Origin of Current Variations in Ultrathin Bipolar Junction Transistors Under Bending Stress

The dc characteristics variations of ultrathin flexible high-speed polysilicon emitter bipolar transistors under uniaxial tensile bending stress have been demonstrated, which shows an apparent increase of base current while a slight decrease of collector current, resulting in the degradation of current gain. Such variations are ascribed to the stress-induced changes in minority carrier mobility and intrinsic carrier concentration, as clarified by the piezojunction effect theory. The base current variation is also related to the generation of recombination traps at poly/crystalline-silicon (poly/c-Si) and SiO2/Si interfaces within the quasineutral emitter and E-B junction space-charge region, respectively, with the former dominating at the operation bias, which however does not affect the low-frequency noise.