Optimization of passivation in AlGaN/GaN heterostructure microwave transistor fabrication by ICP CVD

Abstract

We have studied the effect of silicon nitride (SiN) dielectric passivating film deposition by inductively coupled plasma chemical vapor deposition (ICP CVD) on the parameters of AlGaN/GaN heterostructure high electron mobility transistors (HEMT). Study of the parameters of the dielectric layers has allowed us to determine the effect of RF and ICP power and working gas flow ratio on film growth rate and structural perfection, and on the current vs voltage curves of the passivated HEMT. The deposition rate changes but slightly with an increase in RF power but increases with an increase in ICP power. Transistor slope declines considerably with an increase in RF power: it is the greatest at minimum power RF = 1 W. In the beginning of growth even at a low RF power (3 W) the transistor structure becomes completely inoperable. Dielectric deposition for HEMT passivation should be started at minimum RF power. We have developed an AlGaN/GaN microwave HEMT passivation process providing for conformal films and low closed transistor drain–source currents without compromise in open state transistor performance: within 15 and 100 mA, respectively, for a 1.25 and 5 mm common T-gate (Ug = –8 V and Ud-s = 50 V).