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Physical Origin of Drive Current Enhancement in Ultrathin Ge-on-Insulator n-Channel Metal–Oxide–Semiconductor Field-Effect Transistors under Full Ballistic Transport

2011; Institute of Physics; Volume: 50; Issue: 1R Linguagem: Inglês

10.1143/jjap.50.010110

1347-4065

Shinichi Takagi, Mitsuru Takenaka,

Ferroelectric and Negative Capacitance Devices

Drive current ( I 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 inj ) and inversion-layer capacitance ( C inv ). It is found that v inj on (111) is significantly enhanced with decreasing the GOI thickness ( T GOI ), while no enhancement is observed for (100). This increase in v 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 inv on (111) slightly decreases with a decrease in T GOI , because of the stronger influence of C inv due to the density-of-states. In contrast, C inv on (100) significantly increases with a decrease in T GOI , because of the increase in C 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 sat is compared under a given gate voltage, the optimum surface orientation is dependent on T GOI and gate oxide thickness ( T ox ), because of the trade-off relationship in the effective mass between v inj and C inv . In bulk Ge and GOI with T GOI thicker than around 5 nm, (111) and (110) surfaces can provide the identical and maximum I sat independent of T ox , which is attributed to the higher v inj . In GOI with T GOI thinner than around 5 nm, I sat is higher in the order of (111), (110), and (100) for T ox thicker than around 1 nm, while in the order of (110), (100), and (111) in T GOI thinner than around 1 nm, because of the lowest C inv on (111). Here, the transition T ox is dependent on T GOI . As a consequence, we can conclude that, in a realistic choice of T GOI and T ox , (111) and (110) surfaces can yield higher I sat in GOI n-MOSFETs.