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.     

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-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     

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.     

High-frequency performance of lattice-strained heterojunction GaInAs/GaAs bipolar transistors

The performance of bipolar devices and circuits in the novel lattice-strained GaInAs/GaAs materials system show an improving trend with increased In composition in the base. An fT of 8 GHz has been measured using an 8 ?m emitter stripe width and 8% In. A small-signal model is presented.     

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.     

Improving collector-current uniformity in emitter-gradedAlGaAs/GaAs heterojunction bipolar transistors

To investigate the effect of a graded layer on collector-current uniformity, two types of HBTs were fabricated by metalorganic chemical vapor deposition (MOCVD). One type had a bandgap graded layer at the emitter-based interface to eliminate the conduction-band spike. The other type was a conventional HBT (heterojunction bipolar transistor) with an abrupt heterojunction fluctuated due to the fluctuation of the barrier energy from the emitter to the base. The bandgap-graded layer drastically suppressed the fluctuation of the collector current. the standard deviation of the threshold voltage was improved from 3.03 to 0.42 V by adopting bandgap grading at the emitter-based interface     

Self-aligned InGaP/GaAs heterojunction bipolar transistors formicrowave power application

As an alternative to AlGaAs/GaAs heterojunction bipolar transistors (HBTs) for microwave applications, InGaP/GaAs HBTs with carbon-doped base layers grown by metal organic molecular beam epitaxy (MOMBE) with excellent DC, RF, and microwave performance are demonstrated. As previously reported, with a 700-Å-thick base layer (135-Ω/sq sheet resistance), a DC current gain of 25, and cutoff frequency and maximum frequency of oscillation above 70 GHz were measured for a 2-μm×5-μm emitter area device. A device with 12 cells, each consisting of a 2-μm×15-μm emitter area device for a total emitter area of 360 μm², was power tested at 4 GHz under continuous-wave (CW) bias condition. The device delivered 0.6-W output power with 13-dB linear gain and a power-added efficiency of 50%     

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     

Emitter excess resistance in GaAlAs/GaAs heterojunction bipolar transistors

Numerous studies have shown the influence of parasitic elements, particularly emitter access resistance and inductance, on HBTs DC and dynamic performances. Instead the achievement of low specific contact resistances, the apparent emitter resistance presents high values. To explain this discrepancy the longitudinal distribution of the emitter current has been investigated by using a distributed DC-model, taking into account the metal layers resistance of the different contacts. Thus it is shown that only the emitter contact parameters act on the non equipotentiality along the structure.     

Monolithic ultra-broadband transimpedance amplifiers usingAlGaAs/GaAs heterojunction bipolar transistors

Monolithic ultra-broadband transimpedance amplifiers are developed using AlGaAs/GaAs HBTs. To realize good amplifier performances, two factors are mentioned: an affordable HBT fabrication process using the self-aligned method and an optimized circuit design considering large signal operations. The developed HBT fabrication process achieves excellent uniformity in DC characteristics and the effect on amplifier microwave performances, derived from the discrete device uniformity, is estimated. Amplifier circuit configurations are designed by harmonic balance simulation using the extracted large signal device parameters The fabricated amplifier exhibits a DC to 13.4-GHz bandwidth with an 18.1-dB gain. Fairly good uniformity is also achieved for the amplifier microwave performances. An optical receiver module is constructed mounting the developed HBT amplifier and InGaAs p-i-n photodiode chips. The optical receiver module provides a 9.4-GHz bandwidth and an optical receiver sensitivity of -15.7 dBm at 10-Gb/s data rate     

Multiple-channel GaAs/AlGaAs high electron mobility transistors

Multiple-channel high electron mobility transistors (HEMT's) have been designed and fabricated on GaAs/AlGaAs heterostructural material grown by molecular beam epitaxy (MBE). The sheet carrier density of the two-dimensional electron gas (2-DEG) measured at 77 K was linearly proportional to the number of high mobility electron channels, and reached 5.3 × 10¹²cm^-2for six-channel HEMT structures. Depletion-mode devices of the double-heterojunction HEMT were operated between negative pinchoff voltage and forward-biased gate voltage without any transconductance degradation. A peak extrinsic transconductance of 360 mS/mm at 300 K and 550 mS/mm at 77 K has been measured for a 1-µm gate-length double-heterojunction enhancement-mode device. An extremely high drain current of 800 mA/mm with a gate-to-drain avalanche breakdown voltage of 9 V was measured on six-channel devices.     

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.<>     

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.<>     

Comparison of GaInP/GaAs heterostructure-emitter bipolartransistors and heterojunction bipolar transistors

Carbon-doped GaInP/GaAs heterojunction bipolar transistors (HBT's) and heterostructure-emitter bipolar transistors (HEBT's) grown by MOCVD were fabricated. Experimental comparison of HBT's and HEBT's has been made based on the dc and the RF performance. HBT's have higher current gains than those of HEBT's in the high current regime, while HEBT's offer a smaller offset voltage and better uniformity in dc characteristics across the wafer. The current gain and cutoff frequency of the DEBT with a 150 Å emitter set-back layer are comparable to those of HBT's. DC (differential) current gains of 600 (900) and 560 (900) were obtained at a collector current density of 2.5×10⁴ A/cm² for the HBT and HEBT, respectively. The cutoff frequencies are 37 and 31 GHz for the HBT and HEBT, respectively. It is shown that there is negligible contribution of the diffusion capacitance to the emitter capacitance in HEBT's with a thin emitter set-back layer but not with a thick emitter set-back layer. The behavior of HEBT's both in dc and RF characteristics is similar to that of HBT's     

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.     

A short-term high-current-density reliability investigation ofAlGaAs/GaAs heterojunction bipolar transistors

In high current and power density applications of AlGaAs/GaAs heterojunction bipolar transistors (HBT's), reliability is a critical issue. Therefore, in this letter we show results of a fundamental investigation on the temperature and current dependence of the fast initial rise of the dc-current gain (burn-in), which takes place during stress at current densities beyond those of today's applications. We find that the burn-in occurs at lower device junction temperatures (135°C) than previously reported in literature, and that it depends linearly on the current density. An activation energy of 0.4 eV is extracted for the burn-in effect