JPS6058676A - Driving method for thin film transistor - Google Patents

Abstract

PURPOSE:To reduce the OFF-current by a method wherein a gate electrode is provided also in the lower part of a semiconductor thin film via gate insulation film, and the thin film transistor is driven by application of a constant voltage in the neighborhood of a flat band voltage at the lower part interface on this electrode. CONSTITUTION:When the difference in work functions between the lower gate electrode and the semiconductor thin film is phiMS', the interface level of the lower interface QSS, the static capacitance of the lower gate insulation film C0, the film thickness of the gate insulation film x0, and the charge density in the gate insulation film rho(x), the flat band voltage VFB at the lower interface can be expressed by the formula I . Therefore, application of a voltage in the neighborhood of a constant voltage VFB on the lower gate electrode 41 causes the prevention of the bending of the band and then enables the OFF-current to be kept to a very small value.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a thin film transistor that reduces leakage current between source and drain.

In recent years, research on forming thin film transistors on insulating substrates has been actively conducted. This technology has many applications, including active matrix image display devices that create thin-film displays using inexpensive insulating substrates, and so-called three-dimensional integrated circuits that form active elements such as transistors on regular semiconductor integrated circuits. This is something to look forward to.

Hereinafter, an example in which thin film transistors are applied to an active matrix panel will be explained.

A liquid crystal display device in which a thin film transistor is applied to an active matrix panel is generally composed of an upper glass substrate, a lower thin film transistor substrate, and a liquid crystal sealed between them. An arbitrary character can be created by selecting a liquid crystal driving element arranged in the area by an external selection circuit and applying a voltage to a liquid crystal driving electrode connected to the liquid crystal driving element.

It displays figures or images. A general circuit diagram of the thin film transistor substrate is shown in FIG.

FIG. 1(α) is a matrix arrangement diagram of liquid crystal drive elements on a thin film transistor substrate. The area surrounded by 1 in the figure is the display area, and the liquid crystal driving elements 2 are arranged in a matrix therein. 3 is a data signal line to the liquid crystal driving element 2, and 4 is a timing signal line to the liquid crystal driving element 2. The circuit diagram of the liquid crystal driving element 2 is shown in Figure 1C.
Shown in b). A thin film transistor 5 performs data switching. 6 is a capacitor, which is used for holding data signals. 7 is a liquid crystal panel;
7-1 is a liquid crystal drive electrode formed corresponding to each liquid crystal drive element, and 7-2 is a common electrode on the upper glass substrate.

As can be seen from the above description, thin film transistors are used to switch voltage data applied to liquid crystals. At this time, regarding the characteristics of thin film transistors, 1. The following two items are required.

(1) When the thin film transistor is turned on, sufficient current can flow to charge the capacitor.

(2) When the thin film transistor is turned off, as little current as possible should flow through it.

(1) relates to the characteristics of writing data to a capacitor. Since the liquid crystal display is determined by the potential of the capacitor, the thin film transistor must be able to flow a sufficiently large current so that data can be completely written in a short period of time. The current at this time (hereinafter,
The ON current (referred to as ON current) is determined by the capacitance of the capacitor and the write time, and thin film transistors must be manufactured so as to clear the ON current.

(2) relates to the retention characteristics of data written to the capacitor. Generally, written data must be retained for a much longer time than the write time. Since the capacitance of a capacitor is usually as small as IPF, even a small amount of leakage current (hereinafter referred to as 0FIF current) occurs when the thin film transistor is in the OFF state.
When the current flows, the potential of the drain, that is, the potential of the capacitor, rapidly approaches the potential of the source, and the written data is no longer retained correctly. Keeping the OFF current low is exactly the same requirement when applying thin film transistors to uses other than active matrix panels. For example, when a normal logic circuit is constructed using thin film transistors, static current increases, and when a memory circuit is constructed, this may cause malfunction.

An object of the present invention is to provide a method for driving a thin film transistor that reduces OFF current, thereby making it possible to further expand the field of application of thin film transistors. Hereinafter, after describing the relationship between the conventional thin film transistor driving method and the OFF current, the present invention will be explained in detail.

FIG. 2 is a sectional view showing a conventional general structure of an N-channel thin film transistor. 8 is glass.

