Thin-film transistor circuit

DETAILED DESCRIPTION It is an aim of the present invention to provide a thin-film transistor circuit having an input gate protection device for which the voltage at which conduction occurs is not solely dependent on the threshold voltage of the subsidiary thin-film transistors forming the input gate protection device.
According to the present invention, there is provided a thin-film transistor circuit comprising a main thin-film transistor and an input gate protection device comprising first and second subsidiary thin-film transistors connected in series and to the gate electrode of the main thin-film transistor with the gate and one of the main electrodes of each of the first and second subsidiary thin-film transistors being connected, characterised in that the other main electrodes of the first and second subsidiary thin-film transistors are connected together so that only one of the first and second subsidiary thin-film transistors conducts when a voltage above a threshold voltage is applied to the gate electrode of the main thin-film transistor and in that the first and second subsidiary thin-film transistors have channel regions of a length selected for causing breakdown of the other one of the first and second subsidiary thin-film transistors when a given voltage greater than the threshold voltage is applied to the gate electrode of the main thin-film transistor, thereby rendering both the first and second subsidiary thin-film transistors conducting.
It should, of course, be understood that, as used herein and as is conventional in the art, the term threshold voltage means the voltage applied to the gate electrode at which, with appropriate voltages applied to the main electrodes, current will just begin to flow in the channel region so that above the threshold voltage a conduction channel is established in the channel region.
Using a circuit in accordance with the present invention the voltage at which the input gate protection device conducts to remove an excessively high (negative or positive) voltage from the gate electrode of the main thin-film transistor is determined partly by the threshold voltage of one of the first and second subsidiary thin-film transistors and partly by the voltage at which the other of the first and second subsidiary thin-film transistors breaks down