The importance of bandgap narrowing distribution between theconduction and valence bands in abrupt HBTs

Abrupt heterojunction bipolar transistors (HBTs) show interfaces where discontinuities in the energy levels appear. Currents through these interfaces are controlled by tunneling and thermionic emission. The values of these currents depend on the form and height of the energy barriers, which are disturbed by the heavy doping effects on semiconductor energy band structure. In this work, the real bandgap narrowing is distributed between the conduction and valence bands according to Jain-Roulston model, and its effect on the base and collector currents of Si/SiGe and InP/InGaAs HBTs is analyzed. This analysis is carried out through a numerical model which combines the drift-diffusion transport in the bulk of transistor with the thermionic emission and tunneling at the base-emitter interface, and an empirically determined surface recombination current     

High-gain InGaAlAs/InGaAs heterojunction bipolar transistors andphototransistors

The characteristics of InGaAlAs/InGaAs heterojunction bipolar transistors (HBTs) grown by molecular beam epitaxy are described. A current gain of 15600 at a current density of ~10⁴ A/cm² and an emitter-base heterojunction ideality factor of 1.02 were measured. Appropriately designed InGaAlAs/InGaAs HBTs, when operated as phototransistors, also had high gains. A current gain of 1000 for a collector current of only 10 μA was obtained for phototransistors. Such high gains are due to low recombination currents as a consequence of the good crystalline quality of the InGaAlAs bulk and InGaAlAs/InGaAs interface     

A collector current model for InAlAs/InGaAsSb/InGaAs double heterojunction bipolar transistors with non-ideal effects

A collector current model incorporating electron diffusion in the base and thermionic emission at the abrupt B–E heterojunction is derived and applied to InGaAsSb heterojunction bipolar transistors (HBTs). Parameters extracted from the model include effective base doping concentration, conduction band discontinuity (ΔE_C) at the base-emitter junction, and emitter resistance. The calculated current from the model is consistent with the measured result. It is found that a high collector current ideality factor is resulted from a nonzero ΔE_C and a low effective base doping concentration. Moreover, a nonzero ΔE_C makes the high-injection effect almost invisible in collector current characteristics, even if it actually exists.     

Thermionic saturation of diffusion currents in transistors

Macroscopic diffusion theory is not applicable to large percentage variations in carrier concentration over distances comparable to a scattering path length. It is plausible that the transport velocity for thermionic emission over a barrier of zero height constitutes a limit on the velocity of diffusive current flow at any point in a bulk semiconductor. Not incorporating a thermionic limit, the drift-diffusion current transport equation cannot account for thermionic emission. While a full remedy to these difficulties can only lie in a complete investigation of the relevant statistical physics, we present herein an empirical modification of the current transport equation based on the elementary concept of thermionic saturation of the diffusion current. The modified equation appears capable of accounting, at least in a crude way, for thermionically limited current flow, while permitting a unified treatment of diffusion-drift-thermionic transport. For narrow base transistors (w_b 1000 Å), much larger ratios of stored base charge to collector current are predicted than from diffusion theory. A thermionic emission-diffusion equation is derived for barrier injection in floating base transistors biased into punch-through. It is found for silicon structures that transport is diffusion-drift dominated at base impurity concentrations < 10¹⁷ cm⁻³, and thermionically controlled at higher base doping. A possible small reduction in the thermionic Richardson constant by a factor e is also predicted from the diffusion saturation equation.     

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     

High current bulk GaN Schottky rectifiers

GaN Schottky rectifiers employing guard-ring and SiO₂ edge termination show almost ideal forward current characteristics, with ideality factor 1.08 and specific on-state resistance as low as 2.6×10⁻³ Ω cm². A maximum forward current of 1.72 A at 6.28 V was achieved under pulsed (10% duty cycle) conditions. The reverse breakdown voltage was inversely dependent on rectifier area. The presence of defects in the GaN still dominates the reverse leakage, with both field emission and thermionic field emission contributions present. The parallel-plane breakdown voltage is never reached, even with the use of multiple edge termination methods, but the results show the promise of GaN rectifiers for power conditioning and electric utility applications.     

