An improved extraction method considering ac current crowding effect is investigated to determine intrinsic ( $C_{mu}$) and extrinsic ( $C_{mu}$) base-collector capacitances of bipolar junction transistors separately. The drawbacks of conventional methods are pointed out, and the improved extraction equations are derived from a cutoff mode equivalent circuit with the ac crowding capacitance. The frequency response curves of modeled current and power gains using the extracted values of $C_{mu}$ and $C_{mu}$ have much better agreements with measured ones than those of the conventional methods, verifying the accuracy of the improved technique.