Device Applications of Low-Temperature-Grown GaAs
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DEVICE APPLICATIONS OF LOW-TEMPERATURE-GROWN GaAs
Frank W. Smith Lincoln Laboratory, Massachusetts Institute of Technology Lexington, MA 02173-9108
ABSTRACT Low-temperature-grown (LTG) GaAs is a unique material that has been used in a variety of device applications to achieve record performance. LTG GaAs used as a buffer layer eliminates sidegating and backgating and in GaAs integrated circuits. Record output power density (1.57 W/mm) and superior microwave-switch performance were demonstrated when LTG GaAs was used at a gate insulator in a metal-insulator-semiconductor field-effect transistor. High-speed (0.5 ps) and high-voltage (1 kV) LTG GaAs photoconductive switches have also been demonstrated. Using the same material, researchers have demonstrated highresponsivity (0.1 A/W), wide-bandwidth (- 375 GHz) LTG GaAs photodetectors. Devices incorporating LTG GaAs are currently being optimized for systems applications. LTG GaAs technology can enhance system performance and enable new systems for military and commercial applications in the areas of radar, communications, instrumentation, and highspeed computing. INTRODUCTION Low-temperature-grown (LTG) GaAs was discovered at Lincoln Laboratory by A. R. Calawa in the mid 1980's and has been applied to electronic and photoconductive devices and circuits since that time. LTG GaAs is deposited by molecular beam epitaxy (MBE) at relatively low substrate temperature (- 200°C). Some of the unique features of LTG GaAs are contrasted with conventional GaAs grown by MBE in Table 1. Most importantly for device applications, the large excess As concentration in LTG GaAs results in a large trap density, that in turn results in a high resistivity, a large breakdown field, and an extremely short photoexcited carrier lifetime. Record device and circuit performance achieved using LTG II-V semiconductors has made it clear that this new materials system has a number of unique and desirable properties. LTG 111-V materials that have been demonstrated include LTG GaAs, AlGaAs, InAlAs, InGaAs, and InP. Of these materials, the one most extensively characterized and utilized in
Table 1
Comparison of the properties of LTG GaAs with undoped GaAs grown by MBE. MBE GaAs
LTG GaAs
6000C
0 200 C
STOICHIOMETRIC GaAs < 1015 cm- 3
1 at. % EXCESS As Gao. 495 Aso. 50 5 - 1019 cm-3
GROWTH TEMPERATURE STOICHIOMETRY TRAP DENSITY BREAKDOWN FIELD RESISTIVITY
-
3 x 104 V/cm 0.3-20 Q1- cm
~ 5 x 105 V/cm -
106 ! - cm
PHOTO-EXCITED CARRIER LIFETIME Mat. Res. Soc. Symp. Proc. Vol. 241. @1992 Materials Research Society
device and circuit applications is LTG GaAs. For this reason, this paper will exclusively address LTG GaAs, although other papers in this volume will address the properties and applications of other LTG II-V materials. LTG GaAs has been exploited in electronic devices, photoconductive devices, and integrated circuits (ICs). LTG GaAs used as a buffer layer has eliminated sidegating and backgating in GaAs ICs and has been shown to reduce short-channel effects in GaAs field effect trans
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