A series of eight perylene diimide (PDI)- and naphthalene diimide (NDI)-based organic semiconductors was used to fabricate organic field-effect transistors (OFETs) on bare SiO2 substrates, with the substrate temperature during film deposition (Td) varied from 70–130 °C. For the N,N′-n-octyl materials that form highly ordered films, the mobility (µ) and current on-off ratio (Ion/Ioff) increase slightly from 70 to 90 °C, and remain relatively constant between 90 and 130 °C. Ion/Ioff and µ of dibromo-PDI-based OFETs decrease with increasing Td, while films of N,N′-1H,1H-perfluorobutyl dicyanoperylenediimide (PDI-FCN2) exhibit dramatic Ion/Ioff and µ enhancements with increasing Td. Increased OFET mobility can be correlated with higher levels of molecular ordering and minimization of film morphology surface irregularities. Additionally, the effects of SiO2 surface modification with trimethylsilyl and octadecyltrichlorosilyl monolayers, as well as with polystyrene, are investigated for N,N′-n-octyl dicyanoperylenediimide (PDI-8CN2) and PDI-FCN2 films deposited at Td = 130 °C. The SiO2 surface treatments have modest effects on PDI-8CN2 OFET mobilities, but modulate the mobility and morphology of PDI-FCN2 films substantially. Most importantly, the surface treatments result in substantially increased Vth and decreased Ioff values for the dicyanoperylenediimide films relative to those grown on SiO2, resulting in Vth > 0.0 V and Ion/Ioff ratios as high as 108. Enhancements in current modulation for these high-mobility, air-stable, and solution-processable n-type semiconductors, should prove useful in noise-margin enhancement and further improvements in organic electronics.