We present a simple and effective method to estimate the width (σ) of the density of states (DOS) of the Highest Occupied Molecular Orbital (HOMO) in pentacene using field effect studies. This method can be applied to most organic semiconductors where charge carrier motion is by thermally activated hopping. The charge carrier density dependent mobility acts as the probe. We show from simulations that the variation of mobility, as a function of charge carrier density, is highly sensitive to σ. Thus, σ can be found with low error using this method. We optimize the fabrication process to develop pentacene transistors with greatly diminished non-idealities. We estimate the width of the DOS of the HOMO for pentacene to be 80 ± 10 meV. The contribution of the tail states is explored and we find it to be negligible in the on-state of the transistors studied. © 2019 Elsevier B.V.