High gain operational amplifier using enhancement and depletion mode a-IGZO TFTs

Abstract

An operational amplifier (OpAmp) consisting of twenty-two dual gate amorphous IGZO (a-IGZO) thin-film transistors (TFTs) is fabricated on a glass substrate. To achieve high voltage gains enhancement and depletion mode transistors are used. The enhancement mode TFTs work as drivers and the depletion mode TFTs act as depletion loads in the OpAmp circuit. The created OpAmp has a voltage gain (G) of 54.80 dB, a cutoff frequency (fc) of 75 Hz, a unity gain frequency (fug) of 10 kHz and a slew rate ts(up/down) of 0.15/0.30 V/µs. Transistors with different threshold voltages Uth on the same substrates are realized by using different semiconductor channel thicknesses. This is possible, as an increasing layer thickness of the semiconductor leads to a reduction of the threshold voltage Uth [1].