Base Sheet Research Articles

Effects of annealing on the hydrogen concentration and the performance of InGaP/InGaAsN/GaAs heterojunction bipolar transistors

The effects of rapid thermal annealing (RTA) on InGaP/InGaAsN heterojunction bipolar transistors (HBTs) with a carbon-doped base have been studied. The hydrogen and nitrogen concentrations in the base, as well as the direct current (DC) and radio frequency (RF) device performance, were studied as a function of the annealing temperature. A 30-sec anneal at 650°C and 700°C under N2 ambient effectively eliminates hydrogen from the base. As the annealing temperature is increased, the base sheet resistance decreases, and the corresponding maximum frequency of oscillation increases. For all annealing temperatures studied, we found degradation in the DC gain, presumably caused by the increase of nitrogen concentration in the base region.

Wideband DHBTs using a graded carbon-doped InGaAs base

We report an InP/InGaAs/InP double heterojunction bipolar transistor (DHBT), fabricated using a mesa structure, exhibiting 282 GHz f/sub /spl tau// and 400 GHz f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> . The DHBT employs a 30 nm InGaAs base with carbon doping graded from 8/spl middot/10/sup 19//cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> to 5/spl middot/10/sup 19//cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , an InP collector, and an InGaAs/InAlAs base-collector superlattice grade, with a total 217 nm collector depletion layer thickness. The low base sheet (580 /spl Omega/) and contact (<10 /spl Omega/-μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) resistivities are in part responsible for the high f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> observed.

Elimination of burn-in effect in carbon-doped InGaP/GaAs HBTs by hydrogen lateral diffusion

Hydrogen lateral diffusion by annealing at low temperature was proposed to eliminate the burn-in effect in carbon-doped InGaP/GaAs heterojunction bipolar transistors (HBTs). After a thermal annealing at 480 °C for 30 min, the current gain variations caused by the electrical stress decreased from 42.7% to 2.6% as the emitter width was reduced from 100 to 5 μm. After the annealing process, the sheet resistance was decreased from 194.4 to 162.7 ohm/sq. as the van der Pauw line widths was reduced from 65 to 5 μm. Effective doping concentration in base layer was increased by removal of incorporated hydrogen atoms. Degradation of device characteristics was not obvious after annealing by comparing the ratio difference of current gain to base sheet resistance.

An analytical approach to the modelling of intrinsic base sheet resistance in a SiGe HBT and optimal profile design considerations for its minimization

An analytical model has been developed to calculate the intrinsic base sheet resistance (Rbi), for a given Ge dose and base dopant, with a view to determine the optimal doping profile and Ge profile in the neutral base of a SiGe-base HBT. We have derived an empirical formula for the diffusion constant of holes in a p-type SiGe as a spatial function of mole fraction of Ge as well as base doping concentration. Our model has been verified with experimental and simulated results published in the literature. Our study shows that for a given Ge dose and amount of base dopant, a triangular Ge profile with a retrograde dopant profile yields the lowest Rbi.

Observation of enhanced transport in carbon-doped InGaAsN after in situ anneal and its impact on performance of NpN InGaP/InGaAsN heterojunction bipolar transistors

The incorporation of a low band gap carbon-doped InGaAsN material into a standard GaAs heterojunction bipolar transistor (HBT) has great potential to achieve higher operating efficiency at lower bias conditions. In order to improve the performance of the initial as-grown InGaAsN-based HBTs (with 1% N and 3% In for an energy band gap EG of 1.2 eV), the effects of different thermal treatments on material quality and their impact on dc and rf characteristics on small-area devices are examined in this letter. A degradation of the base transport is observed after a postgrowth anneal associated with lower current gain β and degraded microwave properties. An in situ anneal approach under inert ambient conducted following the emitter growth, increases the active doping level (with a base sheet resistance RSH three times lower than the as-grown structure and demonstrates suitable high frequency performance for a device with such a large amount of nitrogen in the base.

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.

Self-aligned SiGe NPN transistors with 285 GHz f/sub MAX/ and 207 GHz f/sub T/ in a manufacturable technology

This paper reports on SiGe NPN HBTs with unity gain cutoff frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) of 207 GHz and an f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> extrapolated from Mason's unilateral gain of 285 GHz. f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> extrapolated from maximum available gain is 194 GHz. Transistors sized 0.12×2.5 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> have these characteristics at a linear current of 1.0 mA/μm (8.3 mA/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). Smaller transistors (0.12×0.5 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) have an f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> of 180 GHz at 800 μA current. The devices have a pinched base sheet resistance of 2.5 k/spl Omega//sq. and an open-base breakdown voltage BV/sub CEO/ of 1.7 V. The improved performance is a result of a new self-aligned device structure that minimizes parasitic resistance and capacitance without affecting f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> at small lateral dimensions.

