Current Gain Of Heterojunction Bipolar Transistors Research Articles

GaAsP/InGaP HBTs grown epitaxially on Si substrates: Effect of dislocation density on DC current gain

Heterojunction bipolar transistors (HBTs) with GaAs0.825P0.175 bases and collectors and In0.40Ga0.60P emitters were integrated monolithically onto Si substrates. The HBT structures were grown epitaxially on Si via metalorganic chemical vapor deposition, using SiGe compositionally graded buffers to accommodate the lattice mismatch while maintaining threading dislocation density at an acceptable level (∼3 × 106 cm−2). GaAs0.825P0.175 is used as an active material instead of GaAs because of its higher bandgap (increased breakdown voltage) and closer lattice constant to Si. Misfit dislocation density in the active device layers, measured by electron-beam-induced current, was reduced by making iterative changes to the epitaxial structure. This optimized process culminated in a GaAs0.825P0.175/In0.40Ga0.60P HBT grown on Si with a DC current gain of 156. By considering the various GaAsP/InGaP HBTs grown on Si substrates alongside several control devices grown on GaAs substrates, a wide range of threading dislocation densities and misfit dislocation densities in the active layers could be correlated with HBT current gain. The effect of threading dislocations on current gain was moderated by the reduction in minority carrier lifetime in the base region, in agreement with existing models for GaAs light-emitting diodes and photovoltaic cells. Current gain was shown to be extremely sensitive to misfit dislocations in the active layers of the HBT—much more sensitive than to threading dislocations. We develop a model for this relationship where increased base current is mediated by Fermi level pinning near misfit dislocations.

Novel Bipolar-Enhanced Tunneling FET With Simulated High On-Current

We propose and simulate a new device combining a tunneling field-effect transistor (TFET) with a heterojunction bipolar transistor (HBT). The carriers generated in the tunneling junction are used as base current to drive the HBT and obtain a high bipolar current. Owing to the sharp switching of the TFET and high HBT current gain, the CMOS-compatible Si/Si1-xGex device shows a subthreshold swing of <; 60 mV/ dec over seven decades of current, a high ON current, and scaling capability down to 10 nm.

Enhanced performance virtual substrate heterojunction bipolar transistor using strained-Si/SiGe emitter

We have investigated the influence of strained-Si cap layers on n–p–n heterojunction bipolar transistors (HBTs) fabricated on virtual substrates. Using an approximate theoretical model, it is found that the presence of a strained-Si/SiGe (relaxed) heterojunction barrier in the emitter can substantially improve the HBT's current gain, relaxing the need for a high Ge content in the strained base. Furthermore, two-dimensional numerical simulations of a virtual substrate HBT with a realistic geometry demonstrate that, besides the current gain enhancement, a three times improvement in ft and fmax can be realized when a strained-Si/SiGe emitter is incorporated.

Correlation between photoreflectance signal intensity and current gain of InP/InGaAs heterojunction bipolar transistor structures

InP/InGaAs heterojunction bipolar transistor (HBT) wafers grown by metal–organic vapor-phase epitaxy were characterized by photoreflectance (PR) spectroscopy. We found that the intensity of PR signals from the InP emitter and InGaAs collector layers of the HBT wafer decreases with increasing emitter growth temperature and shows a linear positive correlation with the HBT current gain. On the other hand, the intensity of PR signals from the n-InP single layers scarcely changes with increasing InP growth temperature. Similar tendencies in the PR intensity were also observed with changes in postgrowth annealing temperature. The change in the PR intensity of the emitter and collector layers is expected to reflect the crystal quality of the adjacent InGaAs:C base layer, which determines the HBT current gain. The present PR method is eminently suitable for the nondestructive diagnostics of the crystal quality of InP/InGaAs HBT wafers.

Metal-sulfur-based air-stable passivation of GaAs with very low surface-state densities

An air-stable electronic surface passivation for GaAs and other III–V compound semiconductors that employs sulfur and a suitable metal ion, e.g., Zn, and that is robust towards plasma dielectric deposition has been developed. Initial improvements in photoluminescence are twice that of S-only treatments and have been preserved for &amp;gt;11 months with SiOxNy dielectric encapsulation. Photoluminescence and x-ray photoelectron spectroscopies indicate that the passivation consists of two major components with one being stable for &amp;gt;2 years in air. This process improves heterojunction bipolar transistor current gain for both large and small area devices.

