Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-04-18
2002-03-26
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S059000, C257S072000, C257S351000
Reexamination Certificate
active
06362507
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having a circuit structured-by a thin film transistor (hereinafter referred to as TFT), and to a method of manufacturing the same. Specifically the present invention is suitably utilized in electro-optical devices in which pixel section and the driver circuit provided in its peripheral are disposed over the same substrate, typified by a liquid crystal display device, and electronic equipment incorporating such electro-optical devices. Note that, throughout this specification, the semiconductor device indicates general devices that can function by using semiconductor characteristics, and that the electro-optical devices and electronic equipment incorporating the electro-optical devices are all categorized as semiconductor devices.
2. Description of the Related Art
Development of a semiconductor device having integrated circuit that comprises TFTs formed on a substrate having an insulation surface, has been made progressively. An active matrix type liquid crystal display device, an EL display device and a close adhesion type image sensor are known as typical examples. Particularly because TFTs using a crystalline silicon film as an active layer (the TFT will be hereinafter referred to as “crystalline silicon TFT”) have high electric field mobility, they can form a variety of functional circuits, and the electro-optical devices integrally forming these over a same substrate are developed.
In the active matrix type liquid crystal display device, for example, a pixel section for displaying images and driver circuits that performs image display are provided. The driver circuit comprises a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit and so forth, each being based on a CMOS circuit, is formed on one substrate. In the case of the close adhesion type image sensor, a sample-and-hold circuit, a shift register circuit, a multiplexer circuit, and so forth, are formed.
Because these driving circuits do not always have the same operating condition, the characteristics required for the TFTs are different not a few. The pixel section comprises a pixel TFT formed from an n-channel TFT and an auxiliary storage capacitor, which applies voltage to the liquid crystal and drives the liquid crystals. Here, it is necessary to drive the liquid crystal by alternating current, and a system called “frame inversion driving” is widely applied. Accordingly, a required characteristics of the TFT is that an OFF current value (a drain current value that flows when a TFT is in the OFF operation) must be sufficiently lowered. In a buffer circuit of the driver circuit on the other hand, because a high driving voltage is applied, the TFT must have a high withstand voltage such that it does not undergo breakdown even when a high voltage is applied. In order to improve the current driving capacity, it is necessary to sufficiently secure the ON current value (the drain current value that flows when a TFT is in the ON operation).
However, there was a problem that the OFF current is likely to become high in the crystalline silicon TFT. Degradation such as the drop of the ON current value is observed in the crystalline silicon TFT in the same way as in MOS transistors used for ICs, or the like. It is presumed that the main cause is hot carrier injection, and the hot carriers generated by a high electric field in the proximity of the drain presumably invite this degradation.
An LDD (lightly doped drain) structure is known as a structure of the TFT for lowering the OFF current value. This structure forms an impurity region having a low concentration between a channel forming region and a source or drain region to which an impurity is doped in a high concentration. The low concentration impurity region is called the “LDD region”.
A so-called “GOLD (gate-drain overlapped LDD) structure” is also known as a structure for preventing deterioration of the ON current value by hot carrier injection in which the LDD region is so arranged as to overlap with a gate wiring through a gate insulation film. By forming this structure, the high electric field in the proximity of the drain is relieved and hot carrier injection is prevented, and it is effective for preventing degradation phenomenon. For example, Mutsuko Hatano, Hajime Akimoto and Takeshi Sakai, IEDM97 Technical Digest, pp.523-526, 1997, discloses a GOLD structure formed by side walls formed from silicon, and it has been confirmed that this structure provides by far higher reliability than the TFTs having other structures.
However, the required characteristics of the pixel TFT of the pixel section and the required characteristics of the TFTs of the driver circuit such as the shift register circuit and the buffer circuit, are not always the same. For example, a large reverse bias voltage (a negative voltage in n-channel TFT) is applied to the gate wiring in the pixel TFT, but the TFT of the driving circuit is not fundamentally driven by the application of the reverse bias voltage. Further the operation speed of the former may be lower than {fraction (1/100)} of the latter.
The GOLD structure has a high effect for preventing the degradation of the ON current value, it is true, but is not free from the problem that the OFF current value becomes greater than the ordinary LDD structures. Therefore, it was not a preferable structure to be applied to a pixel TFT. On the contrary, the ordinary LDD structures have a high effect for restricting the OFF current value, but is not resistant to degradation due to hot carrier injection.
For these reasons, it is not always preferred to constitute all the TFTs by the same structure in the semiconductor devices having a plurality of integrated circuits such as the active matrix type liquid crystal display device.
SUMMARY OF THE INVENTION
The present invention is a technology for solving the above stated problems and, the present invention aims at improving operation performance and reliability of a semiconductor device by optimizing in accordance with the function of each circuit the structures of the TFT used for each circuit of the semiconductor device.
In order to solve the above stated problems, a constitution of the present invention is characterized as: in a semiconductor device which comprises: a pixel section comprising a n-channel TFT having an active layer, an LDD region provided in the active layer, a gate insulating film provided between the active layer and the substrate, a gate electrode provided between the gate insulating film and the substrate; and its driving circuit, wherein the semiconductor device is characterized in that: at least a portion or all of the LDD region of the n-channel TFT of the pixel section is disposed so as to overlap the gate electrode of the pixel section; LDD region of the n-channel TFT of the driving circuit is disposed so as to overlap the gate electrode of the n-channel TFT of the driving circuit; and an impurity element imparting n-type is included in the LDD region of the n-channel TFT of the driving circuit at a higher concentration than that included in the LDD region of the n-channel TFT of the pixel section.
Another constitution of the present invention is characterized as: in a semiconductor device which comprises: a pixel section comprising a n-channel TFT having an active layer, an LDD region provided in the active layer, a gate insulating film provided between the active layer and the substrate, a gate electrode provided between the gate insulating film and the substrate; and its driving circuit, wherein the semiconductor device is characterized in that: at least a portion or all of the LDD region of the n-channel TFT of the pixel section is disposed so as to overlap the gate electrode of the pixel section; at least a portion or all of the LDD region of the n-channel TFT of the driving circuit is disposed so as to overlap the gate electrode of the n-channel TFT of the driving circuit; and an impurity element imparting n-type is included in t
Kasahara Kenji
Kawasaki Ritsuko
Kitakado Hidehito
Ogawa Hiroyuki
Tomiyasu Kazuhide
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Ngo Ngan V.
Semiconductor Energy Laboratory Co,. Ltd.
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