Influence of the dynamic access resistance in the gm and fT linearity of AlGaN/GaN HEMTs

The decrease of transconductance g_m and current gain cutoff frequency f_T at high drain current levels in AlGaN/GaN high-electron mobility transistors (HEMTs) severely limits the linearity and power performance of these devices at high frequencies. In this paper, the increase of the differential source access resistance r_s, with drain current is shown to play an important role in the fall of g_m and f_T. The increase of r_s occurs due to the quasi-saturation of the electron velocity in the source region of the channel at electric fields higher than 10 kV/cm. This has been confirmed by both experimental measurements and two-dimensional drift-diffusion simulations. Through simulations, we have identified HEMT structures with source implanted regions (or n⁺⁺ cap layers) as good candidates in order to increase the linearity of the g_m and f_T versus current profile.