Thyristors have been used as indispensable devices for large capacity power switching owing to the low ON-state voltage characteristic. Gate Turn-Off (GTO) thyristors, for example, are widely used in these days in high-voltage large-current range applications. The GTO thyristor, however, has revealed drawbacks as follows: first, the GTO thyristor requires large gate current for turn-off, in other words, the thyristor has a relatively small turn-off gain; and secondly, a large-sized snubber is needed to safely turn off the GTO thyristor. In addition, since the GTO thyristor does not show current saturation in its current-voltage characteristic, a passive component, such as a fuse, must be coupled to the thyristor so as to protect a load from short-circuiting. This greatly impedes the reduction in the size and cost of the whole system.
MOS controlled thyristor (hereinafter abbreviated as MCT) as a voltage-driven type thyristor was proposed by V. A. K. Temple in IEEE IEDM Tech. Dig., 1984, p 282. Since then, the characteristics of this type of thyristor have been analyzed and improved in various institutions worldwide. This is because the MCT, which is a voltage-driven type device, requires a far simpler gate circuit than the GTO thyristor, while assuring a relatively low ON-state voltage characteristic. The MCT, however, does not show a current saturation characteristic as in the case of the GTO thyristor, and therefore requires a passive component, such as a fuse, in its practical use.
Dr. Pattanayak and others revealed that an emitter switched thyristor (hereinafter abbreviated as EST) shows a current saturation characteristic, as disclosed in U.S. Pat. No. 4,847,671 (Jul. 11, 1989). Subsequently, M. S. Shekar and others proved by actual measurements that a dual channel type emitter switched thyristor (EST-1) shows a current saturation characteristic even in a high voltage range, as disclosed in IEEE Electron Device Letters vol. 12 (1991), p 387. In Proceedings of IEEE ISPSD '93, p 71 and Proceedings of IEEE ISPSD '94, p 195, the inventors of the present invention disclosed results of their analysis on a forward bias safe operation area (FBSOA) and a reverse bias safe operation area (RBSOA) of this EST, and paved the way to the development of a voltage-driven type thyristor having the safe operation area in which the device operates safely when a load is short-circuited. FIG. 25 shows the structure of this EST device.
In the device as shown in FIG. 25, a first p base region 4, p.sup.+ well region 5 and a second p base region 6 are formed in a surface layer of an n base layer 3 deposited on a p emitter layer 1 via an n.sup.+ buffer layer 2. The p.sup.+ well region 5 forms a part of the first p base layer 4, and has a relatively large diffusion depth. An n source region 7 is formed in a surface layer of the first p base region 4, and an n emitter layer 8 is formed in a surface layer of the second p base region 6. A gate electrode 10 is formed on a gate oxide film 9 over a portion of the first p base region 4 that is interposed between the n source region 7 and an exposed portion of the n base layer 3, and a portion of the second p base region 6 that is interposed between the n emitter region 8 and the exposed portion of the n base layer 3. Each of the n source region 7, n emitter region 8 and gate electrode 10 has a limited length in the Z-direction in FIG. 25, and the first p base region 4 and the second p base region 6 are coupled to each other outside these regions 7, 8 and electrode 10. Further, L-shaped p.sup.+ well region 5 is formed outside the coupled portion of the first and second p base regions 4, 6. A cathode electrode 11 is formed in contact with both a surface of the p.sup.+ well region 5, and a surface of the n source region 7. On the other hand, an anode electrode 12 is formed over the entire area of the rear surface of the p emitter layer 1.
