JP2002090780A - Polysilicon tft liquid crystal display element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly reliable TFT for an analog switch which functions to hold a video signal and write it to pixels in a driving circuit integrated type polysilicon TFT liquid crystal display device. SOLUTION: A panel sample system driving circuit integrated type polysilicon TFT liquid crystal display device is constituted by using a switching transistor composed of a P type TFT single body having its source and drain formed by self-matching by injecting acceptor type ions.

Description

DETAILED DESCRIPTION OF THE INVENTION

[Industrial applications] The present invention is applied to an active matrix type liquid crystal display, a contact type image sensor, and an active matrix type EL display. [Conventional technology]

2. Description of the Related Art In a polycrystalline Si-TFT liquid crystal display device integrated with a driving circuit, an analog switch for holding and writing an analog video signal has conventionally been constructed based on an n-type TFT. Basically, it is a switch, and even if the characteristics change a little, it should be sufficient if it finally meets the specifications. However, since a large-sized transistor is used to secure high channel conductance, capacitance coupling at the time of cutoff increases. Therefore, when the threshold voltage of the TFT changes and the effective channel voltage changes, the video signal level may change. The sampling circuit duty is about 99%, and most of them are off periods. On the other hand, an n-type TFT manufactured by a low-temperature process of 600 ° C. or less is vulnerable to stress in an off state, and may cause deterioration of display quality.

[Means for Solving the Problems] N-type TFT
Is considered to be caused by the injection of holes into the gate insulating film. The mechanism is that the bond of Si—OH contained in SiO ₂ is broken by the holes, and the fixed charges are removed by the film. It is thought to be due to being induced in. Therefore, using a p-type TFT to reduce this effect is considered as a means.

[Operation] In the off-stress state of the p-type TFT, the charge carriers become electrons. Therefore, electrons jump over the barrier at the drain end and jump into the gate insulating film, and the collision cross section with respect to Si—OH is smaller than that of holes, so that the probability of breaking bonds and generating fixed charges is reduced.

[Embodiment] FIG. 1 shows an equivalent circuit diagram of a panel sample-hold type TFT liquid crystal display device. Pixel TF
T1 was made of n-type, and TFT2 for a video signal writing analog switch was made of p-type. Other logic elements for the shift register 3 and the buffer circuit 4 include complementary TFs.
T was used. At this time, the pull-in voltage Vc when the analog switch is off is expressed by the following equation. Vc = α · (Vth−Vgl) · Cgd / (Csl + Cgd) where α = Cgd / (Csl + Cgd), Cgd: gate drain capacity, Csl: pixel and auxiliary capacity, Vth:
The threshold voltage of the TFT, Vgl: a lower gate voltage. A similar pull-in voltage is generated in the pixel TFT, but the TFT size is small, so that the video signal level does not greatly change. FIG. 2 shows that an n-type TFT has Vg = −
7.0 V, Vs = 4.5 V, Vd = -4.5 to 4.5
V, Vg = 7.0 V, Vs = −4.5 as ON state
Vth after applying V and Vd = 4.5V for 1 hour at room temperature
Shows the change. FIG. 3 shows the results of a similar aging test performed on a p-type TFT with the above polarity reversed. The characteristics of the n-type TFT change greatly in the off state,
It can be seen that the change in the ON state is large in the p-type. In the analog switch 2, the ON operation period is extremely short, and the analog switch 2 is exposed to the OFF bias condition.
It becomes 99% or more in the case of driving. Therefore, when an n-type TFT is used for an analog switch, the threshold value between the TFTs greatly varies, and the video signal level varies from source line to source line. This deterioration could be avoided by using a type TFT.

[00010]

[Brief description of the drawings]

FIG. 1 is an equivalent circuit of a TFT-LCD using the present invention.

FIG. 2 is a result of stress aging of an N-type TFT.

FIG. 3 shows a result of stress aging of a P-type TFT.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Pixel TFT 2 ... Analog switch TFT 3 ... Shift register 4 ... Buffer circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification FI FI Theme coat ゛ (Reference) H01L 29/78 616V F-term (Reference) 2H092 GA59 JA24 NA21 PA06 5C058 AA09 BA32 BA35 5C094 AA02 BA03 BA27 BA43 CA19 DB01 DB04 EA04 EA07 EB02 5F110 AA14 BB02 BB10 GG02 GG13 GG37 HJ04 QQ11 5G435 AA00 BB05 BB12 EE37

Claims (3)

[Claims] An analog switch for inputting a video signal is a P-type TF.
A polycrystalline silicon TFT liquid crystal display device integrated with a drive circuit, comprising: 2. The method according to claim 1, wherein an impurity whose source and drain serve as acceptors to Si is formed in a self-aligned manner by ion implantation using the gate electrode as a mask. Drive circuit integrated type polysilicon TFT Analog switch P-type TFT for liquid crystal display device. 3. The method according to claim 1, wherein a P-type impurity having a concentration of 10 ¹⁸ cm ⁻³ or less is doped to a width of 1 μm or less between the drain end and the drain and the channel. Drive circuit integrated type polysilicon TFT Analog switch P-type TFT for liquid crystal display device.