Control of Threshold Voltage in GaN Based Metal--Oxide--Semiconductor High-Electron Mobility Transistors towards the Normally-Off Operation

• Published in: Japanese Journal of Applied Physics 2013-08, Vol.52 (8), p.08JN08-08JN08-5
• Main Authors: Tapajna, Milan, Kuzmik, Jan
• Format: Article
• Language: English
• Subjects: Aluminum gallium nitrides; Aluminum oxide; Charge; Dielectrics; Gallium nitrides; Gates; Hafnium oxide; Oxides; Threshold voltage
• ISSN: 0021-4922; 1347-4065
• DOI: 10.7567/JJAP.52.08JN08

The effect of defect charges on a threshold voltage ($V_{\text{th}}$) of the GaN/AlGaN/GaN metal--oxide--semiconductor heterostructures (MOS-Hs) with the gate stack thickness scaling was investigated by using capacitance--voltage measurements and a comprehensive analytical model [M. Ťapajna and J. Kuzmík: Appl. Phys. Lett. 100 (2012) 113509]. Using the MOS-Hs with Al 2 O 3 and HfO 2 dielectrics grown by plasma-enhanced atomic layer deposition, a high density (${\sim}10^{13}$ cm -2 ) of negative fixed oxide charge ($N_{\text{ox}}$) was extracted for Al 2 O 3 layers, while an order of magnitude lower density (${\sim}10^{12}$ cm -2 ) of positive $N_{\text{ox}}$ was extracted for HfO 2 films. Consequently, despite similar dielectric constant of Al 2 O 3 to that of (Al)GaN, it is advisable to attain normally-off operation by scaling the AlGaN layer thickness rather than the oxide thickness, taking advantage of the negative $N_{\text{ox}}$ offsetting $V_{\text{th}}$ towards the positive voltages. Scaling of the AlGaN layer thickness is found to be effective also in the case of HfO 2 dielectric, however, due to a positive $N_{\text{ox}}$ in HfO 2 , AlGaN layer needs to be scaled to a similar thickness (e.g., ${\sim}3$ nm) to obtain a positive $V_{\text{th}}$ for the same oxide thickness of HfO 2 and Al 2 O 3 (${\sim}5$ nm). On the other hand, scaling of the GaN cap has no effect on $V_{\text{th}}$. Further, our analysis suggests that for MOS-Hs with both Al 2 O 3 and HfO 2 gate dielectric, the Fermi level position at the oxide/barrier interface in equilibrium is located within donor-like interface traps. Therefore, the oxide/GaN cap interface trap charge in MOS-Hs with the given structure parameters should lead to a negative $V_{\text{th}}$ shift.