A short-term high-current-density reliability investigation ofAlGaAs/GaAs heterojunction bipolar transistors

In high current and power density applications of AlGaAs/GaAs heterojunction bipolar transistors (HBT's), reliability is a critical issue. Therefore, in this letter we show results of a fundamental investigation on the temperature and current dependence of the fast initial rise of the dc-current gain (burn-in), which takes place during stress at current densities beyond those of today's applications. We find that the burn-in occurs at lower device junction temperatures (135°C) than previously reported in literature, and that it depends linearly on the current density. An activation energy of 0.4 eV is extracted for the burn-in effect     

Simulation and design of InAlAs/InGaAs pnp heterojunction bipolartransistors

The performance capabilities of InP-based pnp heterojunction bipolar transistors (HBT's) have been investigated using a drift-diffusion transport model based on a commercial numerical simulator. The low hole mobility in the base is found to limit the current gain and the base transit time, which limits the device's cutoff frequency. The high electron majority carrier mobility in the n⁺ InGaAs base allows a reduction in the base doping and width while maintaining an adequately low base resistance. As a result, high current gain (>300) and power gain (>40 dB) are found to be possible at microwave frequencies. A cutoff frequency as high as 23 GHz and a maximum frequency of oscillation as high as 34 GHz are found to be possible without base grading. Comparison is made with the available, reported experimental results and good agreement is found. The analysis indicates that high-performance pnp InP-based HBT's are feasible, but that optimization of the transistor's multilayer structure is different than for the npn device     

Temperature dependent DC characteristics of an InP/InGaAs/InGaAsPHBT

We report the DC characteristics of n-p-n InP/InGaAs/InGaAsP HBT's which have a composite-collector structure designed to improve the breakdown and gain. The devices exhibit common-emitter current gain of greater than 8 for over 9 orders of magnitude of collector current and breakdown voltages greater than 10 V. The DC gain for a typical device decreases from 40 at room temperature to 8 at 90 K. Over the same temperature range the collector-current ideality factor increases from 1.04 to 1.46, and the base current ideality factor is 0.05 to 0.1 larger than these values. We suggest that the high collector-current ideality factor and the lower gain at the lower temperatures is due to the increasing importance of tunneling of current across the emitter-base junction. The devices with the InGaAs/InGaAsP composite-collector structure offer better common-base turn-on behavior than those with InGaAs/InP as the collector structure, without the breakdown behavior being compromised     

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     

Multi-emitter Si/GexSi1-x heterojunctionbipolar transistor with no base contact and enhanced logic functionality

We demonstrate multi-emitter Si/Ge_xSi_1-x n-p-n heterojunction bipolar transistors (HBT's) which require no base contact for transistor operation. The base current is supplied by the additional emitter contact under reverse bias due to the heavy doping of the emitter-base junction. Large-area HBT test structures exhibit good transistor characteristics, with current gain β≈400 regardless of whether the base current is supplied by a test base electrode or one of the emitter contacts. These devices have enhanced logic functionality because of emitter contact symmetry. Since device fabrication does not require base electrode formation, the number of processing steps can be reduced without significant penalty to HBT performance     

Excess noise in AlGaAs/GaAs heterojunction bipolar transistors andassociated TLM test structures

Noise measurements both on transmission line model (TLM) test structures and on associated HBT's are presented. Contact noise is proved to be negligible in the TLM's related to the base structure of transistors. A Hooge parameter for p⁺⁺ doped GaAs is extracted. Activation energies are calculated from results versus temperature. Considering the TLM related to the structure of the emitter, it is shown that the g-r levels observed originate from the AlGaAs layer. Noise measurements on HBT's also exhibit excess noise. A value of the cutoff frequency between the equivalent input current white noise and the 1/f component is given. The base current dependencies associated with different measurement configurations suggest the 1/f noise to come from the base or the emitter-base junction. The g-r components are studied as a function of temperature. Activation energies are deduced. Finally a comparison of the TLM and HBT noise results is presented. The presence of the complex DX center and of g-r levels in the base region are proposed as possible origins for the g-r noise in HBT's     

Scaling issues and Ge profile optimization in advanced UHV/CVD SiGeHBT's

The SiGe heterostructure device simulation tool SCORPIO is used to investigate profile optimization in SiGe HBT's for high-performance analog circuit applications. After calibrating SCORPIO to measured data, the effects of germanium profile shape on current gain, cut-off frequency, Early voltage and maximum oscillation frequency are compared over the temperature range of 200-360 K. The impact of aggressive base profile scaling on device performance is also investigated as a function of SiGe film stability     

Numerical study on the injection performance of AlGaAs/GaAs abruptemitter heterojunction bipolar transistors

