1. Field of the invention:
This invention relates to a lateral thyristor, and more particularly, to a lateral thyristor suitable for use as a driver for flat display panels.
2. Description of the prior art:
In recent years, development of flat display panels for displaying images has been extensively carried out.
A flat display panel has a plurality of X electrodes (acting as scanning lines) disposed parallel to each other and a plurality of Y electrodes (acting as signal lines) disposed perpendicular to the X electrodes. At each of the intersections between the X and Y electrodes on the panel, there is disposed a pixel which is connected to the X and Y electrodes intersecting each other. The pixels are arranged in a matrix form on the panel.
The selection of a specific pixel is accomplished by applying a predetermined voltage between the X and Y electrodes connected to that pixel by means of a driver or the like.
In a self-luminescent flat display panel such as a plasma display panel, current flows between the selected electrodes, thereby emitting light. The current is supplied through the driver.
Because of the advantage of relatively high current capacity, lateral thyristors with the cathode as the input terminal and the anode as the output terminal have come to be used as drivers to drive self-luminescent flat display panels such as plasma display panels.
FIG. 18 is a plan view showing a conventional lateral thyristor. FIG. 19 is a cross section taken along line D--D of the lateral thyristor shown in FIG. 18.
The conventional lateral thyristor has a P-type semiconductor substrate 7, an N.sup.- -type epitaxial layer 4 grown on the semiconductor substrate 7, and a P.sup.+ -type anode diffusion layer 1 and a P.sup.+ -type gate diffusion layer 2 both formed within the epitaxial layer 4, and an N.sup.+ -type cathode diffusion layer 3 formed within the gate diffusion layer 2.
An N.sup.+ -type buried layer 5 is formed below the anode diffusion layer 1 and extending between the semiconductor substrate 7 and the epitaxial layer 4.
In the epitaxial layer 4, a P.sup.+ -type isolating diffusion layer 6 is formed so as to surround the region where the anode diffusion layer 1, the gate diffusion layer 2, and the cathode diffusion layer 3 are formed.
The conventional lateral thyristor has the following problem when used as a driver for a self-luminescent flat display panel having discharge cells.
There is a possibility, when the flat display panel is being driven, that the "on" voltage applied between the anode diffusion layer 1 and the cathode diffusion layer 3 of the lateral thyristor (i.e., the voltage between the anode diffusion layer 1 and the cathode diffusion layer 3 in the conducting state) may rise to a level to cut off the thyristor, causing unstable operation of the lateral thyristor. Such an unstable increase or decrease in the "on" voltage for the lateral thyristor will cause flicker in the images being produced on the flat display panel.
The cause of the above problem will hereinafter be discussed with reference to FIG. 20 showing the transient characteristics of the voltage applied between the anode diffusion layer 1 and the cathode diffusion layer 3 of the conventional lateral thyristor.
First, let us suppose that the lateral thyristor is in the cut-off state. In this state, no current is flowing between the electrode connected to the lateral thyristor (i.e., the X electrode on the flat display panel) and the opposing electrode (i.e., the Y electrode on the flat display panel).
The voltage characteristics in this state are represented by the region "a" in FIG. 20. The potentials at the cathode diffusion layer 3, the isolating diffusion layer 6, the semiconductor substrate 7, and the gate diffusion layer 2 are set at a minimum level, for example, at zero volts, while the potential at the anode diffusion layer 1 is set at a predetermined high level, for example, at 100 volts.
When a voltage is applied to the gate diffusion layer 2 at least 0.7 volt higher than the potential at the cathode diffusion layer 3, the lateral thyristor switches from the cut-off state to the conducting state, causing current to flow from the anode diffusion layer 1 to the cathode diffusion layer 3. This causes the "on" voltage (V.sub.OL) between the anode diffusion layer 1 and the cathode diffusion layer 3 to drop to 2-3 volts (represented by the region "b" in FIG. 20).
The plasma display panel has discharge cells which act as pixels. When the lateral thyristor which is the driver for the plasma display panel becomes conducting, a sufficiently high voltage is applied between the X electrode in the discharge cell (acting as a pixel) connected to the lateral thyristor and the Y electrode disposed within that discharge cell and corresponding to the X electrode. This causes discharge to occur in the discharge cell, causing the plasma therein to emit light for display.
When a predetermined time has elapsed after initiation of the discharge, the discharge current flowing between the electrodes in the discharge cell (i.e., the X and Y electrodes) temporarily reaches the magnitude two to five times greater than that of the current flowing in the steady-state condition. Specifically, a current of 1.times.10.sup.4 -3.times.10.sup.4 amperes/cm.sup.2 or greater may flow into one anode diffusion layer 1. When such an excessive current flows into the anode diffusion layer 1, the "on" voltage (V.sub.OL) temporarily rises by a large extent. The increased value (.DELTA. V.sub.OL) of the "on" voltage momentarily reaches to 30-40 volts (represented by the region "c" in FIG. 20).
Such a high voltage causes the lateral thyristor to be put in the near cut-off state, resulting in an unstable operation of the lateral thyristor.
When the discharge is stabilized to the steady-state condition, since the current flowing into the anode diffusion layer 1 decreases, V.sub.OL drops to a relatively low level (represented by the region "d" in FIG. 20).
After that, when the potential at the gate diffusion layer 2 is lowered to approximately the same level as the potential at the cathode diffusion layer 3, the lateral thyristor switches from the conducting state to the cut-off state. The speed of this change (i.e., switching speed) is dependent on the speed at which the carriers stored in the epitaxial layer 4 vanish. In the above conventional lateral thyristor, since the carriers are absorbed rapidly into the buried layer 5, the switching speed is sufficiently high.
After the thyristor has been switched from the conducting state to the cut-off state, the "on" voltage (V.sub.OL) gradually increases to return to the initial value (i.e., 100 volts) (represented by the region "e" in FIG. 20).
Thus, in the conventional lateral thyristor, when an excessive current flows into the anode diffusion layer 1 with the thyristor being in the conducting state, the "on" voltage (V.sub.OL) increases by tens of volts from the steady-state value. Therefore, in the flat display panel using the conventional lateral thyristor as the driver, the voltage between the electrodes (i.e., the X and Y electrodes) drops by the value corresponding to the increase (.DELTA. V.sub.OL) of the "on" voltage. This makes it impossible to obtain an enough voltage to light the pixel, causing flicker in the images being produced.
Such a problem can be encountered not only in cases where the conventional lateral thyristor is used as the driver of a self-luminescent flat display panel having discharge cells, but also in cases where the lateral thyristor is used as the driver of any other self-luminescent flat display panel. Furthermore, the same problem may be encountered in all cases where the lateral thyristor is used under the condition that a large current flows into the anode diffusion layer 1.