POWER SEMICONDUCTOR DEVICE

Abstract: In a power semiconductor device according to the present invention, a sheet (11), formed of a soft metal such as Ag, is provided on that portion of a pressing control electrode (13a) which is brought into contact with an Aℓ gate electrode (1a) of a pellet (1). By means of this sheet (11), it is possible both to apply a strong pressing power to the Aℓ gate electrode (1a) and to reduce the contact resistance between the two electrodes (1a and 13a). Since an excessive amount of heat is not produced on account of the contact resistance, the semiconductor device can be protected against being damaged.

Description: The present invention relates to a power semiconductor device in which the control electrode section has a solderless contact construction and, more particularly, to an improvement of the solderless contact construction. In recent years, remarkable progress has been made in increasing the power capacity of a power semiconductor device. In the case of a gate turn-off thyristor (hereinafter referred to as a GTO thyristor), the muximum turn-off current I TGQ was 800A in the early 1980s. Thereafter, the muximum turn-off current I TGQ was increased every year, and by the middle of 1980s, a GTO thyristor, the muximum turn-off current of which was increased up to 2,700A, became commercially available. With this increase in muximum turn-off current I TGQ , the muximum value I RGP of the reverse gate current for turning off a GTO thyristor was also increased every year; it increased from 200A to as large as 700A. With an increase in the power capacity of power semiconductor devices such as a power transistor, an ordinary thyristor, a GTO thyristor etc., a solderless contact construction has come to be widely employed for connecting the element-controlling electrode (e.g., the gate electrode of the thyristor, and the base electrode of the transistor) and the external lead to each other. Solderless contact construction is usually adopted in the fabrication of a power semiconductor device with increased power capacity, such as a GTO thyristor the turn-off current of which exceeds 1000A. EP-A-0 158 749 shows a power semiconductor device having pressure loaded contacts, and US-A-3 838 000 shows a semiconductor device with a silver layer of specified roughness in the contact assembly, which enables contact movement, to reduce damaging horizontal mechanical stresses. EP-A-0 158 749 also shows using a ceramic ring as the insulator, and indicates that to avoid cracking this element, the pressure value and contact area should be carefully chosen. Fig. 1 is a sectional view illustrating the solderless contact construction of the gate electrode section of a conventional GTO thyristor. In Fig. 1, reference numeral 1 represents a semiconductor pellet on which a GTO element is formed. Gate electrode 1a formed of Al (aluminium) is located in the center of pellet 1, and cathode element 1b surrounds gate electrode 1a. Reference numeral 2 represents a pressing cathode formed of Cu (copper). Pressing cathode 2 urges cathode element 1b, with buffer plate 2a and foil 2b, both formed of Mo (molybdenum), interposed therebetween. Reference numeral 3 represents a pressing gate which is insulated from pressing cathode 2 by Teflon ring 3A and ceramic disk 3B. Generally, pressing gate 3 is formed of a base material of Cu plated with Ni (nickel) and Ag (silver). The Ag layer generally has a thickness of several m. As is shown in Fig. 1, pressing gate 3 is urged by coil spring 4 placed on spring seat 2c of pressing cathode 2, to urge gate electrode 1a of the GTO thyristor via pressing gate 3. In a GTO thyristor which uses the solderless contact structure shown in Fig. 1 and the maximum turn-off current of which is 2,000A, the pressing gate with a pressing area of 52 mm² is urged, by the coil spring with a force equivalent to 1.5 kg, to urge the gate electrode, formed on the pellet. Accordingly, the pressure is about 0.03 kg/mm². It has recently been found, however, that the conventional solderless contact construction is inappropriate to a GTO thyristor with an increased power capacity. If the GTO thyristor, the maximum value I RGP of the reverse gate current of which exceeds 300A, is used continuously over a long period of time, the contact resistance between the gate and the cathode increases with time. Within about 3-5 years, the gate-cathode resistance increases considerably. In the end, the Aℓ layer, which constitutes gate electrode 1a on the pellet, is melted by the heat generated due to the increased resistance of the pressed sections. As a result, the breakdown voltage between the anode and the cathode of the GTO thyristor is adversely affected. This phenonenon can be observed with respect to not only the GTO thyristor but a power transistor with a pressed base. The object of the present invention is to provide a power