An effective mean-free path of hot electrons in the conduction band of SiO2 in a Si-gate metaloxide-semiconductor (MOS) electron tunneling cathode was measured and found to be about 0.7 nm. Following these observations, we proposed and fabricated a depletion gate MOS electron tunneling cathode. The highest transfer ratio of 13.3% was achieved in the cathode at the low emission current level, which was considerably higher than that of tunneling cathodes studied in the past. However, the ratio decreased drastically at high current due to the hole injection into the depletion region from the gate.