Patent Publication Number: US-2019189854-A1

Title: Semiconductor device, light-emitting device chip, optical print head, and image forming device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device including a light-emitting thyristor, a light-emitting device chip including the semiconductor device arranged on a substrate part, an optical print head including the light-emitting device chip, and an image forming device including the optical print head. 
     2. Description of the Related Art 
     Conventionally, image forming devices for forming an image on a print medium by means of an electrophotographic process are widespread. In the image forming device, an electrostatic latent image is formed on the surface of a photosensitive drum by irradiating the surface with light emitted from an optical print head including a plurality of light-emitting devices arranged in a line, a developing agent image is formed by developing the electrostatic latent image, and the developing agent image is transferred onto a print medium and fixed. As the light-emitting devices included in the optical print head, light-emitting thyristors as three-terminal light-emitting devices are well known (see Japanese Patent Application Publication No. 2010-239084, for example). 
     However, a more excellent light emission property is being required of the conventional light-emitting thyristors. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a semiconductor device including a light-emitting thyristor having an excellent light emission property, a light-emitting device chip including the semiconductor device arranged on a substrate part, an optical print head including the light-emitting device chip, and an image forming device including the optical print head. 
     A semiconductor device according to an aspect of the present invention includes: 
     a light-emitting thyristor including a first semiconductor layer of a first conductivity type, a second semiconductor layer of a second conductivity type arranged adjacent to the first semiconductor layer, a third semiconductor layer of the first conductivity type arranged adjacent to the second semiconductor layer, and a fourth semiconductor layer of the second conductivity type arranged adjacent to the third semiconductor layer; 
     a first electrode electrically connected with the first semiconductor layer; 
     a second electrode electrically connected with the second semiconductor layer or the third semiconductor layer; and 
     a third electrode electrically connected with the fourth semiconductor layer. 
     The first semiconductor layer includes: 
     a first layer electrically connected with the first electrode; 
     a second layer having a first band gap wider than a second band gap of the second semiconductor layer and a third band gap of the third semiconductor layer; and 
     a third layer having a first impurity concentration higher than a second impurity concentration of the second semiconductor layer and a third impurity concentration of the third semiconductor layer, the third layer having a fourth band gap narrower than or equal to the second band gap of the second semiconductor layer and the third band gap of the third semiconductor layer. 
     According to the present invention, a semiconductor device and a light-emitting device chip including a light-emitting thyristor having an excellent light emission property can be provided. Further, the quality of printed images can be improved in an image forming device employing an optical print head including such a light-emitting device chip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the attached drawings: 
         FIG. 1  is a schematic plan view showing the structure of a semiconductor device in a first embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional view showing the structure of the semiconductor device in the first embodiment, namely, cross-sectional structure at sections of  FIG. 1  along the line A-B-C viewed in the directions of arrows in  FIG. 1 ; 
         FIG. 3  is a diagram showing an example of an impurity concentration and an Al composition ratio of each semiconductor layer in a light-emitting thyristor of the semiconductor device in  FIG. 2 ; 
         FIG. 4  is a schematic cross-sectional view showing the structure of a semiconductor device of a first modification of the first embodiment; 
         FIG. 5  is a schematic cross-sectional view showing the structure of a semiconductor device of a second modification of the first embodiment; 
         FIG. 6  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor of the semiconductor device in  FIG. 5 ; 
         FIG. 7  is a schematic cross-sectional view showing the structure of a semiconductor device of a third modification of the first embodiment; 
         FIG. 8  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor of the semiconductor device in  FIG. 7 ; 
         FIG. 9  is a schematic cross-sectional view showing the structure of a semiconductor device of a fourth modification of the first embodiment; 
         FIG. 10  is a schematic cross-sectional view showing the structure of a semiconductor device in a second embodiment of the present invention; 
         FIG. 11  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor of the semiconductor device in  FIG. 10 ; 
         FIG. 12  is a schematic cross-sectional view showing the structure of a semiconductor device of a first modification of the second embodiment; 
