Abstract: Based on an improved energy dispersion relation, the terahertz field induced nonlinear transport of miniband electrons in a short period AlGaN/GaN superlattice is theoretically studied in this paper with a semiclassical theory. To a short period superlattice, it is not precise enough to calculate the energy dispersion relation by just using the nearest wells in tight binding method: the next to nearest wells should be considered. The results show that the electron drift velocity is 30% lower under a dc field but 10% higher under an ac field than the traditional simple cosine model obtained from the tight binding method. The influence of the terahertz field strength and frequency on the harmonic amplitude, phase and power efficiency is calculated. The relative power efficiency of the third harmonic reaches the peak value when the dc field strength equals about three times the critical field strength and the ac field strength equals about four times the critical field strength. These results show that the AlGaN/GaN superlattice is a promising candidate to convert radiation of frequency $\omega$  to radiation of frequency 3$\omega$  or even higher.

Key words: terahertz, GaN, superlattice, nonlinear transport