A simple vertical organic field-effect transistor (VOFET) structure has been fabricated using ambipolar 6, 13-bis (triisopropylsilyl ethynyl) pentacene (TIPSP) with a channel length (L) of 90 nm. This device can operate at-2 V which is much lower than the voltage, reported so far for the organic devices based on TIPSP. The first time, the authors are using transistor efficiency to extract VOFET's parameters. The threshold voltage (V th) of the device has been found to vary between 0.18 and 0.38 V with the current on/off ratio (I on /I off) of 10 4. The mobility (µ) of the device has been calculated as 0.62 cm 2 /Vs. The subthreshold slope, transconductance (g m), output conductance (g d), and early voltage (V E) have been found to be 140 ± 30 mV/ decade, 2 µS, 10 −6 S, and 1.3 ± 2 V, respectively.