Collector Current Density Research Articles

Fully Radiative Current Path Structure (FRACS) for sub-0.1 μm emitter transistor

A new structure is proposed for bipolar transistors - FRACS (Fully Radiative Current Path Structure). A FRACS transistor has a line emitter and a cylindrical base and collector or a point emitter and a spherical base and collector. Device parameters of the FRACS transistor is obtained by extending the conventional one-dimensional transistor model to a two- or three-dimensional model. In this structure, base transit time is reduced as the emitter size is reduced by radiative collector current flow. Using this model, a general bipolar transistor with a shallow link base is found to increase the cutoff frequency as the emitter size is reduced. The Kirk effect is suppressed in this structure because of the small collector current density at the collector-base junction. The effect was experimentally examined. A cylindrical base was fabricated by thermal diffusion of boron to achieve the FRACS transistor. Cutoff frequency was observed to increase as the emitter size was reduced. Maximum cut-off frequency of 64 GHz was achieved by this transistor with a 25-nm thick base formed by rapid vapor-phase diffusion. >

Metalorganic molecular beam epitaxial growth of InP and InGaP with tertiarybutylphosphine for the application of carbon-doped base heterojunction bipolar transistors

Metalorganic molecular beam epitaxy (MOMBE) of InP and In 0.5Ga 0.5P has been studied by using the elemental group III sources (In and Ga) and tertiarybutylphosphine (TBP) as phosphorus source. The catalysis of heated tantalum (Ta) is important to effectively decompose the TBP. Undoped In 0.5Ga 0.5P with n-type background carrier concentration of n = 4 × 10 15 cm -3 was obtained, and the controllability of Si doping in In 0.5Ga 0.5P ranging from 9.7 × 10 17 to 1.3 × 10 19 cm -3 was shown by using cracked Si 2H 6. In the growth of InP with the TBP, carbon was unintentionally incorporated as donor and was well controlled in the range suitable for InP/In 0.53Ga 0.47As HBTs by the growth temperature and the flow rate of TBP. The MOMBE growth of In 0.5Ga 0.5P and InP layers was applied to fabricate carbon-doped base In 0.5Ga 0.5P/GaAs and InP/In 0.53Ga 0.47As HBTs, respectively. A common emitter current gain h fe of 26 at a collector current density J C of 1.4 kA/cm 2 was obtained for In 0.5Ga 0.5P/GaAs HBTs with a hole concentration as high as 1 × 10 20 cm -3 in the base. Carbon-doped base InP/In 0.53Ga 0.47As HBTs were also fabricated by applying the carbon-doping technique for the InP emitter and a h fe value of 16 at J C = 36 A/cm 2 was obtained for the first device.

Effect of base dopant species on heterojunction bipolar transistor reliability

The electrical activation of carbon in GaAs grown by metal-organic molecular beam epitaxy has been shown to be excellent in as-grown material. At very high doping levels, however, the activation can be reduced substantially by annealing, even at rather moderate temperatures. This reduction in hole concentration combined with the reduction in gain observed with increasing dopant level in the base suggests a practical limit of less than 10 20 cm −3 for base doping in GaAs/AlGaAs heterojunction bipolar transistors (HBTs). Using moderate base doping, p ≈ 7 × 10 19 cm −3 , carbon-doped devices do not exhibit short-term degradation at current densities less than or equal to 2.5 × 10 4 A cm −2 , even at operating temperatures up to 200 °C, unlike Be-doped HBTs which typically degrade much more rapidly even at twice lower base doping levels. Operation of carbon-doped devices at higher current densities for longer periods of time, however, can result in current-induced decreases in d.c. gain which are correlated with the amount of hydrogen incorporated in the base layer during growth. During device operation, minority carrier injection induced debonding of hydrogen from neutral C-H complexes leads to an increase in effective base doping level and therefore to a decrease in gain. Post-growth in situ or ex situ annealing eliminates this effect by breaking up the C-H complexes. Properly designed HBTs are stable during operation at 125 °C even for very high collector current densities (10 5 A cm −2).

