JPH0544195B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor thin film transistor having a structure that reduces leakage current between a source and a drain.

In recent years, research on forming thin film transistors on insulating substrates has been actively conducted. This technology can be used to create active matrix panels that use inexpensive insulating substrates to create thin displays, three-dimensional integrated circuits that form active elements such as transistors on regular semiconductor integrated circuits, and inexpensive, high-performance images. It is expected to have many applications, including sensors and high-density memory. Below, we will explain the case where thin film transistors are applied to active matrix panels as an example.
The present invention can be similarly applied to other cases using thin film transistors. This is because the gist of the present invention is to reduce leakage current, which is an essential improvement in the characteristics of thin film transistors.

Liquid crystal display devices when applied to active matrix panels using thin film transistors generally have the following characteristics:
It is composed of an upper glass substrate, a lower thin film transistor substrate, and a liquid crystal sealed between them, and an external selection circuit selects the liquid crystal driving elements arranged in a matrix on the thin film transistor substrate, and selects the liquid crystal driving elements arranged in a matrix on the thin film transistor substrate. By applying a voltage to a liquid crystal drive electrode connected to a drive element, arbitrary characters, figures, or images are displayed. A general circuit diagram of the thin film transistor substrate is shown in FIG.

FIG. 1a is a diagram showing a matrix arrangement of liquid crystal driving elements on a thin film transistor substrate. The area surrounded by 1 in the figure is a display area, in which liquid crystal driving elements 2 are arranged in a matrix. 3
is a data signal line to the liquid crystal drive element 2,
4 is a timing signal line to the liquid crystal driving element 2; A circuit diagram of the liquid crystal driving element 2 is shown in FIG. 1b. A thin film transistor 5 performs data switching. 6 is a capacitor, which is used for holding data signals. The capacitance of this capacitor includes the capacitance of the liquid crystal itself and the capacitance of an intentionally provided capacitor, but in some cases, it may be composed only of the capacitance of the liquid crystal. 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 liquid crystal drive electrode formed corresponding to each liquid crystal drive element.
is the upper glass panel.

As can be seen from the above description, thin film transistors are used to switch voltage data applied to liquid crystals, and the characteristics required of thin film transistors at this time can be broadly classified into the following two types.

(1) Sufficient current must be able to flow to charge the capacitor when the thin film transistor is turned on.

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

(1) relates to the characteristics of writing data to the capacitor. Since the display of a liquid crystal is determined by the potential of a capacitor, thin film transistors 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 referred to as ON current) is determined by the capacitance of the capacitor and the writing time, and thin film transistors must be manufactured to clear this ON current. The ON current that can flow through a thin film transistor greatly depends on the transistor's size (channel length and channel width), structure, manufacturing process, gate voltage, drain voltage, etc.

(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. The capacitance of the capacitor is
Since it is usually a small value of about 1 pF, if even a small amount of leakage current (hereinafter referred to as OFF current) flows when the thin film transistor is in the OFF state, the drain potential (that is, the capacitor potential) will rapidly approach the source potential. The written data will no longer be retained correctly. Therefore,
The OFF current must be kept as small as possible. The mechanism of the OFF current will be described in detail later because it is related to the gist of the present invention.

As can be seen from the above description, reducing the OFF current of a thin film transistor has very important significance. This is because if an attempt is made to obtain a sufficient ON current by decreasing the channel length and increasing the channel width, the OFF current will also increase, which will deteriorate data retention characteristics. Therefore, reducing the OFF current is an urgent need for improving the characteristics of thin film transistors. This is exactly the same when thin film transistors are applied 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 or an image sensor is constructed, it causes malfunction.

The present invention aims to eliminate such drawbacks of conventional thin film transistors, and an object thereof is to provide a thin film transistor having a structure that reduces OFF current. below,
After describing the mechanism of the OFF current in detail, the content of the present invention will be explained based thereon.

FIG. 2 is a cross-sectional view showing the general structure of an N-channel thin film transistor using a semiconductor thin film.
8 is an insulating transparent substrate such as glass or quartz; 9 is a semiconductor thin film such as polycrystalline silicon; 10 is a source region formed by doping impurities such as phosphorus or arsenic into the semiconductor thin film; 11 is a drain region;
2 is a gate film, 13 is a gate electrode, 14 is an interlayer insulating film, 15 is a source electrode, and 16 is a drain electrode. Typical characteristics of a thin film transistor having this structure are shown in FIGS. 3 and 4.