Temperature dependent common emitter current gain andcollector-emitter offset voltage study in AlGaN/GaN heterojunctionbipolar transistors

We have demonstrated state-of-the-art performance of AlGaN/GaN heterojunction bipolar transistors (HBTs) with a common emitter (CE) current gain of 31 at 175 K and 11.3 at 295 K. The increase in collector current and CE current gain at lower temperature can be attributed to the reduced base-emitter interface recombination current. We also observed an increase of collector-emitter offset voltage with decrease of temperature. The increase of V_CEOFF at lower temperature is related to an increase of V_BE as the base bulk current is increased, or to the reduction of the ideality factor n_BE     

Planar AlGaAs/GaAs heterojunction bipolar transistors fabricatedusing selective W-CVD

The authors describe a planar process for the AlGaAs/GaAs HBTs in which collector vias are buried selectively, even to the base layers, with chemical vapor deposited tungsten (CVD-W) films. By using WF₆ /SiH₄ chemistry, W could be deposited on Pt films, which were overlapped 50 nm thick on the AuGe-based collector electrodes, without depositing W on the surrounding SiO₂ layers. Current gains of planar HBTs with 3.5-μm×3.5-μm emitters were up to 150, for a collector current density of about 2.5×10⁴ A/cm²     

Power performance of InP-based single and double heterojunctionbipolar transistors

The microwave and power performance of fabricated InP-based single and double heterojunction bipolar transistors (HBTs) is presented. The single heterojunction bipolar transistors (SHBTs), which had a 5000 Å InGaAs collector, had BV_CEO of 7.2 V and J_Cmax of 2×10⁵ A/cm². The resulting HBTs with 2×10 μm² emitters produced up to 1.1 mW/μm2 at 8 GHz with efficiencies over 30%. Double heterojunction bipolar transistors (DHBTs) with a 3000-Å InP collector had a BV_CEO of 9 V and J_{c max} of 1.1×10⁵ A/cm², resulting in power densities up to 1.9 mW/μm² at 8 GHz and a peak efficiency of 46%. Similar DHBTs with a 6000 Å InP collector had a higher BV_CEO of 18 V, but the J _{c max} decreased to 0.4×10⁵ A/cm² due to current blocking at the base-collector junction. Although the 6000 Å InP collector provided higher f_max and gain than the 3000 Å collector, the lower J_{c max} reduced its maximum power density below that of the SHBT wafer. The impact on power performance of various device characteristics, such as knee voltage, breakdown voltage, and maximum current density, are analyzed and discussed     

Photo carrier generation in bipolar transistors

Anomalous substrate currents have been observed in SiGe bipolar NPN-transistors, dependent on the collector bias, at high current levels. These currents appear to originate from light that is generated in the collector base junction when it is reverse biased. This light generates electron hole pairs in the n⁺ buried layer-substrate diode, yielding a considerable substrate current. This paper will show that these substrate currents can be used as a useful monitor for the occurrence of avalanche multiplication and high-level injection (Kirk effect) in heterojunction bipolar transistors (HBTs)     

Operation of SiGe heterojunction bipolar transistors in theliquid-helium temperature regime

We present the first dc measurements of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) operating in the liquid-helium temperature (LHeT=4.2 K) regime. The current gain of the self-aligned, UHV/CVD-grown SiGe HBT increases monotonically from 110 at 300 K to 1045 at 5.84 K, although parasitic base current leakage limits the useful operating current to above about 1.0 μA at 5.84 K. An aggressively designed base profile (peak N_AB≈8×10¹⁸ cm ^-3) is used to suppress base freeze-out at LHeT (R_bi =18.3 kΩ/□ at 4.48 K). We have also identified a non-ideal minority carrier transport mechanism in the collector current at temperatures below 77 K (I_C is not proportional to exp(qV _BE/kT)) which is unaccounted for in conventional device theory. Preliminary calculations suggest that this phenomenon is due to trap-assisted carrier tunneling from the emitter to the collector through the base potential barrier     

Submicrometer self-aligned AlGaAs/GaAs heterojunction bipolartransistor process suitable for digital applications

