Patent Publication Number: US-2002000572-A1

Title: Soft recovery diode

Description:
[0001] This invention relate to a soft recovery diode.  
       BACKGROUND OF THE INVENTION  
       [0002] Fast operating diodes are widely used in power electronics circuits. For example, there is a DC power supply apparatus which converts a DC voltage obtained by rectifying a commercial AC voltage into a high-frequency voltage in an inverter. The high-frequency voltage is voltage-transformed by a transformer before it is rectified in an output-side rectifier. The resulting DC voltage is applied to a load, e.g. a communication apparatus or a welder, which requires more or less higher DC power. Fast operating diodes may be used in the output-side rectifier of the power supply apparatus of the described type. A diode for use in such output-side rectifier desirably should have a short time during which a reverse recovery current flows after forward-biasing of the diode is switched to reverse-biasing. In other words, such diode should have a prompt recovery characteristic for reducing noise and preventing surge.  
       [0003]FIG. 1 shows an example of prior art prompt-recovery characteristic diodes. The diode shown in FIG. 1 includes a first high-concentration N-type semiconductor layer, e.g. an N-type semiconductor substrate  2 , having an impurity concentration of, e.g. 10 18 /cm 3 . The thickness of the semiconductor substrate  2  is from 250 μm to 400 μm, for example. A second N-type semiconductor layer  4  is epitaxially grown on one of the major surfaces of the substrate  2 . The second N-type layer  4  is a high-concentration, thin layer having a doping concentration of 10 15 /cm 3  and having a thickness of about 5 μm. Over the second N-type semiconductor layer  4 , a third N-type semiconductor layer  6  is epitaxially grown. The third layer  6  has a impurity concentration of, for example, 10 14 /cm 3  and has a thickness of, for example, about 50 μm. A P-type impurity is diffused into the third N-type layer  6  through its surface to thereby change part of the third N-type layer  6  into a P-type semiconductor layer  8  having an impurity concentration of, for example, 10 18 /cm 3  and having a thickness of, for example, about 10 μm. Lattice defects are formed in the remaining third N-type semiconductor layer  6  by, for example, doping it with a heavy metal, e.g. gold or platinum, or by irradiating the layer  6  with an electron beam. A cathode electrode  10  and an anode electrode  12  are formed on the exposed surfaces of the substrate  2  and the P-type layer  8 , respectively, by evaporating, e.g. aluminum.  
       [0004] Let it be assumed that the forward-biased diode of FIG. 1 having its anode  12  placed at a positive potential and having its cathode  10  placed at a negative potential is switched to the reverse-biased condition in which the anode  12  is placed at a negative potential with the cathode  10  being at a positive potential. Holes emitted from the P-type layer  8  in the forward biased state are absorbed by the lattice defects in the third N-type semiconductor layer  6  when the diode is reverse-biased. As a result, current I flowing in the diode rapidly decreases as shown in FIG. 2, and a depletion layer tends to extend in the third N-type layer  6  toward the second N-type layer  4 . Since the concentration of the second N-type layer  4  is higher than that of the third N-type layer  6 , the depletion layer cannot extend into the second N-type layer  4 .  
       [0005] Accordingly, some holes in the second semiconductor layer  4  remain as residual carriers even after the current I reaches its maximum reverse recovery current value I rr  shown in FIG. 2. The residual holes are gradually absorbed into the cathode  10 , so that the reverse recovery current gradually decreases after it reaches the maximum reverse recovery current value of I rr . Then, the voltage V across the diode gradually recovers its nominal value as shown in FIG. 2, and no large oscillations are induced in the voltage V, resulting in reduced noise and no surge.  
       [0006] Two of the semiconductor layers, namely, the second and third semiconductor layers  4  and  6 , of the above-described diode are epitaxially grown. Accordingly, the manufacturing cost is about 1.5 times that of a diode with only one epitaxially grown layer. Furthermore, the manufacture of this type of diode requires doping or electron beam radiation and, therefore, is relatively troublesome. Another problem is increase of a voltage drop V F  across the PN junction formed between the P-type layer  8  and the third N-type layer  6 . Soft recovery can be attained for a diode having a reverse breakdown voltage of 600 V or higher. However, it cannot be attained for a diode having a reverse breakdown voltage of, for example, 400 V or lower. This is because the thickness of the third semiconductor layer  6  is thinner than that of the third layer of a higher reverse breakdown voltage diode, which results in less residual holes or carriers, resulting, in turn, in the current rapidly recovering its nominal value after it reaches the maximum reverse recovery current I rr .  
