The collapse of current gain in multi-finger heterojunction bipolartransistors: its substrate temperature dependence, instability criteria,and modeling

One undesirable phenomenon observed when AlGaAs/GaAs heterojunction bipolar transistors (HBT's) are operated under high power density is the collapse (of current gain). The collapse manifests itself by a distinct abrupt decrease of collector current in the transistor common-emitter current-voltage (I-V) characteristics. In this investigation, we study the substrate temperature dependence of the collapse. A unified equation is introduced to relate the collapse instability criterion with other thermal instability criteria proposed for silicon bipolar transistors. The effects of the thermal instability on the collapse behavior of 2-finger and 1-finger HBT's are examined. We also present a numerical model to adequately describe the collapse in multi-finger HBT's having arbitrary geometry. The I-V characteristics and regression plots of both ballasted and unballasted HBT's are compared     

Thermal coupling in 2-finger heterojunction bipolar transistors

We have previously analyzed the collapse phenomenon in heterojunction bipolar transistors (HBT's) when the mutual couplings among the transistor fingers are negligible. In this investigation, we derive the collapse loci equations in 2-finger HBT's in the presence of thermal coupling. It is found that the collapse loci equations are closely linked to a thermal instability condition best determined from the transistor regression characteristics. Unlike the previous derivation assuming zero thermal coupling, the collapse loci equations derived here are different depending on whether the HBT is driven by constant base current or constant base voltage bias     

Zn-doped InGaAs Base Heterojunction Bipolar Transistors Grown by Low Pressure Metal Organic Chemical Vapor Deposition

High performance InP/InGaAs heterojunction bipolar transistors(HBTs) have been widely used in high-speed electronic devices and optoelectronic integrated circuits. InP-based HBTs were fabricated by low pressure metal organic chemical vapor deposition(MOCVD) and wet chemical etching. The sub-collector and collector were grown at 655 ℃ and other layers at 550 ℃. To suppress the Zn out-diffusion in HBT, base layer was grown with a 16-minute growth interruption. Fabricated HBTs with emitter size of 2.5×20 μm2 showed current gain of 70～90, breakdown voltage(BVCE0)＞2 V, cut-off frequency(fT) of 60 GHz and the maximum relaxation frequency(fMAX) of 70 GHz.     

InP/InGaAs heterojunction bipolar transistors with different g-bridge structures

Several μ-bridge structures for InP-based heterojunction bipolar transistors (HBTs) are reported. The radio frequency measurement results of these InP HBTs are compared with each other. The comparison shows that μ-bridge structures reduce the parasites and double μ-bridge structures have a better effect. Due to the utilization of the double μ-bridges, both the cutoff frequency f_T and also the maximum oscillation frequency f_(max) of the 2×12.5 μm~2 InP/InGaAs HBT reach nearly 160 GHz. The results also show that the μ-bridge has a better effect in increasing the high frequency performance of a narrow emitter InP HBT.     

Temperature dependence of DC characteristics of NpN InP/GaAsSb/InP double heterojunction bipolar transistors: an analytical study

An analytic study of DC characteristics based on the drift-diffusion approach has been performed for the InP/GaAsSb DHBTs. The current transport of InP/GaAsSb/InP DHBTs has been investigated focusing the device temperature dependence. Our simulation results show that, at room temperature, the DC characteristics of the InP/GaAsSb/InP DHBTs similar to the conventional InP-based HBT using InGaAs as the base layer although a type-II energy band alignment is presented in the InP/GaAsSb HBT. However, due to different mechanisms for the electron injection from the emitter induced by the different conduction band alignments, the InP/GaAsSb HBTs may present a different temperature dependent behavior in term of device current gain as compared to the conventional InP/InGaAs HBTs. Higher current gain could be achieved by the InP/GaAsSb HBTs at elevated temperature.     

