A SHORTED ANODE TYPE GATE TURN-OFF THYRISTOR AND A PROCESS OF MANUFACTURING THYRISTORS OF THIS TYPE

Abstract: The present invention relates to a method of manufacturing a shorted anode type gate turn-off thyristor which has an n⁺layer that is a shorted anode portion, comprising the step of forming the n⁺ layer at an impurity concentration of at most l0²⁰ atoms/cm³, and the step of subsequently diffusing lifetime killer sources.

Description: This invention relates to a shorted anode type gate turn-off thyristor and to a process of manufacturing a shorted anode type gate turn-off thyristor having an increased controllable anode current. Fig. l is a sectional view showing a conventional, shorted anode type gate turn-off thyristor, in which numeral ll designates an n-type emitter region (hereinbelow, termed "n E layer"), numeral l2 a p-type base region (hereinbelow, termed "p B layer"), numeral l3 an n-type base region (hereinbelow, termed "n B layer"), numeral l4 a p-type emitter region (hereinbelow, termed "p E layer"), and numeral l5 an n-type region that is a shorted anode portion (hereinbelow, termed "n⁺ layer"). In the shorted anode type gate turn-off thyristor having such a structure, the n⁺ layer l5 of the shorted anode portion is effective to draw out excess carriers stored within a wafer at the time of turn-off. The turn-off process proceeds especially from the edge of the n E layer ll, and an ON region in which the excess carriers are stored is narrowed down to the central position of the n E layer ll. The n⁺ layer l5 is therefor disposed in that part of the anode side portion confronting the n E layer ll, to raise the effect of drawing out the excess carriers. Thus, the shorted anode type gate turn-off thyristor is improved with respect to its turn-off characteristics as compared with a reverse blocking type gate turn-off thyristor which does not have the shorted portion. Meanwhile, when such a shorted anode type gate turn-off thyristor is to have a higher breakdown voltage, the n B layer l3 is made thicker. In this case, it is insufficient for the turn-off characteristics to resort only to the carrier drawing-out effect that is the n⁺ layer l5 of the shorted portion. For this reason, Au, Pt etc., to function as lifetime killer sources are diffused in the p B layer l2 and the n B layer l3 so as to recombine the excess carriers. In the shorted anode type gate turn-off thyristor as described above (and as known from EP-A-0 160 525 falling under Article 54 (3) EPC), the n B layer l3 is thickened to increase the breakdown voltage, and the lifetime killer sources are diffused for the purpose of enhancing the turn-off characteristics. However, when the impurity concentration of the n⁺ layer l5 is high (for example, at least l0²¹ atoms/cm³) on this occasion, the lifetime killer sources cannot sufficiently be diffused on account of the gettering action of the n⁺ layer l5, at the step of diffusing them. Fig. 2 illustrates a spreading resistance R s along a line II-II in Fig. l in the n B layer l3 in the case where the impurity concentration of the n⁺ layer l5 is l0²¹ atoms/cm³ and where Au is diffused as the lifetime killer sources. Since Au functions as an acceptor in silicon, it cancels out a donor in the n B layer l3 and raises the spreading resistance R s of the n B layer l3. The relationship between the Au diffusion concentration and the spreading resistance R s is as shown in a graph of Fig. 3. More specifically, the spreading resistance R s is high in a region 17 in which the lifetime killer sources are sufficiently diffused, whereas it is low in a region 16 in which the lifetime killer sources are not diffused considerably because of the gettering action of the n⁺ layer 15. In the region 16, accordingly, the diffusion concentration of Au to function as the lifetime killer sources is lower than in the region 17, so that the lifetime is not shortened, and the recombination of the excess carriers is unsatisfactory. This effect has led to the problems that the turn-off characteristics are inferior and that the controllable anode current cannot be made large. This invention has been made in order to solve such problems, and has for its object to provide shorted anode type gate turn-off thyristors and a process of manufacturing shorted anode type gate turn-off thyristors according to which lifetime killer sources are sufficiently diffused even in a region adjoining the n⁺ layer that is a shorted portion, whereby turn-off characteristics are enhanced and also a controllable anode current can be made high. The inventive shorted anode type gate turn-off thyristor, has an n⁺ layer as a shorted anode portion with a high impurity concentration of at most 10²⁰ atoms/cm³ and lifetime killer sources diffused in a base region of the thyristor for enhancing the turn-off characteristics. Preferably diffused Au or Pt are provided as lifetime killer sources in a p-type base region and/or in an n-type base region of the thyristor. According to a further aspect of the invention the spreading resistance of the lifetime killer substances within the n-type base region of the thyristor is