A device has been reported recently [M. W. Geis et al., Appl. Phys. Lett. 87, 192115 (2005)] in which electrons transit through the image states of a negative electron affinity glass substrate before being emitted to the vacuum. The external field required for this emission may be as low as 10 V/cm, which is up to three orders of magnitude lower than the fields encountered with other materials and configurations. The authors present a description and analysis of this device, which includes the glass substrate and the metallic contacts. The model also accounts for the ionization of cesium in the glass. Two stages are considered: (i) the activation step in which a 200 V bias is established through the glass and (ii) the operational state in which the transfer of electrons from the metallic contacts to the glass surface is enhanced by positive ions trapped in the glass. The results provide insight into the role of the positive charges trapped in the glass. It is shown that the conditions are encountered, both in the activation step of the device and in its subsequent unbiased working mode, to account for the emission reported experimentally. (c) 2007 American Vacuum Society.