Although theC–Vprofiling technique is usually applied to obtain the doping concentration of lightly doped wells, here we extend it to extract structural information of high‐performance double poly bipolar transistors (BJTs) by comparing profiles taken from two adjacent but distinctC–Vstructures. In particular, the trenching that occurs in unprotected substrate silicon during the gate (base) poly overetch can be measured by the change inp+source/drain implant ton+buried layer depletion width for two such diodes, one with and one without trenching. The diffused extrinsic base junction depth of the double‐poly BJT can be extracted similarly. Both of these physical dimensions must be accurately measured and tracked, to optimize bipolar device performance. Structural measurements by this comparativeC–Vtechnique are nondestructive, simple, and agree with scanning electron microscopy and secondary ion mass spectroscopy within ∼100 Å (5%).