After decades of development, silicon based power devices are approaching their material limits in performance. To further enhance the device performance and thus the power conversion efficiency, power devices based on wide-bandgap semiconductors that possess a higher critical breakdown field are desired. SiC- and GaN-based power devices have been intensively studied in recent years, because of the promise of a significantly lower specific on-resistance (R_on) and a much faster switching speed, compared with the traditional silicon-based power devices for the same voltage rating. However, power devices based on these two material systems still face their respective technical challenges.

In this work, we addressed several critical issuses regarding GaN- and SiC-based power switch...[ Read more ]

After decades of development, silicon based power devices are approaching their material limits in performance. To further enhance the device performance and thus the power conversion efficiency, power devices based on wide-bandgap semiconductors that possess a higher critical breakdown field are desired. SiC- and GaN-based power devices have been intensively studied in recent years, because of the promise of a significantly lower specific on-resistance (R_on) and a much faster switching speed, compared with the traditional silicon-based power devices for the same voltage rating. However, power devices based on these two material systems still face their respective technical challenges.

In this work, we addressed several critical issuses regarding GaN- and SiC-based power switching transistors via proper channel engineering. The work is divided into three parts:

Firstly, an E-mode GaN double-channel MOS-HEMT (DC-MOS-HEMT) is proposed and experimentally demonstrated. The MOS-HEMTs with partially recessed gate is unsuitable for mass production since its V_th is extremely sensitive to the recess depth, although it boasts a high mobility in the gated channel. On the contrary, V_th of the fully recessed gate structure is much less sensitive to the recess depth, but it is suffers from high MOS-channel resistance at gate region and parasitic resistance at the gate recess corners. The proposed GaN DC-MOS-HEMT combines the advantages of the two structures by forming a second channel several nanometers underneath the original one. With the gate recess terminated within the upper channel layer, V_th of the DC-MOS-HEMT is insensitive to the recess depth, while the lower heterojunction channel helps reduce the channel resistance. The DC-MOS-HEMT is first studied with analytical modelling, and numerical simulation. Then, the electrical coupling between two channels of the double-channel heterostructure is experimentally investigated in order to determine the double-channel heterostructure. The fabricated device demonstrates high performances with a positive V_th, low R_on, large breakdown voltage, high current, a steep subthreshold swing, and robustness against process variation.

Secondly, channel engineering of SiC MOSFETs is discussed. The SiC planar MOSFET and SiC trench MOSFET have their respective problems. In the first half of this chapter, we proposed an alternate MSOFET archetecture for SiC: the trench/planar MOSFET (TP-MOS). The TP-MOS combines the advantages of the planar MOSFET and the trench MOSFET, achieving a reduced oxide field, a low R_on and an enhanced switching performance. In the second half of this chapter, the influences of termination of p-shield in a SiC trench MOSFET is studied. The trench MOSFET is an effective approach to lower the channel resistance since its channel density is high. A p-shield is commonly suggested to protect the gate oxide at the trench bottom. It was found in this thesis a floating p-shield induces a series of dynamic performance degradations. These degradations were well explained by a charge storage mechanism. Therefore, a grounded p-shield should be adopted for SiC trench MOSFET.

Thirdly, a GaN/SiC hybrid field effect transistor (HyFET) for power switching applications is proposed. The GaN HEMT boasts a low channel resistance and a low reverse transfer capacitance, but they are unsuitable for high voltage applications (e.g., >1000 V) due to their lateral configuration and significant surface/buffer trapping under high drain voltage. SiC MOSFETs are superior power devices for high voltage applications. The main issue for SiC MOSFETs is the high channel resistance caused by the low channel mobility. Since epitaxial growth of GaN layers on SiC substrate is a rather mature technology, a superior power device is expected if the merits of both SiC devices and GaN HEMTs are combined on a single platform. The GaN/SiC hybrid field-effect transistor (HyFET) utilizes a high mobility enhancement-mode AlGaN/GaN channel to reduce the channel resistance, while a vertical 4H-SiC drift region is used to sustain the high off-state voltage. Remarkable improvements on R_on, C_GD, and gate charges are achieved in the HyFET compared with the conventional SiC MOSFET for the same voltage rating.