Common-base Current Gain Research Articles

Current transport mechanisms and their effects on the performances of InP-based double heterojunction bipolar transistors with different base structures

InP-based double heterojunction bipolar transistors (DHBTs) with different base structures were studied. The base structures are InGaAs with and without graded composition and GaAsSb. Both of the terminal currents of InP/GaAsSb/InP DHBT in forward and reverse modes are limited by the carrier transport across the base layer. This causes the offset voltage to be determined by the difference between the base-collector and base-emitter areas and by the normal common-base current gain. The emitter currents of both graded- and abrupt-base InP/InGaAs/InP DHBTs in the reverse mode are also limited by the carrier transport across the base layer, while their collector currents are limited by the band discontinuity of the base–emitter junction. The different current transport mechanisms of the terminal currents in the forward and reverse modes result in the larger offset voltage.

Bias dependence of high-frequency noise in heterojunction bipolar transistors

Hawkins' isothermal model developed to study noise in bipolar junction transistors (BJTs) is modified to investigate bias-dependent noise in heterojunction bipolar transistors (HBTs) by incorporating thermal effects. It is shown that the inclusion of thermal effects into the high-frequency noise model of HBTs is necessary as the temperature of the device may become very different from the ambient temperature, especially at high bias current. Calculation of the noise figure by including the thermal effect shows that the isothermal calculation may underestimate the noise figure at high bias current. It is observed that noise at low bias is ideality factor n dependent whereas high bias noise is insensitive to the variation of n. Moreover, the common base current gain plays a major role in the calculation of the minimum noise figure. The excellent fit obtained between the theoretical calculation and the measured data are attributed to the inclusion of the bias-dependent junction heating as well as C/sub De/ and C/sub bc/ into the present calculation.

Transistor action in GaP/TmP/GaAs heterostructure

Transistor action in a gallium phosphide/thulium phosphide/gallium arsenide (GaP/TmP/GaAs) structure with a GaP emitter, TmP base, and GaAs collector is reported. The emitter-base junction was constructed through wafer bonding and the base-collector junction was formed by epitaxial growth of TmP on GaAs in a molecular beam epitaxy system. From the I-V measurements, a common base current gain /spl alpha//spl sime/0.55 measured at V/sub CB/=0 was obtained at room temperature.

Molecular beam epitaxy growth of TmP/GaAs and transistor action in GaP/TmP/GaAs heterostructures

The growth of thulium phosphide (TmP) by molecular beam epitaxy (MBE) on GaAs substrate is reported. Good epilayer quality was demonstrated through X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM) analysis. The closely lattice matched TmP layer was n-type with an electron concentration of 1.6/spl times/10/sup 21/ cm/sup -3/ and a room temperature mobility of 4.8 cm/sup 2/V/sup -1/s/sup -1/. The Schottky barrier height determined from 1/capacitance/sup 2/ (1/C/sup 2/) versus voltage (V) measurements is about 0.81 eV which agrees well with the value obtained through the current-voltage (I-V) measurements. In this work, we also report transistor action in a GaP/TmP/GaAs structure, for which chemical bonding techniques were employed. From I-V measurements, a common base current gain /spl alpha//spl ap/0.55 at V/sub CB/=0 was obtained at room temperature.

Direct extraction and numerical simulation of the base and collector delay times in double heterojunction bipolar transistors

A new method is presented to evaluate the base and collector transit times, /spl tau//sub B/ and /spl tau//sub C/ in heterojunction bipolar transistors (BBT's) from the phase and magnitude of the common-base current gain, /spl alpha/(/spl omega/), which itself was directly extracted from measured S-parameter data. The method is applied to InGaP/GaAs single and double HBT's. A smaller cutoff frequency in the latter device is attributed to /spl tau//sub B/ and /spl tau//sub C/ due to two effects: trapping of electrons in the conduction band triangular barrier existing at the base-collector (B-C) heterojunction and smaller saturation velocity of electrons in InGaP as compared to GaAs. Finally, a new B-C design of InGaP/GaAs DNBT's is proposed to partially compensate the transit time effects. Numerical simulation of the cutoff frequency demonstrates the superiority of the proposed structure for high-frequency applications.

Intermodulation analysis of the collector-up InGaAs/InAlAs/InP HBT using Volterra series

The intermodulation (IM) distortion of the collector-up InGaAs/InAlAs/InP heterojunction bipolar transistor (HBT) is analyzed using Volterra-series theory. A T-equivalent circuit is used for this analysis. The contribution and interaction of four nonlinear elements: base-emitter resistance, base-emitter capacitance, base-collector capacitance, and common base-current gain are analyzed. For the particular device under investigation, it is found that the cancellation effect is not significant and the base-emitter resistance nonlinearity dominates the third-order IM.

