| dc.description.abstract | Abstracts
In this dissertation, the applications of insulators/oxides on GaN-based MSM photodectors or GaN-, ZnO-based FETs are performed. Several application cases and theories of insulator on GaN- and ZnO-based devices are introduced here:
GaN-based MSM UV photodetectors with a low-temperature grown GaN (LT-GaN) layer is demonstrated first. It was found that we could achieve a two-order of magnitude smaller, dark-current of GaN MSM photodetector by employing a
LT-GaN surface insulating layer. This result could be attributed to the larger Schottky-barrier height between the Ni/Au metal contact and the LT-GaN surface insulating layer. It was also found that photodetectors with the LT-GaN layer could provide a larger photocurrent to dark-current contrast ratio and a larger UV-to-visible rejection ratio. The maximum responsivity was found to be 3.3 A/W and 0.13 A/W
when the biases were at 5 V and 1 V, respectively.
The performance of AlGaN/GaN heterostructure field-effect transistors (HFETs) with either uncapped free surface or with LT-GaN, SiO2, Si3N4 as gate insulators is examined second. LT-GaN, SiO2 and Si3N4 surface high-resistivity layer disposed on the AlGaN/GaN heterostructures resulted in an increase of sheet carrier concentrations. These observations could be attributed from the passivation effect to passivate the surface states, thereby having a different, maybe lower, electronic density of states compared to the AlGaN free surface. To clarify the effect of these surface insulating
layers on the AlGaN/GaN HFET structures, Hall measurement were performed here. The sheet carrier concentrations of AlGaN/GaN HFETs with any of these surface insulating layers are similar to each other and about 100% higher than that in an AlGaN/GaN HFET structure with a free surface. Due to the better lattice match with
the AlGaN surface layer, the HFET with a LT-GaN layer as the gate insulating layer shows the best DC and RF device performance, demonstrating that this material is an effective insulator for nitrides.
Comparison of ZnO MOSFETs and MESFETs fabricated on the same wafers using either sapphire or glass substrate is report finally. ZnO thin film field effect transistors with 1.5-20um gate width were fabricated using either a metal gate (metal-semiconductor field-effect transistor, MESFET) or a metal-oxide-semiconductor (MOS) gate. In both cases we found that use of a thick (around 0.8~0.9µm) ZnO buffer was necessary on the sapphire or glass substrate prior to growing the active layers in order to reduce gate leakage current. The MOS
structure with a 50-nm-(Ce,Tb)MgAl11O19 gate dielectric showed an order of magnitude lower gate leakage current than the MESFET, due to the relatively high barrier height of MOS structure. Good drain-source current characteristics were
obtained from MOS gate structures using phosphorus-doped ZnO channels, whereas the metal structures showed very poor modulation.
For the general speaking form this dissertation, surface insulating layer could provide device high Schottky barrier height, low metal/semiconductor leakage current, low surface state density and highly stable device performance. One may use surface insulating layer to achieve more stable, even higher, performance of semiconductor device easily. | en_US |