Measurement of Transit Time and Carrier Velocity Under High Electric Field in III-Nitride P-I-N Diodes
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Measurement of Transit Time and Carrier Velocity Under High Electric Field in III-Nitride P-I-N Diodes M. Wraback1, H. Shen1, J.C. Carrano2, T. Li3, and J.C. Campbell3 1 U.S. Army Research Laboratory, Sensors and Electron Devices Directorate, AMSRL-SE-EM, 2800 Powder Mill Road, Adelphi, MD 20783 2 Photonics Research Center, Department of Electrical Engineering and Computer Science, U.S. Military Academy, West Point, NY 10996 3 Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78712
ABSTRACT We present an optically-detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in a p-i-n diode, and show how it may be used to determine the electron transit time and velocity-field characteristic in GaN at room temperature. The transit time drops with increasing electric field E in the intermediate field regime (50-100kV/cm), and the electron velocity possesses a weak, quasi-linear dependence on E attributed to polar optical phonon scattering. In the high field regime the transit time and the electron velocity gradually become independent of E. The peak electron velocity of 1.9 ×107 cm/sec, corresponding to a transit time of ~2.5 ps across the 0.53 µm depletion region, is attained at ~ 225 kV/cm. The experimental results are in qualitative agreement with theoretical steady-state velocity-field characteristics found in the literature. INTRODUCTION Wide bandgap, nitride-based semiconductors have become important materials for the development of high temperature, high power electronics and ultraviolet photodetectors. Theoretical calculations [1-4] of the electron velocity-field characteristic predict a peak steadystate velocity of 2-3×107 cm/sec, implying that these materials may be important candidates for high frequency devices as well. Ultrawide-bandwidth AlGaN/GaN heterostructure field effect transistors [5-8] and very high-speed [9], transit-time-limited GaN metal-semiconductor-metal (MSM) [10] and p-i-n [11] ultraviolet photodetectors have been demonstrated. Nevertheless, direct measurements of electron transit times and carrier velocities in III-nitride materials, which would help to determine the ultimate speed of electronic and optoelectronic devices and provide reliable high field transport data for the design of these devices, cannot be achieved using conventional electronic or optoelectronic methods. Such measurements have been performed in GaAs [12-14] and amorphous hydrogenated silicon [15] using optically-detected time-of-flight techniques. In this paper we present an optically-detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in a III-N p-i -n diode, and show how it may be used to determine the electron transit time and velocity-field characteristic in GaN at room temperature. The experimentally obtained peak steady-state electron velocity of 1.9 ×107 cm/sec, correspondi
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