Abstract
Ultra-thin AlN/GaN heterostructure field-effect transistors (HFETs) with, and without, SiN passivation were fabricated by the same growth and device processes. Based on the measured DC characteristics, including the capacitance-voltage (C-V) and output current-voltage (I-V) curves, the variation of electron mobility with gate bias was found to be quite different for devices with, and without, SiN passivation. Although the AlN barrier layer is ultra thin (c. 3 nm), it was proved that SiN passivation induces no additional tensile stress and has no significant influence on the piezoelectric polarization of the AlN layer using Hall and Raman measurements. The SiN passivation was found to affect the surface properties, thereby increasing the electron density of the two-dimensional electron gas (2DEG) under the access region. The higher electron density in the access region after SiN passivation enhanced the electrostatic screening for the non-uniform distributed polarization charges, meaning that the polarization Coulomb field scattering has a weaker effect on the electron drift mobility in AlN/GaN-based devices.
Highlights
Attributed to the high critical field and electron velocity, nitride heterostructures have attracted great attention because of the excellent potential application in highvoltage and high-power operations at microwave/submicrowave frequency [1,2,3,4,5]
It was deemed worthwhile to investigate the influence of SiN passivation on the ultra-thin AlN barrier layer and the transport properties in AlN/GaN heterostructure field-effect transistors (HFETs)
Rectangular HFET devices with, and without, SiN passivation were fabricated on ultra-thin AlN/GaN heterostuctures
Summary
Attributed to the high critical field and electron velocity, nitride heterostructures have attracted great attention because of the excellent potential application in highvoltage and high-power operations at microwave/submicrowave frequency [1,2,3,4,5]. Thanks to the large bandgap energy and conduction-band offset to GaN, AlN/ GaN heterostructures with ultra-thin barrier layer (~3 nm) are expected to be important in threedimensional device scaling in order to obtain high frequencies, enabling the realization of millimeter-wave and/or even sub-millimeter-wave power devices [6,7,8]. The PCF scattering exerts a dominant influence on electron drift mobility in AlN/GaN HFETs due to the thin barrier layer [26, 27]. Whether the SiN passivation induces additional stress in the AlN barrier layer or just affects the surface properties of the AlGaN barrier layer, the increase in 2DEG after SiN passivation will affect the PCF scattering, which influences electron mobility in AlN/GaN HFETs. As a result, it was deemed worthwhile to investigate the influence of SiN passivation on the ultra-thin AlN barrier layer and the transport properties in AlN/GaN HFETs
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