Diode and method of manufacturing the same

Abstract: A diode has a semiconductor layer which has a predetermined impurity concentration, and in which the rate of extension of a depletion layer during a reverse recovery operation gradually decreases so as to decrease the rate of change in reverse recovery current. In addition, the number of excess carriers accumulated in the semiconductor layer during a forward operation is decreased so as to decrease reverse recovery charge. The diode generates less surge voltage and loss and can comply with operating conditions of various forward current densities, and various reverse voltages.

Description: The present invention relates to a semiconductor diode, and particularly to improvements in characteristics of the diode, and a method of manufacturing the same. Fig. 15(a) is a sectional view showing a structure of a conventional diode 100. Fig. 15(b) is a schematic diagram showing a distribution of impurity concentration in the direction along a longitudinal section of the diode 100. This figure illustrates a relationship between the thickness X in the direction along the longitudinal section and the impurity concentration N in a semi-logarithmic scale. The diode 100 comprises an N- layer 1 of low impurity concentration which is formed on a first main surface of an N+ layer 3 of high impurity concentration by epitaxial growth. A P layer 4 is formed by diffusion on a first main surface of the N- layer 1 to form a PN junction J100. An anode electrode 5 is formed on a first main surface of the P layer 4, and a cathode electrode 6 is formed adjacent to a second main surface of the N+ layer 3. The impurity concentration of the N- layer 1 comprising a layer of low impurity concentration has a uniform distribution and substantially no gradient, as shown in Fig. 15(b). Fig. 16(a) is a sectional view showing a structure of a conventional diode 200, and Fig. 16(b) is a schematic diagram showing a distribution of impurity concentration in the direction along a longitudinal section of the diode 200. The diode 200 comprises an N- layer 1 of low impurity concentration, an N+ layer 3 of high impurity concentration, an N layer 2 which has an intermediate impurity concentration between the low and high impurity concentrations and which is formed between the N- and N+ layers 1 and 3, a P layer 4 formed on the upper main surface of the N- layer 1 to form a PN junction J200, an anode electrode 5 formed on the upper surface of the P layer 4, and a cathode electrode 6 formed on the lower surface of the N+ layer 3. Fig. 17(a) is a sectional view showing a structure of a conventional diode 300, and Fig. 17(b) is a schematic diagram showing a distribution of impurity concentration in the direction along a longitudinal section of the diode 300. The diode 300 comprises an N- layer 1' of low impurity concentration having such a concentration gradient that the impurity concentration increases from the side of the upper main surface to the side of the lower main surface, an N+ layer 3 of high impurity concentration, an N layer 2 which has an intermediate impurity concentration between the low and high impurities concentrations and which is formed between the N- and N+ layers 1' and 3, a P layer 4 formed on the upper main surface of the N- layer 1' to form a PN junction J300, an anode electrode 5 formed on the upper surface of the P layer 5, and a cathode electrode 6 formed on the lower surface of the N+ layer 3. Fig. 14 is a schematic diagram showing an operating current waveform of a diode. After a forward current I F flows, the current decreases to zero at time t0, the current after time t0 serving as a reverse recovery current. After the time the reverse recovery current reaches a peak I RM , there occurs a surge voltage VS which is represented by a rate of change dirr/dt (corresponding to the slope of a tangent to the reverse recovery current shown in Fig. 14) and an inductance component L of a circuit in accordance with the following equation (1): VS = -L dirr/dt As is obvious from the above equation (1), with a reverse recovery current waveform having a high rate of change dirr/dt (so-called hard recovery), as shown by solid line (1) in Fig. 14, the surge voltage VS is increased and sometimes fatally affects a circuit device, for example, destroys an element. When it is demanded to design a circuit device, therefore, it is required for suppressing the surge voltage that a diode has a reverse recovery current waveform having a low rate of change dirr/dt (so-called soft recovery), as shown by broken line (2) in Fig. 14. In order to decrease a switching loss, it is also required to decrease the reverse recovery charge Qrr (corresponding to the area of the shadowed portion shown in Fig. 14) of a diode, which is calculated by time integration of the reverse recovery current. In a diode, the PM junction J100 is formed between the P layer 4 and the N- layer 1 of low impurity concentration, as shown in Fig. 15. Since this layer 1 has a low impurity concentration, a depletion layer easily develops, and high voltage resistance can be realized. During a reverse recovery operation of a diode having a PN-N+ structure, a depletion layer is widened into the P layer and the N- layer from the PN junction while sweeping out the excess carriers accumulated during a forward operation. Finally, the excess carriers accumulated are completely swept out. At a high rate of extension of the depletion layer, the carriers are rapidly swept out, and thus the reverse recovery current becomes zero immediately after it reaches the peak I RM . As a result, the reverse recovery current has a hard recovery waveform having a high rate of change dirr/dt. On the contrary, at a low rate of extension of the depletion layer, the carriers are slowly swept out, and thus the reverse recovery current hardly becomes zero after the current reaches the peak I RM . As a result, the reverse recovery current has a soft recovery waveform having a low rate of change dirr/dt. However, in the conventional diode 100 shown in Fig. 15, since the N- layer 1 of low impurity concentration comprises a uniform layer of low impurity concentration in order to permit easy depletion for realizing high voltage resistance, the depletion layer rapidly extends from the PN junction J100 into the N- layer 1 toward the N+ layer 3 during the reverse recovery operation. The conventional diode thus has the fault that the reverse recovery current has a hard recovery waveform, and the problem of increasing the surge voltage. A method of solving the problem is to increase the thickness of the N- layer 1 of the diode 100. However, this method decreases the width ratio of the depletion layer extending due to the reverse voltage applied during the reverse recovery operation to the N- layer 1, and decreases the rate of disappearance of carriers by an amount corresponding to an increase in number of the carriers accumulated outside the depletion layer, thereby attaining the reverse recovery current having a soft recovery waveform with a low rate of change dirr/dt. However, an increase in the thickness of the N- layer 1 causes the faults of increasing the forward voltage, and increasing the reverse recovery charge Qrr, and the problem of increasing a loss. The conventional diode 200 shown in Fig. 16 has the same structure as the diode 100 except that the N layer 2 is inserted between the N- layer 1 and N+ layer 3 of the diode 100 in order to solve the above problem. The impurity concentration of the N layer 2 is higher than the impurity concentration of the N- layer 1 so that the end of the depletion layer extending due to the reverse voltage stops at an intermediate portion in the N layer 2, and is lower than the impurity concentration of the N+ layer 3 so that excess carriers are accumulated during the forward operation. This decreases the rate of disappearance of the excess carriers accumulated in the remainder of the depletion layer of the N layer 2, which extends due to the reverse voltage applied during the reverse recovery operation, thereby obtaining a reverse recovery current having the soft recovery waveform with a low rate of change dirr/dt. In addition, since the impurity concentration of the N layer 2 is relatively high, the extension distance of the depletion layer is small, and thus soft recovery is attained even if the thickness of the N layer 2 is small. The N layer 2 must not be so thick as the N- layer 1 of the diode 100 in which the depletion layer extends at a high rate and which must be thickened. It is thus possible to restrain the total of the thickness of the N- layer 1 and the N layer 2, and the forward voltage and the reverse recovery charge Qrr are less increased. However, an actual circuit device such as an inverter or the like is sometimes operated with various forward current densities ranging from a small current density to a large current density. Since the impurity concentration of the N layer 2 is fixed after the diode 200 is manufactured, within an operating region of very low forward current density, the density of the carriers accumulated in the N layer 2 is low, thereby eliminating the soft