Homojunction Bipolar Transistors Research Articles

Current Gain Enhancement for Silicon-on-Insulator Lateral Bipolar Junction Transistors Operating at Liquid-Helium Temperature

Conventional homojunction bipolar junction transistors (BJTs) are not suitable for cryogenic operation due to heavy doping-induced emitter band-gap narrowing and strong degradation in current gain (β) at low temperature. In this letter, we show that, on lateral version of the BJTs (LBJTs) fabricated on silicon-on-insulator (SOI) substrate, such β degradation can be mitigated by applying a substrate bias (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sub</sub> ), and a β over unity is achieved in a base current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> ) range over 5 orders of magnitudes at 4.2 K, with a peak β ~ 100 demonstrated. The β improvement is explained by the enhanced electron tunneling through base region as a result of base barrier lowering and thinning by a positive Vsub, which leads to dramatic increase of collector current (IC) while IB is negligibly affected.

2D Material‐Based Vertical Double Heterojunction Bipolar Transistors with High Current Amplification

AbstractThe heterojunction bipolar transistor (HBT) differs from the classical homojunction bipolar junction transistor in that each emitter‐base‐collector layer is composed of a different semiconductor material. 2D material (2DM)‐based heterojunctions have attracted attention because of their wide range of fundamental physical and electrical properties. Moreover, strain‐free heterostructures formed by van der Waals interaction allows true bandgap engineering regardless of the lattice constant mismatch. These characteristics make it possible to fabricate high‐performance heterojunction devices such as HBTs, which have been difficult to implement in conventional epitaxy. Herein, NPN double HBTs (DHBTs) are constructed from vertically stacked 2DMs (n‐MoS2/p‐WSe2/n‐MoS2) using dry transfer technique. The formation of the two P–N junctions, base‐emitter, and base‐collector junctions, in DHBTs, was experimentally observed. These NPN DHBTs composed of 2DMs showed excellent electrical characteristics with highly amplified current modulation. These results are expected to extend the application field of heterojunction electronic devices based on various 2DMs.

Apparent base resistance decomposition by means of small-signal and high-frequency noise analyses of submicron InP/InGaAs HBTs

We present an original and reliable technique to elucidate the different contributions to the apparent base resistance (RB = RBx + XRBi) of double heterojunction bipolar transistors (HBTs) designed by Alcatel-Thales III–V Lab. The extrinsic base resistance (RBx) is quantified using small-signal measurements. The base-collector junction distribution factor (X) and the intrinsic base resistance (RBi) are extracted from high-frequency noise (HFN) measurements. This method was applied to three InP/InGaAs HBTs having different emitter surfaces (SE). The correct determination of RBx, X and RBi may be a useful tool for compact and/or linear electrical modelling and may give some guidelines to designers to improve operation frequencies. Moreover, this strategy can be applied to any layout and technological variation of HBT; it can be also applied to homojunction bipolar transistors. Our results show that HFN analysis should be included to fully characterize bipolar transistors.

Optimal Doping Profiles via Geometric Programming

We first consider the problem of determining the doping profile that minimizes base transit time in a (homojunction) bipolar junction transistor. We show that this problem can be formulated as a geometric program, a special type of optimization problem that can be transformed to a convex optimization problem, and therefore solved (globally) very efficiently. We then consider several extensions to the basic problem, such as accounting for velocity saturation, and adding constraints on doping gradient, current gain, base resistance, and breakdown voltage. We show that a similar approach can be used to maximize the cutoff frequency, taking into account junction capacitances and forward transit time. Finally, we show that the method extends to the case of heterojunction bipolar junction transistors, in which the doping profile, as well as the profile of the secondary semiconductor, are to be jointly optimized.

Stability and electronic structure of ordered Si0.75Ge0.25C alloy

Calculations are performed on the stability and electronic structure of an ordered Si0.75Ge0.25C alloy within the generalized gradient approximation using the first-principles method. The alloy is stable at zero pressure and temperature, with its lattice constant 4.34 Å close to that of cubic SiC and bulk modulus 223 GPa. An analysis of the band structure and density of states shows the cubic alloy to be an indirect semiconductor with a wider band gap compared to SiC and therefore is a candidate material that can function in heterostructure applications. When combined with cubic SiC to form heterostructure bipolar transistors, an enhancement coefficient 2×104 for current gain β would be expected relative to the SiC homojunction bipolar junction transistors at room temperature.

