Photonic generation of millimeter- and sub-millimeter-waves up to THz range using uni-traveling-carrier photodiodes (UTC-PDs) is described. The UTC-PD utilizes only electrons as the active carriers and this unique feature is the key to achieving excellent high-speed and high-output characteristics simultaneously. The obtained performance includes record output powers of over 20mW at 100GHz and more than 10μW at 1 THz. Promising applications, such as fiber radio communications systems, imaging systems, local signal supply for radio telescope, and measurement systems, are also overviewed.