semiconductor device with a large power capactiy, which includes a control electrode section with a solderless contact construction, and in which a semiconductor element can be reliably prevented from being damaged in the cause of the increase of the contact resistance at the solderless contact after long, continuous use of the power semiconductor device. According to the present invention there is provided a power semiconductor device with a solderless contact construction comprising: a first pressing electrode ; a second pressing electrode ; a control electrode provided for a semiconductor element: a member for pressing said first pressing electrode against said control electrode , said pressing member being urged towards said first pressing electrode and said control electrode by said second pressing electrode ; and an insulating element for insulating said first pressing electrode form said second pressing electrode ; characterized by further including an annealed foil member formed of a soft metal such as silver, gold or platinum and interposed between said first pressing electrode and said control electrode; and said insulating element is composed of mica. The inventors of the present invention have studied the causes of the increase in the contact resistance of the conventional power semiconductor device, and, as a result, has come up with the following findings: When Ni plating 3c and Ag plating 3d (both are shown in Fig. 2) are formed on the surface of pressing control electrode 3, the surface of the resultant plating is inevitably rough. This may be the first cause of the increase in contact resistance. Second, the Aℓ particles of the Aℓ gate electrode 1a are coarse. Third, the pressure applied between pressing control electrode 3 and Aℓ gate electrode 1a is comparatively small; it is about 0.02 kg/mm², with the result that the contact between control electrode 3 and Aℓ gate electrode 1 is insufficient at contact portion 52 (Fig. 2). For these reasons, the potential difference at contact portion 52 will be greater than 0.10V if reverse gate current I RGP exceeds 300A. If this occurs, Aℓ atoms will begin to flow from Aℓ gate electrode 1a on the pellet, toward control electrode 3 via electrolyte 51 which is attached, in the vicinity of contact portion 52, to both control electrode 3 and gate electrode 1a. As a result, the resistance between the gate and the cathode will increase considerably after the power semiconductor device has been used continuously for 3-5 years. The characteristic feature of the power semiconductor device of the present invention is that the pressing control electrode is urged to press the Aℓ electrode on the pellet, with a soft metal sheet interposed between the two electrodes and that the insulator is formed of mica. Desirably, the device of the present invention is designed in such a manner that the pressure is greater than 0.14 kg/mm². It is also desirable that the soft metal sheet be formed of a foil of a conductive metal, such as Ag, Au, or Pt (platinum), having a thickness ranging from about 30 to 150 um. In the device of the present invention, the pressing control electrode urges the Aℓ electrode on the pellet, with the soft metal sheet interposed therebetween. Thus, the pressing control electrode and the Aℓ electrode on the pellet can be brought into tight contact with each other. As a result, the contact resistance between the two electrodes will be 0.05 mΩ if the pressure is greater than 0.14 kg/mm². The contact resistance of the semiconductor device according to the present invention is therefore, less than one tenth of the corresponding contact resistance of the conventional device. Even if maximum value I RGP of the reverse gate current is 700A, the potential drop, which may occur at the contact portion of the gate, due to the flow of electric current I e , is not more than 0.028V. Since the flow of Aℓ through electrolyte 51 is not markedly accelerated until the potential drop at the contact portion becomes more than 0.1V, ion current I i (Figs. 2 and 3) does not flow as long as the potential drop is so small as in the present invention. Therefore, it can be expected that the life of the device of the present invention will be around 300 times as long as that of the conventional device. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a cross sectional view illustrating the solderless contact construction of the gate of a conventional GTO thyristor; Fig. 2 is an enlarged view illustrating part of the solderless contact construction; Fig. 3 shows an equivalent circuit of the solderless contact construction shown in Fig. 2; Fig. 4 is a cross-sectional view illustrating the solderless contact construction of the gate of a GTO thyristor according to a first embodiment of the present invention; Fig. 5 is a graph showing the relationship between the pressure per unit area and the contact resistance with respect to a GTO thyristor having the solderless contact construction shown in Fig. 4; Fig. 6 is a cross-sectional view illustrating the solderless contact construction of the gate of a GTO thyristor according to a second embodiment of the present invention; and Fig. 7 is a cross-sectional view illustrating the solderless contact construction of the gate of a GTO thyristor according to a third embodiment of the present invention. Fig. 4 is a cross-sectional view of the solderless contact construction of the gate of a GTO thyristor according to an embodiment of the present invention. In Fig. 4, the same reference numerals as those in Fig. 1 are used to represent the corresponding parts of the prior art. Reference numeral 1 designates a semiconductor pellet on which a GTO element is formed. Gate electrode 1a, which is formed of Aℓ - (aluminium), is located in the center of pellet 1, and cathode element 1b is arranged around gate electrode 1a. Reference numeral 2 designates a pressing cathode formed of Cu (copper). Pressing cathode 2 urges cathode element 1b, with buffer plate 2a and foil 2b, both formed of Mo (molybdenum), interposed therebetween. Reference numeral 13 designates a pressing gate formed of Cu located on the pressing side of the GTO thyristor. The central portion of pressing gate 13 is hollowed to form annular peripheral contact portion 13a. The hollow construction of gate 13 will be effective in ensuring a uniform pressure against pellet 1. Contact portion 13a has a contact area of about 50 mm² which is in contact with Aℓ gate electrode 1a on pellet 1. Although not illustrated in the drawings, pressing gate 13 is formed of a base material of Cu plated with Ni and Ag. Cap member 11, formed of an Ag sheet with a thickness of 100 m and subjected to an annealing process, is fitted around contact portion 13a in such a manner that contact portion 13a is completely covered. Pressing gate 13 urges Aℓ gate electrode 1a on pellet 1 by means of initially coned disk springs 12a and 12b which can produce a combined pressure ranging from 40 kg to 100 kg (0.8 kg/mm² to 2 kg/mm²). Washer 14a is located between spring 12b and pressing gate 13, and washers 14b and 14c, which are stacked on each other, are located between spring 12a and pressing cathode 2. Insulating sheet 15, formed of a mica which can withstand a high pressure, is interposed between washers 14b and 14c. Together with Teflon ring 3a, insulating sheet 15 provides insulation between pressing gate 13 and pressing cathode 2. Fig. 5 shows the characteristics of the GTO thyristor of the first embodiment (Fig. 4) of the present invention. In Fig. 5, the pressure which the pressing gate applies per unit area is plotted along the ordinate, and the contact resistance of the gate is plotted along the abscissa. For the sake of comparison, the characteristics of the case where an Ag cap is not used in the device of the first embodiment (i.e., the construction similar to that of the prior art) are also shown in Fig. 5, by way of the curve indicated as "WITH NO CAP"). As can be seen from Fig. 5, the presence or absence of the cap has virtually no influence as regards the contact resistance if the pressure is less than 0.06 kg/mm². However, if the pressure is increased to more than 0.06 kg/mm², the contact resistance remains substantially constant (about 0.08 mΩ) in the case where the cap is not used, while if continues to decrease further in the case where the cap is provided, as in the first embodiment (Fig. 4) of the present invention. In the latter case, the contact resistance does not become constant (0.05 mΩ) until the pressure is increased up to 0.12 kg/mm². As long as the contact resistance is 0.05 m , potential difference Vd can be represented by the equation below, even if I RGP is 700A. Vd = 700A x 0.05 mΩ = 0.035V. This potential difference is sufficiently lower than 0.1V, at which level Aℓ atoms begin to flow from Aℓ gate electrode 1a on pellet 1. The inventors of the present invention compared the GTO thyristor of the first embodiment (Fig. 4) of the present invention (pressure: 1.3 kg/mm²) with the conventional GTO thyristor (pressure: 0.02 kg/mm²) by continuously driving the two thyristors under the same operating condition. The operating condition was such that the muximum value I RGP of the reverse gate current, used for turning off the GTO thytistors, was determined to be greater than the rated value. In the case of the conventional GTO thyristor, the surface of the Aℓ electrode on the pellet became rough within 3-5 hours, due to the flow of Aℓ atoms. In the case of the GTO thyristor of the first embodiment (Fig. 4) of the present invention, in contrast, the surface of the Aℓ electrode remained unchanged even after continuously driving the thyristor for 1,000 hours. Therefore, the GTO thyristor according to the first embodiment of the present invention is