         FIG. 13  is a schematic cross-sectional view showing the structure of a semiconductor device of a second modification of the second embodiment; 
         FIG. 14  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor of the semiconductor device in  FIG. 13 ; 
         FIG. 15  is a schematic cross-sectional view showing the structure of a semiconductor device of a third modification of the second embodiment; 
         FIG. 16  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor of the semiconductor device in  FIG. 15 ; 
         FIG. 17  is a schematic cross-sectional view showing the structure of a semiconductor device of a fourth modification of the second embodiment; 
         FIG. 18  is a schematic cross-sectional view showing the structure of a semiconductor device in a third embodiment of the present invention; 
         FIG. 19  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor of the semiconductor device in  FIG. 18 ; 
         FIG. 20  is a schematic cross-sectional view showing the structure of a semiconductor device of a first modification of the third embodiment; 
         FIG. 21  is a schematic cross-sectional view showing the structure of a semiconductor device of a second modification of the third embodiment; 
         FIG. 22  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor of the semiconductor device in  FIG. 21 ; 
         FIG. 23  is a schematic cross-sectional view showing the structure of a semiconductor device of a third modification of the third embodiment; 
         FIG. 24  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor of the semiconductor device in  FIG. 23 ; 
         FIG. 25  is a schematic cross-sectional view showing the structure of a semiconductor device of a fourth modification of the third embodiment; 
         FIG. 26  is a schematic perspective view showing the structure of a substrate unit as a principal part of an optical print head in a fourth embodiment of the present invention; 
         FIG. 27  is a schematic cross-sectional view showing the structure of the optical print head in the fourth embodiment; and 
         FIG. 28  is a schematic cross-sectional view showing the structure of an image forming device in a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Semiconductor devices, light-emitting device chips, an optical print head and an image forming device according to embodiments of the present invention will be described below with reference to the accompanying drawings. In the drawings, the same components are assigned the same reference character. The following embodiments are just examples for the purpose of illustration and a variety of modifications are possible within the scope of the present invention. 
     In a first embodiment ( FIG. 1  to  FIG. 9 ), a second embodiment ( FIG. 10  to  FIG. 17 ) and a third embodiment ( FIG. 18  to  FIG. 25 ), the semiconductor devices and the light-emitting device chips will be described. The semiconductor device includes one or more light-emitting thyristors. The semiconductor device may include a plurality of light-emitting thyristors arranged in a line. The light-emitting device chip includes a substrate part and the semiconductor device arranged on the substrate part. The light-emitting device chip may include a semiconductor integrated circuit part (referred to also as a “drive IC part”) as a drive circuit for lighting up and extinguishing the light-emitting thyristors of the semiconductor device. The light-emitting device chip including the semiconductor device and the drive IC part is referred to also as a “semiconductor composite device”. 
     In a fourth embodiment ( FIG. 26  to  FIG. 27 ), the optical print head including the light-emitting device chip in any one of the first to third embodiments will be described. The optical print head includes one or more light-emitting device chips. The optical print head is an exposure device as an electrostatic latent image formation means in an image forming device that forms an image made of a developing agent on a print medium by means of an electrophotographic process. The optical print head may include a plurality of light-emitting device chips arranged in a line. 
     In a fifth embodiment ( FIG. 28 ), the image forming device including the optical print head in the fourth embodiment will be described. The image forming device is a device for forming an image made of a developing agent on a print medium by means of the electrophotographic process, such as a printer, a copy machine, a facsimile machine, a multi-function peripheral (MFP) or the like, for example. 
     (1) First Embodiment 
     (1-1) Configuration 
       FIG. 1  is a schematic plan view showing the structure of a semiconductor device  1000  in the first embodiment.  FIG. 2  is a schematic cross-sectional view showing the structure of the semiconductor device  1000  in the first embodiment, namely, cross-sectional structure at sections of  FIG. 1  along the line A-B-C viewed in the directions of arrows in  FIG. 1 . The semiconductor device  1000  in the first embodiment is arranged on a substrate part  101 . The substrate part  101  includes, for example, a substrate  102  and a planarization layer  103  formed on the substrate  102  as shown in  FIG. 2 . A light-emitting device chip  100  includes the substrate part  101  and the semiconductor device  1000  arranged on the substrate part  101 . 
