Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Patent
1997-06-30
1999-06-29
Jackson, Jr., Jerome
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
G02F 113
Patent
active
059172100
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates in general to thin film transistors (TFTs) and more particularly to a pair of novel TFT structures exhibiting reduced parasitic capacitance.
BACKGROUND OF THE INVENTION
As a consequence of recent rapid development of flat panel display technologies, thin film transistors (TFTs) are being actively utilized in the implementation of two types of large area electronic devices, namely liquid crystal displays (LCDs) and flat-panel imaging devices. These devices generally comprise a large number of TFTs, which act as switches or amplifiers.
As is well known in the art, a typical TFT is constructed using a MOS structure (metal oxide semiconductor) comprising a semiconductor film, a gate electrode, a gate dielectric film, source and drain electrodes. The semiconductor film can be fabricated from amorphous silicon (a-Si), poly-silicon (poly-Si), cadmium selenide (CdSe), or other suitable semiconductor material. The metal material of the electrodes can be fabricated from chromium or aluminium. The material of the dielectric film is fabricated typically from one of either silicon nitride, silicon oxide or various anodic oxide films.
As is well known, MOS transistors are normally provided with an overlapping area between the gate and source and between the gate and drain electrodes, to ensure continuity of the channel formed in the semiconductor layer. Generally, the overlapping area should be no less than the design rule of a particular TFT device. Two parasitic capacitances (Cgs and Cgd) are formed in the overlapping areas between gate and source and between gate and drain, respectively. As a consequence of these known parasitic capacitances, gate control pulses are known to feedthrough the semiconductor layer into the source or drain, thereby deteriorating switching performance. While this is a well known common problem for all MOS transistors, the problem is exacerbated in large area TFT matrix applications where design rules must provide sufficiently large tolerances, corresponding to the lithographic tolerances of the fabrication process on a large size exposure area.
When a TFT switch turns off, the feedthrough charge comes from two components. The first is the differential component of the gate pulse on the parasitic capacitor, and the other results from channel electrons which are split away and squeezed into the source and drain electrodes (Z. S. Huang, Y. Katayama and T. Ando, "The dependence of the parasitic capacitance and the reset potential level in a solid-state imaging sensor," Proceedings of the Joint Meeting of 1989 Electric & Electronic Institutes, Tokai Shibu, Japan, P. 325, October (1989) and Z. S. Huang and T. Ando, "An analysis of reset mechanism in a stacked and amplified imaging sensor," Journal of the Institute of Television Engineers of Japan, Vol. 46, no. 5, pp. 624-631, May (1992)).
For a TFT-LCD, when the TFT turns off, negative charges are left on the pixel capacitor, causing the bias voltage of the liquid crystal to drop. This is equivalent to applying a DC voltage directly on the liquid crystal. This DC bias voltage causes the characteristics of the liquid crystal to shift in one direction, causing crosstalk. Moreover, because the capacitances of a liquid crystal in the ON and OFF states are different, feedthrough charges generate different feedthrough voltage shifts for "white" and "black" pixels. This causes image sticking and flicker noise in the TFT-LCD, a phenomenon referred to as "image persistence" in I-Wei Wu, "High-definition displays and technology trends in TFT-LCD", Journal of the SID, 2/1, pp. 1-14 (1994).
The problem of feedthrough charges in TFT LCD applications is less serious when compared to the problem of feedthrough charges in imaging sensors since the signal voltage is extremely small. Feedthrough charges in imaging applications can result in saturation of the feedback capacitor in the readout charge amplifier of a TFT matrix causing latch-up of the amplifier. One solution to this problem involves incorporating a la
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Huang Zhong Shou
Wright John
Jackson, Jr. Jerome
Litton Systems (Canada) Limited
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