AlGaAs/GaAs Heterojunction bipolar power transistors

An AlGaAs/GaAs heterojunction bipolar transistor with a total emitter periphery of 320 ?m has been developed for power amplifier applications. For the base contact, Zn diffusion was used to convert the n-type emitter material into p-type with a doping of ?1.0 × 1020 cm?3. Because of the highly doped layer, contact resistivity was extremely low (5 × 10?7 ?cm2). At 3 GHz, a CW output power of 320 mW with 7 dB gain and 30% power-added efficiency was obtained. Under pulsed operation, the output power increased to 500 mW with 6 dB gain and 40% power-added efficiency. With further device structure optimisations, the power performance of heterojunction bipolar transistors is expected to rival, or even surpass, that of the GaAs MESFETs.     

High-frequency characteristics of AlGaAs/GaAs heterojunction bipolar transistors

The fabrication and high-frequency performance of MBE-grown AlGaAs/GaAs heterojunction bipolar transistors (HBT's) is described. The achieved gain-bandwidth product fTis 25 GHz for a collector current density Jcof 1 × 10⁴A/cm²and a collector-emitter voltage VCEof 3 V.fTcontinues to increase with the collector current in the high current density region over 1 × 10⁴A/cm²with no emitter crowding effect nor Kirk effect. The limitation on fTin fabricated devices is found to be caused mainly by the emitter series resistance.     

High-frequency output characteristics of AlGaAs/GaAs heterojunction bipolar transistors

A model has been developed which generates the high-frequency i/sub c/-v/sub ce/ output characteristics of bipolar transistors from computed cutoff frequency against current density data. The presented results, which can be used directly for large-signal modelling are the first report of high-frequency output characteristics of bipolar transistors.<>     

Monte Carlo simulation of AlGaAs/GaAs heterojunction bipolar transistors

A first demonstration of one-dimensional Monte Carlo simulations of AlGaAs/GaAs heterojunction bipolar transistors is reported. The electron motion is solved by a particle model, while the hole motion is solved by a conventional hydrodynamic model. It is shown that the compositional grading of AlxGa1-xAs in the base region is effective to cause the ballistic acceleration of electrons in the base region, resulting in a high collector current density of above 1 mA/µm². The current-gain cutoff frequency fT reaches 150 GHz if the size of a transistor is properly designed. Also shown is the relation between the device performances and the electron dynamics investigated.     

Fabrication and characterization of AlGaAs/GaAs heterojunction bipolar transistors

The fabrication and characterization of MBE-grown AlGaAs/GaAs heterojunction bipolar transistors (HBT's) are described, A Be redistribution profile in the HBT epi-layer at the emitter-base heterojunction interface is investigated using secondary ion mass spectrometry, A relatively high substrate temperature of 650°C during growth can be employed by introducing a 100-Å undoped spacer layer between the emitter and base layer. A simple wafer characterization method using phototransistors is demonstrated for accurately predicting current gain in a three-terminal device. A dc current gain of up to 230 is obtained for the fabricated HBT with a heavy base doping of 1 × 10¹⁹/cm³. A gain-bandwidth product fTof 25 GHz is achieved with a 4.5-µm-width emitter HBT.     

GaAs/AlGaAs heterojunction bipolar transistors for integrated circuit applications

Molecular-beam epitaxy (MBE) and ion implantation were used to fabricate GaAs/AlGaAs heterojunction bipolar transistors with buried wide bandgap emitters. Inverted-mode current gains of ∼ 100 were obtained, demonstrating the feasibility of this technology for I²L types of digital integrated circuits.     

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     

AlGaAs/GaAs bipolar transistors with a modulation-doped superlattice emitter

A new emitter-base concept for heterojunction bipolar transistors is proposed and demonstrated. The abrupt or graded emitter-base heterojunction drawbacks are circumvented by means of a modulation-doped stack of ternary compound alternating with binary compound layers. This stack provides efficient barriers to the hole current while preserving good diode characteristics. These concepts are demonstrated by experimental results on small-area devices with a common-emitter gain of ~40 and a collector-emitter voltage offset of ~80mV.     

Very high gain AlGaAs/GaAs pnp heterojunction bipolar transistor

The effect of the emitter Al mole fraction and emitter-base junction type on the performance of AlGaAs/GaAs pnp heterojunction transistors was investigated experimentally. A current gain as high as 1360 was obtained for a structure with an Al/sub 0.4/Ga/sub 0.6/As emitter and compositionally abrupt emitter-base junction; this was attributed to reduced surface recombination.<>     

High-frequency performance of MOVPE npn AlGaAs/GaAs heterojunction bipolar transistors

Base doping densities near 10/sup 20/ cm/sup -3/ and emitter doping densities near 7*10/sup 17/ cm/sup -3/ have been achieved in MOVPE HBT structures and combined with self-aligned processing resulting in f/sub max/=94 GHz and f/sub t/=45 GHz. To the authors' knowledge, these are the first MOVPE HBTs demonstrated to operate at millimetre-wave frequencies.<>     

Collector-top GaAs/AlGaAs heterojunction bipolar transistors for high-speed digital ICs

GaAs/AlGaAs collector-top heterojunction bipolar transistors with magnesium and phosphorus double-implanted external bases were fabricated. A cutoff frequency of 17 GHz and a gate delay time of 63 ps for DCTL were obtained. These results indicate the potential of collector-top HBTs for high-speed ICs.     

