Roger E. Welser

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.

Turn-on voltage investigation of GaAs-based bipolar transistors with Ga/sub 1-x/In/sub x/As/sub 1-y/N y 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.

GaInP/GaAs collector-up tunneling-collector heterojunction bipolar transistors (C-up TC-HBTs): optimization of fabrication process and epitaxial layer structure for high-efficiency high-power amplifiers

This paper describes a novel heterojunction bipolar transistor (HBT) structure, the collector-up tunneling-collector HBT (C-up TC-HBT), that minimizes the offset voltage V/sub CE,sat/ and the knee voltage V/sub k/. In this device, a thin GaInP layer is used as a tunnel barrier at the base-collector (BC) junction to suppress hole injection into the collector, which results in small V/sub CE,sat/. Collector-up configuration is used because of the observed asymmetry of the band discontinuity between GaInP and GaAs depending on growth direction. To minimize V/sub k/, we optimized the epitaxial layer structure as well as the conditions of ion implantation into the extrinsic emitter and post-implantation annealing. The best results were obtained when a 5-nm-thick 5/spl times/10/sup 17/-cm/sup -3/-doped GaInP tunnel barrier with a 20-nm-thick undoped GaAs spacer was used at the BC junction, and when 2/spl times/10/sup 12/-cm/sup -2/ 50-keV B implantation was employed followed by 10-min annealing at 390/spl deg/C. Fabricated 40/spl times/40-/spl mu/m/sup 2/ C-up TC-HBTs showed almost zero V/sub CE,sat/ (<10 mV) and a very small V/sub k/ of 0.29 V at a collector current density of 4 kA/cm/sub 2/, which are much lower than those of a typical GaInP/GaAs HBT. The results indicate that the C-up TC-HBTs are attractive candidates for high-efficiency high power amplifiers.

Role of neutral base recombination in high gain AlGaAs/GaAs HBT's

Neutral base recombination is a limiting factor controlling the maximum gain of AlGaAs/GaAs HBTs with base sheet resistances between 100 and 350 /spl Omega///spl square/. In this work, we investigate five series of AlGaAs/GaAs HBT growths in which the base thickness was varied between 500 and 1600 /spl Aring/ and the base doping level between 2.9/spl times/ and 4.7/spl times/10/sup 19/ cm/sup -3/. The dc current gain of large area devices (L=75 /spl mu/m/spl times/75 /spl mu/m) varies by as much as a factor of two at high injection levels for a fixed base sheet resistance, depending on the growth optimization. One of these series (Series TA) has the highest current gains ever reported in this base sheet resistance range, with dc current gains over 225 (@ 200 A/cm/sup 2/) at a base sheet resistance of 330 /spl Omega///spl square/. A high dc current gain of 220 (@ 10 kA/cm/sup 2/) was also confirmed in small area devices (L=8 /spl mu/m/spl times/8 /spl mu/m). High-frequency tests on a separate set of wafers grown under the same conditions indicate these high current gains can be achieved without compromising the RF characteristics: Both high and normal gain devices exhibit an f/sub t//spl sim/68 GHz and f/sub max//spl sim/100 GHz. By fitting the base current as a sum of two components, one due to recombination in the neutral base and the other in the space charge region, we conclude that an improvement in the minority carrier lifetime is responsible for the observed increase in dc current gain. Moreover, we observe a thickness-dependent variation in the effective minority carrier lifetime as the gains increase, along with a nonlinear dependence of current gain on base doping. Both phenomena are discussed in terms of an increase in Auger and radiative recombination relative to Hall-Shockley-Read recombination in optimized samples.

Implementation of reduced turn-on voltage InGaP HBTs using graded GaInAsN base regions

InGaP/GaInAsN double heterojunction bipolar transistors (HBTs) with compositionally graded bases are presented which exhibit superior dc and radio frequency performance. Reducing the average base layer energy gap and optimizing the emitter-base (e-b) and base-collector (b-c) heterojunctions leads to a 100-mV reduction in the turn-on voltage compared to a baseline InGaP/GaAs process. Simultaneously grading the base layer energy band-gap results in over a 66% improvement in the dc current gain and up to a 35% increase in the unity gain cutoff frequency. DC current gains as high as 250 and cutoff frequencies of 70 GHz are demonstrated. In addition, the InGaP/GaInAsN DHBT structure significantly reduces the common emitter offset and knee voltages, as well as improves the dc current gain temperature stability relative to standard InGaP/GaAs HBTs.