The injection performance of abrupt emitter HBT's and related effects on the device characteristics are studied by taking an Npn Al _0.25Ga_0.75As/GaAs/GaAs HBT as an example. In order to take into account the coupled transport phenomena of drift-diffusion and tunneling-emission processes across the abrupt heterojunction in a single coupled formulation, a numerical technique based on the boundary condition approach is employed. Compared to previous numerical investigations relying on either a drift-diffusion or a tunneling-emission scheme, more complete and accurate characterization of abrupt emitter HBT's has been achieved in this study. It is demonstrated that the presence of abrupt discontinuities of the conduction and valence bands at the emitter-base junction brings several different features to the injection efficiency and recombination characteristics of abrupt emitter HBT's compared to graded emitter HBT's. Based on investigations of the emitter doping effects on the current drive capability and device gain, an optimum emitter doping density is determined for a given structure. When the emitter-base p-n junction of the abrupt emitter HBT is slightly displaced with respect to the heterojunction, significant changes in the electrical characteristics are observed. A small displacement of the p-n junction into the narrow bandgap semiconductor is found to be very attractive for the performance optimization of abrupt emitter HBT's     

SiGe异质结晶体管频率特性模型研究

在分析载流子输运和分布的基础上，建立了SiGe异质结晶体管(HBT)各时间常数模型；在考虑发射结空间电荷区载流子分布和集电结势垒区存在可劝电荷的基础上，建立了SiGe HBT发射结势垒电容模型和不同电流密度下包括基区扩展效应的集电结势垒电容模型。对SiGe HBT特征频率及最高振荡频率与电流密度、Ge组分、掺杂浓度、结面积等之间的关系进行了模拟，对模拟结果进行了分析和讨论。     

Base-collector leakage currents in InP/InGaAs double heterojunctionbipolar transistors

For InP/InGaAs HBT's, base-collector leakage current can restrict their operation to a narrow emitter-collector voltage range. We studied several factors that can degrade the leakage current: the base-collector junction design, the base mesa etching technique, and the base metallization process. A step-graded base-collector heterojunction offered the best results. A leakage free multiple step etching process, combining wet and dry etching, has been developed. Ti/Pt/Au is a suitable base metallization, provided that the platinum layer is not too thick, and that the contact annealing temperature is not too high. Finally, very low leakage current HBT's were fabricated     

Hot electron and hot hole degradation of UHV/CVD SiGe HBT's

We investigate the degradation in current gain and low-frequency noise of SiGe HBT's under reverse emitter-base stress due to hot electrons (forward-collector stress) and hot holes (open-collector stress). Contrary to previous assumptions we show that hot electrons and hot holes with the same kinetic energy generate different amounts of traps and hence have a different impact on device degradation. These results suggest that the accuracy of using forward-collector stress as an acceleration tool and reliability predictor must be carefully examined. We also present, for the first time, the effect of Ge profile shape on the reliability of SiGe HBT's, as well as discuss measurements on SiGe HBT's as a function of device geometry and temperature     

Initial degradation of base-emitter junction in carbon-dopedInP/InGaAs HBTs under bias and temperature stress

Bias-temperature stress tests were performed to examine the stability of base-emitter junction characteristics of carbon-doped InP/InGaAs heterojunction biopolar transistors (HBT's). Two different kinds of degradation modes were observed from the Gummel I-V characteristics. One is characterized by the gradual increase in a nonideal base current. The generation of the nonideal current strongly depends on the crystallographic orientation of the emitter mesa. The other degradation mode was observed when a large current (200 kA/cm²) was injected under a high ambient temperature (180°C). This degradation is characterized by an initial decrease in turn-on voltage and significant drop in current gain     

Measurement of the electron ionization coefficient at low electricfields in GaAs-based heterojunction bipolar transistors

Values of the electron ionization coefficient α_n in 〈100〉 GaAs extending the previously available data by two orders of magnitude, down to 1 cm^-1, are presented. The data are directly extracted from the multiplication factor, M-1, measured in lightly doped collector n-p-n AlGaAs/GaAs heterojunction bipolar transistors (HBT's). It is shown that the sensitivity of the technique is limited by the early effect, whose influence can be reduced by driving the device at constant emitter-base bias and by using heavily doped base regions. HBT's can provide simultaneously high base doping and current gain, and represent therefore an excellent tool for these measurements     

An analysis of space-charge-region recombination in HBT's

The importance of including recombination in the base side of the emitter-base space-charge-region (SCR) in the computation of the current gain in AlGaAs/GaAs HBT's is investigated. Recombination due to Shockley-Read-Hall, Auger and radiative processes is considered. The interaction of the base-side SCR recombination currents with the neutral-base current and the collector current, which occurs via their dependence on the quasi-Fermi level splitting (ΔE_fn) at the base-emitter junction, is not found to be a significant factor in the computation of ΔE_fn. However, it is confirmed that the quasi-Fermi level splitting, as calculated from a balancing of the thermionic/tunnel current with the neutral base and collector currents, must subsequently be included in the computation of the base-side SCR currents if the current gain is not to be severely underestimated. A discussion of why the ideality factor is ≈1 for the base-side SCR currents is given. Finally, simple analytical expressions for ΔE _fn and the SCR recombination currents are presented and should prove useful for HBT device- and circuit-simulation purposes     

