## 3 Results and Discussion  
Figure 2a,b show the typical \(I_{D}\)-\(V_{D}\) (output) characteristics of short-channel P3 HT-based FETs with \(L\) of 5 um, 500 nm, and 30 nm fabricated on 200-nm-thick SiO\({}_{2}\) insulators treated with PETS and CPTS SAMs, respectively. The P3 HT FETs with \(L\) = 5 um show typical output characteristics with clear current saturation. This can be attributed to the high gate electric field relative to the source-drain electric field, which ensures that the gate electric field determines the charge distribution within the channel (i.e., the gradual-channel approximation) [4, 10]. As the channel length is reduced to the sub-micrometer regime, the output characteristics become parabolic, and the drain current at \(V_{G}\) = 0 V increases. Such behavior has been explained by the increase in the current flow to the bulk region of the organic semiconductor due to the SCLC [28]. The P3 HT semiconductor allows forming ohmic contacts with the Au source-drain electrodes because of the small potential difference between the work function of Au (4.8 eV) and the ionization potential of P3 HT (4.9 eV) [29]. Applying the source-drain voltage to the sub-micrometer channel leads to an extremely high source-drain electric field, which results in the SCLC significantly contributing to the drain current. Figure 2c shows the \(I_{D}\)-\(V_{G}\) (transfer) characteristics of the short-channel P3 HT FETs with PETS- and CPTS-treated SiO\({}_{2}\) gate insulators. The transfer characteristics of the devices with CPTS-treated gate insulators shift toward the positive \(V_{G}\) direction compared to that with PETS-treated gate insulators. Such behavior is consistent with the results reported for pentacene-based OFETs with \(L\) = 30-150 um on SAM-treated SiO\({}_{2}\) insulators [15, 16] and has been explained by the change in surface potential of the gate insulator by the permanent dipole of SAMs [14, 15, 16, 17, 18, 19, 20]. As shown in Figure 3, the electric field pointing toward the surface of the gate insulator produced by the dipole moment of CPTS causes the bending of molecular orbital levels of organic semiconductors toward high energy. Figure 1: (**a**) Schematic cross-section of short-channel organic field-effect transistors (OFETs) fabricated in this study. Chemical structures of (**b**) organosilanes and (**c**) polymer semiconductors; PETS: phenethyltrichlorosilane; CPTS: 3-chloropropyltrichlorosilane; P3 HT: poly(3-hexylthiophene); F8T2: poly(9,9-dioctylfluorene-co-bithiophene). The upward bending of energy levels accumulates additional holes in the channel, which leads to the positive shift of transfer characteristics [15; 16]. Figure 3: Schematic energy band diagram for bottom-gate OFETs with SiO\({}_{2}\) gate insulators treated with the CPTS SAM; LUMO: lowest unoccupied molecular orbital; HOMO: highest occupied molecular orbital; SAM: self-assembled monolayer. Figure 2: Output characteristics of P3 HT FETs with \(L\) of 5 \(\upmu\)m, 500 nm, and 30 nm on 200-nm-thick SiO\({}_{2}\) insulators treated with (**a**) PETS and (**b**) CPTS; and (**c**) their transfer characteristics. The field-effect mobility of the P3 HT FET devices with \(L\) = 5 \(\upmu\)m on the CPTS-treated gate insulators was 6.8 \(\pm\) 0.81 \(\times\) 10\({}^{-4}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\), which was slightly lower than that of the devices on the PETS-treated gate insulators (1.4 \(\pm\) 0.13 \(\times\) 10\({}^{-3}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\)). Such a decrease in the mobility of P3 HT FETs might be attributed to the smaller water contact angle of CPTS-treated gate insulators, since the ordering of organic semiconducting molecules in bottom-gate OFETs depends largely on the surface energy of gate insulators and a more hydrophobic surface gives a higher mobility [30; 31]. It can be seen in the figures that the off current of the devices with CPTS-treated gate insulators remarkably increases as the channel length is reduced to 30 nm.