SEMICONDUCTOR RECTIFIER DIODE

Abstract: Herein disclosed is a semiconductor rectifier diode of low loss and high speed type having a p⁺-n-n⁺ structure, in which the p⁺-type layer (3) is constructed of a first layer portion (31) and a second layer portion (32) surrounded by the first layer portion (31) and having a lower impurity concentration and a smaller thickness than the first layer portion (31).

Description: SPECIFICATION TITLE OF THE INVENTION Semiconductor Rectifier Diode BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a semiconductor rectifier diode. Description of the Prior Art In recent years, introductions of ICs or LSIs are abruptly advancing into electronic computers, their various terminal devices or various system devices. A low-voltage d.c. stabilized power source is required as one for those devices. In order to obtain a low-voltage stabilized direct current from a commercial power source, there are a variety of methods, of which a switching regulator system having advantages of small size and high efficiency is excellent. A rectifier diode to be used in the aforementioned power source is required to have a low loss, stability and a high speed. These characteristics have not been satisfied by the diodes which are well known in the art. For example, the Shottky barrier diode, as is disclosed in Japanese Patent Laid-Open No. 55 107272 (1980), has had defects that the leakage current in the backward direction is high and that it is thermally unstable. On the other hand, the junction diode of p-i-n type according to the prior art has had a defect that its loss is not sufficiently low. In this p-i-n type diode, moreover, in order to enhance the low-loss character, there has been proposed an improvement for extremely reducing the thickness of the p- or n-type layer, as has been disclosed in IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL.ED-23, No. 8, AUGUST 1976, p.945 - 949. Likewise the Shottky barrier diode, the p-i-n type diode has had defects that it is difficult in production and bad in blocking voltage, yield and evenness. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a semiconductor rectifier diode which has an improved structure freed from the aforementioned defects. Another object of the present invention is to provide a semiconductor rectifier diode which has such an improved structure as is easy in production and as is excellent in the low-loss, stability and high-speed characteristics. The semiconductor rectifier diode according to the present invention for achieving those objects is characterized in that it is constructed of a semiconductor substrate and a pair of main electrodes respectively ohmicly contacting with the semiconductor substrate, and in that the semiconductor region for providing communication between the paired main electrodes is constructed in the following manner. Specifically, that semiconductor region is formed with at least one main current passage and a region which is formed with such a pn junction between the paired main electrodes as to surround the main current passage and which has a function to pinch off the aforementioned main current passage by such a depletion layer as is established when a voltage having a polantyto reversely bias that pn junction is applied between the paired main electrodes. More specifically, the semiconductor rectifier diode of the present invention is constructed: such that there are formed between a paired main surfaces a first layer of one conduction type disposed adjacent to one of the main surfaces, a second layer of one conduction type disposed adjacent to the first layer and having a lower impurity concentration than the first layer, at least one layer portion of one or the other conduction type disposed adjacent to a selected portion of the second layer and the other of the main surfaces and having a higher impurity concentration than the second layer, and a second layer portion of the other conduction type disposed so adjacent to the second layer and the other of the main surfaces as to surround the first layer portion and having a larger thickness in a direction normal to the main surfaces than the first layer portion and a higher impurity concentration than the second layer; such that a main current passage is constructed of the first layer portion and those portions of the first and second layers, which overlap the first layer portion in a direction normal to the main surfaces; and such that a region having a function to pinch off the main current passage is constructed of the second layer portion and those portions of the first and second layers, which overlap the second layer portion in the direction normal to the main surfaces. In case the first layer portion ismade of the other conduction type, it is preferable that the impurity concentration of the first layer portion be between those of the second layer and the second layer portion and near that of the second layer. In case the first layer portion is made of the one conduction type, on the other hand, it is preferable that the concentration of the first layer portion be as higher as possible than that of the second layer. Moreover, the first and second layer portions may be formed either in contact with or at a distance from each other. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic perspective view showing one embodiment of the present invention; Fig. 2 is an enlarged sectional view showing a portion of Fig. 1; Fig. 3 is a graphical presentation illustrating the impurity concentration distribution of the embodiment shown in Figs. 1 and 2; Figs. 4 to 10 are graphical presentations illustrating the characteristic curves for explaining the characteristics of the embodiment shown in Figs. 1 and 2; Fig. 11 is a schematic sectional view showing another embodiment of the present invention; Fig. 12 is a graphical presentation illustrating the impurity concentration distribution of the embodiment shown in Fig. 11; Fig. 13 is a schematic view for explaining the operations of the embodiment shown in Fig. 11; ; Figs. 14 and 15 are graphical presentations illustrating the characteristic curves for explaining the characteristics of the embodiment shown in Fig. 11; and Figs. 16 to 19 are schematic sectional views showing other embodiments of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail in the following with reference to the accompanying drawings. Fig. 1 shows the overall construction and the section of a diode according to one embodiment of the present invention, and Fig. 2 is an enlarged view showing an essential portion in section. As shown, a semiconductor substrate 100 formed with a pair of main surfaces 101 and 102 has such a laminated structure between the paired main surfaces as is composed in succession of an ++ + n -type layer 51, an n -type layer 5, an n-type layer 4 and a p-type layer 3-. This p-type layer 3 is so formed that first p-type layer portions 31 having a higher concentration and made thicker and second p-type layer portions 32 having a lower concentration and made thinner than the p-type layer portions 31 are arrayed alternately in respective stripe shapes. The second p-type layer portions 32 are substantially surrounded by the first p-type layer portions 31. Figs. 1 and 2 provide sections which are taken in directions perpendicular to the longitudinal directions of those stripe shapes. In Fig. 2, numerals I and II indicate first and second laminated structures, respectively. The p-type layer 3 has its terminal end exposed to the one main surface 101. On the other hand, the n -layer 5 is exposed at the peripheral edge of the semiconductor substrate to the one main surface 101. At the boundary between the p-type layer 3 and the n-type layer 4, there is formed a pn junction which terminates at the one main surface. At the exposed portion of the pn junction at the one main surface, there is formed a SiO2 film 10 for protecting that junction. At the exposed portion of the p-type layer 3, on the other hand, there is formed an anode 1 which is made to ohmically contact with the p-type layer 3. The other main surface 102 of the semiconductor substrate is formed with a cathode 2 which is made to ohmlcally contact with the n type layer 51. In case the n -type layer 5 has a high impurity concentration, the n -type layer 51 may be omitted, and the cathode 2 may be + directly made to ohmicly contact with the n - type layer 5. The impurity concentration distribution of the first laminated structure in a direction normal to the main surfaces is plotted in a solid line in Fig. 3, whereas the impurity concentration distribution of the second laminated structure in the same direction as above is plotted in a broken line in the same Figure. The semiconductor substrate of the diode according to the present embodiment is sized to be a square having one side of about 4 mm. The diode of the present embodiment can be fabricated by making use of the well-known semiconductor device fabricating technique. The summary of one example will be described in the following. First of all, an n-n type silicon wafer is prepared. Phosphor is diffused selectively into one of the main surfaces of the wafer and wholly into the other main surface to form both the exposed portion of the n+ -type layer 5 to the one main surface 101 and the n ++-type layer 51. Next, boron is selectively diffused into the one main surface to form the first p-type layer portion 31. Subsequently, boron ions are implanted into the one main surface, and a heat treatment is executed to form the second p-type layer portion 32. Next, the paired main surfaces have their predetermined portions vacuum-evaporated with At or a laminated film of Cr-Ni-Ag, for example, to form the anode 1 and the cathode 2. Incidentally, the SiO2 film is either formed as a result of the oxidization of silicon during the aforementioned heat treatment such as a diffusion process or deposited by another CVD method. The operations and effects of the diode thus fabricated will be described in the following. Likewise the existing pn junction diode, a main current flows between the paired main electrodes in a state (i.e., a forward bias state), in which a positive voltage is applied to the