A simple shift register circuit for depletion-mode oxide TFTs

► Metal-oxide TFTs operate as n-channel FET but tend to have negative VT. ► A shift register circuit for the depletion-mode oxide TFT is required. ► Two different low levels of clock signals in the circuit can resolve the problem. ► New shift register circuit composed of only 6 TFTs operates over VT range −5–+9V according to simulation. ► Fabricated circuit with depletion-mode IGZO TFTs also works successfully. We report the simplest shift register circuit compatible with metal-oxide thin-film transistors (TFTs). Since the oxide TFT often exhibits depletion-mode characteristic, it is impossible to guarantee stable operation of the shift register when we resort to conventional circuit scheme. In order to resolve this problem, we employed two different low-voltage levels for the clock signals. As a result, we could turn off the depletion-mode oxide TFTs completely with sufficient voltage margin. According to the simulation results, the new shift register composed of only 6 TFTs operates properly over the threshold voltage (VT) range of −5–+9V. The fabricated circuit also worked successfully even though VT of the TFTs ranged from −5V to −3V.