Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1995-03-13
2001-06-05
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S059000, C257S350000
Reexamination Certificate
active
06242777
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to thin film transistors, active matrix assemblies including thin film transistors, and more particularly to improved thin film transistors including polycrystalline silicon or amorphous silicon thin films and active matrix assemblies including the thin film transistors for providing improved thin-type displays.
Applications of thin film transistor technology are wide-spread, including their use as active switching elements in thin matrix panels and three-dimensional integrated circuits and the like. A recurring problem encountered when using thin film transistors is leakage current of the transistor when it is in the OFF state. In the conventional MOS type transistor utilizing monocrystalline silicon, a PN junction is utilized to decrease OFF current where a P-type substrate is used for N-channel and N-type substrate is used for P-channel. When using prior art polycrystalline silicon transistors, the formation of an effective PN junction cannot be attained and hence the OFF current cannot be decreased enough to allow its use in a matrix display. This problem is particularly undesirable when the transistor is being used as a switching element in, for example, an active matrix panel. A MOS transistor with a poly-crystalline-silicon film channel region of approximately 2 microns thickness is disclosed in Solid-State Electronics, 1972, Vol. 15, pp. 789-799. The authors reported on field-effects in polycrystalline-silicon films and threshold voltages of doped films and concluded that these MOS transistors would find limited practical application.
When a thin film transistor (TFT) is used as a switching element in a matrix-type arrangement in a liquid crystal display (LCD) device it selects the data signal to be applied to the liquid crystal material. In such construction, the TFT must have the following characteristics:
(1) Permit enough current to flow into a condenser for charging when the TFT is in the ON state;
(2) Exhibit insignificant current flow in an electrode when the TFT is in the OFF state; and
(3) Show stabilized, reproducible performance and reliability.
Requirement (1) relates to the TFT's inputting a data signal into a condenser. A TFT must accept a large amount of current flow for a short time so as to completely input the data signal to the condenser, since the quality of a liquid crystal display depends in part on the capacitance of the condenser. This amount of current flow (hereinafter referred to as “ON current”) is determined by the capacitance of the condenser and the time elapsed in writing the data signal to the condenser. Compliance with condition (1) depends largely on the TFT's size (especially channel length and width), construction, manufacturing process and input voltage to its gate. A TFT composed of polycrystalline silicon is capable of carrying a sufficient amount of ON current, and satisfying the requirement (1), since the polycrystalline silicon has a large carrier mobility comparing with that of amorphous semiconductors.
Requirement (2) relates to the holding time of written data in a condenser. Generally, written data in a condenser must be kept for an extremely long time compared with the writing time of that data to the condenser. Since the capacitance of the condenser is generally a very small value, such as 1pF, if there is even a small amount of leakage current (“OFF current”) at the TFT, during the OFF state, then the driving voltage of the electrode sharply drops to the level of voltage of the data signal time. As a result, the written data signal cannot be held properly at a condenser during the OFF state, and image sharpness is lost. This problem has been especially so in the case of prior art polycrystalline silicon TFTS, where deep and shallow trap levels are unevenly distributed in a crystal grain, allowing leakage current to flow via these trap levels, as discussed in the above-identified article in Solid State Electronics.
The requirement for insignificant current flow in the OFF state is necessary in other applications of TFTs, for example, in logic circuits using TFTs where stationary current increases and in memory circuits using TFTs.
Requirement (3) relates to stability, reproducibility and reliability of thin film transistor characteristics. Generally, several tens of thousands of thin film transistors are formed on one active matrix substrate and all of them must have uniform characteristics and superior reproducibility with no dispersion among manufacturing lots and remain stable and delivery reliable performance for an indefinitely long term.
Conventional thin film transistors as active elements on a substrate include a compound semiconductor, such as calcium selenium, and the like, or non-crystalline semiconductor, such as amorphous silicon, and the like, as the semiconductor thin film. However, these TFTs cannot satisfy all of the three above-mentioned requirements. For example, a TFT including a semiconductor film satisfies requirement (1) due to the high value of carrier mobility, but cannot meet requirements (2) and (3), since this type of semiconductor exhibits poor stability and reproducibility. A non-crystalline semiconductor has a low carrier mobility, and hence substantially small ON current flows. As noted above, an active matrix panel utilizing such a thin film transistor on a substrate did not exhibit sufficiently good characteristics for providing satisfactory image quality of the above matrix panel.
Accordingly, it is desirable to provide improved thin film transistors having reduced leakage current in the OFF state that overcome deficiencies found in the prior art.
SUMMARY OF THE INVENTION
Generally speaking, in accordance with the invention, thin film transistors of a thin film thickness less than 2500 Å and greater than about 100 Å having reduced leakage current in the OFF state are provided. The thickness of the thin film in the channel region is less than the thickness of the source and drain regions. The channel thickness should also be greater than about 100 Å. When such thin film transistors are utilized in matrix display elements, the data signal line and driving electrodes may be formed with the same transparent conductive film. Thin film transistors are prepared in accordance with the invention by depositing a thin film on an insulating substrate and etching to provide a channel portion having a concave surface. The non-etched portions can be doped to provide source and drain regions. This method allows for secure connection of the connector metal to the source and drain regions without their breaking through the diffusion regions.
Accordingly, it is an object of this invention to provide an improved thin film transistor.
Another object of the invention to provide an improved thin film transistor including a silicon thin film exhibiting reduced current leakage.
A further object of the invention to provide an improved thin film transistor including a silicon thin film between about 100 Å and 2500 Å thick.
Still another object of the invention is to provide an improved thin film transistor including a channel region having a depth of at least about 100 Å and no more than about 2500 Å.
Still a further object of the invention is to provide an improved active matrix assembly including the improved thin film transistor in accordance with the invention as switching elements.
Yet another object of the invention is to provide an active matrix assembly including the film transistors as switching elements having a channel region with a thickness of at least about 100 Å and no more than about 2500 Å.
Yet a further object of the invention is to provide a process for preparing improved thin film transistors.
Another object of the invention is to provide a process for preparing improved active matrix-type arrangements including thin film transistors as switching elements.
A further object of the invention is to provide an active matrix arrangement in which the thin film transistor swit
Kodaira Toshimoto
Mano Toshihiko
Oshima Hiroyuki
Loke Steven
Oliff & Berridg,e PLC
Seiko Epson Corporation
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