The use of forward biasing the source-substrate junction in metal–oxide–semiconductor transistors, to reduce its threshold voltage is a simple method to realize low power complementary metal–oxide–semiconductor integrated circuits. A 2 μm longn-metal–oxide–semiconductor field effect transistor was used as the device under test. The threshold voltage was measured in the temperature range of 300–77 K, using two different methods. The classic long channel threshold voltage model was fitted by optimization to the experimental data of the reverse-biased substrate, and the model with the fitted parameters was used to calculate the threshold voltage under forward-biased substrate conditions. The agreement between the fitted and extrapolated threshold voltage with the experimental values demonstrated the validity of this classic model for a substrate forward bias up to 0.5 V, and for a wide temperature range.