It is an insulating transparent substrate such as quartz. When applied to a three-dimensional integrated circuit, 8 is a normal semiconductor integrated circuit. 9
10 is a semiconductor thin film, and 10 is a substrate insulating film to prevent positive charges such as sodium ions (Ha'')' contained in the insulating transparent substrate 8 from being mixed into the semiconductor thin film, and is usually made of silicon dioxide (810 , ) are used. 11 is a source region of an N-type layer formed by doping impurities such as phosphorus or arsenic into a semiconductor thin film, 12 is a drain region, and 13 is a drain region.
14 is a gate insulating film, 14 is a gate electrode, 15 is an interlayer insulating film, 16 is a source electrode, and 17 is a drain electrode. Conventionally, a constant drain voltage VD8 is applied to the drain 12 with respect to the source 11, and the voltage yGs of the gate 14 with respect to the source 11 is controlled.
A driving method is used in which the thin film transistor is switched by controlling the channel formed at the upper interface of the thin film transistor. For N-channel thin film transistors, VO
When S is set to a positive potential, an N-type layer channel is formed at the upper interface of the semiconductor thin film. Therefore, more drain current flows between the source and drain.

This indicates that the thin film transistor is in the ON state. Next, when VaS is decreased, it decreases more than the drain current and takes a minimum value near vos=ov. This state is the OFF state of the thin film transistor. The minimum value flowing at this time is the above-mentioned OFF current. As mentioned above, in the conventional driving method of the 'fJ film transistor, only the channel formed at the upper interface of the semiconductor thin film is controlled, and the channel formed at the lower interface of the semiconductor thin film is not considered at all. . The N-type layer formed at the lower interface will be described below. The oxide film contains some positive charges such as Na. Therefore, the substrate insulating film 10 also contains positive charges. Due to this positive charge, a constant N-type layer is always induced at the lower interface of the semiconductor thin film, and a channel is formed between the source and drain. A schematic diagram of the upper interface and the lower interface is shown in Figure 2 Ch). 18 is an insulating transparent substrate, 19 is a substrate insulating film, 20 is a semiconductor thin film, 21 is a source region, 22 is a drain region, 23 is a gate insulating film, and 24 is a gate electrode. 25 is the interface state at the upper interface, the positive charge contained in the gate insulating film 23, and the work function difference φM between the gate electrode 24 and the semiconductor thin film 20.
This is an N-type layer at the upper interface induced by 8 and 8. Further, 26 is an N-type layer at the lower interface, which is induced due to the positive charges contained in the substrate insulating film 19 and the interface states at the lower interface, as described above. As described above, in the conventional driving method, the N-type N25 at the upper interface is simply controlled by the gate voltage yGs applied to the gate electrode 24. Therefore, the N-type layer 26 at the lower interface is always formed, and when the thin film transistor is in the OFF state, leakage current flows through this N-type layer at the lower interface, resulting in a large OF'? Current will flow. The band diagram at this time is shown in Figure 2 (CC). 27 is a gate electrode;
28 is a semiconductor thin film, 29 is a gate insulating film, 60 is a substrate insulating film, and 31 is an insulating transparent substrate. 32 is the level HO of the conductor edge of one semiconductor film.
15S also shows the level Z-ma of the lenth band edge, and 34 shows the intrinsic 7 Hermi level.

shows the level Bi. 35 indicates that the Fermi levels ET of the gate electrode, semiconductor thin film, and substrate match. 36 is the upper interface of the semiconductor thin film, 67
(also indicates the lower interface). If the C) VFB flow flowing through the N-type layer at the upper interface is defined as OFF, and the leakage current flowing through the N-type layer at the lower interface is defined as OFF, then the OFF current flow in the conventional thin film transistor driving method is OFF. +1L ′−−−−−■ As mentioned above, since a constant N-type layer is always formed at the lower interface of the semiconductor thin film, the 01?'? current of the thin film transistor increases. It turns out.

The present invention provides a method for driving a thin film transistor that prevents the formation of an N-type layer at the lower interface of such a semiconductor thin film and reduces the OFF current. In order to achieve this, in the present invention, a gate electrode is also provided under the semiconductor thin film via a gate insulating film, and a constant voltage near the flat band voltage of the lower interface is applied to the lower gate electrode to drive the thin film transistor. do. The present invention will be explained below.

