In this article, we fabricated a series of different geometries for pentacene based thin film transistors (TFTs), including top contact (TC) and bottom contact (BC) configurations, to monitor variations in characteristics. The threshold voltage (Vth) in the saturation regime shifted toward a positive voltage, i.e., from 0.5, 3.2, 9.1, 12.1, and 19.5 V for the channel lengths 67, 47, 23, 19, and 15 μm, respectively, in BC TFTs. All of the TFTs were operated in depletion mode. However, the Vth in the linear regime shifted from -9.3, -9.0, -3.8, -1.8, and 1.5 V for the same devices. Most of the TFTs in the linear regime were operated in enhanced mode. The phenomenon is believed to be strongly correlated with the longitudinal electric field (VDS/L). The high VDS induces a high carrier injection, which makes the pentacene TFTs behave like a depletion type transistor. This assumption is evidenced by the low carrier injection with small VDS results. The device configuration and space charge region must be discussed in more detail. Furthermore, the field effect mobility variation and structure configuration effect is discussed in more detail within the full manuscript.