| Summary: | 碩士 === 國立中央大學 === 電機工程研究所 === 100 === Enhancement mode AlGaN/GaN MOSFETs fabricated by gate recessed technique and MIS structure is mentioned in this study. ICP dry etch the gate recessed area with BCl3/Cl2/Ar/O2 (19/50/5/6 sccm) gas can effective reduce the etching rate and surface roughness. Furthermore, Compare with other gate dielectric material the Al2O3 gate dielectric grew by ALD (Atomic Layer Deposited) with high k and wide band gap can reduce the gate leakage and improve transconductance. According to the electrical property of 2μm L¬g single device, we design finger type device compare with circular device. Circular type device can achieve 190 mA/mm current density and 76.3 mS/mm transconductance is better than finger type device with 150 mA/mm current density and 45 mS/mm transconductance. From the slope of Id-Vd curve, we can estimate the on resistance of two devices. The on resistance of circular type device (7.53 mΩ-cm2) is lower than finger type device (8.21 mΩ-cm2). The threshold voltage of two different kinds of device are 1.5V and 1V. According to the I-V characteristic in off state, the gate leakage current of Circular type device is two orders lower than finger type device. The oxide breakdown is about 12V and the maximum breakdown of single device is 118 V.
In order to realize high current property of power device, we design multi-finger device to achieve. Forty finger type device can achieve 0.38 A high current and 1.8V threshold voltage, forty circular type device can achieve 0.44 A high current and 1.3 V threshold voltage. In addition, we find that two-thirds of total resistance is probe resistance and bus line resistance. This indicate that the real injected current is lower than our expectation. We can use wire bonding method to improve the current property of the 40 finger device.
By analyzing the band diagram of GaN MOS capacitance with different bias and trap state, we can find the relationship between interface state and threshold voltage. Furthermore, we also find that more acceptor type interface state will cause leakage current path in gate interface. So, it is important to reduce the interface state in Gate recessed MOSFETs.
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