Charge Induced Instability in 709 Operational Amplifiers

Although the bias induced instability which has been discussed is confined to one specific device fabricated on (100) oriented silicon, the general problem of bias induced instability is more prevalent with linear integrated circuits than with the more numerous digital devices. The requirement of higher operating voltages on these linear devices dictates the use of higher resistivity material which is more susceptible to adverse surface effects resulting from any net charge in the overlying passivation layers. Also, the higher internal fields in these devices make it more likely that charge can be moved and concentrated to the extent that it could cause some shift in individual component characteristics. This drift would be most serious in these high gain linear integrated circuits with all components normally biasei in their active region. Digital integrated circuits, besides being less likely to experience drift of individual component characteristics in the first place, probably would not show any such drift in the static terminal parameters commonly measured due to the saturating mode of operation used in the majority of the existing types of logic. To improve the reliability of linear integrated circuits, it is extremely important that the lower passivation layer be of the highest possible quality to minimize the tendency of the high fields to accumulate electronic charge over the underlying silicon.