Base-collector Heterojunction Research Articles

Application of δ-doped wide-gap collector structure for high-breakdown and low-offset voltage transistors

An In0.5Ga0.5P/GaAs double heterojunction bipolar transistor with a δ-doped wide-gap collector structure has been fabricated and studied. Experimental results show that this device exhibits the advantages of a small offset voltage of 50 mV, a small saturation voltage of 1 V, and a large breakdown voltage of 20 V with a current gain of 20. These good characteristics are mainly due to the complete elimination of potential spike at emitter–base and base–collector heterojunctions. Consequently, the studied device shows a good promise for high-speed, high-power, lower-power consumption and large input signal circuit applications.

Investigation of InGaP/GaAs double-delta-doped heterojunction bipolar transistor

The double-delta-doped heterojunction bipolar transistor is successfully fabricated with improved current-voltage characteristics by employing the insertion of delta-doped sheets at emitter-base (E-B) and base-collector (B-C) heterojunction. Because of the use of delta-doped sheets, the potential spikes at E-B and B-C heterojunction are suppressed substantially. Thus a higher emitter injection efficiency (current gain) and a lower knee voltage are obtained. From experimental results, it is shown that the studied device is a good candidate for high-speed and high-power circuit applications.

Base Transit Time in Abrupt GaN/InGaN/GaN and AlGaN/GaN/AlGaN HBTs

AbstractBase transit time, τb, in abrupt npn GaN/InGaN/GaN and AlGaN/GaN/AlGaN double heterojunction bipolar transistors (DHBTs) is reported. Base transit time strongly depends not only on the quasi-neutral base width, but also on the low field electron mobility, μn, in the neutral base region and the effective electron velocity, Sc, at the edge of base-collector heterojunction. μn and Sc are temperature-dependent parameters. A unity gain cut-off frequency of 10.6 GHz is obtained in AlGaN/GaN/AlGaN DHBTs and 19.1 GHz in GaN/InGaN/GaN DHBTs for a neutral base width of 0.05um. It is also shown that non-stationary transport is not required to study τb for neutral base width in the range of 0.05um for GaN-based HBTs.

Base-collector leakage currents in InP/InGaAs double heterojunction bipolar transistors

For InP/InGaAs HBT's, base-collector leakage current can restrict their operation to a narrow emitter-collector voltage range. We studied several factors that can degrade the leakage current: the base-collector junction design, the base mesa etching technique, and the base metallization process. A step-graded base-collector heterojunction offered the best results. A leakage free multiple step etching process, combining wet and dry etching, has been developed. Ti/Pt/Au is a suitable base metallization, provided that the platinum layer is not too thick, and that the contact annealing temperature is not too high. Finally, very low leakage current HBT's were fabricated.

An exact current partitioning and its effect on base transit time in double heterojunction bipolar transistors

The current transport across the abrupt base-collector heterojunction of double heterojunction bipolar transistors (DHBTs) has been studied by partitioning the total collector current into thermionic and tunneling components within a unified formulation. An exact quantum mechanical calculation, including image force lowering of the potential energy barrier, is performed in calculating the total current. A modified junction velocity is calculated and compared with previously reported results. It is shown that the estimation of junction velocity using the previously reported simplified calculation severely underestimated tunneling, especially at low reverse bias. The modified junction velocity is used to study the base transit time of DHBTs.

High-gain, high-speed InP/InGaAs double-heterojunction bipolar transistors with a step-graded base-collector heterojunction

We show that by using InP for the emitter and collector layers, and a thin high-doped base layer, it is possible to achieve both a high DC current gain and a high maximum frequency of oscillation. We have fabricated InP/InGaAs double heterojunction bipolar transistors (DHBT's) with cutoff frequencies f/sub T/ and f/sub max/ of 92 and 95 GHz, respectively, with a DC current gain of over 100. The maximum cutoff frequencies were 107 and 104 GHz. >

Relation between the collector current and the two-dimensional electron gas stored in the base-collector heterojunction notch of InAlAs/InGaAs/InAlGaAs DHBTs

From the Gummel plots of InAlAs/InGaAs/In 0.52Al x Ga 0.48− x As double heterojunction bipolar transistors (DHBTs), the relationship between the collector current and the density of the two-dimensional electron gas (2DEG) stored in the base-collector ( B- C) heterojunction notch is found to be an exponential function which stems from the thermionic emission escaping process of the 2DEG.

