Local 2DEG Density Control in Heterostructures of Piezoelectric Materials and Its Application in GaN HEMT Fabrication Technology

Abstract

This paper investigates the local control of two dimensional electron gas (2DEG) density in AlGaN/GaN heterostructures by applying external mechanical stress. The proposed 2DEG tuning mechanism relies on a local modification of the piezoelectric contribution to the total 2DEG concentration at an AlGaN/GaN heterostructure interface. Discontinuities of internally stressed passivation films are known to locally modify mechanical stress in their vicinity and may, therefore, transfer mechanical strain to neighboring device regions. If a mechanically stressed passivation laterally terminates at a gate trench on top of a GaN/AlGaN heterojunction, it will, therefore, induce mechanical strain into the semiconductor at the location of the trench sidewalls. This locally affects the piezoelectric polarization vector and, thus, changes 2DEG concentration in this region. As this effect directly modifies electronic properties underneath the gate electrode, a shift of threshold voltage will be the consequence. This phenomenon has been investigated by coupling mechanical and physical device simulation and by experimental verification of the simulation results. Finally, as one of the several possible application examples, normally-off (E-mode) and normally-on (D-mode) transistors were monolithically integrated on the same wafer by embedding the respective gate electrodes into passivation layers with different degrees of mechanical stress. DC measurements of the obtained transistors showed competitive performance levels.