Characterization of AlGaN∕GaNp-n diodes with selectively regrown n-AlGaN by metal-organic chemical-vapor deposition and its application to GaN-based bipolar transistors

p - n junctions formed by selective regrowth of n-(Al)GaN on p-GaN were characterized by I–V measurements and secondary-ion-mass spectrometry (SIMS). Regrown p-n diodes of high quality with low leakage currents were achieved, comparable to as-grown GaN p-n diodes, by optimizing the regrowth conditions and structures. SIMS revealed a sharp Mg profile at the regrowth interface. It confirms that these regrown structures are suitable to serve as emitter-base junctions for GaN-based bipolar transistors. This study offers us insight into analyzing n-p-n GaN homojunction bipolar transistors with a current gain of 10 and Al0.05GaN∕GaN heterojunction bipolar transistors with a current gain of 20, demonstrated using emitter regrowth technique.

Heterostructure unipolar spin transistors

We extend the analogy between charge-based bipolar semiconductor electronics and spin-based unipolar electronics by considering unipolar spin transistors with different equilibrium spin splittings in the emitter, base, and collector. The current of base majority spin electrons to the collector limits the performance of “homojunction” unipolar spin transistors, in which the emitter, base, and collector are all made from the same magnetic material. This current is very similar in origin to the current of base majority carriers to the emitter in homojunction bipolar junction transistors. The current in bipolar junction transistors can be reduced or nearly eliminated through the use of a wide band-gap emitter. We find that the choice of a collector material with a larger equilibrium spin splitting than the base will similarly improve the device performance of a unipolar spin transistor. We also find that a graded variation in the base spin splitting introduces an effective drift field that accelerates minority carriers through the base towards the collector.

TEMPERATURE DEPENDENT I-V CHARACTERISTICS OF AlGaN/GaN HBTS AND GaN BJTS

DC I-V characteristics of AlGaN/GaN heterojunction bipolar transistors (HBTs) and GaN homojunction bipolar transistors (BJTs) are analyzed in the temperature range of 200-450 K. At low current levels, the adverse effects of poor ohmic contacts coupled with paths of high leakage make it difficult to extract intrinsic device operation ["Explanation of anomalous current gain observed in GaN based bipolar transistors", Xing et al. IEEE Elect. Dev. Lett. 24(1) 2003:p.4-6]. At intermediate current levels, owing to enhanced ionization of Mg in the base, the HBTs show an increase in current gain resulting from mitigated current crowding, and the BJTs show a decrease in current gain resulting from reduction of emitter injection coefficient. The offset voltage dependence on temperature is also explained.

InGaP/GaAs/InGaP composite collector double heterojunction bipolar transistor with high breakdown, low offset, and knee voltage

A InGaP/GaAs/InGaP composite collector double heterojunction bipolar transistor (DHBT) has been designed and fabricated to combine the advantages of single heterojunction bipolar transistor (SHBT) and conventional DHBT. The device has low collector–emitter offset voltage (∼60 mV) and knee voltage (&amp;lt;1 V). The breakdown voltage of ∼14 V is as high as that of a conventional DHBT. Comparison was made with a GaAs homojunction bipolar transistor and InGaP/GaAs SHBT. Both homojunction transistor and composite collector DHBT show similar collector–emitter offset voltage dominated by the geometrical factor. The knee voltage of the composite collector DHBT is ∼0.15 V lower than that of the InGaP/GaAs SHBT. The results demonstrate the potential of composite collector DHBTs for practical power amplifier applications.

Superfast high-current switching of GaAs avalanche transistor

A GaAs homojunction bipolar transistor has been developed and tested in high-current avalanche mode. The voltage across the transistor dropped from ∼300 to ∼60 V during a transient time of ∼200 ps. Current pulses of amplitude 120 A were measured across the low-ohmic load and the current risetime of ∼2 ns was limited by the parasitic inductance of the circuit. A number of switching channels of ∼10 µm in diameter were directly observed in the experiment. The switching time is shorter by a factor of ∼15 than that achievable with Si avalanche transistors.