A self-aligned process is developed to obtain submicrometer high-performance AlGaAs/GaAs heterojunction bipolar transistors (HBTs) which can maintain a high current gain for emitter sizes on the order of 1 μm². The major features of the process are incorporation of an AlGaAs surface passivation structure around the entire emitter-base junction periphery to reduce surface recombination and reliable removal of base metal (Ti/W) deposits from the sidewall by electron cyclotron resonance (ECR) plasma deposition of oxide and ECR plasma etching by NF₃. A DC current gain of more than 30 can be obtained for HBTs with an emitter-base junction area of 0.5×2 μm² at submilliampere collector currents. The maximum f_T and f_max obtained from a 0.5×2 μm² emitter HBT are 46 and 42 GHz, respectively at I_C=1.5 and more than 20 GHz even at I_C=0.1 mA     

Novel Delta-Doped InAlGaP/GaAs Heterojunction Bipolar Transistor

The first successful demonstration of a delta-doped InAlGaP/GaAs heterojunction bipolar transistor (HBT) is reported. A comparison to a baseline InAlGaP/GaAs HBT without a delta-doping layer is made. Both of these devices exhibit near-ideal current gain (beta) versus the collector current (I _C) characteristics (i.e., beta independent of I _C) at high currents. The delta-InAlGaP/GaAs HBT exhibits a 40% reduction in offset voltage (V _{CE, offset}) and a 250-mV reduction in knee voltage (V _k) without sacrificing beta compared with the baseline InAlGaP/GaAs HBT. At a higher I _C, the decrease in beta of the InAlGaP/GaAs HBTs with increasing temperature is significantly smaller than the corresponding effect measured in the formerly reported GaAs-based HBTs. The rather temperature-insensitive characteristics of these two InAlGaP/GaAs HBTs originate from their large valence-band discontinuity (DeltaE _V) at the emitter-base (E-B) junction. Furthermore, at intermediate base current I _B levels (0.4-1.6 mA), V _{CE, offset} falls as I _B increases, which is a trend contrary to that of most HBTs in the literature. Finally, the experimental dependence of V _{CE, offset} on temperature, I _B, and the effective barrier height at the E-B junction is explained with reference to an extended large-signal model.     

High/fmax collector-up AlGaAs/GaAsheterojunction bipolar transistors with a heavily carbon-doped basefabricated using oxygen-ion implantation

AlGaAs/GaAs collector-up heterojunction bipolar transistors (HBTs) with a heavily carbon-doped base layer were fabricated using oxygen-ion implantation and zinc diffusion. The high resistivity of the oxygen-ion-implanted AlGaAs layer in the external emitter region effectively suppressed electron injection from the emitter, allowing collector current densities to reach values above 10⁵ A/cm ². For a transistor with a 2-μm×10-μm collector, f_T was 70 GHz and f_max was as high as 128 GHz. It was demonstrated by on-wafer measurements that the first power performance of collector-up HBTs resulted in a maximum power-added efficiency of as high as 63.4% at 3 GHz     

Electrical stress effect on RF power characteristics of SiGe hetero-junction bipolar transistors

In this paper, we investigate the electrical stress effects on both the high-frequency and RF power characteristics of Si/SiGe HBTs. Simultaneously applying a high collector current density and a high collector–base voltage upon the Si/SiGe HBTs, their hot carriers will induce device performance degradation. This stress condition is similar to the DC bias conditions of a current source RF power amplifier, and is termed as a “mixed-mode” stress. We find that not only the maximum oscillation frequency but also the output power performance of Si/SiGe HBTs are suffered by this electrical stress. In addition, the degradations of high-frequency and power characteristics are also worse under a constant base-current measurement than those under a constant collector-current measurement. Finally, we developed a commercial large-signal model to examine the degradations of the parasitic resistances and ideality factors of base and collector currents to explain the RF power and linearity degradations.     