Low damage dry etching of III–V materials for heterojunction bipolar transistor applications using a chlorinated inductively coupled plasma

Etching of InGaAs and InP in inductively coupled plasma (ICP) using SiCl4 was studied for heterojunction bipolar transistor (HBT) applications. Low sample temperature was used to minimize etching isotropy and to reduce group V element desorption. The low ion energy etching process results in a damaged layer thickness of a few Å. Auger electron spectroscopy results on InP demonstrate a very thin layer of nonstoichiometric material. The nature of the etching mask impacts the surface contamination: local contamination effects due to sputtering are observed. For such low ion energy process, the sample preparation before ICP etching is shown to be very important for surface roughness, as observed by atomic force microscopy. Various preparation schemes have been investigated, before ICP etching, for reduction of the surface degradation resulting from ICP etching. It is shown that the best results in terms of roughness and etch-rate are obtained with a silicon nitride mask and surface oxidation by ultraviolet ozonization before a wet desoxidation immediately preceding the ICP etching. An ICP process was used successfully for partly etching the base mesa of HBT structures. No significant difference with wet etching was observed in terms of induced damage and HBT current gain.

Nondestructive Analysis of Current Gain of InP/InGaAs Heterojunction Bipolar Transistor Structures using Photoreflectance Spectroscopy

AbstractInP/InGaAs heterojunction bipolar transistor (HBT) wafers grown by metal-organic vapor phase epitaxy (MOVPE) have been characterized by photoreflectance (PR) spectroscopy. The intensity of PR signals from the InP emitter and InGaAs collector layers of the HBT wafers decreased with increasing emitter growth temperature and showed a linear positive correlation with the HBT current gain. On the other hand, the intensity of PR signals from the n-InP single layers scarcely changed with increasing InP growth temperature. The change in the PR intensity of the emitter and collector layers is expected to reflect the crystal quality of the adjacent InGaAs:C base layer, which determines the HBT current gain. Thus, the present PR method is eminently suitable for the nondestructive analysis of the current gain of InP/InGaAs HBT wafers.

Assessment of transport parameters for the design of high speed Si/SiGe HBTs with compositionally graded base

In heterojunction bipolar transistors current gain cut-off frequencies f t are limited by time constants of electron transport across the base and the collector space charge zone. This point of view favours the use of small widths of the base. However, such a choice would increase the base resistance which is detrimental to the maximum frequency of oscillation f max. A way out of this trade-off is to accelerate minority carriers by using a graded or step base structure. Besides their potential to keep transit times low such sophisticated HBT structures may show, in addition, an active behaviour above the cut-off frequencies due to the transit time effect which is commonly ignored in bipolar transistors. A key parameter for that is the minority diffusion constant of electrons in the SiGe base. Our experimental study on this parameter in the present work indicates a value that is above that of minority carrier transport in pure silicon.

Iodine and carbon tetrabromide use in solid source molecular beam epitaxy

We have studied CBr4 doping of GaAs and In0.53Ga0.47As up to 2×1020/cm3, of InxGa1−xAs across the composition range from x=0 to x=1, and of GaAs and In0.53Ga0.47As on the (211)A, (311)A, (511)A, (511)B, (311)B, and (211)B planes. These data show little evidence of self-compensation in C-doped GaAs, although carbon incorporation is more efficient on the Ga-rich (n11)A surfaces. In0.53Ga0.47As shows a similar dependence of carbon incorporation on surface orientation, and also exhibits self-compensation on the As-rich (n11)B surfaces and at In molar fractions x greater than about 0.7. We have made heterojunction bipolar transistors (HBTs) with abrupt emitter-base junctions and GaAs base layers doped to 1×1020/cm3 which have useful dc current gains, and have also used in situ iodine etching for regrowth over patterned subcollectors to make operating HBTs designed for reduced base-collector capacitance. Despite using CBr4 nearly exclusively as our p-type dopant in solid-source molecular beam epitaxy for more than five years and iodine for more than two years, we have observed no persistant degradation in doped or undoped material quality as measured by photoluminescence intensity, carrier mobility, or npn heterojunction bipolar transistor HBT current gain. Photoluminescence intensity and 77 K carrier mobility have gradually improved coincident with the introduction and continued use of iodine in our system, although there is no direct evidence of causality.