The influence of emitter material on silicon nitride passivation-induced degradation in InP-based HBTs

The influence of emitter material on silicon-nitride (SiN) passivation-induced degradation in InP-based heterojunction bipolar transistors (HBTs) has been studied. It has been found that, compared to InP, InAlAs has a much higher resistance to NH/sub 3/-related plasma-induced damage. InP-based HBTs using InAlAs as the emitter can effectively suppress the degradation of device performance caused by dielectric passivation giving least deterioration on the device characteristics compared to the previously reported results concerning the passivation quality using different passivation schemes. Short-term high temperature and high current electrical stress tests indicates that the SiN-passivated devices using InAlAs as the emitter may have better stability than those with InP emitter. Our results suggest that engineering of emitter layer structures could be an alternative approach to suppress passivation-induced degradation in InP-based HBTs.     

InP基MOCVD及MBE外延生长HBT的制备与分析

设计并研制了用于光电集成(OEIC)的InP基异质结双极晶体管(HBT),介绍了工艺流程及器件结构。分别采用金属有机化学气相沉积(MOCVD)及分子束外延(MBE)生长的外延片,并在外延结构、工艺流程相同的条件下,对两种生长机制的HBT直流及高频参数进行和分析。结果表明,采用MOCVD生长的InP基HBT,直流增益为30倍,截止频率约为38GHz;MBE生长的HBT,直流增益达到100倍,截止频率约为40GHz。这表明,MBE生长的HBT外延层质量更高,在相同光刻条件下,所对应的HBT器件的性能更好。     

Material-based comparison for power heterojunction bipolartransistors

Various materials are studied to determine their potential in power heterojunction bipolar transistors (HBTs). The authors first start by generating an HBT figure of merit (FOM) which is defined as the product of operating frequency and output power of the HBT with 3-dB power gain. By using the FOM and available material parameters, a material-based comparison of HBT performance is done. The general tendency is for use of narrow-bandgap materials, such as Ge or InGaAs, as the base and wide-bandgap materials, such as AlGaAs, InP, SiC, or GaN, as the collector, technology permitting     

Performance comparison of state-of-the-art heterojunction bipolar devices (HBT) based on AlGaAs/GaAs, Si/SiGe and InGaAs/InP

This paper presents a comprehensive comparison of three state-of-the-art heterojunction bipolar transistors (HBTs); the AlGaAs/GaAs HBT, the Si/SiGe HBT and the InGaAs/InP HBT. Our aim in this paper is to find the potentials and limitations of these devices and analyze them under common Figure of Merit (FOM) definitions as well as to make a meaningful comparison which is necessary for a technology choice especially in RF-circuit and system level applications such as power amplifier, low noise amplifier circuits and transceiver/receiver systems. Simulation of an HBT device with an HBT model instead of traditional BJT models is also presented for the AlGaAs/GaAs HBT. To the best of our knowledge, this work covers the most extensive FOM analysis for these devices such as I-V behavior, stability, power gain analysis, characteristic frequencies and minimum noise figure. DC and bias point simulations of the devices are performed using Agilent's ADS design tool and a comparison is given for a wide range of FOM specifications. Based on our literature survey and simulation results, we have concluded that GaAs based HBTs are suitable for high-power applications due to their high-breakdown voltages, SiGe based HBTs are promising for low noise applications due to their low noise figures and InP will be the choice if very high-data rates is of primary importance since InP based HBT transistors have superior material properties leading to Terahertz frequency operation.     

Thermal stability analysis of AlGaAs/GaAs heterojunction bipolartransistors with multiple emitter fingers

A numerical electro-thermal model was developed for AlGaAs/GaAs heterojunction bipolar transistors (HBT's) to describe the base current, current gain and output power dependence on junction temperature. The model is applied to microwave HBT devices with multi-emitter fingers. The calculated results of the common-emitter, current-voltage characteristics in the linear active region show a “current crush” effect due to inherent nonuniform junction temperature, current density and current gain distribution in the device. The formation of highly localized high temperature regions, i.e., hot spots, occur when the device is operating beyond the current-crush point. This thermally induced current instability imposes an upper limit on the power capability of HBT's. The dependence of this effect on various factors is discussed. These factors include the intrinsic parameters such as the base current ideality factor, the “apparent” valence band discontinuity, and the temperature coefficient of the emitter-base turn-on voltage, as well as the extrinsic factors such as the emitter contact specific resistance, the substrate thermal conductivity and the heat source layout     

