The level 2 model is an improvement to level 1. Details are available from Macquarie University. Some important items are:

• The description maintains strict continuity in its high-order derivatives, which is essential for prediction of distortion and intermodulation.
• Frequency dependence of output conductance and transconductance is described as a function of bias.
• Both drain-gate and source-gate potentials modulate the pinch-off potential, which is consistent with S-parameter and pulsed-bias measurements.
• Self-heating varies with frequency.
• Extreme operating regions - subthreshold, forward gate bias, controlled resistance, and breakdown regions - are included.
• Parameters provide independent fitting to all operating regions. It is not necessary to compromise one region in favor of another.
• Strict drain-source symmetry is maintained. The transition during drain-source potential reversal is smooth and continuous.

The model equations are described in this pdf document and in [19].