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A distributed model for a junction transistor has been analyzed to include both dc and ac biasing effects in the active base region, with particular emphasis on a small-geometry diffused base planar transistor. For such devices with extremely narrow base width, dc biasing effects cannot be neglected. At high frequencies, the response of these devices is greatly modified by ac biasing effects which are accentuated by the significant dc biasing at large emitter current levels. Two-dimensional current flow under these biasing conditions was studied with a distributed model of the active base region. From such a model, the expressions for emitter-base diode characteristic, small-signal and large-signal base resistance, and complex base impedance valid for high frequencies have been deduced in terms of physical parameters of the devices like the geometry, base resistivity, etc. This equivalent base impedance and an ideal diode with its diffusion and emitter-base transition capacitance constitute the lumped model of the emitter-base region. For any particular frequency, the base impedance can be represented exactly by a parallel RC network. The distributed model can also predict the pulse response of the device more accurately than a lumped model and show the sensitivity of the transient response to the physical parameters mentioned above. Experimental verifications of the theoretical expressions are found to be satisfactory, and limitations of the earlier works are pointed out in regard to present devices.