       [0007] Diodes useable in low-voltage circuitry include a Schottky-barrier diode. An example of Schottky barrier diode is shown in FIG. 3. The Schottky barrier diode shown in FIG. 3 includes a first, high impurity concentration, N-type semiconductor layer or substrate  14  having a concentration of, for example, 10 18 /cm 3 . The substrate  14  has a thickness of from about 250 μm to about 400 μm. On the top surface of the substrate  14 , a second, low impurity concentration, second N-type layer  16  having a concentration of, for example, 10 14 /cm 3  is epitaxially grown to a thickness of about 10 μm. A Schottky barrier metal layer  18  is formed over the top surface of the second N-type layer  16  by evaporation or sputtering of a metal, e.g. molybdenum, tungsten, chromium or titanium. A cathode electrode  20  is formed on the exposed (bottom) surface of the substrate  14 , and an anode electrode  22  is formed on the top surface of the Schottky barrier metallic layer  18 . The electrodes may be formed by evaporating, sputtering or plating a metal, for example, aluminum or nickel.  
       [0008] When the Schottky barrier diode is forward biased with a positive potential applied to the anode  22  and with a negative potential applied to the cathode  20 , only the majority carriers, electrons, in the portion of the second N-type layer  16  near the substrate  14  act as carriers in the layer  16 . The carrier density in the second N-type layer  16  in the forward-biasing condition is shown in FIG. 4. As represented by a curve to in FIG. 4, the carrier density is lower in the region closer to the anode  22  and is higher in the region closer to the cathode  20 .  
       [0009] When the diode is switched from the forward-biasing condition to the reverse-biasing condition, where a negative potential is placed on the anode  22  and a positive potential is on the cathode  20 , a depletion layer extends from the portion of the second N-type semiconductor layer  16  adjacent to the Schottky barrier metal layer  18  as indicated by a line t 1  in FIG. 4. The carrier density as a whole is lower than the density to in the forward-biasing condition. The length of the depletion layer when the maximum reverse recovery current is flowing is L 1 . In the reverse-biasing condition, residual carriers are present in a region in the vicinity of the substrate  14  as indicated by shading in FIG. 4. The residual carriers are gradually attracted toward the cathode  20 , which provides the diode with a gradual recovery characteristic. The reverse breakdown voltage of Schottky barrier diodes of this type is 60 V at the highest or lower.  
       [0010] Therefore, an object of the present invention is to provide low-cost, fast, high reverse breakdown voltage, soft recovery diodes.  
       SUMMARY OF THE INVENTION  
       [0011] A soft recovery diode according to the present invention has a first semiconductor layer of one conductivity type. A second semiconductor layer of the same conductivity type as the first semiconductor layer is disposed on and in contact with one of the two opposing major surfaces of the first semiconductor layer. The second layer has a lower impurity concentration than the first layer. A plurality of mutually spaced first metallic layers are disposed in one of the opposing major surfaces of the second semiconductor layer opposite to said first semiconductor layer. A second metallic layer is disposed on at least those portions of the one surface of the second semiconductor layer in which the first metallic layers are not disposed. The first and second metallic layers provide barriers of different heights. The barrier provided by the first metallic layers may be higher than the barrier provided by the second metallic layer. Conversely, the barrier formed by the second metallic layer may be higher than the barrier formed by the first metallic layers.  
       [0012] When the soft recovery diode of the above-described arrangement is forward biased, majority carriers in the first semiconductor layer provide carriers for the second semiconductor layer in those portions located between the first semiconductor layer and the metallic layer providing a low barrier. Accordingly, the closer to the first semiconductor layer, the higher the carrier density is in the second semiconductor layer, and the closer to the first and second metallic layers, the carrier density is lower. On the other hand, into the regions of the second semiconductor layer between the first semiconductor layer and the metallic layer providing a higher barrier, minority carriers are injected from the higher-barrier providing metallic layer, and, therefore, the closer to the higher-barrier providing layer, the carrier density is higher, and the closer to the first semiconductor layer, the carrier density is lower.  
       [0013] When the diode is switched from the forward-biasing condition to the reverse-biasing condition, a depletion layer expands in the second semiconductor layer from the region adjacent to the lower-barrier providing metallic layer. After the reverse current reaches the maximum reverse recovery current value, the residual carriers are gradually attracted toward the first semiconductor layer. In contrast, the minority carriers injected from the higher-potential-barrier providing metallic layer recombine with the majority carriers injected from the first semiconductor layer, resulting in extending of the depletion layer toward the first semiconductor layer. When the reverse current reaches the maximum reverse recovery current, there are no minority carriers around the first semiconductor layer, and, therefore, once the maximum reverse recovery current is attained, the reverse current promptly decreases to zero. In other words, a faster reverse recovery characteristic is realized beneath the higher-barrier metallic layer, while the reverse recovery is gradual beneath the lower-barrier metallic layer.  
