Generation of ELF and VLF waves in the HF heating wave modulated polar electrojet is studied. Through the Ohmic heating by the amplitude-modulated HF heating wave, the conductivity and thus the current of the electrojet is modulated to set up the ionospheric antenna current. However, it is shown that a stimulated thermal instability is also excited by the amplitude-modulated HF heating wave. This instability introduces an electron temperature modulation more effectively than that by the passive Ohmic heating process and is expected to improve considerably the intrinsic efficiency of ELF and VLF wave generation by the amplitude-modulated HF heating wave. Moreover, the generation efficiency and signal quality also depend on the HF wave modulation scheme. Thus, four amplitude-modulation schemes are examined and compared.