Combined effects of graded and setback layers on the [formula omitted] HBT current-voltage characteristics

The combined effects of graded and setback layers ( W G and W I) on the AlGaAs GaAs heterojunction bipolar transistor (HBT) d.c. performance are investigated, and an anlaytical model which can describe the behavior of such HBTs is presented. The HBT base and collector currents accounting for the variation of the conduction and valence bands due to the presence of W G and W I are also calculated. It is shown that including W G and W I actually degrades the HBT current gain at low current levels. The current gain at high current levels, on the other hand, can be enhanced if W I = 150 A ̊ and 0 ≤ W G ≤ 300 A ̊ or W I = 0 and 150 A ̊ ≤ W G ≤ 300 A ̊ are used. The model predictions compare favorably with results calculated from a numerical model.

Performance comparison of metalorganic vapor phase epitaxy grown C-doped GaAs/AlGaAs heterojunction bipolar transistor wafers between AsH 3 and trimethylarsenic for the base layer growth

C-doped GaAs layers were grown with AsH 3 and As trimethylarsenic (TMAs) for the As source by a production scale metalorganic vapor phase epitaxy machine. In both cases, highly ( p = (1−4) x 10 19 cm -3) p-type C-doped GaAs layers were grown and no significant difference in their mobilities, carbon activation ratios and electron lifetimes were observed. By means of these methods, GaAs/AlGaAs heterojunction bipolar transistor (HBT) layers were grown and HBT test devices were fabricated. Current gains up to the base layer concentration of 3 x 10 19cm -3 were almost equal for these two methods, but at a concentration of p = 4 x 10 19 cm -3, the current gain of HBT whose base layer was grown with AsH 3 decreased rapidly. On the other hand, a current gain as high as 100 was demonstrated for a HBT whose base layer was grown with TMAs and was 4 x 10 19 cm -3 in concentration.

Quasi-planar GaAs heterojunction bipolar transistor device entirely grown by chemical beam epitaxy

Quasi-planar GaAs/GaInP heterojunction bipolar transistor (HBT) structures, fabricated with selective regrowth of an improved collector contact, are reported. Such devices present a planar surface topology which should allow large scale integration. The initial growth of the new HBT structure and the selective regrowth of the collector contact are performed by chemical beam epitaxy (CBE). In the case of the high C base doping level, the high temperature regrowth process induces some degradation of the HBT current gain which is analysed in terms of a decrease in minority carrier lifetime. Despite this effect, promising microwave performances are obtained with a cut-off frequency and maximum oscillation frequency of 30 and 25 GHz, respectively.

Base-emitter leakage and recombination current in an implant isolated region of a GaAs/AlGaAs heterojunction bipolar transistor

The effects of regions made semi-insulating by ion implantation on the base current of a GaAs/AlGaAs heterojunction bipolar transistor (HBT) have been investigated. The implanted region, which overlaps both ends of the emitter and base fingers in the device structures studied, is intended to reduce parasitic capacitances. It is shown that the current between the base and emitter fingers at these implant isolation edges can be divided into two distinct components: a leakage current which is linear with bias and a recombination current which has an ideality of 2.0. Both of these parasitic current components reduce the HBT current gain, particularly at low current densities. A measurement technique was developed to separate these two currents using dc current-voltage characteristics of the device.

Effect of recombination current on current gain of HBTs

In order to predict more accurately the current gain behaviour of HBTs at low collector current region, surface recombination and interface recombination have to be considered. Recombination current has been obtained by the combination of bulk recombination in the neutral base, interface recombination at the heterointerface, space charge recombination, and surface recombination components. Results from the present model have been compared with experimental results, and the most recent analytical models for abrupt and graded heterojunction bipolar transistors (HBTs). The study shows that this model can predict the device gain more accurately.

Observation of radiative and nonradiative tunneling in GaAs/AlxGa1−xAs heterojunction bipolar transistors with compositionally graded base-emitter heterojunctions

A comparative experimental study of 77-K current gain and electroluminescent (EL) spectra in Npn GaAs/AlxGa1−xAs heterojunction bipolar transistors (HBTs) with either compositionally graded or abrupt base-emitter heterojunctions is reported. Shifting-peak spectra associated with radiative tunneling of carriers into the base-emitter heterojunction space-charge region are observed to dominate the 77-K EL spectra of HBTs with linearly graded emitters. The 77-K EL spectra of the abrupt HBTs are characterized by a peak whose energy position is invariant with respect to base-emitter bias and corresponds to recombination of electrons diffusing across the HBT base. Comparison of EL peak intensity and transistor base current indicates that the graded HBT current gain at 77 K is determined by nonradiative tunneling of electrons and holes to deep levels at the base-emitter junction. The loss of minority carriers to radiative and nonradiative tunneling current mechanisms in the graded HBT causes the 77-K current gain of the graded HBT to be significantly lower than that of the abrupt HBT.