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High Speed Tunneling Transistor based on Metallic Materials

High Speed Tunneling Transistor based on Metallic Materials
Alternative Title
금속 성질을 가지는 물질 기반의 고속 동작 터널링 트랜지스터
Shin, Jeong Hee
Jang, Jae Eun
Lee, Youn Gu
Issued Date
Awarded Date
2018. 2
MIM tunnel devicetunnelinggeometric effectworking efficiencycontrast ratioMIM 소자터널링기하학적 효과효율스위칭 대조비
There has been increasing demand for electronic devices with ultra-high working frequency or speed to process big data quickly, especially in the fields of communications, the military, and aerospace as well as a high performance control process unit (CPU). Although parallel computing or computational method have been studied to process big data, it is limited by Amdahl’s law which describes speed saturation even with large number of processors. Therefore, improvement of fundamental components such as diode and transis-tor can be a basic solution. The p-n junction and Schottky barrier are widely used for switching behavior in diode and transistor structure, so far. The switching speed can be controlled easily by the channel length be-tween source and drain in conventional metal-oxide semiconductor field-effect transistor (MOSFET). The channel should be as short as possible to drive high switching speed, while the electrons cannot be controlled well by gate bias due to short channel effect in the p-n junction and Schottky barrier based MOSFET. Elec-tron mobility was considered instead of physical dimension. Although high electron mobility transistor (HEMT), based on III-V group materials, is expected to increase the speed, its theoretical frequency limit is less than 1THz, even with several nanometer channel length. Therefore, conventional switching mechanisms should be substituted to new mechanism which can drive high speed without electron retardation. Tunneling mechanism in non-semiconductive materials was considered in metal-insulator-metal (MIM) structure. Alt-hough MIM tunneling structure can provide ultra-high working speeds (>THz), the very low contrast ratio between forward and reverse currents results in poor working efficiency. Improving efficiency in MIM tunnel devices is highly motivated for high-speed efficient switching elements.

Improving efficiency in various MIM tunnel structures is the main work of this dissertation. Although the work function differences and multi-stack insulators in MIM structure are widely used to increase effi-ciency, these are limited by low work function differences in metallic materials and increase of uncertainty factors, respectively. In here, we suggested asymmetrically geometric design to obtain different tunneling probabilities in MIM structure. 50% ratio of shifted tunneling distance could be obtained by small angle of pointed shape. By theoretical investigation and simulation, geometric design can induces much shorter or longer tunneling distance, which is effective approach for enhancing contrast ratio. In the vertical structure, over 70% ratio of shifted tunneling distance could be theoretically expected by electrical field simulation (5nm Al2O3 and 20° angle of pointed shape). Resulting from comparison of 20° and 90° angle of pointed shape shows large different tunneling probabilities of angle dependence (0.306% and 0.014%, respectively). Therefore, geometric effect is promising method to increase working efficiency.

The quality of insulator materials is one of the key parameters to enhance geometric effect. The vacu-um which is closed to ideal insulation was used to investigate only geometric effect in MIM transistor struc-ture. Laterally moved tunneling electrons can be controlled by bottom a gate electrode, proved by quite good contrast ratio (12.7) and controlling threshold voltage of FN tunneling (5.25V) under vacuum state (3 × 10-6 torr). Even though these parameters related to efficiency are better than that of conventional metal-insulator-metal diode structure, the threshold voltage should be within 5V for compatible to conventional complemen-tary metal-oxide semiconductor (CMOS). Lateral MIM structure, by using sputtered SiO2 instead of vacuum to decrease threshold voltage and working voltage, shows poor contrast ratio and a little gate effect. While the vacuum guarantees quality of insulation by vacuum pressure, the quality of insulator materials is defined by defect or pinhole, which are normally ranked by deposition methods. Furthermore, selection of insulation materials was also important for improving efficiency. Although the band gap of Al2O3 is larger than that of HfO2, the working efficiency was significantly improved by blocking reverse current well. In general, large band gap insulator materials are inappropriate for a tunneling device, due to the low tunneling current. How-ever, in our approach, since the issue of low tunneling probability is compensated by the sharp tip structure, the larger band gap insulator produced better working efficiency with the appropriate current density.

