B.T. McDermott

High-power 10-GHz operation of AlGaN HFET's on insulating SiC

We report the first high-power RF characterization of AlGaN HFETs fabricated on electrically insulating SiC substrates. A record total power of 2.3 W at 10 GHz was measured from a 1280-/spl mu/m wide HFET at V/sub ds/=33 V. An excellent RF power density of 2.8 W/mm was measured on a 320-/spl mu/m wide HFET. These values are a result of the high thermal conductivity of SiC, relative to the typical substrate, sapphire.

MEASUREMENT OF DRIFT MOBILITY IN ALGAN/GAN HETEROSTRUCTURE FIELD-EFFECT TRANSISTOR

Low-field mobilities for electrons in the channel of an Al0.15Ga0.85N/GaN heterostructure field-effect transistor are derived from direct current transistor characteristics. The dependencies of mobility on gate bias, sheet carrier concentration, and temperature are obtained. For negative gate bias voltages, mobility is found to increase monotonically with increasing sheet carrier concentration, which we interpret as a consequence of increased screening of carrier scattering. For positive gate bias voltages, mobility is found to decrease with increasing gate bias due to the onset of parallel conduction in the AlGaN barrier layer. The mobility varies approximately as T−α with α≈1.6–1.8 for temperature ranging from 200 to 400 K, indicating that phonon scattering is dominant in the two-dimensional electron gas in this temperature range.

Growth and doping of GaAsSb via metalorganic chemical vapor deposition for InP heterojunction bipolar transistors

GaAsSb is a low band gap, lattice matched to InP, alternative to GaInAs. Growth and doping using diethyltellurium and carbon tetrachloride were investigated. Hole concentrations up to 1.3×1020 cm−3 have been achieved in as‐grown carbon‐doped GaAsSb [i.e., no postgrowth annealing was necessary for dopant activation, a key requirement for n‐p‐n heterojunction bipolar transistor (HBT) structures]. This is a sevenfold improvement over the best carbon‐doped InGaAs reported by metalorganic chemical vapor deposition. Hall measurements indicate that GaAsSb’s hole mobility is 55%–60% of GaInAs’s, for a given carrier concentration. InP HBTs with carbon‐doped GaAsSb base are demonstrated.

Influence of surface processing and passivation on carrier concentrations and transport properties in AlGaN/GaN heterostructures

The influence of surface chemical treatments and of deposition of a SiO2 surface passivation layer on carrier distributions and mobility in AlxGa1−xN/GaN heterostructure field-effect-transistor epitaxial layer structures is investigated. Surface chemical treatments are found to exert little influence on carrier distribution and mobility. Deposition of a SiO2 surface passivation layer is found to induce an increase in electron concentration in the transistor channel and a decrease in mobility. These changes are largely reversed upon removal of the SiO2 layer by wet etching. These observations are quantitatively consistent with a shift in Fermi level at the AlxGa1−xN surface of approximately 1 eV upon deposition of SiO2, indicating that the AlxGa1−xN/SiO2 interface has a different, and possibly much lower, density of electronic states compared to the AlxGa1−xN free surface.

GaInP/GaAs HBTs for high-speed integrated circuit applications

The use of GaInP/GaAs heterojunction bipolar transistors (HBTs) for integrated circuit applications is demonstrated. The discrete devices fabricated showed excellent DC characteristics with low V/sub ce/ offset voltage and very low temperature sensitivity of the current gain. For a non-self-aligned device with a 3- mu m*1.4- mu m emitter area, f/sub T/ was extrapolated to 45 GHz and f/sub max/ was extrapolated to 70 GHz. The measured 1/f noise level was 20 dB better than that of AlGaAs HBTs and comparable to that of low-noise silicon bipolar junction transistors, and the noise bump (Lorentzian component) was not observed. The fabricated gain block circuits showed 8.5 dB gain with a 3-dB bandwidth of 12 GHz, and static frequency dividers (divide by 4) were operable up to 8 GHz.<<ETX>>

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.

Modification of GaN Schottky Barrier Interfaces Probed by Ballistic-Electron-Emission Microscopy and Spectroscopy

Ballistic-electron-emission microscopy (BEEM) and spectroscopy have been used to investigate the properties of Au/GaN interfaces. The effects of in situ and ex situ annealing on the starting GaN surface were examined, with the aim of increasing the surprisingly low value of interface electron transmission observed in previous BEEM measurements. BEEM imaging and spectroscopy have demonstrated that much higher, more uniform transmission across the Au/GaN interface can be achieved. However, while methods were identified that increase transmission by more than an order of magnitude, BEEM spectroscopy indicates that annealing can substantially alter the Schottky barrier height. These barrier height changes at moderate temperatures are attributed to vacancy diffusion.