FIG. 3 shows an embodiment of the present invention, and the figure (α)
shows a thin film transistor in which a gate electrode is also provided under the semiconductor thin film via a gate insulating film. 38 is an insulating transparent substrate, 39 is a substrate insulating film, 40 is a lower gate insulating film, 41 is a lower gate electrode, 42 is a semiconductor thin film, 43 is a source region, 44 is a drain region, 45 is an upper gate insulating film, 46 is a Upper gate electrode, 47, interlayer insulating film, 48
indicates a source electrode, and 49 indicates a drain electrode. In order to prevent leakage current flowing through the lower interface, the lower gate electrode 41 is provided with a flat band voltage VF at the lower interface.
A constant voltage near R may be applied. The value of VFB takes different values depending on the semiconductor material, gate electrode material, and gate insulating film. Now, the work function difference between the lower gate electrode and the semiconductor thin film is φ-8*, and the interface state of the lower interface is Q s s
s The capacitance of the lower gate insulating film is OO1 The thickness of the lower gate insulating film is "OsT" If the charge density in the gate insulating film is ρ (3:), then the flat band voltage at the lower interface is v
yi+ is VF R=φJ Q, all-Eng.
It can be expressed as xOo Ooo xO ・・・・・・■. Therefore, it is sufficient to apply a voltage near the constant voltage VFR expressed by the above equation to the lower gate electrode 41. The band diagram at this time is shown in FIG. , 56 is the lower interface, 57 is the level IQ of the contact band edge of the semiconductor thin film, and 5B is the level Bv of the valence band edge.

The band is curved because an N-type layer is induced near the upper interface by positive charges in the upper gate insulating film.

However, since the flat band voltage VFR expressed by equation (2) is applied to the lower gate electrode, bending of the band is prevented. In other words, no N-type layer is formed near the lower interface, and no leakage current flows here. Therefore, the OFF current of the thin film transistor is calculated by the formula ■
It is expressed as the value when L=Q. In this way, if the method for driving a thin film transistor according to the present invention is used, O
Let us consider a case where the present invention, which has the excellent effect of suppressing the FF current to a very small value, is applied to an active matrix panel. Since the lower layer of the semiconductor thin film is an insulating transparent substrate that is uniform over the entire surface, the bending of the band at the lower interface of the semiconductor thin film of each thin film transistor arranged in a matrix is all the same. Therefore, in this case, it is sufficient to apply a constant flat band voltage VFR to the lower gate electrodes of all thin film transistors using the same electrode.

On the other hand, when the present invention is applied to a three-dimensional integrated circuit, the lower layer of the semiconductor thin film is a normal semiconductor integrated circuit with a lower gate insulating film interposed therebetween. Therefore, since the substrate is not uniform depending on the location, the bending of the band at the lower interface of the semiconductor thin film varies depending on the location. Therefore, in this case, the leakage currents of all thin film transistors are considered in total, and the voltage to be applied to the lower gate electrode is determined so that the leakage current flowing through the lower interface is minimized in total. By the way, there is also a method of applying a flat band voltage at the lower interface to the lower gate electrode of each thin film transistor, but this method complicates the drive circuit and is not suitable. In this way, when applying the present invention to a three-dimensional integrated circuit, it is optimal to use the same electrode for all the lower gate electrodes of thin film transistors and to apply a constant voltage close to the flat band voltage of the lower interface to the lower gate electrode. It is. Further, the value of the flat band voltage 7FB can be controlled by the combination of the lower gate electrode and the semiconductor thin film material and the type of the lower gate insulating film. This can be understood by referring to equation (2) as described above. Therefore, when setting the flat band voltage to an arbitrary value, a thin film transistor may be manufactured by appropriately selecting a semiconductor material, a lower gate electrode material, and a lower gate insulating film material.

As described above, the present invention provides OFF control of thin film transistors.
It has the excellent effect of significantly reducing current, and enables the creation of superior circuits such as active matrix panels with excellent retention characteristics, logic circuits with low quiescent current, and memory circuits with fewer malfunctions. It becomes possible to do so.

[Brief explanation of the drawing]

FIGS. 1(α) and 1(h) are general circuit diagrams when thin film transistors are applied to active matrix panels. Figure 2 (α) Ch) (C) is a diagram for explaining the conventional driving method of a thin film transistor, and Figure 3 (α) (
b) is a diagram for explaining the thin film door/raster driving method proposed in the present invention. Applicant Suwa Seikosha Co., Ltd. Agent Patent Attorney Tsutomu Mogami

Claims (1)

[Claims] In a thin film transistor in which a gate electrode is placed above and below a semiconductor thin film through a gate insulating film,
A method for driving a thin film transistor, comprising applying a constant voltage near a flat band voltage to one of the gate electrodes.