Fully-planar AlGaAs/GaAs HBT's achieved by selective CBE regrowth

This paper describes a novel fully planar AlGaAs/GaAs heterojunction bipolar transistor (HBT) technology using selective chemical beam epitaxy (CBE). Planarization is achieved by a selective regrowth of the base and collector contact layers. This process allows the simultaneous metallization of the emitter, base and collector on top of the device. For the devices with an emitter-base junction area of 2/spl times/6 /spl mu/m/sup 2/ and a base-collector junction area of 14/spl times/6 /spl mu/m/sup 2/, a current gain cut off frequency of 50 GHz and a maximum oscillation frequency of 30 GHz are achieved. The common emitter current gain h/sub FE/ is 25 for a collector current density J/sub c/ of 2/spl times/10/sup 4/ A/cm/sup 2/. >

BALLISTIC COLLECTION TRANSISTORS AND THEIR APPLICATIONS

We describe the design, fabrication and application of ballistic collection transistors (BCTs) in which electron velocity overshoot is introduced in the collector of a GaAs-based heterojunction bipolar transistor. The guideline for the BCT design is the effective confinement of electrons to the Γ-valley, as simulated by Monte Carlo analysis, and the control of electron energy is accomplished basically with an i-p+-n+ doping profile. Microwave characterization demonstrates the existence of significant overshoot and cutoff frequencies higher than 100 GHz at collector current densities in the mid 104 A/cm 2 range for a typical BCT structure. Some high speed integrated circuits implemented with BCTs include a selector circuit that operates at bit rates up to 40 Gb/s, a dynamic frequency divider with divide-by-four function up to 50 GHz and a broadband preamplifier having an S21 bandwidth as high as 40 GHz.

The high-speed performance of [formula omitted] Schottky collector HBTs

We investigated the high-speed performance of Si 1−x Ge x/ CoSi 2 Schottky collector heterojunction bipolar transistors (SCHBTs). The fact that a Si/Si 1−x Ge x/ CoSi 2 SCHBT can suppress the base push out and eliminate the collector charging time, which is a delay component that makes a sizable contribution in high-speed heterojunction bipolar transistors (HBTs), makes it possible to obtain very high and current-stable speeds with this structure. Analysis of the different delay components contributing to the emitter-to-collector delay time shows that for Schottky collector transistors with a base thickness of 300 Å or less, the structure should be optimized to minimize the emitter delay time even at high collector current densities in order to further improve the speed. An advantage is the lower power consumption when this SCHBT is compared with conventional HBTs.

GaAs HBT RELIABILITY

This paper reviews recent studies on the reliability of GaAs heterojunction bipolar transistors (HBTs), mainly carbon-doped-base AlGaAs/GaAs HBTs, and discusses degradation modes in these devices. The carbon-doped-base AlGaAs/GaAs HBTs generally have more stable current-voltage characteristics than beryllium-doped-base ones. However, even carbon-doped-base HBTs show a decrease in the current gain due to an anomalous increase in the base current that occurs under high-current stress. The estimated ohmic contacts of these HBTs show that their stability is high enough under actual operating conditions. Bias stress testing has shown that two factors dominate the reliability of carbon-doped-base AlGaAs/GaAs HBTs: the material and the passivation. HBTs with an AlGaAs surface passivation layer on the extrinsic base region exhibit a longer life than those without one. The base current ideality factor of short-lived HBTs increases to larger than 2 during bias stress testing, suggesting that the origin of the excess base current cannot be simply explained by a space-charge recombination process. The reported activation energies are distributed around 0.5 eV, which is almost the same as those for AlGaAs/GaAs laser diodes. Cross-sectional TEM analysis has revealed that current-induced defects are formed in the base layer. The indium-incorporated-base HBTs show a median life of longer than 15,000 hours at a junction temperature of 215°C with a collector current density of 50 kA/cm 2, implying that GaAs HBTs have sufficient reliability for very-high-speed communication systems.

Heterojunction bipolar transistors with emitter barrier lowered by /spl delta/-doping

An npn heterojunction bipolar transistor with a Si-/spl delta/-doped layer at the emitter-base hetero-interface is demonstrated. The Si /spl delta/-doped layer reduces the potential spike at the emitter junction. This design reserves the abruptness of the heterojunction, reduces electron barrier and increases the hole barrier simultaneously. Experimental results show that the HBT's turn-on characteristics are greatly improved while the current gain remains high. The offset voltages as low as 44 mV have been obtained. Very high current gains at very low collector current densities are obtained. >

SiGe-HBTs with high fT at moderate current densities

Silicon-germanium heterojunction bipolar transistors (HBTs) have been developed with regard to high cutoff frequencies within a wide collector current range. Unity current gain frequencies f/sub T/ high as 116 GHz were obtained at a collector current density of 2*10/sup 5/ A/cm/sup 2/. In addition, the FWHM of cutoff frequency f/sub T/ against collector current exceeds one and a half decades. Calculations indicate that the ratio of the collector to emitter area essentially determines the current density range of a high frequency HBT. >

