(Invited) Growth and Characterization of High Power AlInN/GaN HEMTs

AlInN, a material lattice-matched to GaN, is very promising for high current, high transconductance, and high RF performance high-electron-mobility transistors (HEMTs). However, it is challenging to grow high quality AlInN HEMTs due to the large immiscibility and distinct thermal stability of AlN and InN, which causes premature voltage breakdown. Several methods have been proposed to solve this problem, such as using a thin AlInN barrier layer and field plate structure (V_B = 400 V), Schottky source/drain (V_B = 460 V), or 5 % AlGaN back barrier layer (V_B = 3 KV). However, most of these methods are still problematic in maintaining a low R_on and high V_B simultaneously. GaN cap has been widely used to achieve good surface smoothness and reduce current collapse in AlGaN/GaN-based HEMTs. GaN MOS-HEMTs with a thick GaN cap layer to reduce the surface field for high breakdown voltage have also been reported (V_B = 840 V). Our study shows that GaN cap layer also has a positive influence on AlInN HEMTs in reducing current collapse phenomenon, dissipating electric field near the gate edge and enhancing the breakdown voltage. By using a GaN cap layer, AlInN-based HEMTs, exhibiting low Ron and high VB simultaneously, have been demonstrated on 6 inch silicon substrates in this work.

Two AlInN-HEMT structures denoted as sample A and B were prepared by metal-organic chemical vapor deposition on 6 inch Si (111) substrates. Sample A is a typical HEMT structure without a GaN cap, while sample B has an additional 13 nm-thick GaN cap layer on top of the AlInN layer. Room temperature Hall mobility, two-dimensional electron gas (2DEG) concentration and sheet resistance (R_sh) of sample A are 886 cm²/V.s, 2.49x10¹³ cm^-2 and 283 ohm/sq, while those of sample B are 1,450 cm²/V.s, 1.61x10¹³ cm^-2 and 267 ohm/sq, respectively. Due to the reverse polarization effect of GaN cap on sample B, the 2DEG concentration of sample B is lower than that of sample A.

Dynamic Ron of power electronics reflects acceptor-like defect in the material which could be improved by decreasing electric field at the gate edge or decreasing the defect density in the material. Devices were tested under off-state condition with a constant Vd for 1 second, then switched to on-state to observe the variation of Ron. Sample A exhibits serious degradation on dynamic Ron with time. The ratio of dynamic Ron over static Ron of sample A reaches 5 under 20 V of Vd stress, while that of sample B can be maintained at 1.3 under the same test conditions. As the off-state stress Vd increases, the dynamic Ron ratio of sample B also increases significantly and reaches 5 under 60 V of Vd stress. The improvement of dynamic Ron on AlInN HEMT with a GaN cap implies the trap density of sample B is significantly reduced

The on-state I-V characteristics of the HEMTs shows that I_d,max of sample A and B at Vg of 0 V are 480 mA/mm and 660 mA/mm, respectively. Thus, the extracted R_on is improved from 4.6 mohm-cm² of sample A to 3.7 mohm-cm² of sample B. This is consistent with their Hall results. The extrapolated threshold voltage (V_th) of sample B is -9.1 V, which is larger than -6.3 V of sample A due to its thicker intermediate layer and the presence of the GaN cap layer. With the help of a GaN cap layer, which disperses the electrical field near the gate edge, sample B has a lower drain leakage current than sample A. While the V_B of sample A reaches 530 V, indicating the crystal quality of AlInN is comparable to that of the traditional AlGaN-based HEMTs, the GaN cap extends the breakdown voltage of sample B to 675 V.