In recent years, considerable attention has been paid to amorphous indium gallium-zinc-oxide (a-IGZO) thin film transistors (TFTs) for high performance flat panel display, such as liquid-crystal displays (LCDs), active-matrix organic light-emitting diode (AMOLED) display and flexible display. This is because IGZO TFTs are more suitable for pixels and circuit integrations on display panel than the conventional silicon-based devices. The merits of IGZO TFT technology include high mobility, decent reliability, low manufacturing cost, and excellent uniformity over large fabrication area. However, it was reported that the electrical characteristics of IGZO TFT are susceptible to shift after electrical aging measurement under illumination, which is caused by the activation of trapped electrons from sub-gap states to conducting states. Therefore, it is necessary to introduce light shielding layer to suppress the electrical characteristic shift under illumination aging measurements. Lim et al. demonstrated the characteristics of IGZO TFT with additional light shielding metal layer, and proved that the threshold voltage of TFT can be tuned linearly by adjusting the biasing voltage of the light shielding metal. Taking advantage of this tunable threshold voltage, AMOLED pixel circuit with a threshold voltage shift compensation function can be implemented. However, drawback of this method lies in the adding of additional biasing line, which increases the circuit area and restricts the integration of high-resolution pixel circuits. Thus, Zan et al. proposed adopting floating (unbiased) light shielding metal layer to improve the characteristics of device. However, Zeng et al. demonstrated the abnormal output characteristics of the IGZO TFT, as it cannot be saturated due to the introduction of floating light shielding metal layer. It seems that the IGZO TFT with floating metal is different from the conventional double-gate or single gate structure. To date, the current conducting mechanism of IGZO TFT with floating metal has not been discussed yet. In this paper, the distribution of electrical potential in the IGZO TFT with a cross sectional view is thoroughly analyzed. It is confirmed that the abnormal output characteristic of IGZO TFT is caused by the capacitive coupling between the floating gate and the drain electrode of the transistor. On the basis of the voltage distribution relationship between the equivalent capacitances, a threshold-voltage-dependent current-voltage model is proposed. The simulated results by technology computer-aided design tool and those by the proposed model are in good agreement with each other. Therefore, the mechanism of floating gate effect for IGZO TFT is comprehensively demonstrated. The illustrated conducting mechanism and the proposed current-voltage model are helpful in developing the device and process of IGZO TFT with novel structure.