substantially uniform. The process according to the invention for manufacturing a shorted anode type gate turn-off thyristor which has an n-type region of high impuritiy concentration in a shorted anode portion, comprises the step of forming the n-type region of high impurity concentration at an impurity concentration of at most 10²⁰ atoms/cm³, and the step of subsequently diffusing lifetime killer sources. According to a further aspect of the invention, the process is characterized in that Au or Pt are diffused as lifetime killer sources. In this invention, the gettering action of an n⁺ layer during the diffusion of lifetime killer sources can be enfeebled, and the lifetime killer sources are homogeneously diffused within a wafer. The invention will be explained in detail below with reference to the accompanying drawings, wherein Fig. 1 is a sectional view showing a conventional, shorted anode type gate turn-off thyristor; Fig. 2 is a graph showing a spreading resistance in the n B layer (n-type base region) of a conventional shorted anode type gate turn-off thyristor along a line II-II in Fig. 1; Fig. 3 is a graph showing the relationship between the spreading resistance and an Au diffusion concentration in the n B layer in such a conventional thyristor; Fig. 4 is a sectional view showing a shorted anode type gate turn-off thyristor according to an embodiment of this invention; and Fig. 5 is a graph showing the spreading resistance in the n B layer of the shorted anode type gate turn-off thyristor according to the invention along a line I-I in Fig. 4. The same symbols in the drawings indicate identical or corresponding portions. Fig. 4 is a sectional view showing a shorted anode type gate turn-off thyristor produced by an embodiment of a manufacturing process according to this invention. Numeral l designates an n-type emitter region (hereinbelow, termed "n E layer"), numeral 2 a p-type base region (hereinbelow, termed "p B layer"), numeral 3 an n-type base region (hereinbelow, termed "n B layer"), numeral 4 a p-type emitter region (hereinbelow, termed "p E layer), and numeral 5 an n⁺ layer that is a shorted anode portion (hereinbelow, termed "n⁺ layer"). The manufacturing process of the thyristor according to the present invention is similar to that of the prior art, in that lifetime killer sources are diffused after the n⁺ layer 5 has been formed. However, since the impurity concentration of the n⁺ layer 5 is made at most l0²⁰ atoms/cm³ at the step of forming the n⁺ layer 5, the gettering action of this n⁺ layer 5 can be considerably weakened. Therefore, the lifetime killer sources can be homogeneously diffused in the n B layer 3, and the spreading resistance R s along a line I - I in Fig. 4 within the n B layer 3 becomes substantially uniform as illustrated in Fig. 5. In the shorted anode type gate turn-off thyristor thus formed, the lifetime killer sources homogeneously diffused can recombine with excess carriers, so that the lifetime of the carriers can be sufficiently shortened at the time of turn-off, along with the attainment of a very efficient carrier drawing-out effect of the n⁺ layer 5. A turn-off test of this shorted anode type gate turn-off thyristor was conducted. As a result, it has been verified that the thyristor has its controllable anode current raised by about 200 A - 300 A as compared with a conventional example. As described above, this invention comprises the step of forming an n+ layer that is a shorted anode portion whose impurity concentration is at most l0²⁰ atoms/cm³, before the step of diffusing lifetime killer sources. Therefore, when the lifetime killer sources are diffused, they can be homogeneously diffused in a wafer without being gettered by the n⁺ layer, to bring forth the effect that the controllable anode current can be increased.

Claims: A shorted anode type gate turn-off thyristor, having an n⁺ layer (5) as a shorted anode portion with a high impurity concentration of at most l0²⁰ atoms/cm³, and lifetime killer sources diffused in a base region (2, 3) of the thyristor for enhancing the turn-off characteristics. The thyristor according to claim l, characterized in that diffused Au or Pt are provided as lifetime killer sources in a p-type base region (2) and/or an n-type base region (3) of the thyristor. The thyristor according to claim l or 2, characterized in that the spreading resistance (Rs) of the lifetime killer sources within the n-type base region (3) is substantially uniform. A process for manufacturing a shorted anode type gate turn-off thyristor which has an n-type region of high impurity concentration in a shorted anode portion, comprising the step of forming said n-type region of high impurity concentration at an impurity concentration of at most l0²⁰ atoms/cm³, and the step of subsequently diffusing lifetime killer sources. The process according to claim 4, characterized in that Au or Pt are diffused as lifetime killer sources.

Inventors: NAKAGAWA, TSUTOMU C/O KITAITAMI SEISAKUSHO, SUZUKI, MASANORI C/O KITAITAMI SEISAKUSHO
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1986
Main classification: H01L29
IPC codes: H01L29