Figure 3 shows channel length L = 20 μm and channel width W.
1 is a graph showing the characteristics of a thin film transistor having a size of =10 μm. Note that this data is the result obtained through experiments conducted by the applicant. The horizontal axis of this graph is the gate voltage V _GS with respect to the source, and the vertical axis is the drain current _ID . The parameter is the drain voltage V _DS to the source, and the curve A corresponds to V _DS =1V, the curve B corresponds to V _DS =4V, and the curve C corresponds to V _DS =8V. As can be seen from this, the drain current _ID takes a minimum value near V _GS =0V, and increases as the absolute _value of V _GS increases. The increase in drain current in the positive V _GS region means that the transistor
This means changing from an OFF state to an ON state, and it is desirable that the rate of increase in current is as large as possible. On the other hand, an increase in drain current in a negative V _GS region means that the OFF current has gate voltage dependence, which is not desirable as a transistor characteristic. Furthermore, the drain current varies greatly depending on the drain voltage V _DS . The drain current, especially in the region where V _GS is negative,
That is, the OFF current has a greater drain voltage dependence than the ON current.

FIG. 4 is a graph showing the dependence of the characteristics of a thin film transistor with a channel width W=10 μm on the channel length L. Note that this data is also the result of experiments conducted by the applicant. The drain voltage is constant at V _DS =4V, and the parameter is the channel length L. D
The curve of E is L = 10 μm, the curve of E is L = 20 μm,
The F curve is L = 40μm, and the G curve is L = 100μm.
correspond to each. As you can see from this,
In the region where V _GS is positive, the drain current I _D is inversely proportional to the channel length L, which is consistent with the theory of ordinary metal-insulating-film-semiconductor field-effect transistors (MOS FETs). However, in a region where V _GS is negative, as the absolute value of V _GS increases, the dependence on the channel length L becomes smaller, and finally the dependence on L disappears altogether. That is, when V _GS is about -8V or less, the OFF current is constant for any L.

From the data shown in Figures 3 and 4, OFF
The current is thought to be generated by the following mechanism.
In other words, the OFF current at V _GS = 0V is determined by the specific resistance of the semiconductor thin film, but the OFF current when V _GS is negatively biased is determined by the P-type layer induced on the surface of the semiconductor thin film, the source region and It is defined by the current flowing through the PN junction formed between the drain region and the N-type layer. Generally, since there are many traps in a semiconductor thin film, this PN junction is incomplete, and therefore junction leakage current easily flows. The more negative the gate voltage is biased
The OFF current increases because the carrier concentration of the P-type layer formed on the surface of the semiconductor thin film increases.
Because the width of the energy barrier of the PN junction becomes narrower,
This is because electric field concentration occurs and junction leakage current increases. Furthermore, the drain voltage dependence of the OFF current is also due to the same reason. The channel length dependence of the OFF current can also be explained by the junction leakage current. That is, as V _GS is biased more negatively, the OFF current is dominated by the junction leakage current near the drain, and the current flowing due to the resistivity of the semiconductor thin film becomes negligible.

Although the mechanism of the OFF current is as described above, few effective measures have been taken to actually reduce the OFF current. In particular, in order to reduce the OFF current when the gate voltage is negatively biased, it is necessary to reduce the junction leakage current, so little effort has been made to this end.

The present invention reduces such dependence of OFF current on gate voltage, and even when V _GS increases to a negative value,
The present invention provides an epoch-making thin film transistor having a characteristic that OFF current hardly increases.
In order to achieve this, in the present invention, in a thin film transistor using a semiconductor thin film and having a source electrode, a drain electrode, and a gate electrode, a plurality of the thin film transistors are connected in series, and the electrodes at both ends are connected to the source electrode and the drain electrode. In addition, the present invention provides a thin film transistor characterized in that the plurality of thin film transistors all have a common gate electrode. Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 5 shows a circuit diagram of the present invention.
S indicates a source, D indicates a drain, and G indicates a gate. Further, N represents the number of thin film transistors connected in series. As can be seen from the figure, N thin film transistors are connected in series, with one electrode at both ends serving as a source and the other as a drain. Further, the gates of all the N thin film transistors are made common to one gate. The gist of the present invention is to treat a plurality of thin film transistors configured in this manner as a single thin film transistor. The thin film transistor configured in this manner has extremely excellent OFF characteristics. The reason for this will be explained with reference to FIG.

FIG. 6a is a circuit diagram when N=2 in FIG. 5. For the sake of simplicity, the present invention will be described using the case where N=2 as an example. In the figure, the meanings of S, D, and G are the same as in FIG. 5. S, D,
The potentials at G and X are respectively V _S , V _D , V _G , V _X
shall be. Further, the numbers in the figure are numbers assigned to two thin film transistors, and the channel lengths of the respective transistors are assumed to be L ₁ and L ₂ . Also,
In FIG. 6b, the transistor in a is equivalently replaced with one transistor, and its channel length is L ₁ +L ₂ . The drain voltage V _DS1 and gate voltage V GS1 of transistor ₁ and the drain voltage V _DS2 and gate voltage V _GS2 of transistor 2 are given by the following equations.