recovery effect. If the density of excess carriers accumulated in the N layer 2 during the forward operation is increased by decreasing the impurity concentration of the N layer 2 in order to achieve soft recovery within the operating region of very low forward current density, the rate of extension of the depletion layer in the N layer 2 is increased, thereby to eliminate the effect of attaining soft recovery. Namely, the diode 200 has the drawback fault that soft recovery can be attained within only a specified narrow operating region of forward current density, and thus has the problem that, under some operating conditions, hard recovery is caused, and the surge voltage is consequently increased. The conventional diode 300 shown in Fig. 17 has the same structure as the diode 200 except that an N- layer 1' having such a concentration gradient that the impurity concentration increases from the PN junction J300 toward the N layer 2 is formed in place of the N- layer 1 of the diode 200. Since, in the N- layer 1', the impurity concentration gradually increases from the junction J300 toward the N layer 2, the rate of extension of the depletion layer during the reverse recovery operation gradually decreases, thereby attaining soft recovery even within the operating region of low forward current density. Further, in an operation with a large current density, the end of the depletion layer extending due to the reverse voltage applied during the reverse recovery operation stops at an intermediate position in the N layer 2, as in the diode 200, and the rate of disappearance of the excess carriers accumulated in the remainder of the N layer 2 is decreased, thereby attaining soft recovery. However, in an actual circuit device, the reverse voltage applied to the diode sometimes varies with variations in the power supply voltage and operation error. If the reverse voltage applied during the reverse recovery operation varies, the end position of the depletion layer extending due to the reverse voltage also varies. Since the impurity concentration and thickness of the N layer 2 are fixed after the diode 300 is manufactured, therefore, if the reverse voltage is changed to a higher value, the depletion layer extending due to the reverse voltage applied during the reverse recovery operation sometimes passes through the N layer 2 and extends into the N+ layer 3. In this case, the excess carriers accumulated in the N layer 2 are also rapidly swept out by the depletion layer, thereby causing hard recovery. As a measure against this, when the impurity concentration of the N layer 2 is increased so that the depletion layer stops at an intermediate position in the N layer 2 even if the reverse voltage is increased, the density of excess carriers accumulated in the N layer 2 is conversely decreased, thereby hardly attaining soft recovery. When the thickness of the N layer 2 is increased so that the depletion layer stops at an intermediate position in the N layer 2 even if the reverse voltage is increased, the forward voltage and reverse recovery charges are significantly increased. Namely, the diode 300 has the drawback that an increase in the reverse voltage causes hard recovery, and the problem that the surge voltage is increased by a variation in the reverse voltage due to a variation in the power supply voltage and operation error. There is further the problem that if an attempt is made to solve the above problem, the forward voltage and reverse recovery charge are increased, and loss is increased. In the conventional diodes as stated above, there are problems to be solved as follows. In the diode 100 of the above-described conventional diodes, the method of increasing the thickness of the N- layer 1 is employed. However, this method decreases the width ratio of the depletion layer extending due to the reverse voltage applied during the reverse recovery operation to the N- layer 1, and decreases the rate of disappearance of carriers by an amount corresponding to an increase in the number of the carriers accumulated outside the depletion layer. As a result, the reverse recovery current has a soft recovery waveform having a low rate of change dirr/dt. However, the conventional diode has the fault that an increase in the thickness of the N- layer 1 increases the forward voltage and the reverse recovery charge Qrr, and the problem of increasing loss. The diode 200 has the drawback that soft recovery is attained within only a specified narrow operating region of forward current densities, and the problem that, under some operating conditions, hard recovery occurs, and the surge voltage is increased. The diode 300 has the drawback that an increase in the reverse voltage causes hard recovery, and the problem that the surge voltage is increased by a variation in the reverse voltage due to a variation in the power supply voltage and operation error. There is also the problem that an attempt to solve the above problem increases the forward voltage and the reverse recovery charge, and increases loss. A diode according to the preambles of claims 1 and 3 is known from EP-A-0 103 138. The present invention has been achieved for solving the above problems, and an object of the present invention is to provide a diode in which the surge voltage is decreased by attaining soft recovery, in which loss is decreased by suppressing reverse recovery charge, in which soft recovery is maintained under operating conditions of various forward current densities, and the surge voltage is decreased, in which soft recovery is maintained even if the reverse voltage changes, and the surge voltage is decreased, and methods of manufacturing diodes having these characteristics. As to the diodes, this object is solved by the subject matters of claims 1 and 3, and as to the methods of manufacturing such diodes this object is solved by the subject matters of claims 9 to 10. Further advantageous modifications are subject matter of the dependent claims. Other and further objects, features and advantages of the invention will appear more fully from the following description. Fig. 1(a) is a sectional view showing a structure of a diode in accordance with a first embodiment of the present invention, and Fig. 1(b) is a schematic drawing showing a distribution of impurity concentration in the direction along a longitudinal section of the diode. Fig. 2(a) is a sectional view showing a structure of a diode in accordance with a second embodiment of the present invention, and Fig. 2(b) is a schematic drawing showing a distribution of impurity concentration in the direction along a longitudinal section of the diode. Fig. 3 (a) is a sectional view showing a structure of a diode in accordance with a third embodiment of the present invention, and Fig. 3(b) is a schematic drawing showing a distribution of impurity concentration in the direction along a longitudinal section of the diode. Fig. 4(a) is a sectional view showing a structure of a diode in accordance with a fourth embodiment of the present invention, and Fig. 4(b) is a schematic drawing showing a distribution of impurity concentration in the direction along a longitudinal section of the diode. Fig. 5(a) is a sectional view showing a structure of a diode in accordance with a fifth embodiment of the present invention, and Fig. 5(b) is a schematic drawing showing a distribution of impurity concentration in the direction along a longitudinal section of the diode. Figs. 6(a) to (d) are sectional views showing a process of manufacturing a diode in accordance with a sixth embodiment of the present invention, and Figs. 6(a') to (c') are schematic drawings respectively showing distributions of impurity concentration in the direction along a longitudinal section. Figs. 7(a) to (d) are sectional views showing a process of manufacturing a diode in accordance with a seventh embodiment of the present invention, and Figs. 7(a')to (c') are schematic drawings respectively showing distributions of impurity concentration in the direction along a longitudinal section. Figs. 8(a)to (h) are sectional views showing a process of manufacturing a diode in accordance with an eighth embodiment of the present invention, and Figs. 8(a')to (g') are schematic drawings respectively showing distributions of impurity concentration in the direction along a longitudinal section. Figs. 9(a) to (f) are sectional views showing a process of manufacturing a diode in accordance with a ninth embodiment of the present invention, and Figs. 9(a') to (e') are schematic drawings respectively showing distributions of impurity concentration in the direction along a longitudinal section. Figs. 10(a) to (c) are sectional views showing a process of manufacturing a diode in accordance with a tenth embodiment of the present invention, and Figs. (a') and (b') are schematic drawings respectively showing distributions of impurity concentration in the direction along a longitudinal section. Figs. 11(a) to (e) are sectional views showing a process of manufacturing a diode in accordance with an eleventh embodiment of the present invention, and Figs. 11(a') to (d') are schematic drawings showing respectively distributions of impurity