Conventional Heterojunction Bipolar Transistor Research Articles

Design and performance of tunnel collector HBTs for microwave power amplifiers

AlGaAs/GaAs/GaAs and GaInP/GaAs/GaAs n-p-n heterojunction bipolar transistors (HBTs) are now in widespread use in microwave power amplifiers. In this paper, improved HBT structures are presented to address issues currently problematic for these devices: high offset and knee voltages and saturation charge storage. Reduced HBT offset and knee voltages (V/sub CE,os/ and V/sub k/) are important to improve the power amplifier efficiency. Reduced saturation charge storage is desirable to increase gain under conditions when the transistor saturates (such as in over-driven Class AB amplifiers and switching mode amplifiers). It is shown in this paper that HBT structures using a 100-/spl Aring/-thick layer of GaInP between the GaAs base, and collector layers are effective in reducing V/sub CE,os/ to 30 mV and V/sub k/ measured at a collector current density of 2/spl times/10/sup 4/ A/cm/sup 2/ to 0.3 V (while for conventional HBTs V/sub CE,os/=0.2 V and V/sub k/=0.5 V are typical). A five-fold reduction in saturation charge storage is simultaneously obtained.

Heterojunction bipolar transistors implemented with GaInNAs materials

Use of GaInNAs in the base of heterojunction bipolar transistors (HBTs) on GaAs substrates allows a reduction of the turn-on voltage, Vbe,on, of the devices, facilitating their use in applications with low power supply voltage (particularly battery operated power amplifiers for mobile communications). Using GaInNAs with N content below 2% and In content of 1–20%, HBTs have been demonstrated with Vbe,on values lower by 25–400 mV than those of conventional GaAs-based HBTs. The GaInNAs base regions exhibit lower diffusion length than conventional GaAs bases, which reduces current gain and detracts from high-frequency performance, as well as higher base sheet resistance. These adverse effects can be mitigated by proper design tradeoffs of base thickness and nitrogen composition, as well as by compositional grading in the base to provide a built-in quasi-electric field to assist electron transport.

Theoretical investigation of an InGaP/GaAs heterostructure-emitter bipolar transistor with a wide-gap collector

We theoretically analyse the performance of an InGaP/GaAs heterostructure-emitter bipolar transistor (HEBT) with a wide-gap collector. Using an exact simulation, we report on detailed calculations and studies including majority-carrier and minority-carrier profiles, recombination-rate distributions, and dc and ac performances. By introducing an undoped GaAs spacer and a heavily doped transition layer into the conventional base–collector heterojunctions, the field across the inserted layers is high enough to pull down the potential spike drastically; thus the knee-shaped characteristics and reach-through effect are not observed. Moreover, the base–emitter (B–E) structure, which we have used, contains an effective p–n homojunction and a hetero-confinement layer to substantially reduce the potential spike at the B–E junctions. Therefore the studied device has exhibited a relatively smaller offset voltage and higher current gain than those of a conventional heterojunction bipolar transistor. The simulated results particularly reveal that a proposed HEBT with an appropriately designed device structure exhibits higher efficiency, lower offset voltage (≤30 mV), lower saturation voltage (≤0.5 V), uniform current gain (∼25), lower operated voltage, higher breakdown voltage and improved characteristics as compared to those obtained from a conventional double heterojunction bipolar transistor structure.

Performance of New Self-Aligned InP/InGaAs Heterojunction Bipolar Transistors Using Crystallographically Defined Emitter Contact Technology

InP/InGaAs heterojunction bipolar transistors (HBTs), using a new crystallographically defined emitter contact technology for a self-aligned base-to-emitter structure, are fabricated and their performances are investigated. In this technology, a new self-alignment process has been developed based on the crystallographic wet etching characteristics of an InP dummy emitter layer, which is grown on the conventional HBT layer structure. The shape of the emitter contact, which is determined by the crystallographically etched sidewall profile of the InP dummy emitter layer, is used to obtain the desired contact spacing between the emitter mesa and the base contact by controlling only the thickness of the InP dummy emitter layer. The fabricated HBTs show good overall device performance at high frequencies with a current gain cutoff frequency fT of 94 GHz and a maximum oscillation frequency fmax of 124 GHz. The microwave power performance of the device was also measured and characterized.