Structure–property-processing investigations of the tenter-frame process for making biaxially oriented HDPE film. I. Base sheet and draw along the MD

We discuss here the first two stages of the tenter-frame film manufacturing process for HDPE, viz. (a) the extrusion of the polymer onto a cooled cast roll (base sheet) and (b) drawing along the machine direction (MD) using several sets of rolls (referred to as machine draw orientation (MDO) sheet). The structure–property relationships are analyzed for a single resin when the cast-roll temperature, MDO temperature and MD draw ratio are systematically varied. The thick base sheets were made by extrusion onto a cast roll at three different temperatures of Tb, Tb−11°C and Tb+11°C (Tb being a reference temperature for the base-sheet draw). While the WAXD indicated an unoriented system, atomic force microscopy (AFM) and transmission electron microscopy (TEM) revealed spherulites of ca. 3–5μm diameter across the thickness of this base sheet. DSC indicated that the base sheet possessed a broad distribution of lamellar thickness and had a degree of crystallinity (Xc) of ca. 75%. After drawing 6X along the MD at three different drawing temperatures, of TMDO, TMDO−8°C and TMDO+8°C (TMDO being a reference temperature for the MDO-sheet draw), both AFM and TEM revealed a complete change in morphology to well-developed lamellar stacks oriented perpendicular to the MD. The MD draw ratio was then systematically varied from 1.5X, 2.25X, 3X, 4.5X to 6X, while keeping the other processing variables constant to understand how transformation of the spherulites to a stacked lamellar morphology took place. A very systematic change in morphology and orientation of the MDO sheet was observed with changing draw ratios, while the degree of crystallinity did not change. For the conditions utilized, the draw ratio of 2.25X was enough to destroy the general features of the bulk spherulitic superstructure although the broken and deformed lamellar remnants of the spherulites remained. With higher draw ratios, well-stacked lamellar structures oriented perpendicular to MD were gradually introduced at the expense of the deformed lamellae. At a draw ratio of 6X, only stacked lamellar structures were present. We discuss the causes of this transformation from primarily three standpoints: (a) localized melting and recrystallization followed by microfibrillation, (b) bulk melting followed by row nucleation and (c) a combination of the above two mechanisms. Interestingly, both X-ray and refractive index measurements revealed that the MDO process resulted in some level of biaxial orientation of the crystal phase. This arises due to the nature of the MDO process, which places constraints on the narrowing of the transverse dimensions during drawing.

New Thermal Offset Printing Employing Dye Transfer Technology (Tandem TOP-D)

In this paper, we describe the development of a tandemtype new thermal offset printing technology that enables reproduction of high speed and high quality full color images on a pulp-based paper (e.g. ppc, re-cycled ppc paper, RC paper, etc.), employing a newly developed thermal transfer engine, and print media including an acceptor layer sheet, ink sheet, and an intermediate belt.The process consists of three steps:(1) acceptor layer (AL) formation from the AL sheet onto an intermediate belt;(2) dye transfer printing with dye sheets into the AL on the belt; and(3) transfer from the dyed AL onto a sheet of paper.The print head for the AL formation and three heads for dye transfer printing are separately located outside the belt along a large drum.The head for the dyed AL transfer is installed inside the belt. The new acceptor layer (AL) of the AL sheet, which has a heat-resistant sliding layer on the back side of the sheet, is arranged on a releasing layer (RL) on the base sheet, separately from the ink sheets including sublimation dye layers and back coat layers.This report describes studies of the basic print engine and print method on pulp-based papers for full color images, and the specially prepared AL sheet, ink sheets and the intermediate belt. More detailed results are given below of the relationship between the materials' properties and the AL formation and transfer mechanism, release strength between the AL and the other interfaces, print density characteristics, and color fastness properties.

Thermal characteristics of InGaP/GaAs HBT ballasted with extended ledge

A InGaP/GaAs heterojunction bipolar transistor structure is proposed in which the base epi-layer underneath the extended ledge works as a base ballast resistor. The structure eliminates the critical alignment for a passivation ledge formation as well as additional process steps for external base ballast resistors. Both ballasted and unballasted devices were fabricated and compared. A small signal equivalent circuit gives us the magnitude of the effective ballast resistance. The thermal characteristics, including gain-collapsed I-V and V/sub be/ regression curve are shown and modeled. The temperature dependency of base sheet resistance and its influence on the device performance are also discussed.

