POLYSILICON TFT CIRCUIT-DESIGN AND PERFORMANCE

Polysilicon thin-film technology is becoming increasingly attractive for active-matrix liquid-crystal displays (AMLCD's) and other large-area electronic devices, primarily because polysilicon thin-film transistors (TFT's) can be used to build integrated drive and interface circuitry on large-area substrates. Both n- and p-channel polysilicon TFT's can be fabricated, allowing CMOS circuit techniques to be used. However, TFT characteristics are poor in comparison to conventional single-crystal MOSFET's, and relatively coarse design rules must be used to be compatible with processing on large-area glass plates (more than 30 cm x 30 cm), and these limitations present a number of challenges for circuit design. This paper examines these issues and describes the performance of a range of digital and analog circuit elements built using polysilicon TFT's. Digital circuit speeds in excess of 20 MHz are reported, along with operational amplifiers with over 80 dB of gain and more than 1-MHz unity-gain frequency. Several polysilicon TFT switched-capacitor circuits are also reported and shown to have adequate linearity, output swing, and settling time to form integrated data line drivers on an AMLCD.