In general most modeling approaches for organic field-effect transistors (OFETs) are based on the typical MOSFET equations. The threshold voltage is usually a fitting parameter without relation to physical parameters hence the impact of their variability on the threshold voltage is not clear. The presented modeling approach is charge based with a continuous equation for the channel current in organic field-effect transistors from below to above threshold. The model provides a physics based parameter set related to trap states, and a compatible parameter set from a circuit designer's perspective. An expression for the threshold voltage is derived depending on the density of trap states. The model considers a power-law mobility model, parasitic contact resistances and channel length modulation effects and is verified with measurements on OFETs fabricated with small molecules. (C) 2017 Elsevier Ltd. All rights reserved.