Early voltage in double heterojunction bipolar transistors

The Early voltage for abrupt double heterojunction bipolar transistors (DHBTs) has been calculated by using an effective junction velocity (S/sub c/) at the base-collector heterojunction. S/sub c/ is obtained by self-consistently partitioning thermionic and quantum mechanical tunneling currents. Unlike single heterojunction bipolar transistors (SHBTs), the Early voltage varies very rapidly at low reverse bias and approaches the SHBT-limit at sufficiently high reverse bias. This is attributed to the presence of an energy barrier at the b-c heterojunction.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of two-dimensional electron gas on the d.c. characteristics of InAlAs/InGaAs double heterojunction bipolar transistors

In this study, the characteristics of InAlAs/InGaAs double heterojunction bipolar transistors (DHBT's) with abrupt and graded base-collector junction structures are investigated. We find that because of the large conduction band discontinuity between the InGaAs base and the InAlAs collector, the base-collector abrupt heterojunction notch will accumulate a two-dimensional electron gas (2DEG) and the heterojunction spike will be pushed up. Therefore, poor knee-shape and reach-through effects appear in the device even with a 500 Å-thick undoped InGaAs layer sandwiched between the base and collector. This finding has never been observed in DHBT's with small band discontinuity material systems such as InP/InGaAs and AlGaAs/GaAs. The InAlAs/InGaAs DHBT with a graded base-collector junction shows no knee-shape and reach-through effects, because the heterojunction notch has been smoothed out.

Effects of displaced p-n junction of heterojunction bipolar transistors

Two-dimensional simulations that demonstrate the effects of displacements of the p-n junctions from the heterojunctions of symmetrical Al/sub 0.28/Ga/sub 0.72//GaAs double-heterojunction bipolar transistors (DHBTs) are reported. When the emitter and/or collector p-n junctions do not coincide with the AlGaAs/GaAs heterojunctions, the electrical characteristics are shown to be drastically altered due to changes in the potential profiles and to changes in recombination rates both in the neutral base and in the space-charge region of the emitter. The effects of a small displacement of the p-n junction from the emitter-base or the base-collector heterojunctions are examined and results for current gain beta and cutoff frequency f/sub T/ are given that demonstrate enhanced performance for DHBTs with p-n junctions that are not coincident with the heterojunctions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quantum Mechanical Transmission and Reflection of Quasi-Particles at Arbitrary Potential Barriers in the Superconducting Base Transistor

We propose a systematic method for calculating the quasi-particle transmission, T, and reflection, R, probabilities at the arbitrary potential barrier, by using multistep potential approximation. T and R were calculated from first principles using the Bogoliubov equations of motion. For the N/I/S emitter-base tunneling junction and base-collector heterojunction in the superconducting base transistor, T and R were computed as functions of quasi-particle energy, potential barrier height and width. In the case of the YBCO/n-InGaAs base-collector heterojunction with a Schottky barrier height of 70 meV, it was found that T was greater than 0.5. This suggests that a combination of YBCO and n-InGaAs is suitable for use in the base-collector heterojunction of a superconducting base transistor. We also show I-V curves for the N/I/S tunneling junction.

The base-collector heterojunction effect in SiGe-base bipolar transistors

This paper discusses a newly found advantage of Si/SiGe/Si heterostructure bipolar transistors (SiGe-base HBTs). We studied the base-collector heterojunction effect of the SiGe-base HBT using two-dimensional device simulation and circuit simulation, and found the following useful characteristics: First, the base-collector heterojunction has a two-order smaller carrier storage and a 0.3 V lower forward voltage drop, as compared with a silicon homojunction, so a high-speed and low-loss diode can be obtained. Second, the SiGe-base HBT shows little carrier accumulation even when operated in the saturation region, so it is expected to operate at high speed even in saturated mode switching. The SiGe-base HBT will provide highly efficient device elements for LSI circuits.

Effect of junction and band-gap grading on the electrical performance of molecular beam epitaxial grown AlGaAs/GaAs/AlGaAs double-heterojunction bipolar transistors

Current-voltage characteristics of AlGaAs/GaAs/AlGaAs double-heterojunction bipolar transistors (DHBTs) grown by molecular beam epitaxy have been studied. DHBTs with both abrupt and graded heterojunctions at the emitter-base and base-collector heterojunctions have been employed for the study. It is noted that DHBTs with abrupt heterojunctions are better than those with graded heterojunctions. These abrupt heterojunctions, along with a nonuniform doping in the base, lead to significantly improved results in current gain and offset voltage. Physical reasons underlying the improvement in the current gain and offset voltage have been analyzed.

