Influence of dislocations on the DC characteristics of AlGaAs/GaAsheterojunction bipolar transistors

The influence of dislocations on the minority electron lifetime in p-type GaAs layers and on AlGaAs/GaAs HBTs has been investigated. The minority electron lifetime in 1×10¹⁹/cm³ doped p-GaAs decreases significantly when the dislocation density is greater than 10⁷/cm². This result agrees well with analysis of carrier transport in highly dislocated material. Current gain reduction in HBTs with high dislocation density is found to be due to two effects: reduction of the electron lifetime in the base layer and an increase of the recombination current in the emitter-base junction depletion region. These two effects are comparable in reducing the current gain     

Design considerations for millimeter-wave power HBTs based on gainperformance analysis

Critical design issues involved in optimizing millimeter-wave power HBTs are described. Gain analysis of common-emitter (CE) and common-base (CB) HBTs is performed using analytical formulas derived based on a practical HBT model. While CB HBT's have superior maximum-gain at very high frequencies, their frequency limit is found to be determined by the carrier transit time delay. Thus, to fully exploit the potential gain in a CB HBT, it is essential to maintain a high f_T even at high collector voltages. The advantage of using CB HBT's in a multifingered device geometry is also discussed. Unlike CE HBTs, CB HBTs are capable of maintaining a high gain even if the device size is scaled up by increasing the number of emitter-fingers. Moreover, it is found that reducing the wire parasitic capacitance allows emitter ballasting resistance to be used without affecting the gain. Fabrication of HBTs based on these design considerations led to excellent power performance in a CB unit-cell HBT at 25-26 GHz, featuring output power of 740 mW and power-added efficiency of 42%     

MBE-grown AlGaAs/GaAs HBTs on InP substrate

AlGaAs/GaAs HBTs are fabricated on InP substrates for the first time. Preliminary results show a current gain of 5 at a collector current density of 2 × 104A/cm2 in spite of a dislocation density higher than 109/cm2.     

突变异质结双极晶体管中的复合效应与电流传输

建立了带有不掺杂隔离层的突变异质结双极晶体管（ＨＢＴ）模型，在热场发射－扩散（ＴＦＤ）理论的基础上，又考虑了空间电荷区中的复合效应。对ＡｌＧａＡｓ／ＧａＡｓＨＢＴ特性的分析表明，不掺杂隔离层虽可有效地降低导带边的势垒尖峰，提高发射结的注入效率，但也会增大空间电荷区中的复合电流。因此，在实际器件的设计和制作中，应适当选择不掺杂隔离层的厚度，以获得较好的器件特性。还给出了计算突变异质结界面处电子准费米能级不连续的公式。     

Intermodulation in heterojunction bipolar transistors

The modeling of small-signal intermodulation distortion (IMD) in heterojunction bipolar transistors (HBTs) is examined. The authors show that IMD current generated in the exponential junction is partially canceled by IMD current generated in the junction capacitance, and that this phenomenon is largely responsible for the unusually good IMD performance of these devices. Thus, a nonlinear model of the HBT must characterize both nonlinearities accurately. Finally, the authors propose a nonlinear HBT model suitable for IDM calculations, show how to measure its parameters, and verify its accuracy experimentally     

Emitter-metal-related degradation in InP-based HBTs operating at high current density and its suppression by refractory metal

We investigated the reliability of InP-based HBTs with a ledge structure, focusing on emitter-metal-diffusion-induced degradation. Bias-temperature accelerated tests under high temperatures and high current densities of up to 5 mA/μm² were conducted for HBTs with conventional emitter electrodes, whose metal configuration was Ti/Pt/Au, and for HBTs with refractory metal emitters of Ti/Mo-Ti/Pt/Au, Ti/W-Ti/Pt/Au, or W-Ti/Pt/Au. The emitter contact layer was analyzed by transmission electron microscopy, energy dispersive X-ray spectroscopy, and transmission electron diffraction. Severe damage and disruption of uniformity of the atomic composition were observed due to diffusion of Ti and Au in HBTs with conventional emitter, whereas suppression of those degradations was observed in HBTs with refractory emitter. Refractory metals were found to be advantageous for blocking upper metal diffusion. Interstitials of host species generated due to metal diffusion must cause a shift of atomic composition.The time-wise change in the emitter resistance was estimated to compare the speed of the contact layer degradation between different emitter electrode metals. The critical time, which we determined as an emitter resistance increases of 3% from the initial value, increased by one order for HBTs with refractory metal than for HBTs with conventional metal at the same junction temperature, despite the same activation energy of 2.0 and 1.65 eV for J_c of 2 and 5 mA/μm², respectively, for all types of emitter electrode. The advantages of refractory metal for improving the reliability of InP HBTs were confirmed, especially for operation at high current densities.     

