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Overcoming Thermodynamic Limit of Subthreshold Swing in MOSFET: Device Structure and Unconventional Source Metal

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Title
Overcoming Thermodynamic Limit of Subthreshold Swing in MOSFET: Device Structure and Unconventional Source Metal
Other Titles
금속 산화막 반도체 전계효과 트랜지스터에 대한 문턱전압이하 스윙의 열역학적 한계 극복: 소자 구조 및 비 전통적인 소스 금속
Author
Lee, Jung-Yong
Advisor
Park, Kibog
Keywords
MOSFET; Subthreshold swing; Dirac source
Issue Date
2020-02
Publisher
Graduate School of UNIST
Abstract
Subthreshold swing is one of most important parameters in controversial metal-oxide-semiconductor (CMOS) technology, which is related on power consumption. In the metal-oxide-semiconductor field effect transistor (MOSFET), there is thermodynamic limit of subthreshold swing of 60 mV/dec at room temperature. In order to achieve the subthreshold swing, edge-over MOSFET structure is proposed, transistor channel of EO MOSFET is formed on sidewall of insulating pillar. Therefore, transistor channel length increases even though the lateral transistor channel length is maintained. Since the subthreshold swing is deteriorated by the short channel effect, relatively long channel due to existence of insulating pillar has advantage to suppress the subthreshold swing in nano-meter scale. By technology computer aided design (TCAD) modeling, electrical characteristics are demonstrated. Low drain induced barrier lowering (DIBL) of 13.7 mV/V and steep subthreshold swing of 62.6 mV/dec are estimated. Ternary characteristics of EO ternary inverter are investigated by TCAD Mixed mode, the voltage transfer characteristics (VTC) of EO ternary inverter gives an apparent ternary voltage states. In according to structures of EO resistor and EO MOSFET, EO ternary inverter can be formed perpendicular to substrate, therefore, which allows thin lateral dimension of the inverter. Reliability of ternary operation is explained with static noise margin (SNM) and transient response. In the transient response, ternary operation is maintained at 10 MHz frequency, and a propagation delay of 1.69 ns is evaluated. Theoretical approach to thermionic emission at Dirac semimetal source is performed. In the Dirac semimetal, since density of states are determined by linear energy dispersion near the Dirac point, thermionic emission current can be controlled by difference between Dirac point and fermi level and Schottky barrier height. As absence of direct injection of carriers from contact to Si, equation of thermionic emission is different with that of conventional up-down source/substrate structure. In case of graphene, there are singularities at negative infinity, hence the possibility of constant thermionic current exists regardless of the gate biasing of MOSFET. Meanwhile, lowest subthreshold swing of 30 mV/dec for 3 dimensional Dirac semimetal source is discussed.
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Department of Physics
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