High-frequency heterojunction bipolar transistor device design and technology

This paper identifies and reviews those aspects of new materials and device technological advances that have pushed HBT circuits towards a 100 GHz operating frequency. The operating principles of the HBT are initially discussed in relation to their differences from homojunction bipolar transistors. The advantages and disadvantages of the various materials systems available to HBTs, how the particular material properties relate to the device performance and a brief outline of growth technologies are then presented. Those device parameters contributing to the frequency performance figures-of-merit are identified and the resulting design approaches discussed. Current device fabrication technology is then reviewed, with the latest results and the most important design aspects for high-frequency operation identified. This is then followed by examples of achievements in both digital and analogue circuit applications. Finally, an attempt is made to identify those device and materials aspects that are likely to contribute to a further improvement in the frequency performance of HBTs.

A silicon heterojunction bipolar transistor with an amorphous silicon emitter and a crystalline silicon emitter region

In this paper, we investigate an npn silicon heterojunction bipolar transistor (HBT) with an n-doped single crystalline emitter region between the p-doped base and the n-doped heteroemitter. In a previous paper [Garner DM, Amaratunga GAJ. A study of frequency response in silicon heterojunction bipolar transistors with amorphous silicon emitters. IEEE Transactions on Electron Devices 1996;43(11):1890–1899] we have demonstrated how a high heteroemitter material mobility (of the order of 100 cm 2 V −1 s −1) is required to make a traditional silicon HBT with performance better than an equivalent homojunction bipolar transistor (BJT). Here, with emphasis on amorphous silicon as the heteroemitter, we show how the insertion of a crystalline silicon region between the base and the heteroemitter can increase the cutoff frequency from 2.6 GHz for an ordinary silicon HBT with an a-Si:H emitter, to 6.4 GHz for the new structure and, more importantly, how a heteroemitter material with mobility of only 20 cm 2 V −1 s −1 is required with the new structure for it to outperform a comparable silicon BJT.

Physics-based minority charge and transit time modeling for bipolar transistors

It has been well known for many years that the transit time model used in the SPICE Gummel-Poon model (SGPM) is not adequate for reliable design of circuits operating either at high current densities (including quasi-saturation), which is often the case in high-speed integrated circuits, or at low voltages, which is important for low-power applications. In addition, extraction of the SGPM's transit time model parameters is often very difficult and time consuming. Although various proposals for modeling the transit time were published in the past, most of them are not suited for compact transistor models required in circuit simulation from a numerical, parameter extraction and lateral scaling point of view. In this paper, a set of minority charge and transit time equations is derived which are physics-based and laterally scaleable as well as suitable for incorporation into compact models. Experimental results of the new model are presented in terms of transit time and transit frequency versus bias (I/sub C/, V/sub CE/), geometry, and temperature, showing excellent agreement for different types of silicon homojunction bipolar transistors.

Quasiballistic transport in GaAs-based heterojunction and homojunction bipolar transistors

An iterative approach to solving an implicit integral relation for the electron distribution function in the base of a bipolar transistor is exploited to achieve a solution to the field-free Boltzmann transport equation. The method, which is based on one previously applied to Si homojunction transistors, is extended here to hetero- and homojunction transistors in the GaAs material system. This involves incorporating tunneling and reflection into the boundary condition for the injected flux at the emitter end of the quasineutral base, and considering anisotropic and inelastic scattering mechanisms. The ballistic, scattered, and reflected portions of the distribution are examined as the base width is reduced to values where quasiballistic transport is evident. Numerical results are presented for the carrier concentration and velocity profiles, and for the base transit time.

SMALL-SIGNAL ANALYSIS BY MEANS OF TRANSIENT EXCITATIONS AND ITS APPLICATIONS IN BIPOLAR TRANSISTOR MODELING

We investigate the linear response of the terminal currents of a semiconductor device to external voltage perturbations by means of a two-dimensional device-simulation code. The calculation is based on the solution of the time-dependent drift-diffusion equations followed by Fourier decomposition of the voltages and currents. Our approach is especially suited for small-signal analysis at high frequencies since, in contrast to iterative methods, convergence problems do not appear. The memory capacity of the transient excitation method hardly exceeds that of the static problem. We apply our method to Si homojunction bipolar transistors and Si/Si 1–x Ge x/Si heterojunction bipolar transistors.

Non-ideal current - voltage characteristics in MBE-grown heterojunction bipolar transistors

The current - voltage characteristics of heterojunction bipolar transistors and Si homojunction bipolar transistors grown by MBE have been measured with the aim of elucidating the causes of degraded common-emitter gain and low . It is shown that high reverse diode leakage currents arising from Zener tunnelling contribute to low in all structures considered. In forward bias, recombination currents in the space charge layers are the dominant cause of excess base current at temperatures above . Below this temperature a defect-assisted tunnelling mechanism gives rise to diode non-ideality.

