JPH0322483A - Thin-film transistor - Google Patents

Abstract

PURPOSE:To enable realization of a sufficiently low OFF current being necessary for holding data, even when a gate bias is zero, by making a gate insulation film contain a ferroelectric film. CONSTITUTION:Inside an operating semiconductor layer 5, an electric field directed toward a gate insulation film 2 is generated, a spontaneous polarization P0<-> is formed and an energy band for electrons is put in a state being equivalent to the one wherein a negative gate voltage is impressed in a conventional TFT structure. In an address period when data are written in, on the other side, a positive voltage being equivalent to Ec or above is impressed on a gate, the direction of the electric field is reversed and the state of a ferroelectric film P0<+> is brought about. Therefore the energy band is put in a reverse state of storage and TFT is set in an ON operation. As for a concrete method of formation, a Ti film to be a gate electrode 1 is formed on a glass substrate 11 by a sputtering method. Next, a PbTiO3 film as the ferroelectric film is formed by a vacuum evaporation method. Subsequently, a silicon nitride film 4 and an a-Si:H film 5 as the operating semiconductor layer is formed by a plasma CVD method. Moreover, an n<+> a-Si:H film 6 as a contact layer and a Ti film 7 as a metal film are stacked and patterned, so as to form a source electrode S and a drain electrode D.

Description

[Detailed Description of the Invention] [Summary] Regarding the structure of a thin film transistor for driving a liquid crystal, an object of the present invention is to provide a novel structure of a thin film transistor that can make the dark voltage a positive voltage. The gate insulating film interposed between the gate insulating film and the gate insulating film is an insulating film including a ferroelectric film.

[Industrial application field]

The present invention relates to the structure of a thin film transistor (TPT) for driving a liquid crystal.

TPT-driven liquid crystal displays are currently being actively developed as flat displays for office automation terminals and the like because they have a large capacity and can display clear, full-color displays.

[Conventional technology]

In TPT-driven liquid crystal display devices, it is necessary to provide a TPT for each pixel, which complicates the manufacturing process, resulting in problems such as low manufacturing yield and throughput.

In order to solve this problem, the present inventors have applied for a patent application to develop a gate-connected opposed matrix type thin film transistor matrix that has no bus line crossover in principle, on a TPT substrate in which TPTs are arranged in a matrix. Showa 6
No. 1-212696. This was proposed in Japanese Patent Application No. 61-212697 and elsewhere.

FIG. 7 is an equivalent circuit diagram of the thin film transistor matrix of the gate connection method, and as shown, the gates of the thin film transistors T+ and T2 are connected to the scan line Sn and Sn+1, respectively, and the drains are connected to the scan line Sn and Sn+1, respectively. Connected to S n+2.

[Problem to be solved by the invention]

Therefore, in this method, during the non-selection period during which data is retained, all the potentials of the non-selected scan canvas lines are at the same potential, so that all the thin film transistors on the non-selected scan canvas lines have the gate voltage 6 and the drain voltage 2 have the same voltage. Therefore, as a TPT characteristic, it is necessary that the off-state current value be sufficiently low when the gate bias is 0.

In addition, in the same figure, LC is a liquid crystal cell, VS is a source voltage, and Dm
, Dm+1 are data bus lines.

It cannot be said that it is necessarily easy to realize the above requirements with an actual thin film transistor matrix.

Therefore, it is desirable to be able to shift the dark voltage of the thin film transistor matrix in the positive or negative direction depending on the purpose.

SUMMARY OF THE INVENTION In view of the above-mentioned requirements, an object of the present invention is to provide a novel structure of a thin film transistor in which the dark voltage can be set to a positive voltage.

[Means to solve the problem]

In the present invention, as shown in FIG. 1, the gate insulating film 2 of the thin film transistor is a film containing a ferroelectric film.

For example, when the active semiconductor layer 5 is a hydrogenated amorphous silicon (a-Si:H) film, the gate insulating film 2 is a two-layer film of a SiN (silicon nitride) film and a ferroelectric film, and the active semiconductor layer A SiN (silicon nitride) film having good interface characteristics with the active semiconductor 3 4 layer 5 is provided on the side of the active semiconductor 3 4 layer 5 , and a ferroelectric film is provided on the side in contact with the gate electrode 1 .

As described above, one main surface of the active semiconductor layer 5 is in contact with the gate insulating film 2, and the other main surface is laminated with a contact layer 6 such as an n'' a-Si layer and a metal film 7, forming a source electrode. S
There is no difference from the conventional method in that the drain electrode D is formed.

