Bipolar linear integrated circuit reliability

Device constraints peculiar to linear integrated circuits often make this class of integrated circuit particularly sensitive to reliability problems. Operating in the active region, these devices are required to say not merely "yes" or "no," but to answer the question "how much." The circuits demand operation at relatively high voltage and usually involve high gain. Both of these conditions tend to sensitize them to small changes in the component devices. This paper will seek to examine reliability problems which are particularly associated with the linear device, rather than consider generic problems broadly associated with integrated circuits. Many of these problems tend to be surface related. For example, negative charge collected by the fringing fields of the collector-base or collector-isolation Junctions can cause inversion layers over the lightly doped epitaxial material. These inversion layers cause leakage in the input stage of the device which can eventually cause the output of the operational amplifier to latch-up. Even for devices where high threshold voltages prevent actual surface inversion, micro-drifts in offset have been observed due to charge collection in the vicinity of the emitter base Junction. This problem is particularly severe if the device is not adequately protected against intrusion of sodium in this region. Static discharge is another problem to which linear integrated circuits are quite sensitive. This failure results from damage to the emitter base Junction of input transistors which affects the offset voltage of the circuit. On-chip capacitors, included for amplifier compensation, are also sensitive to damage. Incipient defects in the thin dielectric layers of the capacitor structure can cause infant mortalities after a few hundred hours of continuous bias in a completed system.