Effect of bottom SiN thickness for AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors using SiN/SiO2/SiN triple-layer insulators

We fabricated AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) using SiN/SiO2/SiN triple-layer insulators. Our MIS-HEMTs are mechanically stable in the sub-50-nm-gate region and free from damage caused by dry etching. We fabricated two kinds of MIS-HEMTs with different bottom SiN thickness d(B_SiN), i.e., 2.5 and 5 nm. We reduced gate length L-g down to 25 nm. For the MIS-HEMTs with a d(B_SiN) of 5 nm, a cutoff frequency f(T) at L-g = 35 nm starts to decrease with reducing L-g On the other hand, we obtained a maximum fT of 145 GHz for a 35-nm-gate MIS-HEMT with a d(B_SiN) of 2.5 nm. Moreover, the f(T) at L-g = 25 nm is almost same as that at L-g = 35 nm. These results indicate that the short-channel effects are suppressed by decreasing d(B_SiN) to be 2.5 nm. The threshold voltages also indicate the suppression of the short-channel effects by decreasing d(B_SiN).