Low tensile strain GaInAs : uid/GaAs : C superlattice heterostructures grown by LP MOCVD: application to GaInP/GaAs heterojunction bipolar transistor base layer

This paper reports on the investigation of GaInP/GaAs HBT-advanced structures with well-controlled strained base layers based on GaAs : C/GaInAs : uid superlattice heterostructures. High-resolution X-ray diffraction (HRXRD) and TEM experiments performed on test GaAs : C/GaInAs : uid superlattice heterostructures reveal well defined and sharp GaInAs/GaAs well/barrier interfaces with a high structural quality. C– V measurements and SIMS analysis, indicate that a low base sheet resistance is achieved with less carbon concentration than needed in the commonly HBTs C-doped GaAs base layers, together with a large strain reduction as shown by X-ray. HBT test structures consisting of 8–11 (GaAs : C/GaInAs : uid) period superlattice base layers, with GaInAs thickness varying from 0.5 to 3.4 nm have been processed. It is demonstrated that the GaInAs layer thickness must be less than 1 nm to preserve the current gain β of the device.

Carbon doping for the GaAs base layer of Heterojunction Bipolar Transistors in a production scale MOVPE reactor

In this work different approaches for carbon doping of GaAs in MOVPE are compared with respect to their growth- and device-related material properties. Doping levels up to 6×10 19 cm −3 and smooth surface morphologies are achieved with either intrinsically (TMG and AsH 3 or TMAs) or extrinsically (CBr 4) doped layers. Despite comparable structural and majority carrier properties differences in GaInP/GaAs-HBT device performance depending on base doping conditions are obtained. Devices with an intrinsically doped base layer (TMG+AsH 3) show superior transistor performance with a current gain to base sheet resistance ratio ( β/ R sb) exceeding 0.5 for base thicknesses as large as 120 nm. The use of either CBr 4 or TMAs as base growth precursors results in reduced current gains ( β/ R sb⩽0.3). It is shown that the achieved HBT current gain is directly related to recombination centers in the heavily doped base layer depending on doping method.

Turn-on voltage investigation of GaAs-based bipolar transistors with Ga/sub 1-x/In/sub x/As/sub 1-y/Ny base layers

The fundamental lower limit on the turn on voltage of GaAs-based bipolar transistors is first established, then reduced with the use of a novel low energy-gap base material, Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/. InGaP/GaInAsN DHBTs (x/spl sim/3y/spl sim/0.01) with high p-type doping levels (/spl sim/3/spl times/10/sup 19/ cm/sup -3/) and dc current gain (/spl beta//sub max//spl sim/68 at 234 /spl Omega///spl square/) are demonstrated. A reduction in the turn-on voltage over a wide range of practical base sheet resistance values (100 to 400 /spl Omega///spl square/) is established relative to both GaAs BJTs and conventional InGaP/GaAs HBTs with optimized base-emitter interfaces-over 25 mV in heavily doped, high dc current gain samples. The potential to engineer turn-on voltages comparable to Si- or InP-based bipolar devices on a GaAs platform is enabled by the use of lattice matched Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/ alloys, which can simultaneously reduce the energy-gap and balance the lattice constant of the base layer when x/spl sim/3y.

Simulation of GaN/AlGaN heterojunction bipolar transistors: part II – pnp structures

The dc characteristics of pnp GaN/AlGaN heterojunction bipolar transistors are simulated using a quasi-3D-model. The effects of base doping and thickness, contact geometry and device operating temperature on dc current gain have been examined. Maximum gains of ∼50 are expected for high quality materials and an optimized layer design. For similar layer thicknesses, npn devices have higher gains than the pnp structures, but the latter does not suffer from a high base sheet resistance.

Direct-current characteristics of pnp AlGaN/GaN heterojunction bipolar transistors

AlGaN/GaN pnp heterojunction bipolar transistors were fabricated using a low-damage dry-etch process, and the dc characteristics measured up to 250 °C. In the common–base mode, the collector current was approximately equal to the emitter current under all conditions. Although not optimized for power operations, the devices were tested up to power densities of ∼50 kW cm−2. The dc current gain was in the range 20–25 at room temperature. The pnp configuration avoids the problem of high base sheet resistance encountered with npn-AlGaN/GaN devices.

