1.  Field of the Invention
The present invention relates to a semiconductor controlled rectifier which is turned on by a gating signal applied to the gate electrode. 
2.  Description of the Prior Art
A semiconductor controlled rectifier, which is turned on in response to a gating signal applied to the gate electrode, comprises a semiconductor substrate having at least four layers of P and N conductivity types, disposed alternately; a pair of main electrodes kept in ohmic contact with the outer surfaces of two outermost layers; and a gate electrode connected with one of the four layers of the substrate. 
With such a semiconductor controlled rectifier as described above, if a gating signal voltage in the form of a pulse is applied between the gate electrode and one main electrode, with a forward voltage applied to make the other main electrode positive with respect to the one main electrode, then the semiconductor controlled rectifier switches from its OFF state to its ON state.  Namely, upon the application of the pulse gating signal, current begins to flow between the two main electrodes, due to the gate current.  The transition of the semiconductor controlled rectifier (hereafter referred to also as SCR) from its cut-off state to its conductive state is called "the turn-on" of the SCR.  The turn-on of the SCR also takes place, independently of the application of the gating signal voltage, in case where the voltage applied between the main electrodes is higher than the maximum blocking voltage of the device, where the rate at which the voltage rises is great enough while the voltage itself is lower than the maximum blocking voltage, or where the rise in temperature of the device is high.  If an SCR is turned on before the application of the gating signal while the voltage applied between the main electrodes is below the maximum blocking voltage of the device, the application of the device to an inverter, a chopper or other electric circuits is impossible.  Therefore, it is essential for the SCR that the device is seldom turned on by itself even if the rate at which the voltage applied between the main electrodes rises (hereafter referred to for brevity as the "dv/dt") is high, that is, to improve the dv/dt capability, and that the device is seldom turned on by itself even if the temperature thereof is high.  How the SCR is turned on before the application of the gating signal if the dv/dt or the temperature is high, will be explained as follows. 
As the forward voltage applied to an SCR is increased, the width of the depletion layer formed on both the sides of the center PN junction which is to be reverse-biased increases.  Consequently, a displacement current flows, which increases in proportion to the dv/dt of the forward voltage.  On the other hand, the reverse current, approximately proportional to the forward voltage, flows across the center junction.  Due to the combined effect of the displacement current and the reverse current, the PN junctions between the intermediate layers and their adjacent outer layers (hereafter referred to as emitter junctions) are forward-biased to induce the injection of carriers from the outer layers to the intermediate ones.  The degree to which the emitter junctions are forward-biased is great near the periphery of each emitter junction where the displacement current and the reverse current, which are generated in the center junction that does not overlap with the emitter junction when the layers are projected in a direction perpendicular to the layers, concentrate.  Consequently, it happens that the turn-on takes place erroneously in or near the pheriphery of the emitter junction when the dv/dt is high.  On the other hand, if the temperature of the SCR is high, the carriers generated by thermal excitation in the depletion layer of the center junction increase so that the reverse current due to the carriers increases across the center junction.  Thus, an erroneous turn-on is incurred when the temperature of the device is high, just as in case where the dv/dt is high.  In order to improve the dv/dt and temperature capabilities, therefore, it is necessary to prevent the forward biasing of the emitter junctions by the displacement current and the reverse current.  One artifice to attain the object is a shorted emitter configuration in which a portion of an intermediate layer is connected by penetrating the adjacent outermost layer with the associated main electrode.  This configuration indeed improves the dv/dt and temperature capabilities of the SCR to a considerable extent, but it still cannot be free from the following difficulties.  Namely, due to the provision of the gate electrode on the outermost layer, there cannot be only small parts of emitter regions in the neighbourhood of the gate electrode.  Accordingly, in order to increase the dv/dt capability, the number of the shorted emitter portions in the area of the outermost layer faced to the gate electrode must be large. 
On the other hand, the most important characteristics of the SCR are the initial turn-on area and the spreading of the conducting region in the initial stage of turn-on process.  The area and the spreading velocity must be designed to be respectively as large and fast as possible, in order to increase the switching power capability.  In order to fulfill this requirement, the number of the shorted emitter portions in the area of the outermost layer faced to the gate electrode must be decreased or preferably be reduced to zero. 
According to the conventional method and techniques, as described above, a semiconductor controlled rectifier which has a large initial turn-on area and a high dv/dt and temperature capabilities, cannot be obtained.