Voltage‐tunable far‐infrared emission was observed fromn‐channel Si‐MOSFET’s on high‐index surfaces. The emission results from radiative decay of electronic excitations across a minigap in the ground‐state subband of the channel. We have observed emission of 10−9W at 4.4 meV with a linewidth of ∼1 meV. Our results indicate that a voltage‐tunable monochromatic source of several times 10−7W and tuning from ∼1 to ∼20 meV is attainable.