[For production]

Figure 2 is a diagram showing the well-known memory effect of ferroelectric materials, where the horizontal axis is the externally applied electric field and the vertical axis is the spontaneous polarization P.
It is. In other words, when the externally applied voltage is 0,
A ferroelectric substance has two states P. ``'' and P0-, and which state is taken depends on the history of the electric field applied before. If the voltage is increased after applying a voltage equal to or less than the negative critical voltage Ec, route A is used, and if the voltage is lowered after applying a positive critical voltage Ec, route B is routed to avoid hysteresis. draw.

The present invention takes advantage of this, and as shown in FIG. 3, the basic voltage waveform applied to the gate for TPT driving is changed to E
A negative voltage corresponding to c1 or more is applied, and the state of the ferroelectric substance is P.during the non-selection period. −.

The operating principle of the present invention when driven in this manner will be explained with reference to FIG.

That is, due to the relationship a ivf=Q, an electric field directed toward the gate insulating film 2 is generated inside the active semiconductor layer 5 . Therefore, a spontaneous polarization P1 as shown in the figure (a) is formed, and the energy band for electrons is a depletion state that curves upward as shown in (+), that is, when a negative gate voltage is applied in a normal TPT structure. The state is equivalent to that of

On the other hand, during the address period in which data is written, a positive voltage equal to or higher than Ec is applied to the gate, and the direction of the electric field is reversed, so that the ferroelectric film becomes P as shown in a). '', the energy band is in an accumulation state opposite to the above, as shown in (d), and the TPT is turned on.

5 6 As described above, according to the present invention, due to the memory effect of the ferroelectric material, even when the gate bias is -0, it is possible to realize a TPT channel state equivalent to applying a negative gate voltage to a normal TPT. A sufficiently low off-state current necessary for data retention can be achieved.

〔Example〕

An embodiment of the present invention will be described with reference to FIG.

A Ti film, which will become the gate electrode 1, is placed on the glass substrate 11 in a thickness of about 8
It is formed to a thickness of 0 nm by sputtering. Next, a P b Ti O 3 film 3 as a ferroelectric film is formed to a thickness of about 500 nm by vacuum evaporation.

Next, by plasma CVD method, the silicon nitride film 4 is made to have a thickness of about 30 nm as an active semiconductor layer T (Da-S
i: H (hydrogenated amorphous silicon) film 5 is formed to a thickness of about 50 nm. Furthermore, an n"a-Si:H film 6 with a thickness of about 30 nm as a contact layer and a Ti film 7 with a thickness of about 100 nm as a metal film are laminated, and these are punctured to form a source electrode S and a drain electrode D. By applying this, the thin film transistor of this example is completed.

The voltage-current characteristics of the thin film transistor of this example described above are as follows from P. In one state, as shown in curve I in Figure 6, the drain current decreases, the dark value shifts in the positive direction, and Po+
In the ``state, the drain current increases as shown in curve ■,
The darkness value shifts in the negative direction. Therefore, as shown in FIG. 3, by applying a voltage of less than Ec1 during the non-selection period and applying a voltage of Ec'' or more during the address period, the curve I of FIG. 6 is applied during the non-selection period. Therefore, the off-state current when the gate bias is 0 is significantly reduced.

Note that the curve {circle around (2)} is shown for comparison with the characteristics of this example, and represents the characteristics of a conventional thin film transistor.

〔Effect of the invention〕

As explained above, according to the present invention, gate bias=0
However, since a sufficiently low off-state current necessary for data retention can be obtained, a TPT-driven liquid crystal display with clear display can be realized.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the structure of the present invention, Fig. 2 is a memory effect of ferroelectric material, Fig. 3 is a TPT drive voltage waveform, Fig. 4 is an explanatory diagram of the principle of the present invention, and Fig. 5 is an illustration of the principle of the present invention. FIG. 6 is an explanatory diagram of characteristics of an embodiment of the present invention, and FIG. 7 is an equivalent circuit diagram of a gate connection method. In the figure, 1 is a gate electrode, 2 is a gate insulating film, 3 is a ferroelectric film (PbTi03 film), 4 is a SiN film, 5 is an active semiconductor layer (a-St:H film), and 6 is a contact layer (n
” a-S i: H film), 7 is a metal film (Ti film),
11 indicates a glass substrate.

Claims (2)

[Claims] (1) A thin film transistor characterized in that the gate insulating film (2) interposed between the gate electrode (1) and the active semiconductor layer (5) is an insulating film containing a ferroelectric film (3). (2) The active semiconductor layer (5) is made of a hydrogenated amorphous silicon film, and the gate insulating film (2) is made of a silicon nitride film (4) in contact with the active semiconductor layer and a ferroelectric material in contact with the gate electrode. The thin film transistor according to claim 1, wherein the thin film transistor is a laminated film including a film (3).