The existence of a poly-Si control gate in an n-channel FAMOS makes the erase characteristics due to ultraviolet light (UVL) illumination different from those of conventional p-channel FAMOS's without control gates. The difference in erasing times between these two types of FAMOS has been explained by the attenuation of UVL in the control gate. However, it was clarified experimentally and analytically in n-channel FAMOS that UVL is propagated horizontally in an optical guide formed between the control gate and substrate and is then absorbed by the floating poly-Si gate. The absorbed UVL intensity in the floating gate through the proposed optical guide is calculated to be 109times stronger than that transmitted directly through the control gate whose thickness and absorption coefficient are assumed to be 3500 Å and 106cm-1, respectively. The proposed optical guide model is supported in experiments that erase time does not depend on the thickness of the control gate (2400- and 3600-Å devices are compared) and erase time in a device whose optical guides are open only on one side and the other side is covered by the control gate is about 2 times longer than that in a device which has two optical guides open on both sides.