concentration in the direction along a longitudinal section. Figs. 12(a) to (c) are sectional views showing a process of manufacturing a diode in accordance with a twelfth embodiment of the present invention, and Figs. 12(a') and (b') are schematic drawings respectively showing distributions of impurity concentration in the direction along a longitudinal section. Fig. 13 is a schematic drawing showing a distribution of impurity concentration in the direction along a longitudinal section of an N- or N layer of a diode in accordance with the first to twelfth embodiments of the present invention. Fig. 14 is a schematic drawing showing an operating current waveform of a diode. Fig. 15(a) is a sectional view showing a structure of an example of conventional diodes, and Fig. 15(b) is a schematic drawing showing a distribution of impurity concentration in the direction along a longitudinal section of the diode. Fig. 16 (a) is a sectional view showing a structure of another example of conventional diodes, and Fig. 16(b) is a schematic drawing showing a distribution of impurity concentration in the direction along a longitudinal section of the diode. Figs. 17(a) is a sectional view showing a structure of a further example of conventional diodes, and Figs. 17(b) is a schematic drawing showing a distribution of impurity concentration in the direction along a longitudinal section of the diode. The first embodiment of this invention will be shown with reference to the drawings. Fig. 1(a) is a sectional view showing a structure of a diode 10 in accordance with a first embodiment of the present invention, and Fig. 1(b) is a schematic diagram showing a distribution of impurity concentration in the direction along a longitudinal section of the diode 10. Fig. 1(b) shows a relation between the thickness X in the direction along the longitudinal section and the impurity concentration N in a semi-logarithmic scale. The diode 10 shown in Fig. 1 can be manufactured as described below. An N+ semiconductor substrate is first prepared as an N+ layer 13. N-type epitaxial layers are then grown on a first main surface of the N+ layer 13 by controlling doping so that the impurity concentration gradually decreases to form an N layer 12a having an impurity concentration gradient. N-type epitaxial layers are successively grown on a first main surface of the N layer 12a so that the impurity concentration gradually decreases to form an N- layer 11a having an impurity concentration gradient which is greater than that of the N layer 12a. A P layer 14 is then formed by ion implantation or diffusion of P-type impurities from the side of the first main surface of the N- layer 11a to form a PN junction J10. A first electrode 15 comprising an anode electrode and a second electrode 16 comprising a cathode electrode are formed, by metal deposition, on a first main surface of the P layer 14 and a second main surface of the N+ layer 13, respectively, to obtain a structure of the diode 10. In the diode 10, since the N- layer 11a has such an impurity concentration gradient that the impurity concentration gradually increases from the PN junction J10 to the N layer 12a, the impurity concentration at the end of a depletion layer gradually increases as the depletion layer extends in the N- layer 11a with the passage of time during a reverse recovery operation. Thus, the rate of extension of the depletion layer gradually decreases, and the reverse recovery current hardly becomes zero after it reaches the peak IRM, thereby achieving soft recovery. Consequently, occurrence of a surge voltage is suppressed. Under the operating condition of a low forward current density, excess carriers are accumulated in almost only the N- layer 11a during the forward operation, and are hardly accumulated in the N layer 12a and N+ layer 13. Even in this case, since the rate of extension of the depletion layer in the N- layer 11a during the reverse recovery operation gradually decreases, soft recovery is maintained. As a result, occurrence of the surge voltage is suppressed even under the operating condition of a low forward current density. Since the N layer 12a has such an impurity concentration gradient that the concentration gradually increases from the side of the first main surface thereof to the side of the second main surface thereof, the rate of extension of the depletion layer, which extends in the N layer 12a during the reverse recovery operation, gradually decreases due to the concentration gradient, thereby achieving a higher level of soft recovery. As a result, occurrence of the surge voltage is further suppressed. Since the impurity concentration gradient of the N layer 12a is smaller than that of the N- layer 11a, the width of a region of the N layer 12a, which has a low impurity concentration, is substantially large, and a large number of excess carriers are accumulated in the N layer 12a during the forward operation. A large number of excess carriers thus remain at the time the reverse recovery current reaches the peak, and take much time to disappear. Therefore, the reverse recovery current slowly becomes zero after it reaches the peak, thereby achieving soft recovery. As a result, occurrence of the surge voltage is further suppressed. Moreover, even if the distance of extension of the depletion layer is increased by a variation in the reverse voltage during the reverse recovery operation, the depletion layer extending due to the reverse voltage does not reach the N+ layer 13 and stops at an intermediate position in the N layer 12a because the impurity concentration of the N layer 12a gradually increases. The excess carriers accumulated in a remainder of the N layer 12a where the depletion layer does not reach, slowly disappear, thereby achieving soft recovery. In this case, since the rate of extension of the depletion layer gradually decreases due to the concentration gradient of the N layer 12a, a higher level of soft recovery is attained. Further, since the impurity concentration of the N layer 12a gradually changes, soft recovery is achieved at various values of reverse voltages within a range below the voltage resistance value, not a specified voltage value. Consequently, occurrence of the surge voltage is suppressed. The second embodiment of this invention will be shown with reference to the drawings. Fig. 2(a) is a sectional view showing a structure of a diode 20 in accordance with a second embodiment of the present invention, and Fig. 2(b) is a schematic diagram showing a distribution of impurity concentration in the direction along a longitudinal section of the diode 20. The diode 20 shown in Fig. 2 can be manufactured as described below. An N+ semiconductor substrate is first prepared as an N+ layer 13, and N-type epitaxial layers are grown on the first main surface of the N+ layer 13 by controlling doping so that the impurity concentration gradually decreases to form an N layer 12a having an impurity concentration gradient. N-type epitaxial layers are successively grown on the first main surface of the N layer 12a so that the impurity concentration gradually decreases to an N- layer 11b which has an impurity concentration gradient greater than that of the N layer 12a and a thickness Db greater than or equal to the distance W a depletion layer extends due to the reverse voltage. The distance W the depletion layer extends due to the reverse voltage can be calculated by simulation or the like. A P layer 14 is then formed by ion implantation or diffusion of P-type impurities from the side of the first main surface of the N- layer 11b to form a PN junction J20. Anode electrode 15 and a cathode electrode 16 are formed on the first main surface of the P layer 14 and the second main surface of the N+ layer 13, respectively, by metal deposition, to obtain a structure of the diode 20. Like the diode 10, this diode 20 has the function to achieve soft recovery even under operating conditions where the forward current density is low and where the reverse voltage changes. In addition, since the thickness Db of the N- layer 11b is greater than or equal to the distance W the depletion layer extends due to the reverse voltage, the number of the excess carriers accumulated in a remainder of the N- layer 11b and the N layer 12a where the depletion layer extending due to the reverse voltage applied during the reverse recovery operation does not reach, is increased, and the excess carriers thus take much time to disappear, thereby achieving a higher level of soft recovery. Consequently, occurrence of the surge voltage is further suppressed. The third embodiment of this invention will be shown with reference to the drawings. Fig. 3(a) is a sectional view showing a structure of a diode 30 in accordance with a third embodiment of the present invention, and Fig. 3(b) is a schematic diagram showing a distribution of impurity concentration in the direction along a longitudinal section of the diode 30. The diode 30 shown in Fig. 3 can be manufactured as described below. An N+ semiconductor substrate is first prepared as an N+ layer 13, and N-type epitaxial layers are grown on the first main surface of the N+ layer 13 by controlling