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.     

Effect of exponentially graded base doping on the performance ofGaAs/AlGaAs heterojunction bipolar transistors

The authors have fabricated n-p-n GaAs/AlGaAs heterojunction bipolar transistors (HBTs) with base doping graded exponentially from 5×10¹⁹ cm^-3 at the emitter edge to 5×10¹⁸ cm^-3 at the collector edge. The built-in field due to the exponentially graded doping profile significantly reduces base transit time, despite bandgap narrowing associated with high base doping. Compared to devices with the same base thickness and uniform base doping of 1×10¹⁹ cm^-3 , the cutoff frequency is increased from 22 to 31 GHz and maximum frequency of oscillation is increased from 40 to 58 GHz. Exponentially graded base doping also results ill consistently higher common-emitter current gain than uniform base doping, even though the Gummel number is twice as high and the base resistance is reduced by 40%     

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).<>     

Influence of buffer layer thickness on DC performance ofGaAs/AlGaAs heterojunction bipolar transistors grown on siliconsubstrates

The DC performance of GaAs/AlAs heterojunction bipolar transistors (HBTs) grown on silicon substrates with buffer layers ranging from 0 to 5 μm was investigated. Current gain, collector-emitter breakdown voltage, emitter-base and collector-base diode ideality factors, and breakdown voltages were measured as the buffer layer thickness was varied between 0 and 5 μm. The current gain steadily increases with increasing buffer layer thickness until the layer reaches 3 μm. However, the other DC parameters are relatively insensitive to the buffer layer thickness. A small-signal current gain of 60 is typically achieved for devices with 6×6-μm² emitters at a density of 6×10⁴ A/cm² when the buffer layer is ⩾3 μm     

Effect of emitter-base spacing on the current gain of AlGaAs/GaAsheterojunction bipolar transistors

An investigation of the effect of surface recombination and emitter-base-contact spacing on the DC current-gain of AlGaAs/GaAs heterojunction bipolar transistor (HBT) using thin AlGaAs emitter structures is discussed. The selectively-etched, thin-AlGaAs-emitter layer has been used to prevent an exposed extrinsic base region, which has previously limited current gain because of high surface recombination. It is found that a factor of ~50 improvement in the current gain can be achieved by proper surface passivation and emitter-base-contact spacing     

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.     

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.     

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     

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.<>     

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.     

Diode ideality factor for surface recombination current inAlGaAs/GaAs heterojunction bipolar transistors

n-p-n AlGaAs/GaAs heterojunction bipolar transistors of various emitter areas have been fabricated to examine the diode ideality factor for surface recombination. These transistors are fabricated with and without exposed extrinsic base surfaces. Comparison of the measured results indicates that the base surface recombination current increases exponentially with the base-emitter voltage with an ideality factor which is closer to 1 than 2(1<n<1.33). This finding agrees with a published theoretical analysis of base surface recombination in HBTs. This study is compared with other experimental work     

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.     

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.<>     

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     

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.