Physical Origin of Drive Current Enhancement in Ultrathin Ge-on-Insulator n-Channel Metal--Oxide--Semiconductor Field-Effect Transistors under Full Ballistic Transport
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概要
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Drive current ($I_{\text{sat}}$) of ultrathin germanium-on-insulator (GOI) n-channel metal--oxide--semiconductor field-effect transistors (n-MOSFETs) under full ballistic transport is theoretically investigated for (100)-, (110)-, and (111)-oriented Ge surfaces. The physical origin of the current drive enhancement associated with thinning GOI films for each surface orientation is clarified from the viewpoints of injection velocity ($v_{\text{inj}}$) and inversion-layer capacitance ($C_{\text{inv}}$). It is found that $v_{\text{inj}}$ on (111) is significantly enhanced with decreasing the GOI thickness ($T_{\text{GOI}}$), while no enhancement is observed for (100). This increase in $v_{\text{inj}}$ on (111) originates in the increase in the electron occupancy of the lowest subband due to the size effect of ultrathin GOI films. On the other hand, $C_{\text{inv}}$ on (111) slightly decreases with a decrease in $T_{\text{GOI}}$, because of the stronger influence of $C_{\text{inv}}$ due to the density-of-states. In contrast, $C_{\text{inv}}$ on (100) significantly increases with a decrease in $T_{\text{GOI}}$, because of the increase in $C_{\text{inv}}$ due to the inversion-layer thickness, determined by the GOI physical thickness. The (110) GOI surface is found to have intermediate characters between (100) and (111). When $I_{\text{sat}}$ is compared under a given gate voltage, the optimum surface orientation is dependent on $T_{\text{GOI}}$ and gate oxide thickness ($T_{\text{ox}}$), because of the trade-off relationship in the effective mass between $v_{\text{inj}}$ and $C_{\text{inv}}$. In bulk Ge and GOI with $T_{\text{GOI}}$ thicker than around 5 nm, (111) and (110) surfaces can provide the identical and maximum $I_{\text{sat}}$ independent of $T_{\text{ox}}$, which is attributed to the higher $v_{\text{inj}}$. In GOI with $T_{\text{GOI}}$ thinner than around 5 nm, $I_{\text{sat}}$ is higher in the order of (111), (110), and (100) for $T_{\text{ox}}$ thicker than around 1 nm, while in the order of (110), (100), and (111) in $T_{\text{GOI}}$ thinner than around 1 nm, because of the lowest $C_{\text{inv}}$ on (111). Here, the transition $T_{\text{ox}}$ is dependent on $T_{\text{GOI}}$. As a consequence, we can conclude that, in a realistic choice of $T_{\text{GOI}}$ and $T_{\text{ox}}$, (111) and (110) surfaces can yield higher $I_{\text{sat}}$ in GOI n-MOSFETs.
- 2011-01-25
著者
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Takagi Shinichi
Department Of Electrical Engineering And Information Systems The University Of Tokyo
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Takagi Shinichi
Department of Electrical Engineering and Information Systems, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan
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