Post-deposition semiconductor dewetting is the transformation of a (nearly) closed organic-semiconductor monolayer into separated individual islands of multilayer height . We have recently observed this phenomenon in both ultra-thin (1-3 nm) and thin (25-40 nm) films of the small-molecule semiconductor dinaphthothienothiophene (DNTT) [2,3]. Since the gate-field-induced carrier channel is located in close vicinity to the semiconductor-dielectric interface, the accelerated pace of dewetting of ultra-thin semiconductor films is relevant to the performance and stability of organic thin-film transistors (TFTs). We have therefore fabricated bottom-gate, bottom-contact TFTs based on 2 nm and 25 nm-thick DNTT films. Compared to the relatively stable charge-carrier mobility of 1.1 cm2/Vs for the 25-nm-DNTT TFT, the 2-nm-DNTT TFTs show a sharp decrease from 0.2 cm2/Vs to 0.011 cm2/Vs over 72 hours after fabrication. To stabilize the TFT performance, we have explored strategies to prevent ultra-thin DNTT films from dewetting, including substrate cooling and semiconductor encapsulation, and fabricated stable DNTT TFTs with monolayer semiconductor thickness. Encapsulation with vacuum-deposited polytetrafluoroethene (PTFE) or titanyl phthalocyanine (TiOPc) leads to a relative decrease in mobility by only 12% and 44%, compared to 99.6% for TFTs without encapsulation over 28 hours after device fabrication. T. Breuer et al., ACS Appl. Mater. Interfaces, 9, 8384, (2017). K. Takimiya et al., Sci. Technol. Adv. Mater., 8, 273, (2007). U. Zschieschang et al., Organic Eletronics, 12, 1370, (2011).