Directionality of electron movement was considered in geometric designed MIM transistor structure for high-speed and improving efficiency. It is relevant to technical ways to define the channel length. The gap between source (geometric designed electrode) and drain is important parameter to operate high speed in MIM transistor structure. While small gap is not reliable in lateral structure, defined by even electron beam lithography, reliable gap in vertical structure was achieved by deposition system controlled with Å level. Alt-hough the electrons are not effectively controlled in vertical orient structure, applying a floating electrode in vertical MIM transistor can control the tunneling current with little leakage current. Low threshold voltage (0.75V) and considerable efficiency (37 contrast ratio) can be achieved by bias control of floating electrode. It shows reliable I-V characteristics under high temperature environment up to 398K, proved that tunneling phenomenon is dominant. By estimating cut-off frequency (0.82 THz), vertical MIM tunneling transistor with floating electrode can be expected for THz applications in various fields, such as communication devic-es, high speed electrical switches, and high performance control process units (CPUs), or other new concept devices. ⓒ 2017 DGIST
Table Of Contents
Ⅰ. Introduction 1--

1.1 Background and motivation 1--

1.2 Related works 5--

1.3 Objectives 10--

1.4 Thesis organization 11--

ⅠI. Geometric effect in metal-insulator-metal structure13--

2.1 Introduction 13--

2.2 Theory of geometric design in potential equation 13--

2.3 The simulation of geometric effect in MIM structure 17--

2.4 Angle dependence of geometric effect 22--

ⅠII. Lateral MIM transistor 24--

3.1 Introduction 24--

3.2 Fabrication of lateral MIM structure 24--

3.2.1 Fabrication of lateral metal-vacuum-metal (MVM) transistor 24--

3.2.2 Fabrication of lateral metal-insulator-metal (MIM) transistor 26--

3.3 Lateral metal-vacuum-metal (MVM) transistor 28--

3.3.1 Characteristics of source and drain in lateral MVM structure 30--

3.3.2 Gate effect of multi tip electrode in MVM structure 31--

3.4 Lateral metal-insulator-metal (MIM) transistor structure 36--

3.4.1 Simulation of lateral MIM transistor structure 37--

3.4.2 Electrical characteristics of lateral MIM transistor structure 40--

3.5 Limit of lateral MVM and MIM transistor structure 41--

ⅠV. Vertical MIM diode structure 43--

4.1 Introduction 43--

4.2 Fabrication of vertical MIM structure 43--

4.2.1 Fabrication of vertical stack-up MIM diode with a rectangular electrode 43--

4.2.2 Fabrication of vertical stack-up MIM diode with multi sharp electrodes 47--

4.2.3 Fabrication of vertical stack-up MIM diode with 5 sharp electrodes 50--

4.3 Investigation of rectangular type electrode in vertical MIM diode structure 50--

4.4 Investigation of sharp type electrode in vertical MIM diode structure 55--

4.4.1 Theoretical study of sharp electrode in vertical MIM diode structure by simulation 55--

4.4.2 Electrical characteristics of multiple sharp electrodes in vertical MIM diode structure 61--

4.4.3 Investigation of insulator selection for geometric effect 68--

V. Vertical MIM transistor structure 70--

5.1 Introduction 70--

5.2 Fabrication of vertical MIM structure 70--

5.2.1 Fabrication of vertical MIM transistor with bottom or top gate electrode 70--

5.2.2 Fabrication of vertical MIM transistor with floating electrode 73--

5.3 Vertical MIM transistor 75--

5.3.1 Locating electrode in vertical MIM transistor 75-- Bottom gate electrode in vertical MIM transistor 75-- Top gate electrode in vertical MIM transistor 79--

5.3.2 Vertical MIM transistor with a floating electrode 83-- Simulation of vertical MIM transistor with a floating electrode 84-- Electrical characteristics of vertical MIM transistor with a floating electrode 89-- Working mechanism of vertical MIM tunneling transistor with floating electrode 93-- Temperature dependence 100-- Frequency response 102--

VI. Conclusion 104--

6.1 Summary 104--

6.2 Future works 105--

6.2.1 Experimental frequency response using RF measurement 105--

6.2.2 Size or thickness dependence of floating electrode for higher contrast ratio 107--

6.2.3 Vertical stack-up MIM tunneling transistor with graphene inter layer 108--

References 109--
Information and Communication Engineering
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Department of Electrical Engineering and Computer Science Theses Ph.D.


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