Study of the multiple-negative-differential-resistance (MNDR) switching behaviors based on heterojunction bipolar transistor (HBT) structures

In this paper, we demonstrate multiple-negative-differential-resistance (MNDR) switching behaviors based on the InGaP/GaAs heterostructure-emitter bipolar transistor (HEBT) and InGaAlAs/InP heterojunction bipolar transistor (HBT) structures. The devices act like conventional HBTs under forward operation mode. The proposed HEBTs show lower offset voltage due to the correct design of the emitter thickness. On the other hand, MNDR phenomena resulting from avalanche multiplication, confinement effects and the potential redistribution process are observed under inverted operation mode for both devices. In addition, three-terminal NDR characteristics are investigated under the applied base currentIB . Moreover, for the InGaAlAs/InP HBT, anomalous multiple-route and multiple-step current–voltage (I–V) characteristics at 77 K are observed due to the insertion of a InGaAs quantum well (QW) between the base and collector layers.

High-speed AlGaAs/GaAs HBTs with reduced base-collector capacitance

A new layout for high-speed AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is presented. The layout is horseshoe shaped and designed to simultaneously reduce base resistance (RB) and base-collector capacitance (CBC). A horseshoe-shaped HBT and a conventional single-finger HBT with the same emitter width of 2 µm were fabricated and tested. The reduction of RB and CBC using the horseshoe-shaped HBT resulted in a 25% improvement of the maximum oscillation frequency (fmax = 130 GHz).

Temperature-dependent study of a lattice-matched InP/InGaAlAs heterojunction bipolar transistor

In this work, we report the temperature-dependent characteristics of a new InP/InGaAlAs heterojunction bipolar transistor (HBT). In order to improve the dc performance of conventional InGaAs-based single HBTs, the quaternary In/sub 0.53/Ga/sub 0.34/Al/sub 0.13/As with a wider bandgap is employed as the material for both the base and collector layers. Experimentally, the studied device exhibits a relatively high common-emitter breakdown voltage and low output conductance even at high temperature. Based on the breakdown mechanism of avalanche multiplication, the negative temperature dependence of breakdown voltage is attributed to the positive temperature-dependent impact ionization coefficient. Furthermore, the temperature dependence of current gain is investigated and reported. It is believed that the suppression of hole injection current with decreasing temperature is responsible for the opposite variation of current gains at high current levels.

ULTRAHIGH fmax AlInAs/GaInAs TRANSFERRED-SUBSTRATE HETEROJUNCTION BIPOLAR TRANSISTORS FOR INTEGRATED CIRCUITS APPLICATIONS

Transferred-substrate heterojunction bipolar transistors (HBTs) have demonstrated very high bandwidths and are potential candidates for very high speed integrated circuit (IC) applications. The transferred-substrate process permits fabrication of narrow and aligned emitter-base and collector-base junctions, reducing the collector-base capacitance and increasing the device f max . Unlike conventional double-mesa HBTs, transferred-substrate HBTs can be scaled to submicron dimensions with a consequent increase in bandwidth. This paper introduces the concept of transferred-substrate HBTs. Fabrication process in the AlInAs/GaInAs material system is presented, followed by DC and RF performance. A demonstration IC is shown along with some integrated circuits in development.

Fabrication of heterojunction bipolar transistors with buried subcollector layers for reduction of base-collector capacitance by molecular beam epitaxy regrowth

A process for fabrication of heterojunction bipolar transistors (HBTs) with selectively buried subcollectors by molecular beam epitaxy (MBE) regrowth is described. This process can be used to reduce parasitic base-collector capacitance of HBTs and improve the speed of these devices. In situ etching by iodine prior to the regrowth was used for the first time to improve the quality of the substrate epilayer interface in a semiconductor device grown by MBE. The secondary ion mass spectroscopy depth profiles of regrown HBT structures suggest that the in situ surface cleaning by molecular iodine was not sufficient to remove all contamination from the substrate epilayer interface and that the microwave performance of HBTs fabricated by this process may have been affected by that contamination. The dc performance of devices which were fabricated by our process was not affected, however, by the contamination and was comparable to the dc performance of conventional HBTs. Our results suggest that the described process for fabrication of HBTs with selectively buried subcollectors by MBE regrowth would be feasible in conjunction with a more effective surface treatment.

