Abstract
With the rapid depletion of communication-frequency resources, mainly due to the explosive spread of information communication devices for the internet of things, GaN-based high-frequency high-power transistors (GaN-HEMTs) have attracted considerable interest as one of the key devices that can operate in the high-frequency millimeter-wave band. However, GaN-HEMT operation is destabilized by current collapse phenomena arising from surface electron trapping (SET), which has not been fully understood thus far. Here, we conduct quantitative mechanistic studies on SET in GaN-HEMTs by applying element- and site-specific photoelectron nanospectroscopy to a GaN-HEMT device under operation. Our study reveals that SET is induced by a large local electric field. Furthermore, surface passivation using a SiN thin film is demonstrated to play a dual role: electric-field weakening and giving rise to chemical interactions that suppress SET. Our findings can contribute to the realization of high-capacity wireless communication systems based on GaN-HEMTs.
Highlights
A typical GaN-HEMT utilizes the two-dimensional electron gas (2DEG) formed at the GaN/AlGaN interface as the conducting channel
Kelvin probe force microscopy (KPFM) without element-specificity cannot distinguish between information on the surface electronic states of the GaN-HEMT from that of carbon contaminants, which can lead to misinterpretation of the obtained results
Operando photoelectron nanospectroscopy is a powerful tool to clarify surface-related phenomena that influence electronic device performance because of its ability to probe the surface electronic states in an elementand site-specific manner. By combining this method with I–V measurements mediated by device simulation, we succeeded in quantitatively clarifying the mechanism of surface electron trapping responsible for the current collapse phenomena in GaN-HEMTs
Summary
A typical GaN-HEMT utilizes the two-dimensional electron gas (2DEG) formed at the GaN/AlGaN interface as the conducting channel. Understanding the physical mechanisms involved in charge trapping is vital for optimizing the device performance to realize the required stable operation of GaN-HEMTs in the millimeter-wave domain To this end, operando spectromicroscopy, i.e., spatially resolved spectroscopy of the surface electronic states during device operation, is considered an ideal tool to clarify the trapping processes. We have already demonstrated that operando X-ray photoelectron spectromicroscopy can provide information on the electronic states of a graphene transistor in an element-specific and spatially resolved manner This was made possible by site-selectively observing the gate-bias-induced shift of the Fermi level of the channel region obtained through precise measurement of the binding energy, i.e., the energy difference between the Fermi level and the core level, which enables element-specific investigation of the Fermi level position at the solid surface[16]. Operando photoelectron spectromicroscopy is considered adequate for clarifying the surface trapping mechanism that causes the current collapse phenomena in GaN-HEMTs
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