Analysis of Electrothermal Effects in Bipolar Differential Pairs

An extensive experimental and theoretical analysis of bipolar differential pairs subject to radical electrothermal feedback is presented. Measurements demonstrate that considerable thermally-induced degradation of circuit characteristics may occur, eventually turning into the full disappearance of a linear region, which is replaced by a hysteresis behavior under voltage-controlled conditions. An analytical model is derived for a simple yet reliable prediction of the distortion of I- V curves. A more elaborated circuit approach is employed to accurately quantify the concurrent destabilizing action of electrothermal and impact ionization effects, as well as to evaluate the impact of layout asymmetries and examine the beneficial influence of emitter degeneration resistors. Simulation results are found to compare favorably with experiments performed on silicon-on-glass test structures with various layouts and isolation schemes, from which the benefits of thermally coupling the two devices become evident.