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/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-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.     

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.     

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.     

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.<>     

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     

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.     

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-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.     

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     

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.     

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.     

Microwave operation of high power InGaP/GaAs heterojunction bipolar transistors

The first high power demonstration of an InGaP/GaAs heterojunction bipolar transistor is presented. Multifinger selfaligned HBTs were tested at 3 GHz. A maximum output power of 2.82 W CW was obtained for a 600 mu m/sup 2/ emitter area device (4.7 mW/ mu m/sup 2/ power density) with an attendant gain of 6.92 dB; simultaneously, the device exhibited 55.2% power added efficiency, 69.1% collector efficiency and 8.0*10/sup 4/ A/cm/sup 2/ emitter current density.<>     

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     

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     

Recombination current reduction in AlGaAs/GaAs heterojunction bipolar transistors with polyimide deposition

N-p-n AlGaAs/GaAs heterojunction bipolar transistors of various emitter-mesa diameters have been fabricated to examine the effects on the recombination current with polyimide deposition. The recombination current in these devices (when the ideality factor is close to 2) was proportional to the device perimeter before and after polyimide deposition. This suggests that the dominant component of the recombination current in these devices is the perimeter recombination current. A simple model was developed which allowed the contribution from the perimeter and bulk recombination currents to be calculated. The common-emitter d.c. current gains of these devices increased correspondingly as recombination current decreased due to the polyimide deposition.     

Fabrication and characterization of InGaP/GaAs heterojunction bipolar transistors on GOI substrates

In this letter, we report the first demonstration of InGaP/GaAs heterojunction bipolar transistors (HBTs) on germanium-on-insulator (GOI) substrates. We have performed physical characterization of the epitaxial layers to verify the high quality of the III-V epitaxial material grown on the GOI substrates and performed dc characterization of large-area InGaP/GaAs HBTs fabricated on the substrates. The InGaP/GaAs HBTs realized on GOI substrates were compared with identical devices grown on bulk germanium substrates and similar devices on semi-insulating GaAs substrates.     

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.