doping so that the impurity concentration gradually decreases to form an N layer 12b having an impurity concentration gradient. N-type epitaxial layers are successively grown on the first main surface of the N layer 12b so that the impurity concentration gradually decreases to form an N- layer 11a having an impurity concentration gradient which is smaller than the concentration gradient of the N layer 12b. A P layer 14 is then formed by ion implantation or diffusion of P-type impurities from the side of the first main surface of the N- layer 11a to form a PN junction J30. An anode electrode 15 and a cathode electrode 16 are formed on the first main surface of the P layer 14 and the second main surface of the N+ layer 13, respectively, by metal deposition to obtain a structure of the diode 30. In the diode 30, since the N- layer 11a has such an impurity concentration gradient that the concentration gradually increases from the PN junction J30 to the N layer 12b, the rate of extension of a depletion layer gradually decreases as the depletion layer extends in the N- layer 11a during the reverse recovery operation, thereby achieving soft recovery, as in the diode 10. As a result, occurrence of the surge voltage is suppressed. Under the operating coition of a low forward current density, excess carriers are accumulated almost only in the N- layer 11a during the forward operation, and are hardly accumulated in the N layer 12b and the N+ layer 13. However, even in this case, since the rate of extension of the depletion layer in the N- layer 11a during the reverse recovery operation gradually decreases, soft recovery is maintained. As a result, occurrence of the surge voltage is suppressed under the operating condition of a low forward current density. Moreover, since the N layer 12b has such an impurity concentration gradient that the impurity concentration gradually increases from the side of the first main surface thereof to the side of the second main surface thereof, as in the diode 10, the rate of extension of the depletion extending in the N layer 12b during the reverse recovery operation gradually decreases due to the concentration gradient, thereby achieving a higher level of soft recovery. As a result, occurrence of the surge voltage is further suppressed. In addition, even if the extension distance of the depletion layer is increased by a variation in the reverse voltage during the reverse recovery operation, the depletion layer extending due to the reverse voltage does not reach the N+ layer 13 and stops at an intermediate position in the N layer 12b because the impurity concentration of the N layer 12b gradually increases. Thus, the excess carriers accumulated in a remainder of the N layer 12b where the depletion layer does not reach, slowly disappear, thereby achieving soft recovery. Further, in this case, since the rate of extension of the depletion layer gradually decreases due to the concentration gradient of the N layer 12b, a higher level of soft recovery is attained. Since the impurity concentration of the N layer 12b gradually changes, soft recovery is attained at various values of reverse voltages within a range below the voltage resistance value, not at a specified voltage value. As a result, occurrence of the surge voltage is suppressed. Further, since the impurity concentration gradient of the N layer 12b is greater than that of the N- layer 11a, the width of a region of the N layer 12b, which has a low impurity concentration, is substantially small, and the number of the excess carriers accumulated in the N layer 12b during the forward operation is decreased, thereby decreasing the reverse recovery charge Qrr. Consequently, a low loss is achieved. The fourth embodiment of this invention will be shown with reference to the drawings. Fig. 4(a) is a sectional view showing a structure of a diode 40 in accidence with a fourth embodiment of the present invention, and Fig. 4(b) is a schematic drawing showing a distribution of impurity concentration in the direction along a longitudinal section of the diode 40. The diode 40 shown in Fig. 4 can be manufactured as described below. An N+ semiconductor substrate is first prepared as an N+ layer 13, and N-type epitaxial layers are grown on the first main surface of the N+ layer 13 by controlling doping so that the impurity concentration gradually decreases to form an N layer 12b having an impurity concentration gradient. N-type epitaxial layers are successively grown on the first main surface of the N layer 12b so that the impurity concentration gradually decreases to form an N- layer 11b having an impurity concentration gradient smaller than the concentration gradient of the N layer 12b and a thickness Db greater than or equal to the distance W a depletion layer extends due to the reverse voltage. The distance W the depletion layer extends due to the reverse voltage can be calculated by simulation or the like. A P layer 14 is then formed by ion implantation or diffusion of P-type impurities from the side of the first main surface of the N- layer 11b to form a PN junction J40. An anode electrode 15 and a cathode electrode 16 are formed on the first main surface of the P layer 14 and the second main surface of the N+ layer 13, respectively, by metal deposition to obtain a structure of the diode 40. Like the diode 30, this diode 40 has the function to achieve soft recovery even under operating conditions where the forward current density is low and where the reverse voltage changes, and thus occurrence of the surge voltage is suppressed. At the same time, the reverse recovery charge is decreased, and loss is decreased. Further, since the thickness of the N- layer 11b is greater than or equal to the distance W the depletion layer extends due to the reverse voltage, the number of the excess carriers accumulated, during the forward operation, in a remainder of the N- layer 11b and the N layer 12b where the depletion layer extending due to the reverse voltage applied during the reverse recovery operation does not reach, is increased, and thus the excess carriers take much time to disappear, thereby achieving soft recovery. As a result, occurrence of the surge voltage is suppressed. The fifth embodiment of this invention will be shown with reference to the drawings. Fig. 5(a) is a sectional view showing a structure of a diode 50 in accordance with a fifth embodiment of the present invention, and Fig. 5(b) is a schematic drawing showing a distribution of impurity concentrations in the direction along the longitudinal section of the diode 50. The diode 50 shown in Fig. 5 can be manufactured as described below. An N+ semiconductor substrate is first prepared as an N+ layer 13, and N-type epitaxial layers are grown on the first main surface of the N+ layer 13 by controlling doping so that the impurity concentration gradually decreases to form an N layer 12c having an impurity concentration gradient. An N-type epitaxial layer is uniformly grown on the first main surface of the N layer 12c so as to have a low impurity concentration to form an N- layer 11c having a thickness Dc smaller than or equal to the width W IRM of a depletion layer at the time a reverse recovery current reaches a peak. The width W IRM of the depletion layer at the time the reverse recovery current reaches the peak can be calculated by simulation or the like. A P layer 14 is formed by ion implantation or diffusion of P-type impurities from the side of the first main surface of the N- layer 11c to form a PN junction J50. An anode electrode 15 and a cathode electrode 16 are then formed by metal deposition on the first main surface of the P layer 14 and the second main surface of the N+ layer 13, respectively, to obtain a structure of the diode 50. This diode 50 has a structure in which the impurity concentration of the N- layer 11a of the diode 10 or diode 30 is uniformed. At a low impurity concentration, even if the thickness of the N- layer 11c is decreased, therefore, voltage resistance can be maintained, and the number of the excess carriers accumulated during the forward operation can be decreased by an amount corresponding to a decrease in the thickness, thereby decreasing the reverse recovery charge. As a result, loss is decreased. Moreover, since the thickness Dc of the N- layer 11c is smaller than or equal to the distance W IRM of the depletion layer at the time the reverse recovery current reaches the peak, the rate of extension of the depletion layer gradually decreases because the end of the depletion layer extends into the N layer 12c having an impurity concentration gradient after the reverse recovery current reaches the peak, thereby achieving soft recovery. As a result, occurrence of the surge voltage is suppressed. In addition, since the excess carriers accumulated, during the forward operation, in the remainder of the N layer 12c where the depletion layer extending due to the reverse voltage applied during the reverse recovery operation does not reach, slowly disappear, a higher level of soft recovery is achieved. Consequently, occurrence of the surge voltage is further suppressed. Further, even if the distance of extension of the