High transconductance n-type Si/SiGe modulation doped field-effect transistors (MODFETs) are fabricated and characterised using a charge control model. The electron saturation velocity, the low field mobility and the equilibrium carrier concentration of the two-dimensional electron gas in the channel are derived from the characterisation. These values are used for the calculation of the MODFET transconductance for devices with different gate lengths and gate-to-channel distances. Transconductances of 130 mS mm(-1) and 310 mS mm(-1) are measured for MODFETs with 0.5 mu m gate length having 24 nm and 4 nm gate-to-channel distances, respectively. Calculated values of transconductances are in good agreement with these measured data. Estimated upper limit of transconductance for very short gate MODFETs based on characterised SiGe heterostructure is 1020 mS mm(-1).