Importance of collector doping in the design of AlInAs/GaInAs/InP double heterojunction bipolar transistors

AlInAs/GaInAs/InP double heterojunction bipolar transistors (DHBTs) have been made with an InP collector thickness of 0.75 μm and collector dopings of 1.4, 2.4, and 3×1016 cm−3. It is shown that because of the conduction band potential barrier between the GaInAs base and the InP collector in DHBTs and particular velocity-field characteristics of InP, the dc and rf performance of the DHBT is very sensitive to the collector doping level. With a collector doping of 3×1016 cm−3 devices conducted a collector current density of 1×105 A/cm2 without gain compression and had a IC-VCE saturation voltage of about 2 V. Base-collector and collector-emitter breakdown voltages were 22 and 14.5 V, respectively, and fT and fmax were in the range of 65–70 GHz.

Changes in Collector and Base Currents for AlGaAs/GaAs Heterojunction Bipolar Transistors

The changes in the collector and base currents after a 5-min operation with a collector current density of more than 2.5×104 A/cm2 has been investigated for AlGaAs/GaAs heterojunction bipolar transistors (HBTs) with a Be doped base layer. Their dependence on the V/III flux ratio during molecular beam epitaxy (MBE) growth and on ion implantation conditions for emitter isolation were examined. A larger change in the collector current was observed for HBTs which were grown under a lower V/III ratio or fabricated under a heavier implantation condition. A change in the base current has also been found even when the change in the collector current was small. The change has been attributed to a decrease in the non-radiative recombination rate. A high V/III flux ratio growth and a low damage implantation are needed to suppress the changes.

Static and dynamic behavior of a Si/Si0.8Ge0.2/Si heterojunction bipolar transistor using Monte Carlo simulation

A theoretical study of a Si/Si1−xGex/Si heterojunction bipolar transistor using Monte Carlo simulations is reported. The geometry and composition of the emitter-base junction are optimized using one-dimensional simulations with a view to improving electron transport in the base. It is proposed to introduce a thin Si-P spacer layer, between the Si-N emitter and the SiGe-P base, which allows launching hot electrons into the base despite the lack of natural conduction-band discontinuity between Si and strain SiGe. The high-frequency behavior of the complete transistor is then studied using 2D modeling. A method of microwave analysis using small signal Monte Carlo simulations that consists of expanding the terminal currents in Fourier series is presented. A cutoff frequency fT of 68 GHz has been extracted. Finally, the occurrence of a parasitic electron barrier at the collector-base junction is responsible for the fT fall-off at high collector current density. This parasitic barrier is lowered through the influence of the collector potential.

Pnp HBT with 66 GHz f/sub max/

The total emitter to collector delay for a Pnp AlGaAs/GaAs HBT has been reduced to 4.8 ps by employing a thin base (325 /spl Aring/) and collector (2300 /spl Aring/). Simultaneously, a low base sheet resistance of 170 ohms/square was achieved with tellurium doping. A higher collector doping than is typically used permitted operation at collector current densities in excess of 5/spl times/10/sup 4/ A/cm/sup 2/. A single emitter (2/spl times/4 /spl mu/m/sup 2/) and a single base contact device topology has an f/sub t/ and f/sub max/ of 33 and 66 GHz, respectively. >

Analytical base transit time model for high-injection regions

Using a perturbation theory focused on injected electron concentrations, we derived analytical models for collector current density, J n , and base transit time, τ B, for arbitrarily doped bases. These models are valid in all injection regions before the onset of the Kirk effect, include existing models as special cases, and agree well with numerical results. Our model predicts that τ B for the Gaussian-doped base is still smaller than that of a base with a uniform profile, a box-doped base, even in high-injection regions.

Small-sized collector-up Ge/GaAs heterojunction bipolar transistors with high gain, low base resistance, and high f/sub max/

Small-sized collector-up Ge/GaAs HBT's are successfully fabricated and their operation at a high collector current density and at a high frequency is realized for the first time. The current gain of these devices reaches a peak value as large as 200 at a current density 6/spl times/10/sup 4/ Acm/sup /spl minus/2/, and no degradation in the current gain is observed as the collector width is decreased down to 2 /spl mu/m. The capability of lower voltage operation is also shown. Intrinsic and extrinsic base resistances are as low as 180 /spl Omega///spl square/ and 90 /spl Omega///spl square/, respectively. The calibrated values of f/sub T/ and f/sub max/ are 25 GHz and 60 GHz, respectively. The larger value of f/sub max/ compared with f/sub T/ might be attributed to low base resistance and low base-collector capacitance as expected from the collector-up structure. >

