Analysis of the Bipolar Current Mirror Including Electrothermal and Avalanche Effects

An experimental and numerical study of the bipolar current mirror characteristics under strong self-heating and avalanche conditions is presented, and a theoretical model to describe the observed behavior is proposed. It is shown that both electrothermal effects and impact ionization may lead to a marked degradation of the mirroring action, eventually resulting in an instability phenomenon which limits the usable operating range of the circuit. Both the separate and combined actions of these positive-feedback mechanisms are investigated. The model compares favorably with experimental data measured on silicon-on-glass and GaAs current mirrors and allows deriving a theoretical relation for the critical condition corresponding to the onset of the instability. The impact of the most significant technology and design parameters is discussed, and design criteria are given in order to ensure an unconditionally stable behavior.