     For example, a Si (silicon) substrate, an IC (integrated circuit) substrate, a glass substrate, a ceramic substrate, a plastic substrate, a metal substrate or the like is usable as the substrate  102 . In the first embodiment, the substrate  102  is an IC substrate including the drive IC part for driving the light-emitting thyristors as the three-terminal light-emitting devices and an external connection pad  104  used for wiring to an external device. 
     The planarization layer  103  has a smooth surface on which the semiconductor device  1000  is arranged. The planarization layer  103  is an inorganic film or an organic film. In a case where a top surface of the substrate  102  is smooth, it is also possible to provide the semiconductor device  1000  on the top surface of the substrate  102  without providing the planarization layer  103 . 
     As shown in  FIG. 1 , the semiconductor device  1000  includes a plurality of light-emitting thyristors  10  arranged in a line. The semiconductor device  1000  is referred to also as a “light-emitting device array” or a “light-emitting thyristor array”. Further, the light-emitting device chip  100  is referred to also as a “light-emitting device array chip” or a “light-emitting thyristor array chip”. Incidentally, an insulation film  71  (shown in  FIG. 2 ) is not shown in  FIG. 1  for easy understanding of the structure of the semiconductor device  1000 . 
     The light-emitting thyristor  10  is famed on a growth substrate as a manufacturing substrate, for example. In a case where the light-emitting thyristor  10  is formed of an AlGaAs (aluminum gallium arsenide)-based semiconductor material, a GaAs (gallium arsenide) substrate can be used as the growth substrate. The light-emitting thyristor  10  is famed on the growth substrate by means of epitaxial growth, for example. The light-emitting thyristor  10  is formed by, for example, peeling off an epitaxial film, as a semiconductor thin film having a laminated structure of semiconductor layers, from the growth substrate, sticking the epitaxial film on the surface of the planarization layer  103  on the substrate  102 , and processing the epitaxial film. The epitaxial film placed on the surface of the planarization layer  103  is fixed to the planarization layer  103  by intermolecular force or the like. 
     As shown in  FIG. 1 , the semiconductor device  1000  includes the plurality of light-emitting thyristors  10 . As shown in  FIG. 2 , each light-emitting thyristor  10  includes a first semiconductor layer  1040  of a first conductivity type, a second semiconductor layer  1030  of a second conductivity type different from the first conductivity type arranged adjacent to the first semiconductor layer  1040 , a third semiconductor layer  1020  of the first conductivity type arranged adjacent to the second semiconductor layer  1030 , and a fourth semiconductor layer  1010  of the second conductivity type arranged adjacent to the third semiconductor layer  1020 . 
     In the semiconductor device  1000 , the first semiconductor layer  1040  of the first conductivity type is a P-type semiconductor layer, the second semiconductor layer  1030  of the second conductivity type is an N-type gate layer, the third semiconductor layer  1020  of the first conductivity type is a P-type gate layer, and the fourth semiconductor layer  1010  of the second conductivity type is an N-type semiconductor layer. 
     Further, as shown in  FIG. 2 , the semiconductor device  1000  includes an anode electrode  61 A as a first electrode electrically connected with the first semiconductor layer  1040 , a gate electrode  51 G as a second electrode electrically connected with the third semiconductor layer (P-type gate layer)  1020 , and a cathode electrode  41 K as a third electrode electrically connected with the fourth semiconductor layer  1010 . The anode electrode  61 A is electrically connected with an anode terminal  62 A (shown in  FIG. 1 ) of the substrate part  101 . The gate electrode  51 G is electrically connected with a gate terminal  53 G (shown in  FIG. 1 ) of the substrate part  101  by gate wiring  52 G. The cathode electrode  41 K is connected with a cathode terminal  43 K (shown in  FIG. 1 ) of the substrate part  101  by cathode wiring  42 K. 
     The P-type first semiconductor layer  1040  includes an anode layer  1043  as a first layer electrically connected with the anode electrode  61 A, an electron cladding layer (barrier layer)  1042  as a second layer arranged adjacent to the anode layer  1043 , and an active layer  1041  as a third layer arranged adjacent to the electron cladding layer  1042 . 
     The N-type fourth semiconductor layer  1010  includes a cathode layer  1011  electrically connected with the cathode electrode  41 K and a hole cladding layer  1012  arranged between the cathode layer  1011  and the third semiconductor layer (P-type gate layer)  1020 . 
       FIG. 3  is a diagram showing an example of an impurity concentration IM (cm 3 ) and an Al (aluminum) composition ratio CR of each semiconductor layer in the light-emitting thyristor  10  of the semiconductor device  1000 . 
     In the first embodiment, let IMpg represent the impurity concentration of the third semiconductor layer (P-type gate layer)  1020 , IMng represent the impurity concentration of the second semiconductor layer (N-type gate layer)  1030 , and IMac 1  represent the impurity concentration of the active layer (third layer)  1041  of the first semiconductor layer  1040 , the light-emitting thyristor  10  is formed so as to satisfy the following conditional expressions (1) and (2): 
         IMpg&lt;IMac 1  (1)
 
