Voltage comparators implemented with GaAs/(GaAl)As heterojunction bipolar transistors

The first voltage comparators implemented with GaAs/(GaAl)As heterojunction bipolar transistors (HBTs) are reported. The offset voltage of these circuits was found to be generally below 4 mV, and hysteresis below 1 mV; both voltages stayed quite constant over a range of reference voltage of about 1 V. The preliminary circuit yield was high. This work demonstrated the suitability of these HBT comparators for use in a flash A/D convertor of high accuracy (up to 8 bits).     

GaAs/(GaAl)As heterojunction bipolar transistors using a self-aligned substitutional emitter process

This paper describes the first self-aligned heterojunction bipolar transistor (HBT) process which includes ion implantation to reduce the base resistance. With this substitutional emitter technique, the base implant and the emitter contact patterns are defined with the same mask. Arbitrary contact materials can be used allowing optimization of the contact resistances. Transistors with emitter width down to 1.5 µm have been fabricated. Nonthreshold logic (NTL) ring oscillators made with these transistors had propagation delay times down to 27.2 ps. This is the lowest reported to date for bipolar transistors.     

Nonthreshold logic ring oscillators implemented with GaAs/(GaAl)As heterojunction bipolar transistors

The first nonthreshold logic (NTL) ring oscillators implemented with GaAs/(GaAl)As heterojunction bipolar transistors (HBT's) are reported. Propagation delay times down to 52 ps per gate were achieved, using transistors with emitter dimensions of 1.2 µm × 5 µm. Numerical simulations of the circuits were also done, which agreed closely with the experimental results.     

(GaAl)As/GaAs heterojunction bipolar transistors with graded composition in the base

The first fabrication and high-speed operation of a three-terminal (AlGa)As/GaAs heterojunction bipolar transistor with graded bandgap base is reported. Cutoff frequencies up to 16 GHz have been achieved in devices fabricated from material made by molecular beam epitaxy. This work demonstrates the feasibility of using a graded (AlGa)As base to enhance the speed of heterojunction bipolar transistors.     

GaAs/(Ga,Al)As heterojunction bipolar transistors with buried oxygen-implanted isolation layers

A simple self-aligned technique using oxygen implants to reduce the extrinsic base-collector capacitance in GaAs/(Ga,Al)As heterojunction bipolar transistors (HBT's) is described. This technique has been used to achieve nonthreshold logic ring-oscillators with propagation delays down to 30 ps per gate, the lowest reported to date for any bipolar transistor circuit.     

Transparent, highly sensitive GaAs/(GaAl)As photodiode

The letter describes the performance and properties of a transparent GaAs/(GaAl)As photodiode. The diodo can be inserted directly into an optical-fibre transmission line. By this means, it becomes possible to couple out a small amount of the light power guided within the fibre in tho form of an electrical signal.     

Numerical simulation of GaAs/GaAlAs heterojunction bipolar transistors

A numerical analysis program for the 1-dimensional simulation of GaAs/GaAlAs heterojunction bipolar transistors is described. Calculations for a representative transistor structure indicate that a cutoff frequency, ft, of 100 GHz is obtainable. To attain such a high value, the transistor design incorporated a graded-bandgap base region and relatively high emitter doping (5 × 10¹⁷cm^-3), and the device was operated at high current density (5 × 10⁴A/cm²). Transient electron velocity overshoot effects were also included.     

Channelled substrate narrow stripe GaAs/(GaAl)As lasers with quarter-wavelength facet coatings

Channelled substrate narrow stripe GaAs/(GaAl)As lasers with quarter-wavelength antireflection coatings are described. These devices have low thresholds, broad spectra, high external quantum efficiencies and operate up to 10 mW in the fundamental transverse mode.     

An emitter guard-ring structure for GaAs high-gain heterojunction bipolar transistors

An emitter guard-ring structure is adopted for GaAs/AlGaAs heterojunction bipolar transistors (HBT's). The guard ring is added around the central emitter and has nearly the same dc bias as the base, so that the carrier injection as well as the surface recombination at the emitter periphery is blocked. As a result, the current gain at low operating currents is improved remarkably. It remains almost constant at 600 as the collector current is decreased from 20 mA down to 5 µA. This is the best result to date for a GaAs HBT at low currents. This technique also makes it possible to investigate the interface quality without interference from surface effects.     

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.     

High-speed, low-noise InGaP/GaAs heterojunction bipolar transistors

We have demonstrated self-aligned InGaP/GaAs heterojunction bipolar transistors (HBT's) with excellent dc, microwave, and noise performance. A 3×10 μm² emitter finger device achieved a cutoff frequency of f_T=66 GHz and a maximum frequency of oscillation of f_max=109 GHz. A minimum noise figure of 1.12 dB and an associated gain of 11 dB were measured at 4 GHz. These results are the highest combined f_T+f_max and the lowest noise figure reported for an InGaP/GaAs HBT and are attributed to material quality and the use of self-aligned base contacts. These data clearly demonstrate the viability of InGaP/GaAs HBT's for high-speed, low-noise circuit applications     

Reliability characteristics of GaAs and InP-based heterojunction bipolar transistors

The reliability characteristics of Heterojunction Bipolar Transistor made on GaAs and InP substrates are reviewed and ways of improving them by design, growth and processing are described. Materials for HBTs are grown by a variety of techniques (MBE, MOCVD, CBE). Their choice is made based on considerations such as satisfaction of design requirements and manufacturing but also dopant incorporation without risk of diffusion, as well as, minimization of hydrogen incorporation which may result in device degradation. The minority carrier lifetime in the base is influenced by hydrogen incorporation in C-InGaAs. Conventional alloyed and non-alloyed, as well as, refractory metallization schemes are considered for best reliability performance. The dielectric deposition scheme used for passivation plays a major role on device reliability. Good reliability performance is reported for GaAs but also for InP-based HBTs. A correlation is finally reported to exist between the low-frequency noise properties of HBTs and their reliability characteristics.     

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.     

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.     

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     

GaAlAs/GaAs heterojunction bipolar transistors: issues andprospects for application

Issues important for the manufacturing of GaAlAs/GaAs heterojunction bipolar transistors (HBTs) and their prospects for application in various areas are discussed. The microwave and digital performance status of HBTs is reviewed. Extrapolated values of maximum frequency of oscillation above 200 GHz and frequency divider operation at 26.9 GHz are reported. Key prospects for further device development are highlighted