺ Si/Si and SiGe/Si junctions grown by rapid thermal epitaxial chemical vapor deposition (CVD) compared with previously reported results in Si junctions fabricated by ion implantation. These results demonstrate the high quality of the epitaxial interface. The low tunneling currents allow higher limits to transistor base and emitter doping levels, yielding higher gains, reduced bias resistances, and higher Early voltages for scaled bipolar devices as well as Si/SiGe/Si heterojunction bipolar transistors     

Research progress of Si-based germanium materials and devices

Si-based germanium is considered to be a promising platform for the integration of electronic and photonic devices due to its high carrier mobility, good optical properties, and compatibility with Si CMOS technology. However, some great challenges have to be confronted, such as: (1) the nature of indirect band gap of Ge; (2) the epitaxy of dislocation-free Ge layers on Si substrate; and (3) the immature technology for Ge devices. The aim of this paper is to give a review of the recent progress made in the field of epitaxy and optical properties of Ge heterostructures on Si substrate, as well as some key technologies on Ge devices. High crystal quality Ge epilayers, as well as Ge/SiGe multiple quantum wells with high Ge content, were successfully grown on Si substrate with a low-temperature Ge buffer layer. A local Ge condensation technique was proposed to prepare germanium-on-insulator (GOI) materials with high tensile strain for enhanced Ge direct band photoluminescence. The advances in formation of Ge n⁺p shallow junctions and the modulation of Schottky barrier height of metal/Ge contacts were a significant progress in Ge technology. Finally, the progress of Si-based Ge light emitters, photodetectors, and MOSFETs was briefly introduced. These results show that Si-based Ge heterostructure materials are promising for use in the next-generation of integrated circuits and optoelectronic circuits.     

High efficiency triple junction thin film silicon solar cells with optimized electrical structure

We report on improving the performance of pin‐type a‐Si:H/a‐SiGe:H/µc‐Si:H triple‐junction solar cells and corresponding single‐junction solar cells in this paper. Based on wet‐etching sputtered aluminum‐doped zinc oxide (ZnO:Al) substrates with optimized surface morphologies and photo‐electrical material properties, after adjusting individual single‐junction solar cells utilized in triple‐junction solar cells with various optimization techniques, we pay close attention to the optimization of tunnel recombination junctions (TRJs). By means of the optimization of individual a‐Si:H/a‐SiGe:H and a‐SiGe:H/µc‐Si:H double‐junction solar cells, we compensated for the open circuit voltage (V_oc) loss at the a‐Si:H/a‐SiGe:H TRJ by adopting a p‐type µc‐Si:H layer with a low activation energy. By combining the optimized single‐junction solar cells and top/middle, middle/bottom TRJs with little electrical losses, an initial efficiency of 15.06% was achieved for pin‐type a‐Si:H/a‐SiGe:H/µc‐Si:H triple‐junction solar cells. Copyright © 2014 John Wiley & Sons, Ltd.