In this paper,we report anomalous behavior of transconductance(gm) in halo implantedMOSFET for linear and saturation regions across both gate and body biases. The gm characteristics undergo sharp change of slope in saturation which cannot be modeled by conventional compact models. The cause of such behavior is identified and explained using the TCAD simulations of source side halo, drain side halo (DH), both side halos, and uniformly doped transistors. An analytical model, based on the equivalent conductance of the halo device, is developed to understand the gm behavior. It is shown that the commonly used approach where only the DH region is considered in saturation, is insufficient to model the atypical gm behavior. The effect of oxide thickness (Tox) variation on gm is also studied, which demonstrates a deviation from the conventional gm behavior for halo implanted devices with thicker Tox. A computationally efficient SPICE model is proposed to model gm characteristics which shows excellent matching with the measured data. © 1963-2012 IEEE.