anode 1 relative to the cathode 2, but is blocked in a state (i.e., in a backward bias state) in which a negative voltage is applied to the anode 1 relative to the cathode 2. The main-current + ++ passage is established mainly in the p-n-n -n diode region (i.e., the first laminated structure) including the second p-type layer portion 32. During the backward or reverse bias, on the other hand, a depletion layer is formed even at the pn junction between the second p-type layer portion 32 and the n-type layer 4, but the depletion layer extending mainly from the pn junction between the first p-type layer portion 31 and the n-type layer 4 pinches off the main current passage to block the main current and to bear the blocking voltage during the reverse bias. Thus, during the forward bias, the second p-type layer portion 32 having a lower concentration and made thinner acts as an anode (or emitter) region to invite advantages such as a low loss and a high speed. During the backward bias, on the other hand, the first ptype layer portion 31 made thicker bears the blocking voltage to invite advantages such as a high blocking voltage and stable characteristics. Moreover, since the depletion layer for pinching off the main current passage is formed of not the thinner second p-type layer portion 32 but the thicker first p-type layer portion 31, the design of the second p-type layer portion 32 of the main current passage can be advantageously conducted exclusively from the standpoint such as the low loss or the high speed, which is contrary to the high blocking voltage. Even if the second p-type layer portion 32 has defects such as pin holes, furthermore, there can be attained an effect that the reverse blocking voltage is not dropped. As a result, the production yield is enhanced. Fig. 4 plots the forward voltage drop of the p-type layer portion 32 of the diode of the present embodiment against a width Wch, and Fig. 5 plots the blocking voltage against the width Wch. Here, those Figures correspond to the case in which: the current density is about 60 A/cmê the thickness Lp of the second p-type layer portion 32 is 2 pm; the thickness of the first p-type layer portion 31 is 5 pm; and the thickness of the n-type layer 4 between the bottom of the first p-type layer portion 31 and the n -type layer 5 is 5 pm. Symbols a, o and o appearing in those Figures correspond to the cases in which the impurity concentration Nn of the ntype layer 4 and the surface impurity concentration Nsp of the second p-type layer portion 32 at the one main surface 101 take such values as are tabulated in the following Table 1: : Table 1 -3 Nn(cm -3 Symbols Nn(cm ) Nsp(cm A 3 x 1013 5 x 1015 o 1 x 1014 5 x 1015 e 3 x 3 x 1013 x 1015 In view of Figs. 4 and 5, it is found that the forward voltage drop is merely increased by about 0.02 V but the blocking voltage can be raised by about 70 V by reducing the width of the second p-tye layer portion 32 from about 10 pin to about 2 jim, for example, even if the second p-type layer portion 32 has the low surface -3 impurity concentration of 5 x 1015 cm 3 and the small thickness of 2 pm. Next, the influences of the surface impurity concentration of the second p-type layer portion 32 upon the characteristics of the diode of the present embodiment will be described in the following. Figs. 6, 7 and 8 plot the forward voltage drop, the backward restoration time trr and the reduction rate diR/dt of the backward restoration current, respectively, in case the thickness Lp of the second p-type layer portion 32 is 1 to 2 jim whereas the impurity concentration Nn of the 14 15 -3 n-type layer 4 is 1 x 10 to 1 x 10 cm Those plots correspond to the case in which the forward current density is about 60 A/cm2 and the forward current reduction rate during the backward restoration is 30 A/#s. Here, the thickness Lp and the impurity concentration Nn of those respective Figures take such values as are tabulated in Table 2: Table 2 Figure No. Symbols Lp (m) Nn (cm ) o 2 1 x 1014 A 6 o 2 1 x 1015 B 1 1 x 1014 A 2- 1 x 1014 7 B 1 1 x 1014 A 2 1 x 1014 8 B 2 1 x 1015 In view of those Figures, it is found that the forward voltage drop can be reduced, the backward restoration can be accelerated and the backward restoration characteristics can be made soft (in other words, the rate diR/dt can be decelerated) by reducing the surface impurity concentration. If the second p-type layer portion 32 is made to have a thickness of 1 to 2 um and a surface impurity concentration of 15 16 to 1 x 1016 -3 1 x 1015 to 1 x 1016 cm-3 for example, the forward voltage drop, the backward restoration time trr and the backward restoration current reduction rate diR/dt can be reduced to 0.68 to 0.69 V, 35 to 40 ns, and 12 to 20 respectively, thereby to provide a high-speed rectifier diode having a high performance. As the impurity concentration of the second p-type layer portion 32 is lowered, the positive holes to be injected into the n-type layer 4 from the second p-type layer portion 32 in the forward bias state so that the current through the n-type layer 4 is a flow of the electrons acting as