depletion layer is increased by a variation in the reverse voltage during the reverse recovery operation, the depletion layer extending due to the reverse voltage does not reach the N+ layer and stops at an intermediate position in the N layer 12c because the impurity concentration of the N layer 12c gradually increases. Thus, the excess carriers accumulated in the remainder of the N layer 12c where the depletion layer does not reach, slowly disappear, thereby achieving soft recovery. Moreover, in this case, since the rate of extension of the depletion layer gradually decreases due to the concentration gradient of the N layer 12c, a higher level of soft recovery is attained. In addition, since the impurity concentration of the N layer 12c gradually changes, soft recovery is achieved at various values of reverse voltages within a region below the maximum tolerable voltage, not at a specified voltage value. As a result, occurrence of the surge voltage is suppressed. The sixth embodiment of a first manufacturing method for manufacturing the diodes 10, 20, 30 and 40 in accordance with the present invention will be described with reference to Fig. 6. Figs. 6(a'), (b') and (c') are schematic drawings showing distributions of impurity concentrations in the direction along the longitudinal sections shown in Figs. 6(a), (b) and (c), respectively. As shown in Fig, 6(a), an N+ semiconductor substrate is prepared as the N+ layer 13, and a plurality of N-type epitaxial layers are grown on the first main surface of the N+ layer 13 so that the impurity concentration gradually decreases to form a semiconductor region which is approximately considered as having an impurity concentration which gradually decreases from the side of the second main surface to the side of the first main surface side thereof, i.e., an impurity concentration gradient G2, as shown in Fig. 6(a'). Thus-formed semiconductor region forms the N layer 12. As shown in Fig. 6(b), a plurality of N-type epitaxial layers are further grown on the first main surface of the N layer 12 to form a semiconductor layer which is approximately considered as having an impurity concentration which gradually decreases from the side of the second main surface to the side of the first main surface side thereof, i.e., an impurity concentration gradient G1, as shown in Fig. 6(b'). The thus-formed semiconductor layer forms the N- layer 11. The P layer 14 is formed by ion implantation or diffusion of P-type impurities from the side of the first main surface of the N- layer 11, as shown in Fig. 6(c). The anode electrode 15 and the cathode electrode 16 are then formed, by metal deposition, on the first main surface of the P layer 14 and the second main surface of the N+ layer 13, respectively, as shown in Fig. 6(d). If the thickness of the N- layer 11 is D, and the distance the depletion layer extends in the N- layer 11 due to the reverse voltage applied to each of the diodes during the reverse recovery operation is W, the diodes 10, 20, 30 and 40 respectively have the following relations between the impurity concentration gradient G1 of the N- layer 11 and the impurity concentration gradient G2 of the N layer 12. (1) If the N- layer 11 and the N layer 12 are formed so as to have the relation G1 ≥ G2, the diode 10 is obtained. (2) If the N- layer 11 and the N layer 12 are formed so as to have the relations G1 ≥ G2 and D ≥ W, the diode 20 is obtained. (3) If the N- layer 11 and the N layer 12 are formed so as to have the relation G1 < G2, the diode 30 is obtained. (4) If the N- layer 11 and the N layer 12 are formed so as to have the relations G1 < G2 and D ≥ W, the diode 40 is obtained. The distance the depletion layer extends due to the reverse voltage can be calculated by simulation or the like. The seventh embodiment of a second manufacturing method for manufacturing the diode 50 in accordance with the present invention will be described with reference to Fig. 7. Figs. 7(a'), (b') and (c') are schematic drawings showing distributions of impurity concentrations in the directions of the longitudinal sections shown in Figs. 7(a), (b) and (c), respectively. As shown in Fig. 7(a), an N+ semiconductor substrate is prepared as the N+ layer 13, and a plurality of N-type epitaxial layers are grown on the first main surface of the N+ layer 13 so that the impurity concentration gradually decreases to form a semiconductor region which is approximately considered as having an impurity concentration which gradually decreases from the side of the second main surface thereof to the side of the first main surface side thereof, i.e., an impurity concentration gradient, as shown in Fig. 7(a'). The thus-formed semiconductor region forms the N layer 12c. As shown in Fig. 7(b), an N-type epitaxial layer is uniformly grown with a low impurity concentration on the first main surface of the N layer to form the N- layer 11c having no impurity concentration gradient, as shown in Fig. 7(b'). The P layer 14 is formed by ion implantation or diffusion of P-type impurities from the side of the first main surface of the N-layer 11c, as shown in Fig. 7(c). At this time, the N- layer 11c is formed so as to have a thickness Dc smaller than or equal to the width W IRM of the depletion layer at the time the reverse recovery current reaches the peak. The width W IRM of the depletion layer at the time the reverse recovery current reaches the peak can be calculated by simulation or the like. As shown in Fig. 7(d), the anode electrode 15 and the cathode electrode 16 are then formed, by metal deposition, on the first main surface of the P layer 14 and the second main surface of the N+ layer 13, respectively, to obtain the diode 50. The eighth embodiment of a third manufacturing method for manufacturing the diodes 10, 20, 30 and 40 in accordance with the present invention will be described with reference to Fig. 8. Figs. 8(a'), (b'), (c'), (d'), (e'), (f') and (g') are schematic drawings showing distributions of impurity concentrations in the directions along the longitudinal sections shown in Figs. 8(a), (b), (c), (d), (e), (f) and (g), respectively. As shown in Figs. 8(a) and (a'), a semiconductor substrate is first prepared, and the N- layer 11 is formed by ion implantation or diffusion of N-type impurities from the side of the second main surface of the semiconductor substrate so as to have such an impurity concentration gradient that the concentration gradually decreases from the second main surface to the first main surface thereof. The N- layer 11 is processed to desired impurity concentration gradient G1 and thickness, as shown in Fig. 8(b'), by polishing or etching the first and second main surfaces thereof. Similarly, another semiconductor substrate is prepared, and the N layer 12 is formed by ion implantation or diffusion of N-type impurities from the side of the second main surface of the semiconductor substrate, as shown in Figs. 8(c) and (c'). The N layer 12 is processed to desired impurity concentration gradient G2 and thickness, as shown in Fig. 8(d'), by polishing or etching the first and second main surfaces thereof, as shown in Fig. 8(d). An N+ semiconductor substrate is prepared as the N+ layer 13, and the second main surface of the N layer is laminated to the first main surface of the N+ layer 13, as shown in Fig. 8(e). The second main surface of the N- layer 11 is further laminated to the first main surface of the N layer 12, as shown in Fig. 8(f). As shown in Fig. 8(g), the P layer 14 is formed by ion implantation or diffusion of P-type impurities from the side of the first main surface of the N- layer 11. As shown in Fig. 8(h), the anode electrode 15 and the cathode electrode 16 are then formed on the first main surface of the P layer 14 and the second main surface of the N+ layer 13, respectively, by metal deposition. If the thickness of the N- layer 11 is D, and the distance of the depletion layer which extends in the N- layer 11 due to the reverse voltage applied to each of the diodes during the reverse recovery operation is W, the diodes respectively have the following relations between the impurity concentration gradient G1 of the N- layer 11 and the impurity concentration gradient G2. (1) If the N- layer 11 and the N layer 12 are formed so as to have the relation G1 ≥ G2, the diode 10 is obtained. (2) If the N- layer 11 and the N layer 12 are formed so as to have the relations G1 ≥ G2 and D ≥ W, the diode 20 obtained. (3) If the N- layer 11 and the N layer 12 are formed so as to have the relation G1 < G2, the diode 30 is obtained. (4) If the N- layer 11 and the N layer 12 are formed so as to have the relations G1 < G2 and D ≥ W, the diode 40 obtained. The distance the depletion layer extends due to the reverse voltage can be calculated by simulation of the like. The ninth embodiment of a fourth manufacturing method for manufacturing the diode 50 in accordance with the present invention will be described with reference to Figs. 9. Figs. 9(a'), (b'), (c'), (d') and (e') are schematic drawings showing the distributions of impurity concentrations in the directions along the longitudinal sections shown in Figs. 9(a), (b), (c), (d) and (e), respectively. As