voltage is applied between electrodes 16 and 26 so as to reverse bias p n junction 24 ideally there is little current flow through the p n junction 24 and thus surface 22a does not have a passivation layer formed thereon under these conditions the surface portion 22a of the p type layer 22 etches away until a surface of the n type region 20 is exposed to the chemical etchant 14 as soon as a surface of n type region 20 becomes exposed to the chemical etchant 14 the p n junction 24 is eliminated and current begins to flow from electrode 16 through the n type region 20 and into the chemical etchant 14 if there is current flow through the n type region 20 into the chemical etchant 14 and the voltage of exposed surface of region 20 is greater than the passivation potential of n type silicon then a protective silicon dioxide layer forms on the exposed surface of region 20 this silicon dioxide layer prevents further etching of region 20 since it is resistant to etching one serious problem with this prior art method of operation of the apparatus 10 of fig1 is the existence of reverse bias leakage through the p n junction this reverse bias leakage is difficult to predict and is indicative of the reverse bias resistance of the p n junction being lower in some cases than is desired as a result of the decreased reverse bias resistance the voltage drop across the p n junction decreases and thereby results in the voltage at the surface 22a becoming more positive than the passivation potential of p type.