Measurement and comparison of 1/f noise and g-r noise in siliconhomojunction and III-V heterojunction bipolar transistors

This paper experimentally determines and compares the 1/f noise and the g-r noise, as components of the base noise current spectral density, in Si homojunction and III-V heterojunction bipolar transistors (HBTs) in common-emitter configuration. The noise spectra for each of these devices are obtained as functions of the base bias current (I_B), and the 1/f noise has been found to depend on I_B as I_B^γ, where γ~1.8 for the silicon BJT's and InP/InGaAs HBT's with high current gains (β~50), and γ~1.1 for the AlGaAs/GaAs HBTs with low current gains (4<β<12). The nearly constant current gain and the near square-law and inverse-square emitter area dependence of 1/f noise in silicon devices are indicative of the dominant base bulk recombination nature of this noise. The 1/f noise in the InP based HBTs has been found to be lowest among all the devices we have tested, and its origin is suggested to be the base bulk recombination as in the Si devices. For the AlGaAs/GaAs HBTs, the low current gain and the near unity value of γ, arise most likely due to the combined effects of surface, bulk, and depletion region recombinations and the base-to-emitter injection. The dependence of the 1/f noise on the base current density in the devices tested in this work, and those tested by others are compared to find out which HBT's have achieved the lowest level of 1/f noise     

Ultra-high-speed InP/InGaAs heterojunction bipolar transistors

We report on the microwave performance of InP/In_0.53Ga _0.47As heterojunction bipolar transistors (HBT's) utilizing a carbon-doped base grown by chemical beam epitaxy (CBE). The f_T and f_max of the HBT having two 1.5×10 μm² emitter fingers were 175 GHz and 70 GHz, respectively, at I_C=40 mA and V_CE=1.5 V. To our knowledge, the f _T of this device is the highest of any type of bipolar transistors yet reported. These results indicate the great potential of carbon-doped base InP/InGaAs HBT's for high-speed applications     

基区设计对InGaP/GaAs HBT热稳定性的影响

研究了不同基区设计对多发射极指结构功率InGaP/GaAs异质结双极型晶体管热稳定性的影响。以发生电流增益崩塌的临界功率密度为热稳定性判定标准,推导了热电反馈系数Φ、集电极电流理想因子η和热阻Rth与基区掺杂浓度NB、基区厚度dB的理论公式。基于TCAD虚拟实验,观测了不同基区掺杂浓度和不同基区厚度分别对InGaP/GaAs HBT热稳定性的影响。结合理论公式,对仿真实验曲线进行了分析。结果表明,基区设计参数对热稳定性有明显的影响,其影响规律不是单调变化的。通过基区外延层参数的优化设计,可以改进多指HBT器件的热稳定性,从而为多指InGaP/GaAs HBT热稳定性设计提供了一个新的途径。     

InGaAsP/InP准平面异质结双极晶体管及其与光器件的集成

本文采用计算机辅助分析等方法,讨论了InGaAs/InP异质结双极晶体管(HBT)结构参数与其性能的关系.在此基础上,提出了一种准平面、双集电区HBT,及其相应的制作工艺.初步测试了器件的性能,就其与材料质量的关系作了讨论.文章还提出和制作了一种采用这种HBT为电子器件的光电子集成电路(OEIC).     