Reliability of AlInAs/GaInAs heterojunction bipolar transistors

The reliability of high-performance AlInAs/GaInAs heterojunction bipolar transistors (HBTs) grown by molecular beam epitaxy (MBE) is discussed. Devices with a base Be doping level of 5×10¹⁹ cm^-3 and a base thickness of approximately 50 nm displayed no sign of Be diffusion under applied bias. Excellent stability in DC current gain, device turn-on voltage, and base-emitter junction characteristics was observed. Accelerated life-test experiments were performed under an applied constant collector current density of 7×10⁴ A/cm² at ambient temperatures of 193, 208, and 328°C. Junction temperature and device thermal resistance were determined experimentally. Degradation of the base-collector junction was used as failure criterion to project a mean time to failure in excess of 10⁷ h at 125°C junction temperature with an associated activation energy of 1.92 eV     

Si/Si1-xGex heterojunction bipolartransistors produced by limited reaction processing

Si/Si_1-xGe_x heterojunction transistors (HBTs) fabricated by a chemical vapor deposition (CVD) technique are reported. A rapid thermal CVD limited-reaction processing (LRP) technique was used for the in situ growth of all three device layers, including a 20-mm Si_1-xGe_x layer in the base. The highest current gains observed (β=400) were for a Si/Si_1-x Ge_x HBT with a base doping of 7×10¹⁸ cm^-3 near the junction and a shallow arsenic implant to form ohmic contacts and increase current gain. Ideal base currents were observed for over six decades of current and the collector current remained ideal for nearly nine current decades starting at 1 pA. The bandgap difference between a p-type Si layer doped to 5×10¹⁷ cm^-3 and the Si_1-xGe_x(x=0.31) base measured 0.27 eV. This value was deduced from the measurements of the temperature dependence of the base current and is in good agreement with published calculations for strained Si_1-xGe_x layers on Si     

Carbon-doped base GaAs-AlGaAs HBT's grown by MOMBE and MOCVDregrowth

High-quality GaAs-AlGaAs heterojunction bipolar transistors (HBTs) in which the carbon-doped base layers (p=10¹⁰-10²⁰ cm^-3, 400-800 Å thick) and Sn-doped collector and subcollector layers are grown by metalorganic molecular-beam epitaxy (MOMBE) and a subsequent regrowth using metalorganic chemical vapor deposition (MOCVD) is used to provide the n⁺ AlGaAs emitter and GaAs/InGaAs contact layers are discussed. A current gain of 20 was obtained for a base doping of 10¹⁹ cm^-3 (800 Å thick) in a 90-μm-diameter device, with ideality factors of 1.0 and 1.4 for the base-collector and emitter-base junctions, respectively, demonstrating the excellent regrowth-interface quality. For a base doping of 10²⁰ cm^-3 (400 Å thick), the current gain decreased to 8     

异质结耗尽层基区侧复合对突变HBT重要性分析

基于热场发射.扩散载流子输运模型,在电流连续性方程中包含异质结(BB结)耗尽层基区侧复合电流的前提下,推导出了描述突变HBT电流特性的新解析方程.在此基础上,探讨了对BB结耗尽层基区侧复合电流各不同考虑情况下的HBT电流计算结果的差异程度.结果表明:在较高集电极电流密度处,E-B结耗尽层基区侧的复合电流很重要;此外,在电流连续性方程中包含E-B结耗尽层基区侧的复合电流,这在更高集电极电流密度处也是必要的.     

Electrical transport properties of crystalline siliconcarbide/silicon heterojunctions

The electrical transport properties of β-SiC/Si heterojunctions were investigated using current-voltage (I-V) and capacitance-voltage (C-V) characteristics. The heterojunctions were fabricated by growing n-type crystalline β-SiC films on p-type Si substrates by chemical vapor deposition (CVD). The I-V data measured at various temperatures indicate that at relatively high current, the heterojunction forward current is dominated by thermionic emission of carriers and can be expressed as exp(-qV_bi/kT ) exp(V/ηkT), where V_bi is the built-in voltage of the heterojunction and η(=1.3) is a constant independent of voltage and temperature. At lower current, defect-assisted multitunneling current dominates. The effective density of states and the density-of-states effective mass of electrons in the conduction band of SiC are estimated to be 1.7×10²¹ cm ^-3 and 0.78m₀, respectively. This study indicates that the β-SiC/Si heterojunction is a promising system for heterojunction (HJ) devices such as SiC-emitter heterojunction bipolar transistors (HBTs)