A Semi-Analytical Interpretation of Transient Electron Transport in Gallium Nitride, Indium Nitride, and Aluminum Nitrid
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ABSTRACT The energy dependent momentum and energy relaxation times, and the effective single valley energy dependent effective mass, are extracted from Monte Carlo simulations of gallium nitride, indium nitride, and aluminum nitride. A simple semi-analytical energy model, which uses these dependencies, is in good agreement with the results of transient Monte Carlo simulations. Both the Monte Carlo and the semi-analytical simulations show that the overshoot effects are most pronounced when the electric field abruptly changes from a value below a critical field to one above. This is attributed to the relatively large difference between the effective energy and momentum relaxation times for such a variation of electric field. Our calculations indicate that gallium nitride and indium nitride should have the most pronounced transient effects. A calculation of the transit times as a function of the gate length shows that an upper bound for the maximum expected cut-off frequencies are 260 GHz and 440 GHz for 0.2 /Lm gallium nitride and indium nitride field effect transistors, respectively. INTRODUCTION The III-V nitride semiconductors, GaN, InN, AIN, and their alloys have already found applications in blue-green light-emitting diodes, lasers, and other small wavelength optoelectronic devices due to their wide-band gap [1]. Their large 2-D electron gas carrier concentrations and high electron velocities have made it possible to fabricate high frequency/high power field effect transistors (FETs). Cut-off frequencies as high as 53 GHz [2] and microwave power in excess of 3 W/mm [3] have been demonstrated. An understanding of electron transport is an important part of optimizing these devices. Recently, Monte Carlo simulations of the velocity-field characteristics have shown these materials might exhibit very high peak drift velocities, 4.3 x 10' cm/s for InN [4] and 3.1 x 10 7 cm/s for GaN [5]. Furthermore, it has also been demonstrated that transient effects such as velocity overshoot may be significant in these materials as well [6, 7]. In this paper, we study overshoot effects in the III-V nitrides using a simple semianalytical energy-momentum balance model similar to the one used by Shur [8] to study velocity overshoot in GaAs. Using this model, overshoot effects in GaN, InN, and AIN are studied and the results are found to be in good agreement with Monte Carlo simulations. Both the Monte Carlo simulations and the semi-analytical model show that the most pronounced overshoot effects occur when the final applied field is above a threshold value. We also find that the transient effects in AIN are much less pronounced than those in GaN or 555 Mat. Res. Soc. Symp. Proc. Vol. 512 ©1998 Materials Research Society
InN and that this difference can be explained by the fact that AIN has a much smaller energy relaxation time in the semi-analytical model. Finally, the results of these calculations are used to estimate an upper bound on the cut-off frequency performance of nitride based FETs as a function of gate length. These resu
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