Heterostructure-based high-speed/high-frequency electronic circuit applications

Abstract With the growth of wireless and lightwave technologies, heterostructure electronic devices are commodity items in the commercial marketplace [Browne J. Power-amplifier MMICs drive commercial circuits. Microwaves & RF, 1998. p. 116–24.]. In particular, HBTs are an attractive device for handset power amplifiers at 900 MHz and 1.9 GHz for CDMA applications [Lum E. GaAs technology rides the wireless wave. Proceedings of the 1997 GaAs IC Symposium, 1997. p. 11–13; “Rockwell Ramps Up”. Compound Semiconductor, May/June 1997.]. At higher frequencies, both HBTs and p-HEMTs are expected to dominate the marketplace. For high-speed lightwave circuit applications, heterostructure based products on the market for OC-48 (2.5 Gb/s) and OC-192 (10 Gb/s) are emerging [http://www.nb.rockwell.com/platforms/network_access/nahome.html#5.; http://www.nortel.com/technology/opto/receivers/ptav2.html.]. Chips that operate at 40 Gb/ have been demonstrated in a number of research laboratories [Zampardi PJ, Pierson RL, Runge K, Yu R, Beccue SM, Yu J, Wang KC. hybrid digital/microwave HBTs for >30 Gb/s optical communications. IEDM Technical Digest, 1995. p. 803–6; Swahn T, Lewin T, Mokhtari M, Tenhunen H, Walden R, Stanchina W. 40 Gb/s 3 Volt InP HBT ICs for a fiber optic demonstrator system. Proceedings of the 1996 GaAs IC Symposium, 1996. p. 125–8; Suzuki H, Watanabe K, Ishikawa K, Masuda H, Ouchi K, Tanoue T, Takeyari R. InP/InGaAs HBT ICs for 40 Gbit/s optical transmission systems. Proceedings of the 1997 GaAs IC Symposium, 1997. p. 215–8]. In addition to these two markets, another area where heterostructure devices are having significant impact is for data conversion [Walden RH. Analog-to digital convertor technology comparison. Proceedings of the 1994 GaAs IC Symposium, 1994. p. 217–9; Poulton K, Knudsen K, Corcoran J, Wang KC, Nubling RB, Chang M-CF, Asbeck PM, Huang RT. A 6-b, 4 GSa/s GaAs HBT ADC. IEEE J Solid-State Circuits 1995;30:1109–18; Nary K, Nubling R, Beccue S, Colleran W, Penney J, Wang KC. An 8-bit, 2 gigasample per second analog to digital converter. Proceedings of the 1995 GaAs IC Symposium, 1995. p. 303–6.]. In this paper, we will discuss the present and future trends of heterostructure device applications to these areas.

Ballistic-electron-emission microscopy and spectroscopy of metal/GaN interfaces

Ballistic-electron-emission microscopy (BEEM) spectroscopy and imaging have been applied to the Au/GaN interface. In contrast to previous BEEM measurements, spectra yield a Schottky barrier height of 1.04 eV that agrees well with the highest values measured by conventional methods. A second threshold is observed in the spectra at about 0.2 V above the first threshold. Imaging of the Au/GaN interface reveals transmission in nearly all areas, although the magnitude is small and varies by an order of magnitude. BEEM of other GaN material shows no transmission in any areas.

Carbon doped InP/GaAsSb HBTs via MOCVD

Heterojunction bipolar transistors (HBTs) need heavily doped p-type regions. However, at high concentrations (>10/sup 19/ cm/sup -3/), most p-type dopants diffuse into other regions of the device, ruining performance and preventing stable, reliable operation. The discovery that carbon as a p-type dopant does not significantly diffuse has lead to reliable operation of GaAs-based HBTs. For the first time, carbon-doped double heterojunction InP/GaAsSb/InP HBTs have been fabricated (emitter area of approximately 70/spl times/70 /spl mu/m/sup 2/). The base doping was P=4/spl times/10/sup 19/ cm/sup -3/. Base thickness was varied from 350 /spl Aring/ to 1500 /spl Aring/, giving sheet resistances of 850 to 200 /spl Omega//sq. The devices had DC current gains ranging from 5 to 45 that scaled sheet resistance. While these gains are low, they are comparable to the best InP/GaInAs HBTs fabricated, where the gain is limited due to Auger recombination in highly doped bases. BV/sub ceo/ is on the order of 6 volts. The typical turn-on voltage for both emitter-base and base-collector junctions is approximately 0.4 V, even with the emitter-base junction grown nominally abrupt: i.e., no undoped setbacks or intentionally graded layers were used for these structures. The authors report on the growth, fabrication, and properties of the MOCVD-grown carbon-doped InP/GaAsSb HBTs.