AlGaAs/Ge/GaAs heterojunction bipolar transistors grown by molecular beam epitaxy

An N-Al/sub 0.22/Ga/sub 0.78/As 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/cm/sup 2/ and a collector voltage of 4 V. As the device area is reduced from 50*50 to 10*40 mu m, the current gain did not show significant changes, suggesting a low surface recombination velocity in the Ge base.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analytical base-collector heterojunction capacitance model including collector current effects

The base-collector heterojunction space-charge-region capacitance of double heterojunction bipolar transistors is treated. This capacitance cannot be modeled as the conventional heterojunction depletion capacitance because it is heavily influenced by the collector current, which is neglected in the depletion capacitance model. The analysis includes heterojunction effects as well as high current phenomena such as base pushout and space-charge-limited flow. Large discrepancies are found when comparing the present model with the depletion model at high current densities. Comparison of the base-collector junction capacitances calculated from a single heterojunction bipolar transistor and a double heterojunction bipolar transistor is also included.

Monte-Carlo study of the double heterojunction bipolar transistor

A study of HJBT's by using Monte-Carlo simulation will be presented. The main results are: a thermionic injection, A quasiballistic transport in the base and an improvement of the collection when the spike at the base-collector heterojunction is erased by a reverse bias voltage. An illustration of these results will be presented in comparison with the experimental characteristics of InGaAlAs/InGaAs/InGaAlAs HJBT's by the way of an analytical model deduced from the Monte-Carlo results.

The effect of the base—collector potential spike on the common-emitter I—V characteristics of AlGaAs double-heterojunction bipolar transistors

Knee-shaped common-emitter current-voltage characteristics are always observed on AlGaAs double-heterojunction bipolar transistors and phototransistors when a potential spike is present at the base-collector heterojunction and the spike height is bias-dependent. This phenomenon is explained successfully using a new transport theory that is based on the current-balancing concept. In addition, many important heterojunction parameters, such as the potential spike height and the p-n junction position with respect to the metallurgical junction, can be extracted. These parameters provide a valuable means of evaluating the initial growth condition of the heterojunction during the epitaxial process. On the other hand, the transport mechanism of nonequilibrium electrons injected over the heterojunction is modeled using a simplified random-walk process. Its effect on the common-emitter current-voltage characteristics is presented.

Transport theory of the double heterojunction bipolar transistor based on current balancing concept

The effect of the two-interface conduction-band spikes of a double heterojunction bipolar transistor on the current transport was theoretically studied. The model was based on the current-balancing concept at both the emitter-base and base-collector heterojunctions. The analytical expressions for the current-voltage characteristics and the offset voltage are derived and relate to material parameters. It is found that the back-and-forth bouncing of carriers between two potential spikes and the associated recombination loss represented by an important parameter ΔE0 play an important role in determining the transistor characteristics. The theory also reveals that in order to eliminate the offset voltage of a double heterojunction bipolar transistor, the emitter and the collector must have the same area, the electron injection efficiency at the base-collector junction must be made close to unity, and the potential spike energy ΔE1 must be smaller than ΔE0.

A gallium arsenide ballistic transistor?

The authors review the work done in 1985 at the IBM Thomas J. Watson Research Center on measuring the ballistic transport of electrons in a gallium arsenide heterostructure hot-electron transistor. The device is referred to as the tunneling hot-electron transfer amplifier (Theta). The way in which the transistor functions as its own spectrometer is explained. The experimental results obtained with Theta are summarized. It was found that when no voltage is applied across the base-collector heterojunction, essentially only ballistic electrons are collected. These account for about 40% of the total number of electrons injected from the emitter at low temperatures. As the voltage across the collector is increased positively, up to 75% of them are collected.

Electrical behavior of double heterojunction NpNGaAlAs/ GaAs/ GaAlAs bipolr transistors

This paper deals with a theoretical and experimental study of double heterojunction NpN GaAlAs/GaAs/GaAlAs bipolar transistors fabricated by liquid phase epitaxy. Starting from the general transport equations of the macroscopic theory of diffusion, a charge-control-type model is first established. The compact analytical expressions which determine the transfer characteristics ( J C′ V BE) and intrinsic current gain h FE ∗ are expressed as a function of the technological or geometrical parameters of the structure. A study of the sensitivity to these parameters highlights in particular the influence of the zone including the conduction band spike of the base collector heterojunction. Such an influence may assume the form of a substantial degradation of the device's electrical characteristics (reduction of the base transfer factor, increase in transit time). The experimental behavior observed on characterized devices confirms the theoretical predictions.