Editing Laser printing (section)

===Charging===
[[File:Corona charging.svg|thumb|Applying a negative charge to the photosensitive drum]]
In older printers, a [[Corona discharge|corona wire]] positioned parallel to the drum or, in more recent printers, a primary charge roller, projects an [[electrostatic]] charge onto the photoreceptor (otherwise named the photoconductor unit), a revolving photosensitive drum or belt, which is capable of holding an electrostatic charge on its surface while it is in the dark.

An [[alternating current|AC]] bias voltage is applied to the primary charge roller to remove any residual charges left by previous images. The roller will also apply a [[direct current|DC]] bias on the drum surface to ensure a uniform negative potential.

Numerous patents{{Specify|date=January 2010}} describe the photosensitive drum coating as a [[silicon]] "sandwich" with a photocharging layer, a charge leakage barrier layer, as well as a surface layer. One version{{Specify|date=January 2010}} uses [[amorphous silicon]] containing [[hydrogen]] as the light-receiving layer, [[boron nitride]] as a charge leakage barrier layer, as well as a surface layer of [[Doping (semiconductor)|doped silicon]], notably silicon with [[oxygen]] or [[nitrogen]] which at sufficient concentration resembles machining [[silicon nitride]].