major carriers. As a result, the backward restoration time t is shortened. As the rr impurity concentration of the second p-type layer portion 32 is lowered, on the other hand, the diffusion potential at the pn junction is accordingly lowered-to reduce the forward voltage drop. Here, a cursory description will be made upon the reduction rate diR/dt. Fig. 9 illustrates the waveforms of a current i and a voltage v when the diode is reversed from its forward state. A backward restoration current iR flows, too, even after the bias has been reversed. Although it is not unimportant that the backward restoration current iR itself small, it is important for the low power loss that the power loss (v x i) during the period, as indicated at tdt be low. Especially large consideration should be taken into that power loss because this loss grows higher than any other loss during a high-frequency operation higher than about 50 Hz. For that importance, it is desired that the reduction rate diR/dt be low (in other words, to have soft recovery characteristics). Since the reduction rate diR/dt is low according to the present embodiment, the aforementioned power loss is so low that the present embodiment finds its suitable application in a switching regulator. Next, the effects of the present embodiment will be described in the following in comparison with the Shottky barrier diode. Fig. 10 plots the relationship in the forward voltage drop and the forward current between the present embodiment (A) and the Shottky barrier diode (B). Here, the sample A corresponds to the case, in 14 = 3 m, Nn = Ix 1O14 -3 which #Wch = 3 m, Nn = 1 x 1014 cm -3 and Nsp = 5 x 10-3 and has its blocking voltage designed at 150 V. On the other hand, the sample B has its base layer thickness adjudted to have a blocking voltage of 150 V. In view of Fig. 10, the sample A exhibits a lower forward voltage drop than the sample B in the range where the current density in the forward direction is equal to or higher than about 50 A/cm2. Within the range of the practically used current density (equal to or higher than 100 A/cmê ), therefore, the diode of the present emebodiment has a lower loss than the Shottky barrier diode. Although not illustrated, moreover, the diode of the present embodiment is far superior in the backward leakage current characteristics to the Shottky barrier diode. Other embodiments of the present invention will be described in the following. Fig. 11 is a sectional view showing an essential portion of another embodiment, and Fig. 12 plots the impurity concentration distribution in the direction normal to the main surfaces of Fig. 11. In Fig. 12, a solid line corresponds to a first laminated structure I, and a broken line corresponds to a second laminated structure II. ,Incidentally, such portions appearing in Fig. 11 as are identical to those of Fig. 2 are indicated at the same reference characters as those of Fig. 2, and their detailed description is omitted. ++ In the present embodiment, moreover, the n - type layer 51 of Fig. 2 is omitted from being + shown. The p -type layer portion 31 of the present embodiment has its impurity concentration distributed substantially evenly at about 1 x 10 cm . That p -type layer portion 31 can be formed by the well-known selective epitaxial growth, for example. In Fig. 11, numeral 33 indicates an n type layer portion, and numeral 7 indicates a portion which will be called a "channel" in the following description. The overall construction of the present embodiment is substantially similar to that shown in Fig. 1, for example. Fig. 13 is a view for explaining the operations of the rectifier diode according to the present embodiment. Fig. 13(a) corresponds to the case in which a power source Es has its positive termianl connected with the anode 1 of the diode and its negative terminal connected with the cathode 2. A load current iL first + + flows through an n -n-n region which is composed of the n+-type layer portion 33, the n-type layer 4 and the n+-type layer 5. If the load current iL gradually augments so that the voltage applied to the p n junction between the p -type layer portion 31 and the n-type layer 4 exceeds the diffusion potential at that p n junction, the load current #iL begins to flow + the + through the p+-n-n+ region which is composed of the p -type layer portion 31, the n-type layer 4 and the n -type layer 5. As a result, in case the anode-cathode voltage (which will be shortly referred to as the A-K voltage) is equal to or lower than the diffusion potential of the p n junction (i.e., about 0.8 V), the load current + + flows through the n -n-n region so that the forward voltage and the backward restoration characteristics of the element are determined + + by the n -n-n region. Consequently, the low- loss and the high-speed characteristics can be achieved. Fig. 13(b) corresponds to the case in which the power source Es has its positive terminal connected with the cathode 2 of the diode and its negative terminal connected with the anode 1. Since, in this case, the aforementioned p n junction is reversely biased by the power