Analytic expressions for evaluating minimum propagation delay time tpd are derived, using a model which is effective for analysis of the device with a merged structure. Calculated results are in good agreement with experimental ones. Junction modifying coefficient k is newly introduced for the theory. k affects tpd through a variable function of current gains, and a deviation from the Klaassen's theory is pointed out. Role of the pnp transistor on the performance of the I2L gate is emphasized.