InGaP/GaAs heterojunction bipolar transistor and RF power amplifier reliability

Electrothermal stress on advanced InGaP/GaAs heterojunction bipolar transistors (HBTs) was carried out experimentally. It showed a long-term stress-induced base current instability and a decrease in the DC current gain. A class-AB RF power amplifier (PA) was also considered to study the stress effect on the amplifier’s RF performance. The SPICE Gummel–Poon (SGP) model parameters were extracted from the pre- and post-stress HBT data and used in Cadence SpectreRF simulation. The amplifier’s post-stress RF characteristics, such as the output power and power-added efficiency (PAE), remained almost unchanged even though the post-stress HBT’s DC current gain had dropped to 73.6% of its initial value.     

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     

Modulating HBT's current gain by using externally biased on-ledgeSchottky diode [GaAs devices]

The current gain of heterojunction bipolar transistors (HBT's) can be effectively modulated through Schottky diodes that contact the emitter passivation ledge directly. The behavior of the gain modulation is determined by the degree of the emitter ledge depletion. If the ledge is fully depleted, HBT's current gain can be modulated in the whole base-emitter bias voltage (V_BE) range up to 1.6 V. If the ledge is partially depleted, HBT's current gain can be modulated only in the low V_BE range (<1.35 V). This discovery leads to a simple method for monitoring the effectiveness of HBT's emitter ledge passivation and offers new insights to the mechanism of HBT gain degradation. It also creates a four-terminal HBT with an extra ledge electrode biased to control and modulate device's current gain at microwave frequencies     

A comparative study of metamorphic InP/InGaAs double heterojunction bipolar transistors with InP and InAIP buffer layers grown by molecular beam epitaxy

We present a comparison of material quality and device performance of metamorphic InGaAs/InP heterojunction bipolar transistors (HBTs) grown by molecular beam epitaxy (MBE) on GaAs substrates with two different types of buffer layers (direct InP and graded InAlP buffers). The results show that the active layer of InP-MHBT has more than one order of magnitude more defects than that of the InAlP-MHBT. The InAlP-MHBTs show excellent direct current (DC) performance. Low DC current gain and a high base junction ideality factor from the InP-MHBT are possibly due to a large number of electrically active dislocations in the HBT active layers, which is consistent with a large number of defects observed by cross-sectional transmission electron microscopy (TEM) and rough surface morphology observed by atomic force microscopy (AFM).     

Current gain collapse in microwave multifinger heterojunctionbipolar transistors operated at very high power densities

The rapid development of heterojunction bipolar transistor (HBT) technologies has led to the demonstration of high power single-chip microwave amplifiers. Because HBTs are operated at high power densities, the ultimate limits on the performance of HBTs are imposed by thermal considerations. The authors address a thermal phenomenon observed when a multifinger power HBT is operating at high power densities. This phenomenon, referred to as the collapse (of current gain), occurs when suddenly one finger of the HBT draws most of the collector current, leading to an abrupt decrease of current gain. A quantitative model and the condition separating the normal operation region and the collapse are presented. Critical difference of the collapse in the constant l _b and constant V_be modes of operation is discussed for the common-emitter l-V characteristics. The collapse in the common-base l-V characteristics and its relationship with avalanche breakdown are also described. A solution to eliminate the collapse is experimentally verified     

Thermal characteristics of InP, InAlAs, and AlGaAsSb metamorphic buffer layers used in In0.52Al0.48As/In0.53Ga0.47As heterojunction bipolar transistors grown on GaAs substrates

In_0.52Al_0.48As/In_0.53Ga_0.47As heterojunction bipolar transistors (HBTs) were grown metamorphically on GaAs substrates by molecular beam epitaxy. In these growths, InAlAs, AlGaAsSb, and InP metamorphic buffer layers were investigated. The InAlAs and AlGaAsSb buffer layers had linear compositional grading while the InP buffer layer used direct binary deposition. The transistors grown on these three layers showed similar characteristics. Bulk thermal conductivities of 10.5, 8.4, and 16.1 W/m K were measured for the InAlAs, AlGaAsSb, and InP buffer layers, as compared to the 69 W/m K bulk thermal conductivity of bulk InP. Calculations of the resulting HBT junction temperature strongly suggest that InP metamorphic buffer layers should be employed for metamorphic HBTs operating at high power densities.