V _DS1 = V _X −V _S V _GS1 = V _G −V _S V _DS2 = _{V D −V X V GS2} = V _{G −V} The potential at point X as
V _X is determined. At this time, V _S < V _X < V _D holds, and
Therefore, the drain voltage V _D -V _S is divided and applied to the two transistors. Therefore, the drain current should decrease, but if a certain relationship holds between the drain current and the channel length, the drain current will increase by the amount of the shorter channel length compared to FIG. 6b, In the end, the current values of the transistor shown in FIG. 6a and the transistor shown in FIG. 6b become equal. In fact, when V _G −V _S >0, this relationship holds true and the ON current does not change. In other words, the current value does not change no matter how the channel length is divided.

However, the situation is different when V _G −V _S <0.
This is because, as shown in FIG. 4, when the gate voltage is negatively biased, the dependence of the drain current on the channel length disappears. In other words, as the gate voltage increases in the negative direction,
Since the OFF current becomes independent of the channel length, the effect of the difference in channel length between Figure 6 a and b becomes smaller. Therefore, the OFF current in a decreases by the amount that the drain voltage applied to each transistor decreases. This effect is
The more negative the gate voltage is biased, the more pronounced this becomes.

The above phenomenon can also be explained in terms of physical properties as follows. When a transistor is on, a channel is formed on the surface of the semiconductor thin film, and a nearly uniform potential gradient (electric field) is generated from the source to the drain. However, the drain current remains unchanged. On the other hand, when the transistor is off, as mentioned above,
Most of the electric field is concentrated in the PN junction near the drain, so dividing the transistor can weaken the concentration of electric field applied to each individual PN junction, reducing junction leakage current, or OFF current.

Next, experimental data will be shown to demonstrate the effects of the present invention.

FIG. 7 is a graph showing the characteristics of the thin film transistor according to the present invention. In Figure 6a, L ₁ =
These are transistor characteristics when L ₂ =10 μm and W ₁ =W ₂ =10 μm. This transistor is equivalently the transistor shown in FIG. Note that this data is also the result obtained through experiments conducted by the applicant. The parameter is the drain voltage, the H curve is V _DS = 1V, the I curve is
The curve J corresponds to V _DS = 4V, and the curve J corresponds to V _DS = 8V. As can be seen from this graph, the region where V _GS is positive, that is, the ON current, almost matches the data in Figure 3, but the region where V _GS is negative, that is, the ON current almost matches the data in Figure 3.
The OFF current is significantly different from Fig. 3, and is a low and almost constant value. In other words, while maintaining the same ON current as conventional thin film transistors,
The current is significantly reduced. In addition, the applicant performed computer simulation based on conventional transistor characteristics and calculated the OFF characteristics of the thin film transistor according to the present invention, and the results agreed very well with the graph in Figure 7. did.

In the above explanation, for the sake of simplicity, the case where N=2, that is, the case where two thin film transistors are connected in series, has been described, but the same explanation can be given to the case where three or more thin film transistors are connected. When the number of thin film transistors connected in series is increased, the improvement in the OFF current when the drain voltage is high becomes noticeable. This is because the drain voltage applied to each transistor decreases as the number of transistors increases. Therefore, the number N may be selected depending on the purpose of the thin film transistor and the level of required OFF current. When applied to active matrix panels, the drain voltage is usually low (about 10 V or less), so N
=2 to 3 is sufficient. When configuring a logic circuit with thin film transistors, there is usually sufficient ON
Applying a high gate voltage to obtain current,
Since the drain voltage is also almost as high as that, increasing the value of N is more effective in reducing the OFF current.

As described above, by adopting the above-mentioned configuration, the present invention suppresses a decrease in on-current, and
Significantly reduces off-state current.

[Brief explanation of the drawing]

FIG. 1 is a general circuit diagram when thin film transistors are applied to an active matrix panel. FIG. 2 is a sectional view showing the general structure of an N-channel thin film transistor using a semiconductor thin film. FIGS. 3 and 4 are graphs showing the characteristics of conventional thin film transistors. FIG. 5 is a circuit diagram showing the general configuration of the present invention. FIG. 6 shows, as an example of the present invention, a circuit diagram in which two thin film transistors are connected in series and a single thin film transistor equivalent thereto. Figure 7 shows
7 is a graph showing the characteristics of the thin film transistor according to the present invention shown in FIG. 6.

Claims (1)

[Claims] 1 It has a plurality of pixel electrodes arranged in a matrix on a substrate, thin film transistors are connected to the pixel electrodes, N (N≧2) thin film transistors are formed in one pixel, and the N The thin film transistors connected in series are connected in series, one end of the series connected thin film transistors is electrically connected to a video signal line, the other end is electrically connected to a pixel electrode, and the gate electrodes of the series connected thin film transistors are common. A thin film transistor characterized by being