Heterojunction Bipolar Transistor with an Additional Minority Carrier Reflection Barrier in the Emitter

A heterojunction bipolar transistor (HBT) structure that provides an additional minority carrier reflection/confinement barrier within the quasi-neutral emitter region has been proposed and investigated by means of numerical simulations. The proposed device structure has been demonstrated to perform better than conventional HBTs–a higher current gain with better uniformity, which has been achieved without affecting the high frequency performance. In addition, the current gain was found to be relatively insensitive to temperature changes in the range of 250 K to 450 K, thus making the new device structure more suitable for high power applications.

Fabrication and characteristics of a GaInP/GaAs heterojunction bipolar transistor using a selective buried sub-collector

A C-doped GaInP/GaAs heterojunction bipolar transistor (HBT) with a selective buried sub-collector has been fabricated by two growth steps. The active HBT region was made on the selective buried sub-collector layer with minimum overlap of the extrinsic base and the sub-collector region resulting in substantial reduction of the base-collector capacitance. The experiment shows that the base-collector capacitance is reduced to about half of that of a conventional HBT while the base resistance remains unchanged resulting in a 40-50% increase in the maximum oscillation frequency. Both DC and RF characteristics are investigated and compared with a conventional HBT. A current gain of 40, cutoff frequency of 50 GHz and maximum oscillation frequency of 140 GHz were obtained for the GaInP/GaAs HBT. It is demonstrated that the selective buried sub-collector provides an effective means for enhancing RF performance of an HBT.

Thermal characterization of thermally-shunted heterojunction bipolar transistors

Heterojunction Bipolar Transistors (HBTs) are potentially useful in a number of microwave applications, but they are severely limited by a current distribution instability caused by electrothermal interaction and the use of a low thermal conductivity substrate. A novel thermal management technique called "thermal shunting" has been developed to reduce thermal resistance and junction temperature non-uniformity. Thermal resistance measurements for thermally-shunted devices are presented. Specific thermal resistance measurements as low as 2.6×10/sup -4//spl deg/C-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /W (147/spl deg/C/W at 0.1 W for a device with a 177 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> emitter area) have been obtained. Thermal resistance values obtained for thermally-shunted HBTs are substantially lower than those reported for conventional HBTs.

Monolithic integration of AlGaAs/GaAs HBT and GaAs junction-gate floated electron channel field effect transistor using selective MOCVD growth

A novel GaAs BiFET structure based on AlGaAs/GaAs HBT and GaAs junction-gate floated electron channel field effect transistor (J-FECFET) has been developed. Selective metalorganic chemical vapor deposition (MOCVD) growth is extensively used for the BiFET. Structural advantage of the BiFET is that the epitaxial layers of the J-FECFET are identical to the lower part of a conventional heterojunction bipolar transistor (HBT). Transconductance of the fabricated J-FECFET with 1×200 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> gate is 102 mS/mm with f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> of 10.7 GHz and 27.3 GHz, respectively. DC current gain of HBT is 21 at a collector current density of 50 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with emitter area of 3×2 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The new integration technology offer a foundation for development of various multifunction monolithic microwave integrated circuits (MMIC's).

Electrical properties of a heterojunction bipolar transistor structure with a resistive gate

A conventional heterojunction bipolar transistor with a resistive gate, formed by placing the third location in the depletion region of the collector layer instead of the base layer, is investigated. The influences of the gate voltage on the device performance then strongly depend on the applied voltage. The results show that the device operated under voltage-controlled mode exhibits novel N-shaped negative differential resistance (NDR) characteristics, which have large room temperature NDR peak-to-valley current ratios (e.g. 140 at a gate bias of 1.5 V) and large voltage extension in N-shaped NDR region (about 9 V). However, the device exhibits the transistor characteristics with the current gain of 4 under current-injected mode. The above discrepancies between voltage-controlled and current-injected modes are attributed to the existence of the resistive gate.

RECENT DEVELOPMENTS IN GaInP/GaAs HETEROJUNCTION BIPOLAR TRANSISTORS

We review the recent developments of GaInP/GaAs heterojunction bipolar transistors (HBTs) used for microwave power, low noise, linear amplification and high temperature applications. Special attention is paid in the comparison of the GaInP HBT results with the more conventional AlGaAs HBTs. The material and processing advantages, as well as the electronic and thermal properties of GaInP are described.

