A 40-Gb/s preamplifier using AlGaAs/InGaAs HBTs with regrown basecontacts

A preamplifier for 40-Gb/s optical transmission systems incorporating AlGaAs/InGaAs heterojunction bipolar transistors (HBTs) with p⁺ regrown extrinsic base layers is described. The HBTs have a heavily doped regrown p⁺-GaAs layer in the extrinsic base regions and a thin graded InGaAs strained layer for the intrinsic base. Their measured peak f_max is above 200 GHz. The developed preamplifier provides a bandwidth of 38.4 GHz and a transimpedance gain of 41.1 dB Ω. Moreover, the frequency response as an optical receiver has a bandwidth of 32 GHz. These characteristics make the preamplifier suitable for use in a 40-Gb/s optical receiver. These results show that AlGaAs/InGaAs HBTs with p⁺ regrown extrinsic base layers are very promising for use in 40-Gb/s optical transmission systems     

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.     

Design study of AlGaAs/GaAs HBTs

The frequency performance of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) having different layouts, doping profiles, and layer thicknesses was assessed using the BIPOLE computer program. The optimized design of HBTs was studied, and the high current performances of HBTs and polysilicon emitter transistors were compared. It is shown that no current crowding effect occurs at current densities less than 1×10⁵ A/cm² for the HBT with emitter stripe width S_E<3 μm, and the HBT current-handling capability determined by the peak current-gain cutoff frequency is more than twice as large as that of the polysilicon emitter transistor. An optimized maximum oscillation frequency formula has been obtained for a typical process n-p-n AlGaAs/GaAs HBT having base doping of 1×10 ¹⁹ cm^-3     

An electrical method for measuring the difference bandgap acrossthe neutral base in SiGe HBT's

This paper describes an electrical method for measuring the bandgap difference across the neutral base of SiGe heterojunction bipolar transistors (HBT's). It measures the effective bandgap difference due to differences in germanium concentration including the effects of heavy doping on bandgap reduction. Numerical device simulation was used to investigate the use of the proposed technique on high performance transistors with graded and uniform germanium profiles. Experimental verification of the technique is conducted on SiGe HBT devices fabricated using LPCVD     

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.     

Effect of collector design on the d.c. characteristics of In0.49Ga0.51P/GaAs heterojunction bipolar transistors

InGaP/GaAs heterojunction bipolar transistors with various collector structures are compared. The dependence of d.c. device characteristics on the thickness of the n⁻ GaAs spacer in the collector of composite collector devices is presented. Results indicate that the spacer thickness significantly affects the performance of the transistor. An n⁺ doping spike on the InGaP side of the collector heterojunction is included in the collector design of the composite collector devices. Standard single-heterojunction d.c. results are compared to abrupt double- and composite collector heterojunction devices. Optimization of the spacer thickness, in conjunction with the n⁺ doping spike, eliminates most of the detrimental effects associated with a double-heterojunction device while retaining the beneficial properties of a wide-gap collector. As expected, the composite collector structure produces devices with higher breakdown voltages and lower offset voltages than single heterojunction devices. In addition, optimizing the spacer thickness can reduce the collector current saturation voltage of the composite collector device below that of a single-heterojunction device. These characteristics make composite collector heterojunction bipolar transistors ideal candidates for high power microwave device applications.     

Device characteristics of the GaAs/InGaAsN/GaAs p-n-p doubleheterojunction bipolar transistor

We have demonstrated the dc and rf characteristics of a novel p-n-p GaAs/InGaAsN/GaAs double heterojunction bipolar transistor. This device has near ideal current-voltage (I-V) characteristics with a current gain greater than 45. The smaller bandgap energy of the InGaAsN base has led to a device turn-on voltage that is 0.27 V lower than in a comparable p-n-p AlGaAs/GaAs heterojunction bipolar transistor. This device has shown f_T and f_MAX values of 12 GHz. In addition, the aluminum-free emitter structure eliminates issues typically associated with AlGaAs     

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%     

Reduction of p+-n+ junction tunneling currentfor base current improvement in Si/SiGe/Si heterojunction bipolartransistors

The authors report a three-order-of-magnitude reduction in parasitic tunneling current at heavily doped p⁺-n⁺ Si/Si and SiGe/Si junctions grown by rapid thermal epitaxial chemical vapor deposition (CVD) compared with previously reported results in Si junctions fabricated by ion implantation. These results demonstrate the high quality of the epitaxial interface. The low tunneling currents allow higher limits to transistor base and emitter doping levels, yielding higher gains, reduced bias resistances, and higher Early voltages for scaled bipolar devices as well as Si/SiGe/Si heterojunction bipolar transistors     