Analysis of the large-signal characteristics of powerheterojunction bipolar transistors exhibiting self-heating effects

The large-signal microwave characteristics of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) are modeled using the conventional Gummel-Poon-based bipolar junction transistor (BJT) model and extending it to include self-heating effects. The model is incorporated as a user-defined model in a commercial circuit simulator. The experimental microwave characteristics of HBTs are analyzed using the new model and harmonic balance techniques and the impact of self-heating effects on the device large-signal characteristics is investigated. Use of constant base voltage rather than constant current is more suitable for achieving maximum output power. Self-heating induced by RF drive is reduced under constant base current conditions. Increased thermal capacitance values result in gain enhancement at high power levels     

SiGe HBT低频噪声PSPICE模拟分析

对SiGe HBT低频噪声的各噪声源进行了较全面的分析,据此建立了SPICE噪声等效电路模型,进一步用PSPICE软件对SiGe HBT的低频噪声特性进行了仿真模拟.研究了频率、基极电阻、工作电流和温度等因素对低频噪声的影响.模拟结果表明,相较于Si BJT和GaAs HBT,SiGe HBT具有更好的低频噪声特性;在低频范围内,可通过减小基极电阻、减小工作电流密度或减小发射极面积、降低器件的工作温度等措施来有效改善SiGe HBT的低频噪声特性.所得结果对SiGe HBT的设计和应用有重要意义.     

Polycrystal isolation of InGaP/GaAs HBTs to reduce collectorcapacitance

InGaP/GaAs heterojunction bipolar transistors (HBTs) with polycrystalline GaAs buried under the base electrode have been fabricated using low-temperature gas-source molecular beam epitaxy on SiO₂-patterned substrates. A cutoff frequency of 120 GHz and a maximum oscillation frequency of 230 GHz were obtained for three parallel 0.7×8.5 μm HBTs. Compared to HBTs without the polycrystal, the collector capacitance was reduced by 28% and the maximum stable gain was improved by 1.2 dB due to complete carrier depletion in the polycrystal under the base electrode. These results show the high potential of the proposed HBTs for high-speed digital and broadband-amplifier applications     

基于图形衬底生长的GaN位错机制分析

采用缺陷选择性腐蚀法结合光学显微镜及原子力显微镜(AFM)对金属有机化合物气相外延(MOVPE)在蓝宝石图形衬底(PSS)上生长的非掺杂GaN体材料的位错产生机制进行了研究,分析结果表明,位错来源于三个方面:一是"二步法"生长机制引入的位错;二是是由于图形衬底上不同区域GaN晶体相互连接时由于晶面不连续所造成的位错群;三是由于图形衬底制作工艺过程中引入的表面污染与损伤.     

Push-pull circuits using n-p-n and p-n-p InP-based HBT's for poweramplification

P-n-p heterojunction bipolar transistors (HBTs) have been combined with n-p-n HBTs in a push-pull amplifier in order to obtain improved linearity characteristics. Simulations of common-collector push-pull amplifiers demonstrate an improvement of 14 dB in second harmonic content at the onset of power saturation under class-B operation. Further improvement of 14 dB in the third harmonic content is shown by moving to class-AB operation at an expense of 4% decreased efficiency. A common-emitter push-pull amplifier was fabricated using both n-p-n and p-n-p HBTs with external matching and couplers. Testing of the circuit under class-AB conditions showed better third-order intermodulation (by ~9 dBc) and smaller second harmonic content (by ~11 dBc) compared with n-p-n HBTs alone. While the second harmonics were shown to combine destructively in the push-pull amplifier, total cancellation of the second harmonic was prevented by the wide difference in linearity characteristics of the n-p-n and p-n-p HBTs. In addition, the circuit produced over 2 dBm more output power than the n-p n HBT alone at 1 dB of gain compression     

Design consideration of the thermal and electro stability of multi-finger HBTs based on different device structures

The thermal and electro stability of multi-finger heterojunction bipolar transistors (HBTs) with different structures were analyzed and discussed simultaneously. The thermal stability of the devices with different layout struc-tures was assessed by the DC-Ⅳ test and thermal resistance calculation. Their electro stability' was assessed by the calculation of the stability factor K based on the S parameter of the HBT. It is found that HBTs with higher thermal stability are prone to lower electro stability. The trade-off relationship between the two types of stability was explained and discussed by using a compact K-factor analytic formula which is derived from the small signal equivalent circuit model of HBT. The electro stability of the device with a thermal ballasting resistor was also discussed, based on the analytic formula.     

Modeling and analysis of ZnSe-Ge HBTs

A Gummel-Poon model is developed for ZnSe-Ge-GaAs heterojunction bipolar transistors (HBTs). In this structure, undoped Ge spacers are placed at the emitter-base and collector-base junctions. Injected current components as well as bulk, spacer, and space charge recombination current components are modeled. Early voltage and bandgap narrowing effects are included in the model. The device performance was simulated and compared with the experimental results. The paper shows a good agreement between our model and the experimental results. The paper shows also that using spacers would improve the device performance. The advantages of this model is that it is analytical, compact, and can be easily implemented in CAD tool programs to simulate single or double HBTs with similar or dissimilar materials structure for the emitter and collector.     

