Comment on "Mathematical proof of the validity of reciprocity in one-dimensional bipolar transistors with arbitrary base parameters"

De Man et al. recently analytically demonstrated the validity of reciprocity in bipolar transistors having position-dependent base parameters (impurity concentration, lifetime, mobility, and bandgap). However, the authors did not make reference to an early transistor paper by Shockley et al. which provided an elegant general proof of the reciprocity principle.