Single ungridded silicon tips have been studied by field emission current--voltage characterization, field emission imaging, field ion imaging, and pulsed laser atom probe (PLAP) analysis. Changes in field emission characteristics were observed as a thin contaminant layer (oxygen, hydrogen, carbon, and carbon monoxide) formed on the surface of clean field evaporated silicon tips. Removal of the contaminant layer by pulsed laser-assisted field desorption restored the original field emission characteristics. Clean silicon tips that were oxidized in air began to emit at half the volt ge required for unoxidized tips. As the voltage was increased. a sudden irreversible increase in field emission current was observed. PLAP analysis and field emission imaging showed that this effect was caused by the disruption of the oxide layer shortly after the onset of field emission. After the oxide disruption. field emission occurred from re-ions of sharp local geometry around the circumference of the Lip. Removal of these regions by field evaporation or PLAP produced field emission characteristics very similar to those obtained from clean silicon tips. This explains a long-standing problem in understanding the "switch-on" behavior of silicon emitters.