An original method for the extraction of FET parameters at low drain bias is presented. It is based on a simple linear charge-control model and on a power-law dependence (with an exponent k > 0 for GaAs/AlGaAs MODFET's and k < 0 for Si-MOSFET's) of the low-field mobility-mu on the two-dimensional electron gas (2DEG) concentration n(S), valid in a certain range of gate voltages. Simple analytical expressions for the transfer characteristics I(ds)-V(gs) and g(m)-V(gs) at low drain bias are combined to extract reliable values of the threshold voltage Vt, the power exponent k, the total parasitic series resistance (R(s)+R(d) and an important control parameter-beta'. We verified that the values of beta' and k extracted on a test MODFET at 300 and 77 K agree very well with those deduced directly from Hall measurements on gated Hall-bridge structures. Our analysis is also applied to extract MOSFET parameters at room temperature.