When the cathode electrode 11 of this device is grounded, and positive voltage is applied to the gate electrode 10 while positive voltage is applied to the anode electrode 12, an inversion layer (partial accumulation layer) is formed under the gate oxide film 9, and a lateral MOSFET is thus turned on. As a result, electrons are supplied from the cathode electrode 11 to the n base layer 3, through the n source region 7, and the inversion layer (channel) formed in the surface layer of the first p base region 4. These electrons function as base current for a pnp transistor which consists of the p emitter layer 1, n.sup.+ buffer layer 2 and n base layer 3, and the first and second p base regions 4, 6 and p.sup.+ well region 5. This pnp transistor operates with this base current. Then, holes are injected from the p emitter layer 1, and flow into the first p base region 4 through the n.sup.+ buffer layer 2 and n base layer 3. A part of the holes flow into the second p base region 6, and then flow under the n emitter region 8 in the Z direction to the cathode electrode 11. Thus, the device operates in an IGBT (insulated gate bipolar transistor) mode. With a further increase in the current, the pn junction between the n emitter region 8 and the second p base region 6 is forward biased, and a thyristor portion consisting of the p emitter layer 1, n.sup.+ buffer layer 2, n base layer 3, second p base region 6 and n emitter region 8 latches up. In this case, the device operates in a thyristor mode. To turn off the EST, the MOSFET is turned off by lowering the potential of the gate electrode 10 below the threshold of the lateral MOSFET. As a result, the n emitter region 8 is potentially separated from the cathode electrode 11, and the device stops operating in the thyristor mode.
FIGS. 26 and 27 are cross sectional views of improved ESTs as disclosed in U.S. Pat. No. 5,317,171 issued May 31, 1994 and No. 5,319,222 issued Jun. 7, 1994 to M. S. Shekar et al. The improved EST of FIG. 27, in particular, is different from the EST shown in FIG. 25, and is designed so as to provide a further lowered ON-state voltage characteristic.
FIG. 28 is a cross sectional view of a FET controlled thyristor as disclosed in U.S. Pat. No. 4,502,070 issued Feb. 26, 1985 to L. Leipold et al. This thyristor is characterized in that the electrode is not in contact with the second p base region 6.
As is understood from the above description, the EST as shown in FIG. 25 utilizes the holes flowing in the second p base region 6 in the Z direction so as to forward bias the pn junction between the second p base region 6 and the n emitter region 8, and therefore a degree or strength of the forward bias decreases in the Z direction toward a contact area of the second p base region 6 with the cathode electrode 11. Namely, the amount of electrons injected from the n emitter region 8 is not uniform over the length of the pn junction in the Z direction. If this EST is switched from this ON state to the OFF state, a weakly biased portion of the pn junction near the contact area with the cathode electrode 11 initially resumes its reverse-blocking ability, and a deeply biased portion of the pn junction remote from the contact area with the cathode electrode 11 slowly resumes the same ability. This tends to cause current localization or concentration upon turn-off, resulting in lowered capability to withstand breakdown of the EST during turn-off.
Although the EST shown in FIG. 26 operates in a similar manner to the EST of FIG. 25, the EST of FIG. 26 can be turned off more quickly since the cathode electrode 11 extends in the Y direction, to be in direct contact with the surface of the second p base region 6. Further, the EST of FIG. 26 shows a uniform turn-off characteristic due to the absence of the hole current flowing in the Z direction. In the operation of this thyristor, however, minority carriers are not uniformly injected in the horizontal direction (Y direction) when the pn junction between the n emitter region 8 and the second p base region 6 is turned on, and therefore the ON-state voltage cannot be lowered as expected. For example, if the impurity concentration of the second p base region 6 is reduced to increase its resistance so as to solve this problem, a depletion layer is punched through the n emitter region 8 during withstanding of the voltage applied in the forward direction. Thus, the conventional EST cannot achieve a sufficiently high withstand voltage.
In the device shown in FIG. 27, the n emitter region 8 extends beyond the second p base region 6 so as to further lower the ON-state voltage. This structure, however, is unable to withstand the voltage applied in the forward direction.
In the device shown in FIG. 28, the n emitter region 8 and second base region 6 are completely separated from the cathode electrode 11, thus preventing the non-uniform operation of the thyristor. This structure, however, has drawbacks as follows: first, the device shows lowered capability to withstand breakdown since the hole current flows through the device such that it concentrates on the side of the first p base region; and secondly, the conductance in the operation of the transistor in the IGBT mode is reduced due to the contact-type FET effect.
In addition, both the EST and FET controlled thyristor suffer from such problems that the maximum current (limit current) that can flow through the device is large, and the device exhibits low capability to withstand breakdown upon short-circuiting of a load.