A study of L-band GaAlAs/GaAs HBTs for high-voltage RF-power

The feasibility of GaAlAs/GaAs heterojunction bipolar transistors as power output transistors for 24–28V cellular base station applications was studied. A 1W, 25V test vehicle was fabricated, with predictable and uniform electrical parameters. Maximum output power was 1.02W with 9.1dB gain at 2GHz and 18V supply voltage. Characterization at more than 22V destroyed all devices, even if emitter ballasting resistors were used for thermal balance. Analysis indicated that the failures were caused by too wide emitters in combination with the high supply voltage, causing current crowding and large temperature spikes. Small (<2-3µm) horizontal emitter dimensions are required even with low base sheet resistivity, otherwise leading to destructive temperature non-uniformities, unless special arrangements to drastically reduce the thermal resistance can be applied.

High DC current gain InGaP/GaAs HBTs grown by LP-MOCVD

The high current gain of InGaP/GaAs heterojunction bipolar transistors (HBTs) grown under optimised growth conditions using MOCVD is demonstrated. Large area (60 µm × 60 µm) samples of InGaP/GaAs HBTs are grown and fabricated for DC characterisation. These devices show Gummel plots with nearly ideal I-V characteristics (nc = 1.00 and nb = 1.09). Measured current gain of the devices with a base sheet resistance Rb of 236 Ω/sq is 130 at a collector current Ic of 1 mA and 147 at the collector current density of 1 kA/cm2 (Ic = 39.1 mA). The current gain to base sheet resistance ratio is 0.623 at 1 kA/cm2, which is the highest value ever reported. The optimised growth condition improves the current gain in the entire range of the collector current. The current gain is as high as 92 at an Ic of 10 µA. These results are the best that have ever been demonstrated with InGaP/GaAs HBTs. The data show that the MOCVD growth condition is an important factor in achieving high current gain in InGaP/GaAs HBTs.

Assessment of layer structures for GaInP/GaAs-heterojunction bipolar transistors

GaAs-based heterojunction bipolar transistors (HBTs) are expected to take an increasing share of the currently expanding market for mobile communications products. The GaInP/GaAs-system offers advantages for device performance and fabrication in comparison with the already established AlGaAs/GaAs-system. However, HBT fabrication is still not mature technology. The main obstacle is the reproducible supply of high quality HBT epitaxial layer structures. An appropriate evaluation of the HBT layer structures is essential to successfully establish the epitaxial growth technology. The ratio of current gain to base sheet resistance is known to be one of the most meaningful figures of merit for the HBT. Therefore, the development of proper epitaxial growth procedures for HBT layer structures includes a device qualification. To the end of the evaluation of GalnP/GaAs-HBT layer structures we have established a fast HBT process that additionally provides windows for material analysis in different depths of the structure. The fundamental device performance and the wafer uniformity are assessed by dc parameter mapping of large area devices and conventional test structures. Several material diagnostic techniques were applied to assess layer properties. Some of the capabilities of these methods are discussed. Our fast HBT process has been instrumental in the development of GalnP/GaAs HBT layer structures at Epitaxial Products International. Current gain of 161 at the base sheet resistance of 273 Ω sq −1 with a standard deviation of the current gain across a 3 in. wafer of 1% proves that these GalnP/GaAs-HBT layer structures are comparable with currently available state-of-the-art AlGaAs/GaAs-HBT layer structures. Parts of the work were performed within the EC-project ESPRIT 21315 GAMMA.

Graded base profiles on the performance of AlGaAs HBTs

Effects of graded base profiles on the performance of AlGaAs HBTs have been examined. The exponential graded base gives highest electron current density for given base-emitter voltage. The parabolic base profile results in lowest base transit time followed by exponential, Gaussian, square root and linear base profiles. For the base resistivity, however, the linearly graded base gives lowest base sheet resistance for given base concentration.

Carrier freezeout in strained p-type Si 1− xGe x and its influence on the low temperature characteristics of SiGe heterojunction bipolar transistors

A physically based model for carrier freezeout in strained p-type Si 1− x Ge x is presented using the band parameters of coherently strained Si 1− x Ge x alloy on 〈001〉 Si substrate. It is found that the carrier freezeout is mitigated at low temperatures with the increasing of Ge fraction. The proposed model proved to give consistently accurate values for base sheet resistance and other device characteristics of SiGe heterojunction bipolar transistors (HBTs) at low temperatures. The results may provide a rational basis for the design of low temperature operation SiGe HBTs.