Minority carrier properties of carbon-doped GaInAsN bipolar transistors

We have developed an InGaP/GaInAsN/GaAs double heterojunction bipolar transistor technology that substantially improves upon existing GaAs-based HBTs. Band-gap engineering with dilute nitride GaInAsN alloys is utilized to enhance a variety of key device characteristics, including lower operating voltages, improved temperature stability and increased RF performance. Furthermore, GaInAsN-based HBTs are fully compatible with existing high-volume MOVPE and IC fabrication processes. While poor lifetimes have limited the applicability of dilute nitride materials in photovoltaic applications, we achieve minority carrier characteristics that approach those of conventional GaAs HBTs. We have found that a combination of growth algorithm optimization and compositional grading are critical for improving minority carrier properties in GaInAsN. In this work, we characterize the impact of both carbon and nitrogen doping on minority carrier lifetimes in GaInAsN base layers. Minority carrier lifetimes are extracted from direct measurements on bipolar transistor device structures. Specifically, lifetime is derived from the DC current gain, or β, taken in the bias regime dominated by neutral base recombination. Lifetimes extracted using this technique are observed to be inversely proportional to both carbon and nitrogen doping. As with conventional C-doped GaAs HBTs, current soaking (i.e.xa0burn-in) is found to have a significant impact on GaInAsN HBTs. While we can replicate poor as-grown lifetimes consistent with those reported in photovoltaic dilute nitride materials, our best material to date exhibits nearly 30 × higher lifetime after current soaking.

Design and performance of tunnel collector HBTs for microwave power amplifiers

AlGaAs/GaAs/GaAs and GaInP/GaAs/GaAs n-p-n heterojunction bipolar transistors (HBTs) are now in widespread use in microwave power amplifiers. In this paper, improved HBT structures are presented to address issues currently problematic for these devices: high offset and knee voltages and saturation charge storage. Reduced HBT offset and knee voltages (V/sub CE,os/ and V/sub k/) are important to improve the power amplifier efficiency. Reduced saturation charge storage is desirable to increase gain under conditions when the transistor saturates (such as in over-driven Class AB amplifiers and switching mode amplifiers). It is shown in this paper that HBT structures using a 100-/spl Aring/-thick layer of GaInP between the GaAs base, and collector layers are effective in reducing V/sub CE,os/ to 30 mV and V/sub k/ measured at a collector current density of 2/spl times/10/sup 4/ A/cm/sup 2/ to 0.3 V (while for conventional HBTs V/sub CE,os/=0.2 V and V/sub k/=0.5 V are typical). A five-fold reduction in saturation charge storage is simultaneously obtained.

High performance Al/sub 0.35/Ga/sub 0.65/As/GaAs HBT's

AlGaAs emitter heterojunction bipolar transistors (HBTs) are demonstrated to have excellent dc and RF properties comparable to InGaP/GaAs HBTs by increasing the Al composition. Al/sub 0.35/Ga/sub 0.65/As/GaAs HBTs exhibit very high dc current gain at all bias levels, exceeding 140 at 25 A/cm/sup 2/ and reaching a maximum of 210 at 26 kA/cm/sup 2/ (L=1.4 /spl mu/m/spl times/3 /spl mu/m, R/sub sb/=330 /spl Omega///spl square/). The temperature dependence of the peak dc current gain is also significantly improved by increasing the AlGaAs mole fraction of the emitter. Device analysis suggests that a larger emitter energy gap contributes to the improved device performance by both lowering space charge recombination and increasing the barrier to reverse hole injection.

Impact of compositionally graded base regions on the DC and RF properties of reduced turn-on voltage InGaP-GaInAsN DHBTs

Built-in drift fields are employed to enhance the performance of GaAs-based heterojunction bipolar transistors (HBTs) with reduced turn-on voltage. Specifically, we explore in detail the dc and RF device property improvements enabled by using compositionally graded GaInAsN base layers. Experimental results are compared to predictions of the standard drift-diffusion base transport model employing a finite exit velocity. In large area devices, graded base samples with built-in fields of /spl sim/7 kV/cm (i.e. 40 meV over 500 /spl Aring/) typically have a dc current gain 1.8/spl times/ larger than constant base composition samples. In small area devices, the peak cut-off frequency is typically 10%-15% higher than constant composition samples. These results are shown to agree reasonably well with predictions, thereby demonstrating that analytical drift-diffusion based models can be extended to HBTs with GaInAsN base layers.

Metalorganic chemical vapor deposition of AlGaAs and InGaP heterojunction bipolar transistors

Abstract Heterojunction bipolar transistors (HBT) are now beginning to be widely incorporated as power amplifiers, laser drivers, multiplexers, clock data recovery circuits, as well as transimpedance and broadband amplifiers in high performance millimeter wave circuits (MMICs). The increasing acceptance of this device is principally due to advancements in metalorganic chemical vapor deposition (MOCVD), device processing, and circuit design technologies. Many of the DC electrical characteristics of large area devices can be directly correlated to the DC performance of small area RF devices. A precise understanding of the growth parameters and their relationship to device characteristics is critical for ensuring the high degree of reproducibility required for low cost high-yield volume manufacturing. Significant improvements in the understanding of the MOCVD growth process have been realized through the implementation of statistical process control on the key HBT device parameters. This tool has been successfully used to maintain the high quality of the device characteristics in high-volume production of 4″ GaAs-based HBTs. There is a growing demand to migrate towards 6″ diameter wafer size due to the potential cost reductions and increased volume production that can be realized. Preliminary results, indicating good heterostructure layer characteristics, demonstrate the feasibility of 6″ InGaP-based HBT devices.