When larger responsivities are required at low light intensities for small area devices, the drain bias can be increased which will simultaneously increase the responsivity and noise current, but leaves the specific detectivity unaffected within an ohmic operational range. Figure illustrates the photosignal of the phototransistor as a function of the irradiance, while Figure shows the corresponding responsivity dependence, both obtained at a gate voltage of 11 V. By reducing the light intensity down to a signal-to-noise ratio of unity, the noise equivalent power (NEP: 1.4×10 -12 W) is derived as marked in Figure . This is an alternative method to measure the specific detectivity according to Equation I, resulting in 3.5×10 Jones with an equivalent noise bandwidth of 0.094 Hz (defined by the lock-in settings), which is highly similar to the value calculated from the responsivity and noise measurements obtained under the same measurement conditions (2.9×10 8 Jones).