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RF Characteristics of Graphene-Channel Device
그래핀을 채널로 갖는 소자의 RF 특성분석

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Authors
이재호
Advisor
신형철
Major
전기·컴퓨터공학부
Issue Date
2012-02
Publisher
서울대학교 대학원
Abstract
Radio-Frequency (RF) performance of devices with graphene grown using low temperature Inductively-Coupled Plasma Chemical Vapor Deposition (ICP-CVD) method on 6-inch wafer was measured. To remove the coupling of electrode in-plane, locally-embedded-back-gate using TiN metal was introduced. The symmetric structure of 2-gate fingers was adopted to reduce misalign issue during fabrication of the structure with underlap between Gate and Source/Drain, which was also adopted for the reduction of parasitic capacitance due to gate oxide with high dielectric constant. Cutoff frequency (fT) increase is moderately obtained with the decrease of gate length. Despite the low transconductance (gm) due to underlap region, we obtained fT =80 GHz.
To apply graphene device in RF circuit design, the exact modeling of device should be needed. The equivalent circuit of device is constructed and device parameters are extracted. The capacitance of quantum effect is induced by the density of state (DOS) of graphene which is called quantum capacitance. If graphene is incorporated in a capacitor, it contributes significantly into the total capacitance due to the electronic compressibility. Graphene has an ambipolar trend which continuously increases from dirac point. Quantum capacitance of graphene is measured in MIM capacitor. However, it is not obtained from RF measurement of graphene device. In this thesis, quantum capacitance is extracted using two-step process. First, the equivalent circuit of graphene device is acquired exactly and Y-parameter equations of device are derived. Then, quantum capacitance is acquired from RF measurement and compared to capacitance of MIM pattern. The equivalent circuit of graphene device and Y-parameter equations are suggested. Device patterns are also investigated to measure RF and DC characteristics.
그래핀은 육각형 벌집 구조의 탄소로 이루어진 한 층의 물질을 말하며 unique band-structure에 의한 높은 carrier mobility, ambipolar 거동을 보이는 band-gap이 없는 semi-metal의 특성을 갖게 된다. 특히 높은 carrier mobility를 갖기 때문에 고속 동작이나 증폭 분야 응용이 모색되고 있으며 SiC 기판의 소자나 nano-wire gate를 사용한 소자들로 RF 특성을 평가한 내용들이 보고되고 있다. 각각의 경우, cutoff frequency 100 GHz, 300 GHz 성능을 보인다. 반면, on-off ratio가 10 미만이기 때문에 digital application 을 위해서는 좀 더 특성 개선이 필요하며 off-current가 크기 때문에 logic 분야에도 응용을 위해서는 좀 더 연구가 필요하다. RF 소자의 경우, 크게 두 가지의 이슈가 있다. 기존 Si 기반 CMOS 공정에 기반하여 wafer scale의 균일도를 갖도록 제작하는 것과 소자의 RF 특성을 높이기 위한 설계이다. RF 특성을 높이기 위해서는 gain loss를 줄이기 위한 parasitic capacitance 억제와 transconductance 향상, 아울러 정확한 측정이 필요로 한다.
본 연구에서는 이런 이슈 해결을 위해 photo, ebeam 등의 CMOS 호환공정을 사용 및 RF 특성향상을 위한 back-gate 형태의 소자설계와 그래핀 층의 성능저하를 막기 위한 공정설계를 진행하였다. 아울러 정확한 성능확인을 위해 측정패턴 고려 및 de-embedding process를 최적화에 대한 연구를 통한 소자성능 향상 및 특성에 대해 기술하였다. 또한 그래핀의 특성 중 하나인 quantum capacitance를 소자에서 추출하는 방법을 제안하여 quantum capacitance를 확인하여 보았다.
Language
eng
URI
http://hdl.handle.net/10371/156618

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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Electrical and Computer Engineering (전기·정보공학부)Theses (Ph.D. / Sc.D._전기·정보공학부)
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