GaAs/AlGaAs heterojunction emitter-down bipolar transistors fabricated on GaAs-on-Si substrate

GaAs/AlGaAs heterojunction bipolar transistors with emitter-down structure were fabricated on GaAs-on-Si substrate for the first time. A maximum current gain of 25 was measured at a collector current density of 6250 A/cm². This value is comparable with that from similar devices fabricated on GaAs substrates. This result, along with previous work on large-scale integration of emitter-down transistors, demonstrates the potential for high-level integration of bipolar devices on GaAs-on-Si substrates.     

High-speed frequency dividers using self-aligned AlGaAs/GaAs heterojunction bipolar transistors

A divide-by-four frequency divider and ring oscillators have been fabricated employing self-aligned AlGaAs/GaAs heterojunction bipolar transistors (HBT's). Maximum toggle frequency of 13.7 GHz and propagation delay time of 17.2 ps are achieved in ECL gate circuitry. These values are the highest and the lowest in ECL circuits and in bipolar circuits, respectively, ever reported.     

AlGaAs/Ge/GaAs heterojunction bipolar transistors grown bymolecular beam epitaxy

An N-Al_0.22Ga_0.78As emitter, p-Ge base, and n-GaAs collector (AlGaAs/Ge/GaAs) heterojunction bipolar transistor (HBT) in the emitter-up configuration grown by molecular beam epitaxy is discussed. Devices exhibited common-emitter current gains of as high as 300 at a collector current density of 2000 A/cm² and a collector voltage of 4 V. As the device area is reduced from 50×50 to 10×40 μm, the current gain did not show significant changes, suggesting a low surface recombination velocity in the Ge base     

AlGaAs/InGaAs/GaAs strained-layer heterojunction bipolar transistors by molecular beam epitaxy

The growth and characteristics of the first AlGaAs/InGaAs/GaAs HBTs are reported. Layers with up to 10% indium content appeared to be free of misfit dislocations, and resulted in HBTs with good I/V characteristics which scaled with In content according to theory.     

AlGaAs/GaAs heterojunction bipolar transistors fabricated using a self-aligned dual-lift-off process

This paper describes a self-aligned heterojunction-bipolar-transistor (HBT) process based on a simple dual-lift-off method. Transistors with emitter width down to 1.2 µm and base doping up to 1 × 10²⁰/cm³have been fabricated. Extrapolated current gain cutoff frequency ftof 55 GHz and maximum frequency of oscillationf_{max}of 105 GHz have been obtained. Current-mode-logic (CML) ring oscillators with propagation delays as low as 14.2 ps have been demonstrated. These are record performance results for bipolar transistors. The dual-lift-off process is promising for both millimeter-wave devices and large-scale integrated circuit fabrication.     

High-performance p-n-p AlGaAs/GaAs heterojunction bipolar transistors: A theoretical analysis

The AlGaAs/GaAs P-n-p heterojunction bipolar transistor (HBT) is shown by a simple analysis to exhibit millimeter wave and digital switching performance comparable to similar N-p-n structures. For example, a P-n-p HBT with a 1-µm emitter stripe and 34-µm²total area yieldsf_{tau} = 31GHz,f_{max} = 94GHz, and an intrinsic switching speedtau_{s} = 8ps. A similar N-p-n structure exhibitsf_{tau} = 56GHz,f_{max} = 102GHz, andtau_{s} = 8ps.     

On the estimation of base transit time in AlGaAs/GaAs bipolar transistors

Electron diffusion across quasi-neutral p-type base regions representative of those used in n-p-n AlGaAs/GaAs/GaAs heterojunction bipolar transistors is investigated. Monte-Carlo simulation results demonstrate that for realistic base widths ≲ 1000 Å) electron transport cannot be described by Fick's Law. As a result, the conventional estimate of base transit time,tau_{B} = Wmin{B}max{2}/2D_{n}, will produce substantial errors for base widths typical of those employed in heterojunction bipolar transistors. Estimates based on the ballistic transport of electrons across the base are shown to significantly underestimate base transit time-even for base widths substantially narrower than those presently employed.     

Accurate modeling of AlGaAs/GaAs heterostructure bipolar transistors by two-dimensional computer simulation

An accurate modeling of Al0.3Ga0.7As/GaAs heterostructure bipolar transistors (HBT's) has been carried out using a two-dimensional numerical simulator. By comparing an HBT with a GaAs homotransistor, the potential advantages of the HBT, such as a high injection efficiency, a low voltage drop in the base region, and an improvement in high-frequency operation, were well confirmed, not only by the calculated transistor performance but also by the carrier and the potential distributions. It was also demonstrated that the injected electrons spread more and more conspicuously into the extrinsic base region, even for the HBT's with the increase in the collector current. Taking into account the near ballistic transport in the base region, it is possible to realize an abrupt HBT with a maximum cutoff frequency of above 130 GHz and current gains of up to 3200. The HBT was confirmed to be a promising device for the realization of ultrahigh-frequency integrated circuits.     

Microwave properties of self-aligned GaAs/AlGaAs heterojunction bipolar transistors on silicon substrates

(Al,Ga)As/GaAs heterojunction transistors (HBT's) grown on Si substrates have been characterized at microwave frequencies and have been found to perform extremely well. For emitter dimensions of 4 × 20 µm², current gain cutoff frequenciesf_{T} = 30GHz and maximum oscillation frequenciesf_{max}of 11.5 GHz have been obtained in a mesa-type structure. These values compare very well with thef_{T} = 40GHz andf_{max} = 26GHz which are the highest reported for HBT's on GaAs substrates in a nonmesa structure with an emitter width of ∼ 1.5 µm. These results clearly demonstrate the potential of HBT's in general at microwave frequencies, as well as the viability of GaAs on Si technology.