High-speed InGaP/GaAs HBTs using a simple collector undercuttechnique to reduce base-collector capacitance

High-speed InGaP/GaAs HBTs were fabricated using a simple collector undercut (CU) technique to physically remove the collector material underneath the extrinsic base region by selective etching for reducing base-collector capacitance (C_BC). The best HBTs achieved a f_T of 80 GHz and a f_max (MSG/MAG) of 171 GHz. To our knowledge, this is the highest f_max (MSG/MAG) ever reported for the InGaP/GaAs HBTs. Compared to the HBTs without CUs, the CU HBTs showed a factor of 1.38 times improvement in the highest achievable f_max (MSG/MAG) due to the significant reduction of the C_BC     

The use of base ballasting to prevent the collapse of current gainin AlGaAs/GaAs heterojunction bipolar transistors

We propose the use of base-ballasting resistance to guarantee absolute thermal stability in AlGaAs/GaAs heterojunction bipolar transistors (HBTs). Base-ballasted HBTs are fabricated and the measured I-V, regression and S-factor characteristics are discussed. We present a numerical model which elucidates the reasons why the base-ballasting scheme is helpful to HBTs but is damaging to silicon bipolar transistors. We compare measured small-signal and large-signal performances of unballasted, emitter-ballasted, and base-ballasted HBTs     

Velocity saturation in the collector of Si/GexSi1-x/Si HBT's

The effects of velocity saturation on the unity gain-bandwidth product f_t and transconductance g_m of n-p-n and p-n-p heterojunction bipolar transistors (HBTs) with Ge_xSi_1-x bases are described and simulated. For the n-p-n device, velocity saturation combined with a valence-band offset at the base-collector junction causes accelerated g_m and f_t rolloff for current densities greater than the knee current for the Kirk effect. For the p-n-p device, the g_m and f_t are degraded for all current densities. These limitations combine with the limits imposed by dislocation formation due to strain in the pseudomorphic layer to impose constraints on the design of Si/Ge_x Si_1-x/Si HBTs     

Thermal performance of collector-up HBTs for small high-power amplifiers with a novel thermal via structure underneath the HBT fingers

We will describe the thermal performance of a special heterojunction bipolar transistor (HBT) structure for mobile communication systems, called a collector-up HBT. We calculated the thermal resistance between the HBT fingers and the bottom surface of a GaAs substrate using a finite element method (FEM). The results suggest that the thermal resistance of collector-up HBTs with thermal via structures can be reduced by 64% compared to the thermal resistance of ordinary emitter-up HBTs. They also show that the thickness of the InGaP emitter layer effects the thermal resistance of, and the temperature distribution in, the collector fingers of collector-up HBTs. Even though the thermal resistance of collector-up HBTs can be much smaller than that of emitter-up HBTs, a thermal interaction between the collector fingers still exists in multi-finger structures. We analyzed the temperature distribution in the collector fingers of a four-finger HBT structure and found that the thickness of the plated heat sink (PHS) was not sufficient to reduce the thermal interaction between the HBT fingers, and that optimization of the HBT location was needed to minimize the thermal interaction. We also found that the thickness of the InGaP emitter layer was the most important parameter for reducing thermal resistance, even in four-finger HBT structures. These calculation results can be used to reduce the temperature of collector-up HBTs and the temperature differences between the HBT fingers in the development of power amplifiers with collector-up HBTs.     

A physics-based high-injection transit-time model applied to barrier effects in SiGe HBTs

A physics-based cutoff frequency model considering high-injection heterojunction barrier effects in SiGe HBTs is derived. Compared with other compact modeling approaches, the present model accurately captures the cutoff frequency behavior at very high current densities for SiGe HBTs with deep Si-SiGe heterojunctions. The model also offers better insight into the charge density distribution under Kirk and barrier effect in SiGe HBTs.     

New collector undercut technique using a SiN sidewall for low basecontact resistance in InP/InGaAs SHBTs

A new collector undercut process using SiN protection sidewall has been developed for high speed InP/InGaAs single heterojunction bipolar transistors (HBTs). The HBTs fabricated using the technique have a larger base contact area, resulting in a smaller DC current gain and smaller base contact resistance than HBTs fabricated using a conventional undercut process while maintaining low C_bc. Due to the reduced base contact resistance, the maximum oscillation frequency (f_max) has been enhanced from 162 GHz to 208 GHz. This result clearly shows the effectiveness of this technique for high-speed HBT process, especially for the HBTs with a thick collector layer, and narrow base metal width     

SOI SiGe HBT集电结渡越时间模型研究

根据器件实际工作情况,找出SOI器件与传统器件的不同,建立并研究了SOI SiGe HBT集电结渡越时间模型。结果表明,模型与集电区掺杂浓度、集电结偏置电压、传输电流有关,电流的增加恶化了渡越时间,进一步恶化了器件性能。所建模型与仿真结果一致。SOI SiGe HBT集电结渡越时间模型的建立和扩展为SOI BiCMOS工艺的核心参数,如特征频率的设计,提供了有价值的参考。