  Electrically driven on-chip electron sources that do not need to be heated
have been long pursued because the current thermionic electron sources show the
problems of high power consumption, slow temporal response, bulky size, etc.,
but their realization remains challenging. Here we show that a nanogap formed
by two electrodes on a silicon oxide substrate functions as an
electron-emitting nanodiode after the silicon oxide in the nanogap is
electrically switched to a high-resistance conducting state. A nanodiode based
on graphene electrodes can be turned on by a voltage of ~7 V in ~100 ns and
show an emission current of up to several microamperes, corresponding to an
emission density of ~10^6 A cm^-2 and emission efficiency as high as 16.6%. We
attribute the electron emission to be generated from a metal-insulator-metal
tunneling diode on the substrate surface formed by the rupture of conducting
filaments in silicon oxide. An array of 100 nanodiodes exhibits a global
emission density of 5 A cm^-2 and stable emission with negligible current
degradation over tens of hours under modest vacuum. The combined advantages of
a low operating voltage, fast temporal response, high emission density and
efficiency, convenient fabrication and integration, and stable emission in
modest vacuum make silicon oxide electron-emitting nanodiodes a promising
on-chip alternative to thermionic emission sources.