A study of high-speed normally off and normally on Al0.5Ga0.5As heterojunction gate GaAs FET's (HJFET)

The dc, small-signal microwave, and large-signal switching performance of normally off and normally on Al0.5Ga0.5As gate heterojunction GaAs field-effect transistors (HJFET) with submicrometer gate lengths are reported. The structure of both types of devices comprises an n-type 10¹⁷-cm^-3Sn-doped active layer on a Cr-doped GaAs substrate, a p-type 10¹⁸-cm^-3Ge-doped Al0.5Ga0.5As gate layer and a p⁺-type 5 × 10¹⁸-cm^-3Ge-doped GaAs "contact and cap" layer on the top of the gate. The gate structure is obtained by selectively etching the p⁺-type GaAs and Al0.5Ga0.5As. Undercutting of the Al0.5Ga0.5As layer results in submicrometer gate lengths, and the resulting p⁺-GaAs overhang is used to self-align the source and the drain with respect to the gate. Normally off GaAs FET's with 0.5- to 0.7-µm long heterojunction gates exhibit maximum available power gains (MAG) of about 9 dB at 2 GHz. Large-signal pulse measurements indicate an intrinsic propagation delay of 40 ps with an arbitrarily chosen 100-Ω drain load resistance in a 50-Ω microstrip circuit. Normally on FET's with submicrometer gate lengths (∼0.6 µm) having a total gate periphery of 300 µm and a corresponding dc transconductance of 20-30 mmhos exhibit a MAG of 9.5 dB at 8 GHz. The internal propagation delay time measured under the same conditions as above is about 20 ps.     

Carbon-doped base GaAs-AlGaAs HBT's grown by MOMBE and MOCVDregrowth

High-quality GaAs-AlGaAs heterojunction bipolar transistors (HBTs) in which the carbon-doped base layers (p=10¹⁰-10²⁰ cm^-3, 400-800 Å thick) and Sn-doped collector and subcollector layers are grown by metalorganic molecular-beam epitaxy (MOMBE) and a subsequent regrowth using metalorganic chemical vapor deposition (MOCVD) is used to provide the n⁺ AlGaAs emitter and GaAs/InGaAs contact layers are discussed. A current gain of 20 was obtained for a base doping of 10¹⁹ cm^-3 (800 Å thick) in a 90-μm-diameter device, with ideality factors of 1.0 and 1.4 for the base-collector and emitter-base junctions, respectively, demonstrating the excellent regrowth-interface quality. For a base doping of 10²⁰ cm^-3 (400 Å thick), the current gain decreased to 8     

Measurement of the electron ionization coefficient at low electricfields in GaAs-based heterojunction bipolar transistors

Values of the electron ionization coefficient α_n in 〈100〉 GaAs extending the previously available data by two orders of magnitude, down to 1 cm^-1, are presented. The data are directly extracted from the multiplication factor, M-1, measured in lightly doped collector n-p-n AlGaAs/GaAs heterojunction bipolar transistors (HBT's). It is shown that the sensitivity of the technique is limited by the early effect, whose influence can be reduced by driving the device at constant emitter-base bias and by using heavily doped base regions. HBT's can provide simultaneously high base doping and current gain, and represent therefore an excellent tool for these measurements     

Uniform, high-gain AlGaAs/In0.05Ga0.95As/GaAsP-n-p heterojunction bipolar transistors by dual selective etch process

AlGaAs/InGaAs/GaAs P-n-p heterojunction bipolar transistors (HBTs) have been fabricated using a dual selective etch process. In this process, a thin AlGaAs surface passivation layer surrounding the emitter is defined by selective etching of the GaAs cap layer. The InGaAs base is then exposed by selective etching of the AlGaAs emitter. The resulting devices were very uniform, with current gain varying by less than ±10% for a given device size. Current gain at a given emitter current density was independent of device size, with gains of over 200 obtained at current densities above 5×10⁴ A/cm ²     

A planar-doped 2D-hole gas base AlGaAs/GaAs heterojunction bipolartransistor grown by molecular beam epitaxy

