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.     

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.     

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.     

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.     

Ideality factor of extrinsic base surface recombination current in AlGaAs/GaAs heterojunction bipolar transistors

Various structures of npn AlGaAs/GaAs heterojunction bipolar transistor have been fabricated to examine the ideality factor for extrinsic base surface recombination. Comparison of the measured results indicates that the base surface recombination current increases exponentially with the base-emitter voltage with an ideality factor of 1, rather than 2. This finding agrees with a published theoretical analysis (see Tiwari et al., J. Appl. Phys., vol.64, p.5009, 1988).<>     

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.     

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.     

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.     

Experimental comparison of base recombination currents in abrupt and graded AlGaAs/GaAs heterojunction bipolar transistors

The relative importance of the base bulk recombination current and the base-emitter junction space charge recombination current is examined for AlGaAs/GaAs HBTs with different grading schemes in the base-emitter junction. Experimental results demonstrate that, in abrupt HBTs, the base bulk recombination current is larger, and the base current increases with the base-emitter bias with an ideality factor of approximately 1. In contrast, in graded HBTs, the space charge recombination current dominates and the base current ideality factor is approximately 2. These experimental results agree well with a published theoretical calculation.<>     

Reduction of extrinsic base resistance in GaAs/AlGaAs heterojunction bipolar transistors and correlation with high-frequency performance

Improved high-frequency performance in GaAs/AlGaAs heterojunction bipolar transistors (HBT's) by reduction of extrinsic base resistance is demonstrated. A new self-aligned process which is very simple, yet capable of producing 0.25-µm emitter-to-base contact gaps, is described. By the use of AuBe, we have also been able to produce contact resistances to p-type GaAs (p = 5 × 10¹⁸) as low as 1.2 × 10^-7Ω.cm². This is the lowest value reported to p-type GaAs considering the relatively low doping levels used. By employing these techniques, we have produced HBT's with 2.5-µm-wide emitters having current gain cutoff frequencies fTthat appear to be greater than 35 GHz and maximum oscillation frequenciesf_{max}of 22 GHz.     

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.<>     

Very high-power-density CW operation of GaAs/AlGaAs microwaveheterojunction bipolar transistors

Thermal instability of multi-emitter high-power microwave heterojunction bipolar transistors (HBTs) was eliminated using a novel heat spreading technique that regulates internal device currents to avoid the formation of hot spots. Devices with 2- and 3-μm minimum emitter sizes and no intentional ballast resistors showed unconditionally stable CW operation up to the device electronic limitations. A record 10-mW/μm² power density was obtained at 10 GHz with 7-dB gain and 60% power-added efficiency. The highest efficiency was 67.2% at 9.3-mW/μm² power density. It was shown that stable high-power-density operation can be maintained at multiwatt output power levels     

The effect of base grading on the gain and high-frequencyperformance of AlGaAs/GaAs heterojunction bipolar transistors

A comprehensive one-dimensional analytical model of the graded-base Al_xGa_1-xAs/GaAs heterojunction bipolar transistor is presented and used to examine the influence of base grading on the current gain and the high-frequency performance of a device with a conventional pyramidal structure. Grading is achieved by varying the Al mole fraction x linearly across the base to a value of zero at the base-collector boundary. Recombination in the space-charge and neutral regions of the device is modeled by considering Schockley-Read-Hall, Auger, and radiative processes. Owing to the different dependencies on base grading of the currents associated with these recombination mechanisms, the base current is minimized, and hence the gain reaches a maximum value, at a moderate level of base grading ( x=0.1 at the base-emitter boundary). The maximum improvement in gain, with respect to the ungraded base case, is about fourfold. It is shown that the reduction in base transit time due to increased base grading leads to a 30% improvement in f_T in the most pronounced case of base grading studied (x=0.3 at the base-emitter boundary). The implications this has for improving f _max via increases in base width and base doping density are briefly examined     

The use of base ballasting to prevent the collapse of current gainin AlGaAs/GaAs heterojunction bipolar transistors

We propose the use of base-ballasting resistance to guarantee absolute thermal stability in AlGaAs/GaAs heterojunction bipolar transistors (HBTs). Base-ballasted HBTs are fabricated and the measured I-V, regression and S-factor characteristics are discussed. We present a numerical model which elucidates the reasons why the base-ballasting scheme is helpful to HBTs but is damaging to silicon bipolar transistors. We compare measured small-signal and large-signal performances of unballasted, emitter-ballasted, and base-ballasted HBTs     

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     

A mechanism for hydrogen-related transient effects in carbon-dopedAlGaAs/GaAs heterostructure bipolar transistors

The transient phenomenon in carbon-doped AlGaAs/GaAs HBT's has been found to reoccur after a brief thermal annealing under no bias. The temperature and the current dependencies have been studied with HBT's made with MOCVD grown wafers. The experimental data ran be explained by a model based on thermal decomposition of C-H complexes during annealing and electron captures by hydrogen ions under minority-carrier injection     

A proposed structure for collector transit-time reduction in AlGaAs/GaAs bipolar transistors

The impact of velocity overshoot in the collector space-charge region on carrier transport is explored. The effects of overshoot on transit time for conventional collector structures are found to be minor. A new collector structure which exhibits extended velocity overshoot is proposed. This structure promises both simple fabrication and significant improvements (≃ 25 percent) in carrier transit time over conventional collector structures as demonstrated by Monte Carlo simulation.     

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.     

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.