       [0014] The total area of those portions of the first metallic layers which are in contact with the second semiconductor layer may be larger than the total area of the portions of the second metallic layer contacting the second semiconductor layer. Conversely, the total area of the second metallic layer portions contacting the second semiconductor layer may be larger than the total area of those portions of the first metallic layers which are in contact with the second semiconductor layer. In other words, the total area of the higher-barrier providing metallic layer contacting the second semiconductor layer may be larger or smaller than the total area of the lower-barrier providing metallic layers contacting the second semiconductor layer. Then, if the total area of the high-barrier providing layer portions contacting the second semiconductor layer is larger, a resulting diode exhibit relatively fast recovery and has a high breakdown voltage, and, if the total area of the high-barrier providing layer portions contacting the second semiconductor layer is smaller, a resulting diode exhibits soft recovery and has a relatively low breakdown voltage. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015]FIG. 1 is a cross-sectional view of a prior art soft recovery diode;  
     [0016]FIG. 2 shows the voltage and current characteristics of the diode of FIG. 1;  
     [0017]FIG. 3 is a cross-sectional view of a prior art Schottky barrier diode;  
     [0018]FIG. 4 shows the carrier distribution of the Schottky barrier diode of FIG. 3;  
     [0019]FIGS. 5A, 5B,  5 C and  5 D are cross-sectional views in different manufacturing steps of a soft recovery diode according to one embodiment of the present invention;  
     [0020]FIG. 6 illustrates the carrier distribution of the soft recovery diode manufactured by the process illustrated in FIGS. 5A through 5D; and  
     [0021]FIG. 7 is a cross-sectional view of a modification of the soft recovery diode shown in FIGS. 5A through 5D. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
     [0022]FIG. 5D shows a completed soft recovery diode according to a first embodiment of the present invention. As shown, the soft recovery diode according the first embodiment has a first semiconductor layer, e.g. a semiconductor substrate  30 . The substrate  30  is of one conductivity type, e.g. N-type, has a high impurity concentration of, for example, 10 18 /cm 3 , and has a thickness of, for example, between about 250 μm and about 400 μm. A second semiconductor layer  32  is epitaxially grown on one of the major surfaces of the substrate  30 . The second layer  32  is of the same conductivity type as the substrate  30 , i.e. N-type in the illustrated example, and has an impurity concentration of, for example, 10 14 /cm 3 , which is lower than that of the substrate  30 . The thickness of the second layer  32  is, for example, between about 10 μm and about 20 μm, which is smaller than the thickness of the substrate  30 .  
     [0023] A plurality of first metallic layers  34  are formed to extend into the second semiconductor layer  32  from its major surface opposite to the substrate  30 . The material of the first metallic layers  34  is a metal forming a high barrier with the second layer  32 . The metal of the first metallic layers  34  may be, for example, platinum which can form a barrier as high as 0.8 eV. The thickness of the first metallic layers  34 , i.e. the depth to which the first metallic layers  34  extend into the second semiconductor layer  32 , is, for example, about 0.01 μm. The first metallic layers  34  are spaced from each other by a predetermined spacing.  
     [0024] A second metallic layer  36  is disposed on the surface of the second semiconductor layer  32  in which the first metallic layers  34  are formed, so that the exposed surface of the second layer  32  and the exposed surfaces of the first metallic layers  34  are covered by the second metallic layer  36 .  
     [0025] The metal of the second metallic layer  36  is a metal which can form a relatively low barrier of a height of, for example, between 0.6 eV and 0.7 eV with respect to semiconductor. Such metal may be molybdenum, tungsten, chromium or titanium. The second metallic layer  36  provides a Schottky barrier metallic layer.  
     [0026] A cathode electrode layer  38  is disposed on the major surface of the substrate  30  opposite to the second semiconductor layer  32 . An anode electrode layer  40  is disposed on the surface of the second metallic layer  36  opposite to said second semiconductor layer  32 .  
     [0027] This soft recovery diode may be manufactured in a following manner.  
     [0028] First, as shown in FIG. 5A, on one of the two opposing major surfaces of the semiconductor substrate  30 , the second semiconductor layer  32  is disposed by epitaxial growing. Then, an oxide film is disposed on the surface of the second semiconductor layer  32  opposite to the substrate  30 , and openings are formed in the oxide film at locations where the first metallic layers  34  are to be formed. Platinum is, then, evaporated or sputtered onto the assembly including the substrate  30 , the second semiconductor layer  32  and the oxide film with openings formed therein, and, thereafter, the assembly is heated to embed the evaporated or sputtered platinum into the second semiconductor layer  32 , which results in the first metallic layers  34  embedded in the second semiconductor layer  32 . After that, the oxide film and the metal adhering to the surface of the exposed surface portions of the second semiconductor layer  32  are removed. Then, the exposed surface portions of the second semiconductor layer  32  and the surfaces of the first metallic layers  34  are leveled off. See FIG. 5B.  
