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An analytic model for the I-V characteristic of the lambda bipolar phototransistor (LBPT) is developed in which the characteristic parameters, such as the peak voltage, peak current, differential negative resistance, valley voltage, and valley current, are expressed in terms of the known device parameters and photocurrent. It is shown that the valley current of the new device in the dark is much smaller than the saturation dark current of the conventional bipolar phototransistor and that the differential negative resistance region can be varied for wide range by controlling the fabrication processes. Comparisons between the characteristics of the fabrication devices and the developed model have been made and satisfactory agreement has been obtained. Moreover, some interesting applications of this new device in photodetection are also presented and discussed.