A novel type of AlGaAs/GaAs heterojunction bipolar transistor (HBT) which uses a two-dimensional (2-D) hole gas base formed by planar doping using molecular-beam epitaxy (MBE) has been demonstrated. The base consists of a submonolayer of Be atoms of sheet concentration 0.5-5×10¹³ cm^-2 which is deposited during growth interruption by MBE. The transistor structure exhibits DC current gains up to 700. The effective base transit time is negligible in these transistors and it is postulated that very high-speed nonequilibrium transport may occur in the collector region     

Investigation of AlGaAs/GaAs superlattice-emitter resonanttunneling bipolar transistor (SE-RTBT)

A bipolar transistor with an i-Al_0.5Ga_0.5As/n⁺-GaAs superlattice emitter as both hole reflection barriers and electron tunneling barriers has been fabricated successfully. The AlGaAs/GaAs potential spike is eliminated by moving the heterointerface away from the emitter-base junction. Both the turn-on voltage of emitter-base and base-collector junctions are almost identical for the same current level. The room-temperature common-emitter current gain is over 60, and a collector-emitter offset voltage of 55 mV has been obtained with a base-to-emitter doping ratio of 10. Multiple differential negative resistance phenomena and different transistor operating regimes have been observed due to the tunneling effects in the AlGaAs/GaAs superlattice at 77 K. Calculated results are in agreement with experimental ones. Because of the existence of high peak-to-valley current ratios as well as current gain over 65, the SE-RTBT is suitable for multivalued logic circuit applications with relatively reduced complexity     

Demonstration of a 77-GHz heterojunction bipolar transferredelectron device

We demonstrate for the first time a heterojunction bipolar transferred electron device (HBTED), a device with a bipolar transistor-like structure in which Gunn oscillations occur. The use of a graded doping profile in the collector region is, we believe, a key factor in the device design. AlGaAs/GaAs HBTEDs fabricated on semi-insulating GaAs substrates exhibit free-running oscillations at a frequency of around 77 GHz. The third (emitter) terminal enables this device to be injection-locked and to function as a self-oscillating mixer     

Current transport mechanism in GaInP/GaAs heterojunction bipolartransistors

GaInP/GaAs heterojunction bipolar transistors (HBTs) and both graded and abrupt AlGaAs/GaAs HBTs were fabricated. A total of 20 wafers were analyzed. Comparisons of the experimental results establish that the dominant carrier transport mechanism in GaInP/GaAs HBTs is the carrier diffusion through the base layer. This suggests that the conduction-band barrier across the GaInP/GaAs emitter-base junction is so small that the barrier spike does not affect the carrier transport. This result differs from other published results which, by studying device structures other than HBTs, determined the conduction band barrier to be as large as ~50% of the bandgap difference. The findings of the present investigation, however, agree well with another published work which also examined an HBT structure. The difference between these works is discussed     

Near-ideal I-V characteristics of GaInP/GaAsheterojunction bipolar transistors

GaInP/GaAs heterojunction bipolar transistors (HBTs) have been fabricated and these devices exhibit near-ideal I-V characteristics with very small magnitudes of the base-emitter junction space-charge recombination current. Measured current gains in both 6-μm×6-μm and 100-μm×100-μm devices remain constant for five decades of collector current and are greater than unity at ultrasmall current densities on the order of 1×10^-6 A/cm². For the 6-μm×6-μm device, the current gain reaches a high value of 190 at higher current levels. These device characteristics are also compared to published data of an abrupt AlGaAs/GaAs HBT having a base layer with similar doping level and thickness     

Microwave low-noise AlGaAs/InGaAs HBT's with p+-regrownbase contacts

This paper reports low-noise AlGaAs/InGaAs heterojunction bipolar transistors (HBT's) with p⁺-regrown base contacts. To reduce the thermal and shot noises, we have reduced R_B by using a p ⁺-regrown base contact and have reduced τ_B by using a compositionally-graded thin base layer. As a result, F_min values of 0.9, 1.1, 1.2, and 1.6 dB were obtained at 2, 6, 12, and 18 GHz, respectively. These low-noise characteristics of our HBT's show high potential for low-noise application     

An anisotype GaAs/InxGa1-xAs heterojunctionfield-effect transistor for digital logic applications

An anisotype heterojunction field-effect transistor (A-HJFET) for GaAs digital integrated circuit applications is proposed. A thin, highly doped, strained In_xGa_1-xAs (x⩽0.2) n-channel is employed for improved transconductance while a p⁺-GaAs cap is used to enhance the dynamic gate voltage range of the device. Prototype devices with 5-μm gate lengths show a maximum transconductance of 80 mS/mm at V_ds=2 V and a forward gate bias voltage of up to +2 V without significant leakage current