The Role of Hydrogen in Current-Induced Degradation of GaAs/AlGaAs Heterojunction Bipolar Transistors

ABSTRACTCarbon-doped base GaAs/AlGaAs HBTs display current-induced decreases in dc gain which are correlated with the amount of hydrogen incorporated in the base layer during growth by Metalorganic Molecular Beam Epitaxy (MOMBE). During device operation, minority carrier injection induced debonding of hydrogen from neutral C-H complexes leads to an increase in effective base doping level and therefore to a decrease in gain. Post-growth in-situ or ex-situ annealing eliminates this effect by breaking up the C-H complexes. Properly designed HBTs are stable even for very high collector current densities (105 A · cm−2)

Fabrication and characterization of high-performance InP/InGaAs double-heterojunction bipolar transistors

This paper is on high-performance InP/InGaAs double-heterojunction bipolar transistors (DHBT's) utilizing compositionally step-graded InGaAsP layers between the InGaAs base and InP collector to suppress the current blocking effect. These DHBT's exhibit current gains of 200 and excellent breakdown behavior. Moreover, the DHBT's permit collector current density levels J/sub C/ up to 3/spl times/10/sup 5/ A/cm/sup 2/ at V/sub CE/=1.5 V. A current gain cutoff frequency of 155 GHz and a maximum oscillation frequency of 90 GHz have been successfully obtained at J/sub C/=1.6/spl times/10/sup 5/ A/cm/sup 2/. We have also investigated electron transport properties in the InP collector using a set of DHBT's with different injection energies into the InP collector. By increasing the injection energies, electron velocity is found to decrease from 3.5/spl times/10/sup 7/ cm/s to 1.6/spl times/10/sup 7/ cm/s, due to increased population of upper valleys. This result clearly demonstrates the significant role of nonequilibrium /spl Gamma/-valley transport in determining the high-speed performance of InP/InGaAs DHBT's. >

An ultra-high-speed ECL-BiCMOS technology with silicon fillet self-aligned contacts

We have developed a half-micron super self-aligned BiCMOS technology for high speed application. A new SIlicon Fillet self-aligned conTact (SIFT) process is integrated in this BiCMOS technology enabling high speed performances for both CMOS and ECL bipolar circuits. In this paper, we describe the process design, device characteristics and circuit performance of this BiCMOS technology. The minimum CMOS gate delay is 38 ps on 0.5 /spl mu/m gate and 50 ps on 0.6 /spl mu/m gate ring oscillators at 5 V. Bipolar ECL gate delay is 24 ps on 0.6 /spl mu/m emitter ring oscillators with collector current density of 40 kA/cm/sup 2/. A single phase decision circuit operating error free over 8 Gb/s and a static frequency divider operating at 13.5 GHz is demonstrated in our BiCMOS technology. >

Significant time constants defined by high-current charge dynamics in advanced silicon-based bipolar transistors

Analytical expressions for the time constants in advanced silicon-based bipolar transistors defined by the charge dynamics in the base-collector junction space-charge region (/spl tau//sub C/) and by the charge modulation in the quasi-neutral base (/spl tau//sub BM/) are derived based on an accounting for the high-current-induced perturbation of the space-charge region. The derivations show that voltage drops in the intrinsic and extrinsic collector regions and in the extrinsic emitter region are important in defining /spl tau//sub C/ and /spl tau//sub BM/, and that /spl tau//sub BM/ is approximately proportional to collector-current density. Application of the results to an aggressive SiGe-base HBT technology shows that /spl tau//sub C/ and /spl tau//sub BM/ are comparable to the base transit time, and hence that they are significant in defining high-current speed of the HBT.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Lattice-Mismatched InGaAs Double Heterojunction Bipolar Transistors Grown on GaAs Substrates

We propose lattice-mismatched InGaAs double heterojunction bipolar transistors (HBTs) fabricated on a GaAs substrate as a new structure for simultaneously reducing power dissipation and improving the electron transport properties. Increasing the In composition in the base and collector layers is effective for reducing the turn-on voltage but causes an increase in the generation current in the collector depletion region and a softening of the breakdown characteristics in the base-collector junction. By optimizing the layer structure for HBTs, the current gain reaches 20 with a base doping of 1?1019/cm3 in spite of the existence of a relatively high dislocation density. The turn-on voltage for an In0.3Ga0.7As base layer at a collector current density of 103 A/cm2 is approximately 0.9 V. The InGaAs HBT can be used also as a phototransistor covering the 1.2-1.5 ?m wavelength region.