Low 1/f noise characteristics of AlGaAs/GaAs heterojunction bipolartransistor with electrically abrupt emitter-base junction

It is shown that the use of an electrically abrupt emitter-base junction considerably reduces the 1/f noise of self-aligned AlGaAs/GaAs heterojunction bipolar transistor (HBT). Although this device does not have depleted AlGaAs ledge passivation layer, the low-frequency noise spectra show a very low 1/f noise corner frequency of less than 10 kHz, which is much lower than previously reported value of about 100 kHz from conventional passivated or unpassivated AlGaAs/GaAs HBT's. Except for a residual generation-recombination (g-r) noise component, the noise power is comparable to that of Si BJT. It is also found that the low-frequency noise power of the AlGaAs/GaAs HBT is proportional to the extrinsic GaAs base surface recombination current square. Unlike the other HBT's reported, the noise sources associated with interface state and emitter-base (E-B) space charge region recombination are not significant for our device     

Neutral base recombination and its influence on the temperaturedependence of Early voltage and current gain-Early voltage product inUHV/CVD SiGe heterojunction bipolar transistors

We present the first comprehensive investigation of neutral base recombination (NBR) in ultra-high vacuum/chemical vapor deposited (UHV/CVD) SiGe heterojunction bipolar transistors (HBT's), and its influence on the temperature characteristics of Early voltage (V_A ) and current gain-Early voltage product (βV_A). We show that a direct consequence of NBR in SiGe HBT's is the degradation of V_A when transistors are operated with constant-current input (forced-I_B) as opposed to a constant-voltage input (forced-V_BE). In addition, experimental and theoretical evidence indicates that with cooling, V_A in SiGe HBT's degrades faster than in Si bipolar junction transistors (BJT's) for forced-I_B mode of operation. Under the forced-V_BE mode of operation, however, SiGe HBT's exhibit a thermally-activated behavior for both V_A and βV_A, in agreement with the first-order theory. The differences in V_A as a function of the input bias and temperature for SiGe HBT's are accurately modeled using a modified version of SPICE. The performance of various practical SiGe HBT circuits as a function of temperature, in the presence of NBR, is analyzed using this calibrated SPICE model     

112-GHz collector-up Ge/GaAs heterojunction bipolar transistorswith low turn-on voltage

This paper reports small-sized collector-up Ge/Ga/As heterojunction bipolar transistors (HBT's) operating at low power and high frequency. A heavily B-doped Ge base-layer and a newly-developed self-aligned process reduce the base resistance and the parasitic elements. Intrinsic base resistance is 50 Ω/□; this is the lowest value reported for bipolar transistors. With limiting the active emitter area through B ion implantation, these collector-up HBT's with a collector size of 2×5 μm² exhibit a current gain of 60. They exhibit a maximum oscillation frequency f_max of 112 GHz with an associated current gain cutoff frequency f_T of 25 GHz. The large value of f_max, exceeding 100 GHz, is attributed to the extremely low base resistance caused by the heavily B-doped base-layer and the self-aligned process and to the low base-collector capacitance expected from the collector-up structure. The turn-on voltage of these HBT's is approximately 0.7 V smaller than that of AlGaAs/GaAs HBT's. These results show that these HBT's have excellent potential for low-power dissipation circuits     

Temperature dependent study of carbon-doped InP/InGaAs HBT's

We report on a temperature dependent study of the dc and the microwave performance of carbon-doped InP/In_0.53Ga_0.47 As heterojunction bipolar transistors (HBT's). The turn on voltage increased 114% and the dc current gain decreased 25% as the temperature was reduced from 300 K to 33 K. Under high-current injection, there was a 29% increase in the current gain cutoff frequency of these devices as the temperature was lowered from 300 K to 77 K. By investigating the operation of HBT's at cryogenic temperatures, increased understanding of the mechanisms of carrier transport in these devices can be obtained, and this may lead to improvements in device performance     

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     

Modeling of current gain's temperature dependence inheterostructure-emitter bipolar transistors

The temperature dependence of the current gain is investigated for GaAs-based heterostructure-emitter bipolar transistors (HEBT's). With the separation of the p-n junction and the heterojunction, the mechanism of hole injection from the base to emitter in the HEBT is different from that of a conventional HBT. Theoretical results demonstrate that the thermionic emission current plays an important role for the hole current which results in a smaller negative or even positive temperature coefficient for the current gain. Experimental data show that the base current for HEBTs is indeed dominated by thermionic emission as predicted. This finding indicates that the HEBT structure is the suitable choice for high power and high speed applications