The inductance and coupling effects in interconnects and non-linear receiver loads has resulted in complex input signals and output loads for gates in the modern deep sub-micron CMOS technologies. As a result, the conventional method of timing characterization, which is based on lookup tables with input slew and output load capacitance as indices, is no longer adequate. The focus has now shifted to current source based standard cell models which are based on the fundamental property of transconductance of MOSFETs. In this paper we propose a systematic methodology for obtaining a current based delay model for gates, which can accommodate both single (SIS) and multi-input (MIS) switching signals of arbitrary shape and complex non-linear output loads. We use an analytical model for the gate output current expressed as a function of the node voltages. This results in an average error less than 0.5% with maximum standard deviation of 2.5% in error when compared with SPICE for a large number of standard cells. When compared with SPICE, using the proposed models gives stage delay and output slew with an average error of less than 3% and 2% respectively for arbitrary inputs and output load combinations.