a process for fabricating zener diodes that does not require the use of photomasks . an oxide layer is grown on a silicon substrate which is doped with an n - type dopant . the substrate is subsequently implanted with a p - type dopant , forming a pn junction . the substrate is then metallized for connecting the zener diode to other circuit components . advantageously , the substrate may be scribed after processing , before processing , or anytime during processing . back - to - back zener diodes formed in this manner are used as shunt circuits across individual lamp sockets in series - wired christmas light strings to maintain current flow to each of the lamps of the light string when one or multiple lamps fail .