Insulating, dipolar and semiconducting molecular phosphonic acid (PA) self-assembled monolayers (SAMs) have been developed for applications in organic field-effect transistors (OFETs) and graphene transistors for low-power, low-cost flexible electronics. Multifunctional SAMs on ultrathin metal oxides, such as hafnium oxide and aluminum oxide, are shown to enable (1) low-voltage (sub 2V) OFETs through dielectric and interface engineering on rigid and plastic substrates, (2) simultaneous one-component modification of source/drain and dielectric surfaces in bottom-conatct OFETs, (3) SAM-FETs based on molecular monolayer semiconductors, and (4) systematic doping control of graphene transistors with dipolar SAMs. The combination of excelent dielectric and interfacial properties results in high-performance OFETs with low-subthreshold slopes down to 75 mV/dec, high Ion/Ioff ratios of 10^5-10^7, contact resistance down to 700 Ω cm, charge carrier mobilities of 0.1-4.6 cm2/(V s), and general applicability to solution-processed and vacuum-deposited n-type and p-type organic and polymer semiconductors. It has been also demonstrated that SAMs can be used to reliably and predictably control the charge carrier concentration of graphene transistors without negatively impacting charge carrier mobility while taking into account the total doping environment.