shown in Figs. 9(a) and (a'), a semiconductor substrate is first prepared, and the N layer 12c is formed by ion implantation or diffusion of N-type impurities from the side of the second main surface of the semiconductor substrate so as to have such an impurity concentration gradient that the concentration gradually decreases from the second main surface thereof to the first main surface thereof. The N layer 12c is processed to desired impurity concentration gradient and thickness, as shown in Fig. 9(b'), by polishing or etching the first and second main surfaces, as shown in Fig. 9(b). An N- semiconductor substrate is prepared as the N- layer 11, and an N+ semiconductor substrate is prepared as the N+ layer 13. As shown in Fig. 9(c), the second main surface of the N layer 12c is laminated to the first main surface of the N+ layer 13, and, as shown in Fig. 9(d), the second main surface of the N- layer 11c is laminated to the first main surface of the N layer 12c. As shown in Fig. 9(e), the P layer 14 is formed by ion implantation or diffusion of P-type impurities from the side of the first main surface of the N- layer 11c. At this time, the P layer 14 is formed so that the thickness Dc of the N- layer 11c is smaller than or equal to the width W IRM at the time the reverse recovery current reaches the peak. The width W IRM of the depletion layer at the time the reverse recovery current reaches the peak can be calculated by simulation or the like. As shown in Fig. 9(f), the anode electrode 15 and the cathode electrode 16 are then formed by metal deposition on the first main surface of the P layer 14 and the second main surface of the N+ layer 13, respectively to form the diode 50. The tenth embodiment of a fifth manufacturing method for manufacturing the diode 50 in accordance with the present invention will be described with reference to Figs. 10. Figs. 10(a') and (b') are schematic drawings showing distributions of impurity concentrations in the directions along the longitudinal sections shown in Figs. 10(a) and (b), respectively. The first half of the method in accordance with this embodiment comprises the same steps as those in the ninth embodiment shown in Figs. 9(a) and (b). Namely, an N+ semiconductor substrate is prepared as the N+ layer 13, and, as shown in Fig. 9(c), the second main surface of the N layer 12c is laminated to the first main surface of the N+ layer 13. As shown in Fig. 10(a), the N- layer 11c is then formed by growing an N-type epitaxial layer on the first main surface of the N layer 12c so as to have a constant impurity concentration. As shown in Fig. 10(b), the P layer 14 is formed by ion implantation or diffusion of P-type impurities from the side of the first main surface of the N- layer 11c. At this time, the P layer 14 is formed so that the thickness Dc of the N- layer 11c is smaller than or equal to the width W IRM of the depletion layer at the time the reverse recovery current reaches the peak. The width W IRM of the depletion layer at the time the reverse recovery current reaches the peak can be calculated by simulation or the like. As shown in Fig. 10(c), the anode electrode 15 and the cathode electrode 16 are formed on the first main surface of the P layer 14 and the second main surface of the N+ layer 13, respectively, to obtain the diode 50. In the ninth embodiment of the fourth manufacturing method, when the thickness of the N- semiconductor substrate used as the N-layer 11c is excessively small, lamination is impossible. However, the method of the tenth embodiment can manufacture a diode even if the thickness of the N- layer 11c is excessively small, and is thus effective as a substitute for the method of the ninth embodiment in such a case. The eleventh embodiment of a sixth manufacturing method for manufacturing the diode 50 in accordance with the present invention will be described with reference to Figs. 11. Figs. 11(a'), (b'), (c') and (d') are schematic diagrams showing the distributions of impurity concentrations in the directions along the longitudinal sections shown in Figs. 11(a), (b), (c) and (d), respectively. As shown in Figs. 11(a) and (a'), an N- semiconductor substrate is first prepared, and N-type impurities are introduced up to an intermediate portion of the N- semiconductor substrate by ion implantation or diffusion of N-type impurities from the side of the second main surface of the substrate to form the N layer 12c having such an impurity concentration gradient that the concentration gradually decreases from the second main surface thereof to the first main surface thereof, and the N- layer 11c having a constant impurity concentration. As shown in Fig. 11(b), the N layer 12c is processed to desired impurity concentration gradient and thickness, as shown in Fig. 11(b'), by polishing or etching the second main surface of the N layer 12. As shown in Fig. 11(c), the N+ layer 13 is formed by growing an N-type epitaxial layer on the second main surface of the N layer 12c so as to have a constant impurity concentration. As shown in Fig. 11(d), the P layer 14 is formed by ion implantation or diffusion of P-type impurities from the side of the first main surface of the N- layer 11c. At this time, the P layer 14 is formed so that the thickness of Dc of the N- layer 11c is smaller than or equal to the width W IRM of the depletion layer at the time the reverse recovery current reaches the peak. The width W IRM of the depletion layer at the time the reverse recovery current reaches the peak can be calculated by simulation or the like. As shown in Fig. 11(e), the anode electrode 15 and the cathode electrode 16 are formed on the first main surface of the P layer and the second main surface of the N+ layer 13, respectively, by metal deposition to obtain the diode 50. The twelfth embodiment of a seventh manufacturing method for manufacturing the diodes 30 and 40 in accordance with the present invention will be described with reference to Figs. 12. Figs. 12(a) and (b) are schematic drawings showing the distributions of impurity concentrations in the directions along the longitudinal sections shown in Figs. 12(a) and (b), respectively. The first half of the method in accordance with this embodiment comprises the same steps as those in the eighth embodiment of the third manufacturing method shown in Figs. 8(a) and (b). Namely, the N- layer 11 having such an impurity concentration gradient that the concentration gradually decreases from the second main surface to the first main surface is obtained. As shown in Figs. 12(a) and (a'), N-type impurities are introduced up to an intermediate portion of the N- layer 11 by ion implantation or diffusion from the second main surface side of the N- layer to form the N layer 12b having such an impurity concentration gradient G2 that the concentration gradually decreases from the-second main surface thereof to the first main surface side thereof, and the N+ layer 13 which is considered as having a substantially constant high impurity concentration near the peak of the impurity distribution. As shown in Fig. 12(b), the P layer 14 is then formed by ion implantation or diffusion of N-type impurities from the side of the first main surface of the N- layer 11. As shown in Fig. 12(c), the anode electrode 15 and the cathode electrode 16 are formed on the first main surface of the P layer 14 and the second main surface of the N+ layer 13, respectively, by metal deposition. If the thickness of the N- layer 11 is D, and the distance the depletion layer extends in the N- layer 11 due to the reverse voltage applied to each of the diodes during the reverse recovery operation is W, the diodes 30 and 40 have the following relations between the impurity concentration gradient G1 of the N- layer 11 and the impurity concentration gradient G1 of the N layer 12. (1) If the N- layer 11 and the N layer 12 are formed so as to have the relation G1 < G2, the diode 30 is obtained. (2) If the N- layer 11 and the N layer 12 are formed so as to have the relations G1 < G2 and D ≥ W, the diode 40 is obtained. The distance the depletion layer extends due to the reverse voltage can be calculated by simulation or the like. In the above all embodiments, the forms of the impurity concentration gradients of the N- layer and N layer may be linear as shown by (1), upwardly curved as shown by (2), or downwardly curved as shown by (3) or may have few infection points as shown by (4) in Fig. 13. Diffusion of a heavy metal such as gold, platinum or the like, irradiation with a radiation of high-energy electron beams or the like, irradiation with a neutral beam of protons or neutrons may be performed for controlling the life time. A structure in which P type and N type are replaced with each other can produce the same effects as those described above. Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims: Diode, comprising: [a] a first semiconductor layer (13) of a first conductivity type having a first main surface and a second main surface opposite said first main surface; [b] a second semiconductor layer (12a; 12b) of said first conductivity type having a first main surface formed adjacent to said second main surface of said first semiconductor layer (13); [c] a third semiconductor layer (11a; 11b) of said first conductivity type having a first main surface formed adjacent to said second main surface of said second semiconductor layer (12a; 12b) and having an impurity concentration gradient such that an impurity concentration thereof gradually decreases from said first main surface thereof to a second main surface thereof opposite said first main surface; [d] a fourth semiconductor layer (14) of a second conductivity type provided adjacent to said second main surface of said third semiconductor layer (11a; 11b); [e] a first main electrode (15) provided adjacent to said fourth semiconductor layer (14); and [f] a second main electrode (16) provided adjacent to said first main surface of said first semiconductor layer (13), wherein the impurity concentration of said first semiconductor layer (13) is greater than that at said first main surface of said second semiconductor layer (12a; 12b), characterized in that: said second semiconductor layer (12; 12b) having an impurity concentration gradient such that an impurity concentration thereof gradually decreases from said first main surface thereof to a second main surface thereof opposite said first main surface, the impurity concentration at said first main surface of said third semiconductor layer (11a;11b) is equal to that at said second main surface of said second semiconductor layer (12a; 12b), and the impurity concentration gradient of said third semiconductor layer (11a; 11b) is either greater or smaller than that of said second semiconductor layer (12a; 12b). Diode according to Claim 1, characterized in that a thickness (D b ) of said third semiconductor layer (11a; 11b) is greater than or equal to a distance (W) a depletion layer extends therein due to a predetermined reverse voltage which is applied to the diode during a reverse recovery operation. Diode, comprising: [a] a first semiconductor layer (13) of a first conductivity type having a first main surface and a second main surface opposite said first main surface; [b] a second semiconductor layer (12c) of said first conductivity type having a first main surface formed adjacent to said second main surface of said first semiconductor layer (13); [c] a third semiconductor layer (11c) of said first conductivity type having a first main surface which is formed adjacent to said second main surface of said second semiconductor layer (12c) and having a second main surface opposite said first main surface, the impurity concentration of said third semiconductor layer (11c) being constant; [d] a fourth semiconductor layer (14) of a second conductivity type provided adjacent to said second main surface of said third semiconductor layer (11c); [e] a first main electrode (15) provided adjacent to said fourth semiconductor layer (14); and [f] a second main electrode (16) provided adjacent to said first main surface of said first semiconductor layer (13), wherein the impurity concentration of said first semiconductor layer (13) is greater than that at said first main surface of said second semiconductor layer (12c); characterized in that: said second semiconductor layer (12c) having an impurity concentration gradient such that an impurity concentration thereof gradually decreases from said first main surface thereof to a second main surface thereof opposite said first main surface, said impurity concentration of said third semiconductor layer (11c) corresponding to the impurity concentration at the second main surface of said second semiconductor layer (12c); and a thickness (D c ) of said third semiconductor layer (11c) is smaller than or equal to a width (W IRM ) of a depletion layer therein at the time a reverse recovery current reaches a peak when a predetermined reverse voltage is applied to the diode during a reverse recovery operation. Method of manufacturing a diode according to claim 1 or 2, comprising the following steps: [a] preparing a semiconductor substrate of a first conductivity type (N) as said first semiconductor layer (13); [b] forming said second semiconductor layer (12a; 12b) by growing a plurality of epitaxial layers of said first conductivity type such that an impurity concentration at a second main surface thereof adjacent to a first main surface of said first semiconductor layer (13) is smaller than that of said first semiconductor layer (13), and the impurity concentration gradually decreases from the second main surface thereof to a first main surface opposite said second main surface; [c] forming said third semiconductor layer (11a; 11b) by growing a plurality of epitaxial layers of said first conductivity type such that an impurity concentration of a second main surface thereof adjacent to said first main surface of said second semiconductor layer (12a; 12b) is equal to that of said first main surface of said second semiconductor layer (12, 12b), and the impurity concentration gradually decreases from the second main surface thereof to a first main surface opposite said second main surface; [d] forming said fourth semiconductor layer (14) of a second conductivity type adjacent to said first main surface of said third semiconductor layer (11a; 11b); [e] providing said first main electrode (15) adjacent to said fourth semiconductor layer (14); and [f] providing said second main electrode (16) adjacent to said second main surface of said first semiconductor layer (13). Method of manufacturing a diode according to claim 3, comprising the following steps: [a] preparing a semiconductor substrate of a first conductivity type as said first semiconductor layer (13); [b] forming said second semiconductor layer (12c) by growing a plurality of epitaxial layers of said first conductivity type such that an impurity concentration of a second main surface thereof adjacent to a first main surface of said first semiconductor layer (13) is smaller than that of said first semiconductor layer (13), and the impurity concentration gradually decreases from the second main surface thereof to a first main surface opposite said second main surface; [c] forming said third semiconductor layer (11c) by growing a single epitaxial layer of said first conductivity type such that an impurity concentration at a second main surface thereof adjacent to said first main surface of said second semiconductor layer (12c) is equal to that at said first main surface of said second semiconductor layer (12c), and impurity concentration is constant from the second main surface thereof to a first main surface opposite said second main surface, a thickness (D c ) of said third semiconductor layer (11c) being selected to be smaller than or equal to a width (W IRM ) of a depletion layer therein at the time a reverse recovery current reaches a peak when a predetermined reverse voltage is applied to the diode during a reverse recovery operation; [d] forming said fourth semiconductor layer (14) of a second conductivity type adjacent to said first main surface of said third semiconductor layer; [e] providing said first main electrode (15) adjacent to said fourth semiconductor layer (14); and [f] providing said second main electrode (16) adjacent to said second main surface of said first semiconductor layer (13). Method of manufacturing a diode according to claim 1 or 2, comprising the following steps: [a] forming a semiconductor layer of a first conductivity type by ion implantation or impurity diffusion from the side of a second main surface of a first semiconductor substrate so that said semiconductor layer has an impurity concentration gradient such that an impurity concentration thereof decreases from said second main surface thereof to a first main surface opposite said second main surface thereof to form said third semiconductor layer (11) in which the impurity concentration of each of said first and second main surfaces is a predetermined value; [b] forming a semiconductor layer of said first conductivity type by ion implantation or impurity diffusion from the side of a second main surface of a second semiconductor substrate so that said semiconductor layer has an impurity concentration gradient such that the impurity concentration thereof decreases from said second main surface thereof to a first main surface thereof opposite said second main surface to form said second semiconductor layer (12) in which the impurity concentration of each of said first and second main surfaces is a predetermined value; [c] forming said first semiconductor layer (13) of said first conductivity type having an impurity concentration higher than that at said second main surface of said second semiconductor layer (12), said first semiconductor layer (13) being formed from a third semiconductor substrate; [d] laminating said second main surface of said second semiconductor layer (12) to a first main surface of said first semiconductor layer (13); [e] laminating said second main surface of said third semiconductor layer (11) to said first main surface of said second semiconductor layer (12); [f] forming said fourth semiconductor layer (14) of a second conductivity type adjacent to said first main surface of said third semiconductor