     [0029] After that, as shown in FIG. 5C, the second metal is evaporated or sputtered onto the leveled surfaces of the semiconductor layer  32  and the first metallic layers  34  to dispose the second metallic layer  36 .  
     [0030] After that, the cathode and anode electrode layers  38  and  40 , respectively, of a metal, for example, aluminum or nickel, are disposed on the major surface of the substrate  30  opposite to the second semiconductor layer  32  and on the exposed major surface of the second metallic layer  36 , respectively, by evaporation, sputtering or plating, to complete the diode shown in FIG. 5D.  
     [0031] When the Schottky barrier diode of the above-described arrangement is forward biased by placing a positive potential on the anode electrode  40  and a negative potential on the cathode electrode  38 , electrons or majority carriers from the semiconductor substrate  30  are distributed in those portions of the second semiconductor layer  32  between those portions of the second metallic layer  36  which are not in contact with the first metallic layers  34  and the semiconductor substrate  30 . Since the first metallic layers  34  form a high barrier, minority carriers, i.e. holes, are injected from the first metallic layers  34  into the second semiconductor layer  32 . Then, the carrier distribution as represented by a line to in FIG. 6 is produced in the second semiconductor layer  32  between each of the first metallic layers  34  and the semiconductor substrate  30 , in which the carrier density is higher in the portion closer to the first metallic layer  34  and decreases toward the substrate  30 .  
     [0032] When the above-described forward-biased diode is switched to a reverse-biased condition, by applying a negative voltage to the anode electrode  40  and a positive potential to the cathode electrode  38 , a depletion layer extends from the second metallic layer  36  side toward the semiconductor substrate  30  side in the portions of the second semiconductor layer  32  between the portions of the second metallic layer  36  which are not in contact with the first metallic layers  34  and the substrate  30 , for a reason similar to the one described with reference to FIGS. 3 and 4. After the maximum reverse recovery current I rr  flows, the residual carriers gradually move into the semiconductor substrate  30 , so that the current becomes zero. The holes injected from the first metallic layers  34  recombine with the electrons injected from the substrate  30  to disappear, and the depletion layer extends toward the substrate  30 . When the reverse current reaches its maximum reverse recovery current value, there are no residual holes in the portion of the second semiconductor layer  32  near the substrate  30  as is seen from a carrier distribution curve to in FIG. 6, and, therefore, the current rapidly reaches zero.  
     [0033] As described above, in the portions of the second semiconductor layer  32  beneath the first metallic layers  34 , a fast reverse recovery characteristic is realized, while, in the portions of the layer  32  directly contacting the second metallic layer  36  with no intervening first metallic layers  34 , gradual recovery takes place.  
     [0034] With the total area of the portions of the first metallic layers  34  contacting the second semiconductor layer  32  being greater than the total area of the portions of the second metallic layer  36  which are not in contact with the first metallic layer  34 , a diode with a relatively fast recovery and with a high breakdown voltage, e.g. 400 V, results. On the other hand, if the total area of the portions of the first metallic layers  34  contacting the second semiconductor layer  32  is smaller than that of the portions of the second metallic layer  36  which are not in contact with the first metallic layers  34 , the diode exhibits a soft recovery characteristic and has a high breakdown voltage of, e.g. 200 V, which is high relative to a prior art Schottky barrier diode.  
     [0035] In the above-described embodiment, a high barrier providing metal, platinum, is used for the first metallic layers  34  and a low barrier metal for the second metallic layer  36 . Instead, a low barrier metal may be used for the first metallic layers  34  with a high barrier metal used for the second metallic layer  36 .  
     [0036] In the above-described embodiment, the second metallic layer  36  is disposed to overlie both the top surfaces of the first metallic layers  34  and the exposed top surface portions of the second semiconductor layer  32 , but second metallic layers  36   a  may be disposed only on the exposed top surface portions of the second semiconductor layer  32  where the fist metallic layers  34  are not disposed, as shown in FIG. 7. The second metallic layers  35   a  are formed of a metal providing a low barrier, such as molybdenum, tungsten or chromium. After that, the anode electrode layer  40  is disposed to overlie the first metallic layers  34  and the second metallic layers  35   a . In this modification, too, the first metallic layers  34  may be of a lower-barrier forming metal with the second metallic layer  35   a  formed of a higher-barrier forming metal.