Effects of quasi-ballistic base transport on the high-frequency characteristics of bipolar transistors

High-frequency transport in bipolar transistors with quasi-ballistic base widths (on the order of a minority-carrier scattering length) is examined by using the approach of Grinberg and Luryi (1992) to solve the Boltzmann transport equation (BTE). By considering the phase angle of the dynamic distribution function in wave-vector space, it is shown that the ballistic mechanism of decay in the common-base current gain becomes important even for base widths in the quasi-ballistic regime. Simple expressions, which correctly yield both the magnitude and phase of all the forward characteristics, as predicted by the BTE, up to the intrinsic transit frequency, are found by combining the results from a one-flux approach with the well-known expressions of Thomas and Moll (1958). Expressions for the reverse small-signal parameters are also found by applying a "moving boundary condition" to the basic one-flux equations of Shockley (1962).

Preparation and Characteristics of a Superconducting Base Transistor with an Au/Ba1-xKxBiO3/Niobium-Doped SrTiO3 Structure

We have observed the fundamental operation of a superconducting base transistor using a Ba1-xKxBiO3 (BKBO) oxide superconductor. The transistor had a structure consisting of planar-type Au/natural-barrier/BKBO/niobium-doped SrTiO3 (STO:Nb), where Au, BKBO and STO:Nb correspond to the emitter, base and collector in the transistor, respectively. The Au/natural-barrier/BKBO emitter-base junction showed good tunneling characteristics, reflecting the superconductivity of BKBO. The BKBO/STO:Nb base-collector junction showed asymmetric I-V characteristics. Electrical measurements during transistor operation revealed collector current increment as the emitter current increased at 4.5 K. Furthermore, a transistor with a 100-nm thick base layer showed a maximum common-base current gain of over 0.5 at 4.5 K.

The physics of the two-dimensional electron gas base vertical hot electron transistor

The authors describe a vertical hot electron transistor structure in the GaAs/AlGaAs materials system, in which the base is a two-dimensional electron gas in GaAs. They undertake initial studies of the hot electron spectroscopy at low temperature, and corroborate the high common base current gain measured to be >100 at low temperature, the best result ever achieved for a unipolar transistor. They compare these attractive results with the less satisfactory results achieved earlier with a uniformly doped (3D) GaAs base. A number of devices have been made which allow magnetotransport and Hall measurements to be performed on the base while the transistor is operating.

A full adder using resonant-tunneling hot electron transistors (RHETs)

Full adders are demonstrated using InGaAs-In(AlGa)As RHETs. The RHET's emitter and base electrodes were self-aligned using a SiO/sub 2/ sidewall and angled beam ion milling. The common-base current gain was about 0.9 and the emitter current peak-to-valley ratio was 10. The RHET full adder was constructed using a three-input exclusive-OR logic gate and a three-input majority logic gate. The authors confirmed normal operation of the full adder at 77 K. Only seven RHETs were needed for the full adder, about one-quarter of bipolar transistors that would have been required. >

Integrated Si/CoSi2/Si-heterotransistors at high current densities

We have fabricated integrated Si/CoSi 2 /Si-heterotransistors by MBE. The common-base current gain of the devices is correlated to pinhole size and density. The electrical behaviour is investigated up to an emitter current density of 6000A/cm 2 . A decrease of current gain at high current densities is observed for devices with higher pinhole density. For devices with lower pinhole density, space charge effects will lead to an increase of emitter-to-collector delay time at high current densities.

Oxygen implantation to suppress parasitic bipolar action in CMOS

Oxygen implantation and subsequent epitaxial silicon deposition have been developed to improve CMOS latchup prevention through reducing the current gains of parasitic bipolar transistors. The buried oxygen implanted layer is well confined, and defects do not extend into the epitaxial silicon layer. The device characteristics of the n- and p-MOSFETs fabricated on a wafer with the oxygen implantation are therefore not affected by the buried implanted layer. The oxygen implanted layer can reduce the minority-carrier lifetime and hence decrease the current gain of the lateral parasitic bipolar transistor. In addition, it introduces a potential barrier which decreases the current collected at the frontside contact of the vertical parasitic bipolar transistor. The common base current gain is reduced by 50% and 80% for the lateral and the vertical parasitic bipolar transistors, respectively. As a consequence, the CMOS latchup immunity is significantly improved.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Rotation of base region hall field in magnetotransistors

