Student Number 89222013 Author Chi-Joe Kao(°ªªv¦à) Author's Email Address No Public. Statistics This thesis had been viewed 2346 times. Download 1493 times. Department Physics Year 2004 Semester 2 Degree Ph.D. Type of Document Doctoral Dissertation Language English Title The application of surface insulating layer on wide-bandgap semiconductor devices Date of Defense 2005-05-21 Page Count 108 Keyword FETs GaN HFETs insulators MSM photodectors ZnO 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.
Table of Content ºKn
Chapter 1: Introductions-1
1.1 The overview of insulator applications on GaN and ZnO-based field-effect transistors and photodetectors-1
1.2 The features in this dissertation-5
Chapter 2: The theories of insulator on AlGaN/GaN heterostructure semiconductor materials-14
2.2 Carrier concentration in AlGaN/GaN HFETs-15
2.2.1 Free electron distribution in AlGaN/GaN HFETs-15
2.2.2 The effect of polarization fields on carrier properties in AlGaN/GaN HFETs-18
2.3 The influence of insulators on GaN-based semiconductor materials-21
2.3.1 Influence of surface states on the electron concentration in AlGaN/GaN HFETs-21
2.3.2 The influence of insulators on Surface state-22
Chapter 3: The applications of LT-GaN on GaN-based MSM photodectors-27
3.2 MSM photodetector device structures and fabrication processes-28
3.2.1 Structures growing and preparing-28
3.2.2 Device fabrication and processes-29
3.3 Device measurement-31
3.3.1 Dark-current measurement-31
3.3.2 Photocurrent measurement-32
3.4 The influence of LT-GaN surface insulating on GaN-based MSM photodetectors-33
3.4.1 The influence on electrical characteristics-33
3.4.2 The influence on optical characteristics-35
3.5 The comparisons of device characteristics-36
3.5.1 Dark-current and photocurrent behaviors and comparisons-36
3.5.2 The photo-responsivity performance and discussions-39
Chapter 4: The comparisons of LT-GaN, SiO2 and Si3N4 as gate insulators on AlGaN/GaN HFETs-46
4.2 HFET device structures preparing and fabrication processes-48
4.2.1 Heterostructure FETs growing-48
4.2.2 Insulators growing and preparing-48
4.2.3 Devices fabrication and processes-51
4.3 Materials Hall analysis and devices DC Measurement-54
4.3.1 The influence of insulators on GaN-based HFET structures-54
4.3.2 DC measurement-57
4.3.3 DC performance comparisons-60
4.4 Results and comparisons from pulsed measurement-64
4.4.1 Pulsed measurement-64
4.4.2 Device behaviors and discussions-66
4.5 Results and comparisons from RF measurement-68
Chapter 5: The comparisons of ZnO MOSFET and MESFET structures grown by PLD-79
5.2 Device structures preparing and fabrication processes-82
5.2.1 Material grown by Pulsed Laser Deposition-82
5.2.2 Fabrication and processes-83
5.3 The electrical comparison between ZnO MOSFET and MESFET grown on sapphire-85
5.3.1 The Ohimc contacts and insulations electrical characteristics-85
5.3.2 The schottky behaviors comparison-87
5.3.3 C-V characteristics-88
5.3.4 The DC characteristics comparisons and discussions-90
5.4 ZnO MOS-gate devices with phosphorus-doped channels and ITO-coated glass substrates-92
5.4.1 The DC performance-93
5.4.2 The Transfer DC performance-94
Chapter 6: Conclusions-101
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Advisor Gou-Chung Chi(¬ö°êÄÁ)
89222013.pdf Date of Submission 2005-07-04