Gallium arsenide (GaAs) transit-time diodes with tunnel injection of electrons (TUNNETT) with transit-time layer thickness of 100 and 150 nm were fabricated with molecular layer epitaxy (MLE). Continuous-wave fundamental-mode oscillation in the frequency range of 240 to 325 GHz in metal rectangular resonant of 0.86 x 0.43 mm size (WR-3) cavities Was obtained. Output power of -13 dBm was generated at 322 GHz. The fundamental mode operation, as well as experiments on different impedance matching configurations, suggest that it is possible to develop fundamental mode TUNNETT generators for the frequency range of 350 GHz to 1 THz. Operation of the TUNNETTs confirms device quality of the MLE.