source Es, a depletion layer 70 is formed in the vicinity of the p n junction so that the channel portion 7 comes into a pinchedoff state thereby to allow no load current 1R to flow. The present embodiment will be described in more detail with reference to Figs. 14 and 15. Fig. 14 plots the potential distribution at the central portion of the channel 7 in the state, in which a negative voltage of - 65 V is applied between the anode and the cathode, in case the channel has a width Wch of 2 pm and lengthes Lch of 7.5 pin (in a solid line) and 10 pm (in a broken line). In view of Fig. 14, by the application of the backward voltage, there is formed in the channel a negative potential peak, i.e., a barrier against the electrons to flow from the cathode 2 to the n-type layer 4 thereby to invite the blocking state. The barrier thus formed is raised by reducing the width Wch of the channel and by enlarging the length Lch of the channel. Fig. 15 plots the reverse voltage - reverse current characteristics of the diode of the present embodiment and corresponds to the case of Fig. 14. In case the channel length is 10 pin (as plotted in a broken line), the reverse current at the reverse voltage of 150 V is 2 x 10 A. In the case of the channel length of 7.5 um (as plotted in a solid line), the reverse current at the reverse voltage of 150 V is 5 x 10 A. The reverse currents for the two cases are low. On the other hand, the forward voltage in the forward direction (with the current density of 60 A/cm2) took values of 0.7 V and 0.71 V, respectively, in cases the channel length was 7.5 pin and 10 pm. The diode according to the embodiment shown in Figs. 1 and 2 will be compared with that shown in Fig. 11 in the following. Since the former diode has both its first and second laminated structures formed with the pn junction, it can enjoy a higher blocking voltage than the latter diode in case its channel width and the thickness of its first p-type layer portion 31 are made equal to those of the latter. On the other hand, since the latter diode has its first laminated structure made of the layers of the identical conduction type, it substantially provides a multi-carrier device so that it can enjoy a higher speed than the former diode. Figs. 16 to 19 show further embodiments of the present invention. The device of Fig. 16 has a construction, in which the first and second p-type layer portions 31 and 32 are so separated from each other that the n-type layer 4 is exposed through between the two portions to the one main surface 101, and can achieve substantially the same effects as those of the embodiment shown in Figs. 1 and 2 The device of Fig 17 has a construction, in which the first p-type layer portion 31 and the n -type layer portion 33 are so separated from each- other that the n-type layer 4 is exposed through between the two portions to the one main surface 101, and can achieve substantially the same effects as those of the embodiment shown in Fig. 11. The device of Fig. 18 is constructed such that the end layer of the first laminated structure I is composed of the the second p-type layer portion 32 and the n - type layer portion 33, and can achieve the effects like those of both the embodiments shown in Figs. 1 and 2 and in Fig. 11. The device of Fig. 19 has a construction, in which the n-type layer 4 extends through substantially the central portion of the second p-type layer portion 32 until it is exposed to the one main surface 101, and has its high-speed effect enhanced while slightly sacrificing the effect of the high blocking voltage more than the embodiment shown Figs. 1 and 2. Although the present invention has been described hereinbefore in connection with the representative examples thereof, it should not be limited thereto but can be modified in various manners within the scope of the sprit thereof. The modifications conceivable are, for example: (1) one in which the conduction types are reversed in the embodiments shown in the respective Figures; (2) one in which the n-type layer 4 of Fig. 16 extends through the generally central portion of the second p-type layer portion 32, too, until it is exposed to the one main surface 101; (3) one in which the n-type layer 4 of Fig. 17 extends through the generally central portion of the n -type layer portion 33, too, until it is exposed to the one main surface; (4) one, in which the second p-type layer portion 32 is formed between the first p-type layer portion 31 and the n -type layer portion 33 of Fig. 18, too, and one in which the first p-type layer portion 31 and the n -type layer portion 33 are separated from each other; and (5) one, in which such a portion of the n-type layer 4 of Fig. 19 as is exposed to the one main surface is made into a high ceontration layer n and one in which the p-type layer portion 32 is replaced by the n -type layer portion 33. On the other hand, the second p-type layer portion 32 and the n+ -type layer portion 33 may be arranged not only in the stripe shapes, as viewed from the one main surface 101, but also in concentric, radial or matrix shaped and in their combined shapes.