expected to have a life at least 300 times longer than that of the conventional GTO thyristor. In the present invention, the shape of the control electrode is not limited to the one shown in Fig. 4. Fig. 6 is a cross-sectional view illustrating the solderless contact of the gate of a GTO thyristor according to a second embodiment of the present invention. As illustrated, pressing gate 33 is not hollowed and is pressed against the Aℓ electrode on the pellet. Pickup section 33a extends upward from the center of the upper side of the pressing gate, and is bent in the horizontal direction at the location where it enters the interior of pressing cathode 2. Insulating member 33b is formed on the surface of pickup section 33a. Cap 31, formed of Ag and having a thickness of about 150 µm, is fitted around pressing gate 33 such that the entire contact area is covered. Washers 32a 32b, 32c, and 32d are provided such that they are brought into contact with pressing gate 33, with a pressure of more than 0.14 kg/mm² per unit area. In Fig. 6, the pellet is not shown, for the sake of simplicity. Fig. 7 is a cross-sectional view of the solderless contact of the gate of a GTO thyristor according to a third embodiment of the present invention. In this embodiment, a pressing gate in the form of a small block is used. Cap 41, formed of Ag and having a thickness of about 50 µm, is fitted around the pressing surface of pressing gate 43. Two initially coned disk springs, 42a and 42b, are brought into contact with pressing gate 43, with a pressure of 0.14 kg/mm² per unit area. In Fig. 7 as well, the pellet is not shown, for the sake of simplicity. As in the first embodiment (Fig. 4), the GTO thyristors according to the second and third embodiments (Figs. 6 and 7) can be expected to have a longer life than that of conventional GTO thyristors. Not only Ag but other soft metals, such as Au and Pt, can be used for forming caps 11, 31, and 41, and it is preferable that the sheets of these caps be in the range of about 30 to 150 µm. In the power semiconductor device of the present invention, the control electrode can be brought into tight contact with the electrode of the semiconductor element, with soil metal foil interposed between the two electrodes. Since the contact resistance between the two electrodes is reduced, the contact potential difference can be decreased to less than 0.1V, even if a large amount of current flows through the control electrode. As a result, hardly any metal flows from the electrode of the element, and thus the contact resistance does not increase, even after the device has been driven for a long period of time. Therefore, the life of the power semiconudctor can be increased considerably.

Claims: A power semiconductor device with a solderless contact construction comprising: a first pressing electrode (13, 33, 43); a second pressing electrode (2); a control electrode (1a) provided for a semiconductor element: a member (14a, 14b; 32a-32d; 42a, 42b) for pressing said first pressing electrode (13, 33, 43) against said control electrode (1a), said pressing member (14a, 14b; 32a-32d; 42a,42b) being urged towards said first pressing electrode (13, 33, 43) and said control electrode (1a) by said second pressing electrode (2); and an insulating element (15) for insulating said first pressing electrode (13, 33, 43) from said second pressing electrode (2); characterized by further including an annealed foil member (11, 31, 41) formed of a soft metal such as silver, gold or platinum and interposed between said first pressing electrode (13, 33, 43) and said control electrode (1a), and said insulating element (15) is composed of mica. A power semiconductor device according to claim 1, characterized in that the first pressing electrode (13, 33, 43) is a pressing gate electrode (13, 33, 43) located on a pressing side of a gate-turnoff thyristor, and the control electrode (1a) provided for the semiconductor element is a gate electrode (1a) of the gate-turnoff thyristor. A power semiconductor device according to claim 1, characterized in that the first pressing electrode (13, 33, 43) is a pressing base electrode of a power transistor, and the control electrode (1a) provided for the semiconductor element is a base electrode of the power transistor. A power semiconductor device according to claim 1, characterized in that the foil member has a thickness ranging from about 30 to 150 um. A power semiconductor device according to claim 1, characterized in that the pressing member (14a, 14b; 32a-32d, 42a; 42b) presses said electrodes against each other, with pressure of more than 0.14kg/mm².

Inventors: FUJIWARA, TAKASHI C/O PATENT DIVISION, OKAMURA, HIROSHI C/O PATENT DIVISION, UETAKE, YOSHINARI C/O PATENT DIVISION, USUI, YASUNORI C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1987
Main classification: H01L23
IPC codes: H01L23, H01L21, H01L29, H05K3