Effects of Si₃N₄ passivation on the dc and RF characteristics of metamorphic high-electron-mobility transistors depending on the gate-recess structures

Effects of the Si₃N₄ passivation on the dc and RF characteristics of a 0.1 νm metamorphic high-electron-mobility transistor (HEMT) are investigated for narrow and wide gate-recess structures. Maximum drain-source saturation current (I_dss,max) and maximum extrinsic transconductance (g_m,max) are reduced by ∼14.8 and ∼11.6%, respectively, in the wide gate-recess structure after the passivation; on the other hand, only ∼5.7 and ∼4.9% reductions are measured from I _dss,max and g_m,max, respectively, in the narrow gate-recess structure. We examine the passivation-induced degradation by using a modified charge control model assuming the charged surface states on the Si₃N₄ interface and a comparative study of the hydrodynamic device simulation with the experimental measurement. From the analysis, it is proposed that the difference of degradation in two different gate structures is due to an approximately three times higher charged surface state density of ∼4.5 × 10¹¹ cm^-2 in the wide gate-recess structure than ∼1.6 × 10¹¹ cm^-2 in the narrow gate-recess structure. The cut-off frequency (f_T) of the wide gate-recess structure also exhibits a greater reduction of ∼14.5%, while the f_T of the narrow gate-recess structure is reduced by only ∼6.6% after the passivation. This is mainly due to the passivation-induced surface states of a higher density in the wide gate-recess structure. A great increase of the gate-to-drain parasitic capacitance in the wide gate-recess structure makes a major contribution to ∼13.5% degradation of the maximum frequency of oscillation.