Self-aligned InAlAs/InGaAs heterojunction bipolar transistor with a buried subcollector grown by selective epitaxy

We report the first microwave characterization of an In/sub 0.52/Al/sub 0.48/As/In/sub 0.53/Ga/sub 0.47/As heterojunction bipolar transistor (HBT) with a buried InGaAs subcollector grown by selective epitaxy. The study compares two HBT's having identical 2/spl times/10 /spl mu/m/sup 2/ self-aligned emitter fingers but different subcollectors. Improvement in microwave performance of the selectively-grown HBT over the conventional HBT was observed due to the reduced parasitic base-collector capacitance achieved by incorporating the selectively-grown buried subcollector. >

A COMPOUND EMITTER HETEROJUNCTION BIPOLAR TRANSISTOR

A compound emitter heterojunction bipolar transistor (HBT) structure that incorporates an additional heterojunction within the emitter for minority carrier confinement has been proposed. In this new device configuration, the single wide band‐gap emitter layer in a conventional HBT is replaced by two sub‐layers of wide band‐gap material, with the sub‐layer nearer the base having a narrower band‐gap. By means of numerical simulations, the compound emitter HBT was found to perform better than comparable conventional HBTs. With the AlGaAs(n) / GaAs heterostructure system, the optimum compound emitter HBT structure was found to be Al0.3Ga0.7As(n) ‐ Al0. 2Ga0.8As(n) / GaAs with grading at the two hetero‐interfaces. It has a low turn‐on voltage that is almost identical to that of a homojunction GaAs bipolar transistor with similar doping conditions. Compared with a conventional single emitter layer Al0.3Ga0.7As/GaAs HBT, the optimum compound emitter HBT has an enhancement in the current gain by approximately 2 folds, an improvement in the uniform current gain region from 2 to 4 decades of collector current density, and a slight increase in the unity‐gain cut‐off frequency fT by about 7 %.

A physics-based, analytical heterojunction bipolar transistor model, including thermal and high-current effects

A detailed, analytical model for predicting the DC and high-frequency performance of AlGaAs/GaAs graded heterojunction bipolar transistors (HBTs) is presented. The model is developed based on the relevant device physics, such as current-induced base pushout and thermal effects. The current gain, cutoff frequency, and maximum frequency versus the collector current density, which is a function of the applied voltage as well as the corresponding temperature in the HBT, are calculated. The results suggest that the conventional HBT model, which assumes the HBT temperature is the same as that of the ambient, can overestimate the three figures of merit considerably when the collector current density is high. Furthermore, it is shown that the present model correctly explains such experimentally observed HBT high-current behavior as the rapid falloff of the current gain and cutoff frequency. The model predictions compare favorably with the results obtained from a model which solves numerically the Poisson and continuity equations coupled with the lattice heat equation. >

Does velocity overshoot reduce collector delay time in AlGaAs/GaAs HBTs?

The effect of velocity overshoot on the collector delay in an AlGaAs/GaAs heterojunction bipolar transistor (HBT) is examined. A new impulse response technique to rigorously evaluate the total transit-time delay using transient Monte-Carlo simulation is introduced. By applying the technique to a conventional HBT and comparing the result with a similar calculation ignoring the effects of velocity overshoot, it is found that velocity overshoot effects are difficult to observe in the normal operating range of base-collector bias. Impulse response analysis predicts that velocity overshoot should reduce the collector delay in AlGaAs/GaAs HBTs but the effect should rapidly diminish with increasing bias. It is also shown that impulse response is sensitive to the field profile. Intentional, unintentional, or current-induced variations in the collector field profile may have substantial effects on collector delay. >

A Novel GaAs Bipolar Transistor Structure with GaInP-Hole Injection Blocking Barrier

ABSTRACTGaAs bipolar transistors of different emitter types have been fabricated from MOCVD grown lattice matched Ga0.5In0.5 P/GaAs layer structures using carbon for heavy base doping (p=2×1019 cm−3). Besides conventional heterojunction bipolar transistors we also investigated tunneling emitter bipolar transistors having 2 and 5 nm thin GalnP layers between emitter and base, which act as a hole repelling potential barrier in the valence band. Current gains up to 115 have been obtained at collector current densities of 104 A/cm2 even for this heavy base doping. All devices show an almost ideal output characteristics with large Early voltage and small offset voltage. From the temperature dependence of the collector current a small effective conduction band barrier at the heterointerface is determined which hardly affects electron injection into the base.