We present measurement results of Hall field rotation in the base region of npn lateral magnetotransistors operated in common base configuration at various emitter currents. The measured Hall voltages are small ( V H ∼μV) and the direction of the corresponding Hall field progressively rotates with increasing collector bias voltages, V CB . In terms of device parameters, the Hall fields were found to behave in the following manner. In the regime where the common base current gain h FB <1, the Hall field is relatively large (due to the high base current) and is mainly due to majority carriers. Beyond a certain critical V CB where h FB ⩾1, the Hall field saturates to some finite value for a given emitter current and is opposite in direction. Its Hall coefficient is opposite in sign and is due to the excess minority charge injected into the base (n ≳ p). The switch in sign happens where h FB ≲ 1 when n ≳ p (μ p */μ n *) 2 in the base region.

Enhanced ballistic transport in InGaAs/InAlAs hot-electron transistors

Hot-electron transistors fabricated using a 600-Å InGaAs base and a 500-Å InAlAs barrier have shown a peak ballistic common base current gain of 0.82 at 77 K despite the large injection energies. Reduction of injection energies by lowering the emitter and collector barriers should lead to an even higher ballistic transport ratio due to the reduced Γ-L scattering.

Effects of nonequilibrium quasiparticles on characteristics of Nb/AlOx/Nb/n-InSb triodes

A triode with a Nb/AlOx/Nb Josephson junction emitter and an n-InSb collector was fabricated and tested. The base layer was superconducting Nb film. The collector current of the triode was controlled by the injection of low-energy quasiparticles with an energy near the gap energy of the base layer. Common-base small-signal current gain was 10−3 to about 0.1 at 1.7 K. The characteristics of the triode were calculated. The analysis used a model where the collector-base junction is treated as a tunnel junction, and the quasiparticle density was calculated by using the rate equations given by Rothwarf and Taylor [Phys. Rev. Lett. 19, 27 (1967)]. The calculated characteristics agreed qualitatively with the measured results.

High-frequency characteristics of inverted-mode heterojunction bipolar transistors

It is shown that in emitter-down heterojunction bipolar transistors (HBT's), parasitics can be reduced sufficiently that intrinsic transit-time delays become the dominant limitations to high-frequency performance. In this situation it is found that the dependence of the unilateral gain on frequency can be significantly different from the simple 6-dB/octave decrease usually assumed. It is found that the device exhibits negative output conductance over certain bands of frequencies, and that when this occurs a series of resonances are observed in the gain versus frequency characteristics. Explanations of this behavior are given in terms of the phase delay of the common-base current gain. The generality and relevance of these observations to other types of transistors, and the utilization of the negative output conductance to enhance high-frequency operation are also discussed.

Induced base transistor fabricated by molecular beam epitaxy

A novel three-terminal hot-electron device, the induced base transistor (IBT), has been fabricated by molecular beam epitaxy (MBE). Two-dimensional electron gas (2-DEG) induced by the applied collector field in an undoped GaAs quantum well is used as the base of the IBT. The common-base current gain α has been achieved as high as 0.96 under a collector bias of 2.5 V and an emitter current of 3 mA.

Study of ballistic transport in Si-CoSi2-Si metal base transistors

The hot-electron transfer ratio from the Schottky emitter diode to the Schottky collector diode through the thin base layer of Si-CoSi2-Si metal base transistors has been measured in devices for which the pinhole contribution to the current gain is shown by transconductance measurements to be negligible. The common-base current gain α of the transistors, measured as a function of temperature and base thickness, exhibits an exponential dependence on CoSi2 film thickness, from which a ballistic mean free path is deduced. This value is in good agreement with the mean free path deduced from conductivity data, both at 77 and 300 K. The behavior of the pre-exponential coefficient, i.e., the gain extrapolated to zero base thickness, is not completely understood and shows evidence for hot-electron and space-charge effects in the epitaxial overgrown Si material of the emitter.

Transistor action in Si/CoSi2/Si heterostructures

We report transistor action in a Si/CoSi2/Si structure. The thin silicide layer (&amp;lt;100 Å), which acts as the base, is a single-crystal metal, essentially continuous and locally exhibiting atomically perfect interfaces with Si. The transistor action is manifested by a common base current gain α as high as 0.6 and a voltage gain greater than 10.

An induced base hot-electron transistor

A novel three-terminal device is proposed in which the base represents an undoped quantum well in a graded-gap heterostructure. The base conductivity is provided by a two-dimensional electron gas induced by the collector field. The intrinsic delay time is estimated to be about 1 ps at room temperature with a common-base current gain close to unity.