Shuzhen You

On the origin of the two-dimensional electron gas at AlGaN/GaN heterojunctions and its influence on recessed-gate metal-insulator-semiconductor high electron mobility transistors

It is commonly accepted that interface states at the passivation surface of AlGaN/GaN heterostructures play an important role in the formation of the 2DEG density. Several interface state models are cited throughout literature, some with discrete levels, others with different kinds of distributions, or a combination of both. The purpose of this article is to compare the existing interface state models with both direct and indirect measurements of these interface states from literature (e.g., through the hysteresis of transfer characteristics of Metal-Insulator-Semiconductor High Electron Mobility Transistors (MISHEMTs) employing such an interface in the gate region) and Technology Computer Aided Design (TCAD) simulations of 2DEG densities as a function of the AlGaN thickness. The discrepancies between those measurements and TCAD simulations (also those commonly found in literature) are discussed. Then, an alternative model inspired by the Disorder Induced Gap State model for compound semiconductors is proposed. It is shown that defining a deep border trap inside the insulator can solve these discrepancies and that this alternative model can explain the origin of the two dimensional electron gas in combination with a high-quality interface that, by definition, has a low interface state density.

Direct comparison of GaN-based e-mode architectures (recessed MISHEMT and p-GaN HEMTs) processed on 200mm GaN-on-Si with Au-free technology

Gallium nitride transistors are going to dominate the power semiconductor market in the coming years. The natural form of GaN-based devices is “normally-on” or depletion mode (d-mode). Despite these type of devices can be used in power semiconductor systems by means of special drivers or in a cascode package solution, yet the market demands for normally-off or enhancement mode (e-mode) devices. In this work, we directly compare and analyze the two most common approaches to obtain GaN-based e-mode devices: recessed gate MISHEMTs and p-GaN HEMTs. Both approaches have their pro’s and con’s as well as their critical process steps.

Stability evaluation of Au-free Ohmic contacts on AlGaN/GaN HEMTs under a constant current stress

Abstract The reliability of Au-free Ohmic contacts has been evaluated under a constant current stress (500xa0mA/mm) in a high temperature environment (150xa0°C, 175xa0°C, and 200xa0°C). Two Ohmic contact schemes with different Ti/Al thicknesses (5xa0nm/100xa0nm and 10xa0nm/100xa0nm) have been tested. The results showed that the degradation of the resistance (Rincrease) is accelerated at a higher temperature condition. Moreover, Rincrease has a square root dependence with the time. This indicates that a diffusion process could be responsible for the observed degradation. This has been confirmed by means of physical failure analyses (FIB/TEM) performed on a highly degraded device: an apparent roughness of the interface between TiN and the Ohmic metal layer was observed. On top of this, Nitrogen out-diffusion into the Ohmic metal was observed in the EDS/EELS analyses, suggesting that a material diffusion phenomenon might play a role. In addition, we observed that the devices with a lower initial contact resistance (Rc) showed lower degradation with respect to devices with a larger initial Rc. This suggests that obtaining lower Rc is beneficial not only for the performance but also for the reliability.