Claims: WHAT IS CLAIMED IS: 1. A semiconductor rectifier diode, characterized in that it comprises: a semiconductor substrate (100) including a first semiconductor layer (5) of first conduction type, a second semiconductor layer (4) of one conduction type disposed adjacent to said first semiconductor layer (5) and having a lower impurity concentration than said first semiconductor layer (5), a first layer portion (31) of the other conduction type disposed adjacent to a selected portion of said second semiconductor layer (4) and having a higher impurity concentration than said second semiconductor layer (4), and at least one second layer portion (32) of selected conduction type disposed so adjacent to said second semiconductor layer (4) that it is surrounded by said first layer portion (31) and having a smaller thickness in a direction normal to the exposed surface of said first layer portion (31) than said first layer portion (31) and a higher impurity concentration than said second semiconductor layer (4); a first main electrode (2) ohmically contacting with the exposed surface of the first semiconductor layer (5) of said semiconductor substrate (100); and a second main electrode (1) contacting with the first and second layer portions (31, 32) of said semi conductor substrate (100). 2. A semiconductor rectifier diode as set forth in Claim 1, wherein said second layer portion (32) is of the other conduction type. 3. A semiconductor rectifier diode as set forth in Claim 1, wherein said second layer portion (33) is of the one conduction type. 4. A semiconductor rectifier diode, characterized in that it comprises: a semiconductor substrate (100) including a pair of main surfaces (101, 102) positioned at the opposite sides to each other, a first semiconductor layer (5) of one conduction type interposed between said main surfaces adjacent to one (102) of said main surfaces (101, 10.2), a second semiconductor layer (4) of one conduction type disposed adjacent to said first semiconductor layer (5) and having a lower impurity concentration than said first semiconductor layer (5), a first layer portion (31) adjacent to selected portions of both said second semiconductor layer (4) and the other t101) of said main surfaces (101, 102) and having a higher impurity concentration than said second semiconduction layer (4), and at least one second layer portion (32, 33) of selected conduction type disposed so adjacent to said second semiconductor layer (4) and the other (101) of said main surfaces (101, 102) that it is surrounded by said first layer portion (31) and having a smaller thickness in a direction normal to the other (101) of said main surfaces (101, 102) than said first layer portion (31) and a higher impurity concentration than said second semiconductor layer (4); ; a first main electrode (2) ohmically contacting at the one (102) of said main surfaces (101, 102) of said semiconductor substrate (100) with said first semiconductor layer (5); and a second main electrode (1) contacting at the other (101) of said main surfaces (101, 102) of said semiconductor substrate (100) with said first and second layer portions (31; 32, 33). 5. A semiconductor rectifier diode as set forth in Claim 4, wherein the distance of said first layer portion (31) between the side ends of said second layer portion (32, 33) is made sufficient to be pinched off by a depletion layer which is formed when the p-n junction between said second semiconductor layer (4) and said first layer portion (31) is reversely biased by a voltage used. 6. A semiconductor rectifier diode as set forth in Claim 5, wherein said second layer portion (32) is of the other conduction type. 7. A semiconductor rectifier diode as set forth in Claim 6, wherein said second layer portion has a plurality of stripes (33, 32, 33) juxtaposed in one direction. 8. A semiconductor rectifier diode as set forth in Claim 5, wherein said second layer portion (33) is of the one conduction type.

Inventors: MURAKAMI, SUSUMU, NAITO, MASAYOSHI, SHIMIZU, YOSHITERU, TERASAWA, YOSHIO
Applicants: HITACHI, LTD.
Application year: 1982
Main classification: H01L29
IPC codes: H01L29