Measurement and comparison of 1/f noise and g-r noise in silicon homojunction and III-V heterojunction bipolar transistors

This paper experimentally determines and compares the 1/f noise and the g-r noise, as components of the base noise current spectral density, in Si homojunction and III-V heterojunction bipolar transistors (HBTs) in common-emitter configuration. The noise spectra for each of these devices are obtained as functions of the base bias current (I/sub B/), and the 1/f noise has been found to depend on I/sub B/ as I/sub B//sup /spl gamma//, where /spl gamma//spl sim/1.8 for the silicon BJT's and InP/InGaAs HBT's with high current gains (/spl beta//spl sim/50), and /spl gamma//spl sim/1.1 for the AlGaAs/GaAs HBTs with low current gains (4</spl beta/<12). The nearly constant current gain and the near square-law and inverse-square emitter area dependence of 1/f noise in silicon devices are indicative of the dominant base bulk recombination nature of this noise. The 1/f noise in the InP based HBTs has been found to be lowest among all the devices we have tested, and its origin is suggested to be the base bulk recombination as in the Si devices. For the AlGaAs/GaAs HBTs, the low current gain and the near unity value of /spl gamma/, arise most likely due to the combined effects of surface, bulk, and depletion region recombinations and the base-to-emitter injection. The dependence of the 1/f noise on the base current density in the devices tested in this work, and those tested by others are compared to find out which HBT's have achieved the lowest level of 1/f noise.

A study of frequency response in silicon heterojunction bipolar transistors with amorphous silicon emitters

A detailed physical model of amorphous silicon (a-Si:H) is incorporated into a two-dimensional device simulator to examine the frequency response limits of silicon heterojunction bipolar transistors (HBT's) with a-Si:H emitters. The cutoff frequency is severely limited by the transit time in the emitter space charge region, due to the low electron drift mobility in a-Si:H, to 98 MHz which compares poorly with the 37 GHz obtained for a silicon homojunction bipolar transistor with the same device structure. The effects of the amorphous heteroemitter material parameters (doping, electron drift mobility, defect density and interface state density) on frequency response are then examined to find the requirements for an amorphous heteroemitter material such that the HBT has better frequency response than the equivalent homojunction bipolar transistor, We find that an electron drift mobility of at least 100 cm/sup 2/ V/sup -1/ s/sup -1/ is required in the amorphous heteroemitter and at a heteroemitter drift mobility of 350 cm/sup 2/ V/sup -1/ s/sup -1/ and heteroemitter doping of 5/spl times/10/sup 17/ cm/sup -3/, a maximum cutoff frequency of 52 GHz can be expected.

An analytical expression for base transit time in an exponentially doped base bipolar transistor

An analytical expression of base transit time for exponentially doped base, which is applicable to both homojunction and heterojunction bipolar transistors, is developed. The present treatment includes dopant dependent mobility variation, bandgap narrowing and finite velocity saturation effects in the calculation of base transit time and thus is a generalization of the results reported recently by Rosenfield and Alterovitz[6]. These effects degrade base transit time significantly and must be incorporated in the calculation. the finite velocity saturation effect will progressively play a pivotal role as the base width is scaled down and for double heterostructure bipolar transistor. The transition of base transit time from the ballistic and the drift-diffusion limit will occur at a longer base width if the base is exponentially doped with a sufficiently high doping index.

Improved switch time of I 2L at low power consumption by using a SiGe heterojunction bipolar transistor

The total switch time of bipolar integrated injected logic (I 2L) is simulated. The simulations are carried out at three different current levels: 0.1, 1, and 10 μA. For each current the Ge fraction in the base is varied from 0 to 20%. It is shown that the introduction of Ge in the base makes it possible to reduce the amount of current needed to switch the transistor at a desired speed. At least 90% of the original current can be saved by using a well-designed SiGe heterojunction bipolar transistor instead of a likewise well-designed homojunction bipolar Si transistor. The necessity to reduce the amount of current used per computation is one of the most important problems that needs to be solved before evolution towards higher computational speed and the development of hand-held equipment can progress. Therefore, the possibility to save 90% of the current makes SiGe HBT an interesting device for future digital bipolar technology.