IBIC (Ion Beam Induced Charge ) in its "lateral " version is a powerful method for mapping the drift length of carriers in a depletion region of a semiconductor device. This technique has been recently used in order to measure the drift length and to profile the electric field in a p+/n Si junction and in a totally depleted CdTe nuclear detector. Problems in data analysis arise when the device is not totally depleted and diffusion regions are present. These problems are generally solved by a sharp separation between the two regions, which is not the real case, since a smooth transition exists between high and low electric field regions and a back electrode induced field could also be present. In order to overcome these difficulties, a new theoretical approach based on the extended Ramo’s theorem has been developed. This theory allows diffusion length to be accurately evaluated taking into account the effects of the finite integration time of the electronic chain.