layer (11) by ion implantation or impurity diffusion from said first main surface of said third semiconductor layer (11); [g] providing said first main electrode (15) adjacent to said fourth semiconductor layer (14); and [h] providing said second main electrode (16) adjacent to a second main surface of said first semiconductor layer (13). Method of manufacturing a diode according to claim 3, comprising the following steps: [a] forming said third semiconductor layer (11c) of a first conductivity type having an impurity concentration which is constant from a first main surface thereof to a second main surface thereof opposite said first main surface, said third semiconductor layer (11c) being formed from a first semiconductor substrate, a thickness (D c ) of said third semiconductor layer (11c) being selected to be smaller than or equal to a width (W IRM ) of a depletion layer therein at the time a reverse recovery current reaches a peak when a predetermined reverse voltage is applied to the diode during a reverse recovery operation; [b] forming a semiconductor layer of said first conductivity type by ion implantation or impurity diffusion from the side of a second main surface of a second semiconductor substrate so that said semiconductor layer has an impurity concentration gradient such that an impurity concentration thereof decreases from said second main surface thereof to a first main surface opposite said second main surface to form said second semiconductor layer (12c) in which the impurity concentration of each of said first and second main surfaces is a predetermined value; [c] forming said first semiconductor layer (13) of said first conductivity type having an impurity concentration higher than that at said second main surface of said second semiconductor layer (12c), said first semiconductor layer (13) being formed from a third semiconductor layer; [d] laminating said second main surface of said second semiconductor layer (12c) to a first main surface of said first semiconductor layer (13); [e] laminating said second main surface of said third semiconductor layer (11c) to said first main surface of said second semiconductor layer (12c); [f] forming said fourth semiconductor layer (14) of a second conductivity type adjacent to said first main surface of said third semiconductor layer (11c) by ion implantation or impurity diffusion from said first main surface of said third semiconductor layer (11c); [g] providing said first main electrode (15) adjacent to said fourth semiconductor layer (14); and [h] providing said second main electrode (16) adjacent to a second main surface of said first semiconductor layer (13). Method of manufacturing a diode according to claim 3, comprising the following steps: [a] forming a semiconductor layer of a first conductivity type by ion implantation or impurity diffusion from the side of a second main surface of a first semiconductor substrate so that said semiconductor layer has an impurity concentration gradient such that the impurity concentration thereof decreases from said second main surface thereof to a first main surface opposite said second main surface to form said second semiconductor layer (12c) in which the impurity concentration of each of said first and second main surfaces is a predetermined value; [b] forming said first semiconductor layer (13) of said first conductivity type having an impurity concentration higher than that at said second main surface of said second semiconductor layer (12c), said first semiconductor layer (13) being formed from a second semiconductor substrate; [c] laminating said second main surface of said second semiconductor layer (12c) to a first main surface of said first semiconductor layer (13); [d] forming said third semiconductor layer (11c) by growing a single epitaxial layer of said first conductivity type so that an impurity concentration at a second main surface thereof is equal to that of said first main surface of said second semiconductor layer (12c) adjacent thereto, and the impurity concentration is constant from said second main surface thereof to a first main surface opposite said second main surface, a thickness (D c ) of said third semiconductor layer (11c) being selected to be smaller than or equal to a width (W IRM ) of a depletion layer therein at the time a reverse recovery current reaches a peak when a predetermined reverse voltage is applied to the diode during a reverse recovery operation; [e] forming said fourth semiconductor layer (14) of a second conductivity type adjacent to said first main surface of said third semiconductor layer (11c) by ion implantation or impurity diffusion from said first main surface of said third semiconductor layer (11c); [f] providing said first main electrode (15) adjacent to said fourth semiconductor layer (14); and [g] providing a second main electrode (16) adjacent to a second main surface of said first semiconductor layer (13). Method of manufacturing a diode according to claim 3, comprising the following steps: [a] forming said third semiconductor layer (11c) of a first conductivity type having an impurity concentration which is constant from a first main surface thereof to a second main surface thereof opposite said first main surface, a thickness (D c ) of said third semiconductor layer (11c) being selected to be smaller than or equal to a width (W IRM ) of a depletion layer therein at the time a reverse recovery current reaches a peak when a predetermined reverse voltage is applied to the diode during a reverse recovery operation; [b] forming a semiconductor layer of said first conductivity type by ion implantation or impurity diffusion from a side of said second main surface of said third semiconductor layer (11c) so that a first main surface of said semiconductor layer is formed adjacent to said second main surface of said third semiconductor layer (11c), said semiconductor layer having an impurity concentration gradient such that an impurity concentration thereof increases from said first main surface thereof to a second main surface opposite said first main surface to form said second semiconductor layer (12c) in which said second main surface thereof has a predetermined impurity concentration; [c] forming said first semiconductor layer (13) by growing an epitaxial layer of said first conductivity type so that a first main surface thereof is formed adjacent to said second main surface of said second semiconductor (12c) layer so as to have an impurity concentration higher than that at said second main surface of said second semiconductor layer (12c), and said first semiconductor layer (13) having an impurity concentration which is constant from said first main surface thereof to a second main surface thereof opposite said first main surface; [d] forming said fourth semiconductor layer (14) of a second conductivity type adjacent to said first main surface of said third semiconductor layer (11c) by ion implantation or impurity diffusion from said first main surface of said third semiconductor layer (11c); [e] providing said first main electrode (15) adjacent to said fourth semiconductor layer (14); and [f] providing said second main electrode (16) adjacent to said second main surface of said first semiconductor layer (13). Method of manufacturing a diode according to claim 1 or 2, comprising the following steps: [a] forming said third semiconductor layer (11b) of a first conductivity type by ion implantation or impurity diffusion from a side of a second main surface of a first semiconductor substrate so that said third semiconductor layer (11b) has an impurity concentration gradient such that an impurity concentration thereof decreases from said second main surface to a first main surface thereof opposite said second main surface, and the impurity concentration of each of said first and second main surfaces is a predetermined value; [b] forming a semiconductor layer of said first conductivity type by ion implantation or impurity diffusion from said second main surface of said third semiconductor layer (11b) so that a first main surface thereof is formed adjacent to said second main surface of said third semiconductor layer (11b), and said semiconductor layer has an impurity concentration gradient such that an impurity concentration thereof increases from said first main surface thereof to a second main surface opposite said first main surface to form said second semiconductor layer (12b) and said first semiconductor layer (13); [c] forming said fourth semiconductor layer (14) of a second conductivity type adjacent to said first main surface of said third semiconductor layer (11b) by ion implantation or impurity diffusion from a side of said first main surface of said third semiconductor layer (11b); [d] providing said first main electrode (15) adjacent to said fourth semiconductor layer (14); and [e] providing said second main electrode (16) adjacent to a second main surface of said first semiconductor layer (13).

Inventors: SOEJIMA NORIYUKI, SOEJIMA, NORIYUKI, Soejima, Noriyuki, Fukuryo Semi-con Eng. Corp.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1996
Main classification: H01L29
IPC codes: H01L21, H01L29