[Nowl-93] have shown that this recombination Collector causes an increase in base current Ib, Minority Carriers Recombination Current proportional to exp ( Nox2 where Nox is the total net charge trapped in the spacer oxide (in the notations of this Author, Nox= Qit). This increase in Ib then results in the reduction Figure 69. Diagram of the recombination currents of minority carriers causing the of the BJT gain (I/Ib). degradation of the gain and simple law versus dose. In addition to this pure electrostatic effect of controling surface potential by the oxide charge Nox, interface states play also the role of recombination centers. As the total charge Nox shifts the surface potential S, the recombination current changes exponentially, in accordance with the Shockley-Read-Hall statistics [Emil-96]. The expression of the base current can be obtained, by recalling the Poisson's law in the depleted base of (assumed) constant doping level NA. By considering that the depletion is entirely caused by the radiation-induced charge Nox, we can express the surface potential S with respect to Nox and NA (in cm-2 and cm-3 (110) A This gives finally give, with Wdep the surface depletion width [Pers-97]: III-81 (111) ) For PNP transistors, phenomena in the oxide are similar, producing generally a net positive charge Nox, but electric fields are inverted in the oxide. Another major dirrerence comes from the doping type of the base itself: it ensues that the net positive charge Nox accumulates the N type base instead of depleting it. [Pers-97] noted that the expression of the recombination current is different for a PNP transistor, and this dependence should be in 1/Nox 2 instead of exp ( Nox2). In vertical NPN and PNP, most of the recombination current flows from the base to the perimeter of the emitter junction, as reviewed in [Mess-92] and illustrated, e.g. in [Nowl-92] and [Flam- 94]. In this case, the variation of the reciprocal current gain can be expressed as follows: (112) where: recombination velocity (function of Nox as