         IMng&lt;IMac 1  (2)
 
     In  FIG. 3 , the following numerical examples are shown as the impurity concentrations: 
         IMac 1≈1×10 19 (cm −3 )
 
         IMpg≈ 5×10 17 (cm −3 )
 
         IMng≈ 2×10 17 (cm −3 )
 
     However, the impurity concentrations are not limited to the example of  FIG. 3 . 
     In the light-emitting thyristor  10 , the reason for setting the impurity concentration IMpg of the third semiconductor layer (P-type gate layer)  1020  and the impurity concentration IMng of the second semiconductor layer (N-type gate layer)  1030  at low values and setting the impurity concentration IMac 1  of the active layer  1041  of the first semiconductor layer  1040  at a high value is to increase the luminous efficiency by lowering the occurrence probability of recombination of an electron and a hole in the third semiconductor layer  1020  and the second semiconductor layer  1030  and raising the occurrence probability of the recombination of an electron and a hole in the active layer  1041 . 
     Further, in the first embodiment, let CRpg represent the Al composition ratio of the third semiconductor layer (P-type gate layer)  1020 , CRng represent the Al composition ratio of the second semiconductor layer (N-type gate layer)  1030 , CRac 1  represent the Al composition ratio of the active layer (third layer)  1041 , and CRcl 1  represent the Al composition ratio of the electron cladding layer (second layer)  1042 , the light-emitting thyristor  10  is formed so as to satisfy the following conditional expression (3): 
         CRac 1 =CRng=CRpg&lt;CRcl 1  (3)
 
     However, CRac1=CRng=CRpg in the expression (3) does not necessarily have to be satisfied. The light-emitting thyristor  10  may also be famed so as to satisfy the following conditional expressions (4) and (5) instead of the conditional expression (3): 
         CRac 1 ≤CRpg&lt;CRcl 1  (4)
 
         CRac 1 ≤CRng&lt;CRcl 1  (5)
 
     The Al composition ratio CR of each semiconductor layer of the light-emitting thyristor  10  corresponds to a band gap BG of each semiconductor layer. Put another way, the band gap BG of each semiconductor layer of the light-emitting thyristor  10  increases with the increase in the Al composition ratio CR of the semiconductor layer, and the band gap BG of each semiconductor layer decreases with the decrease in the Al composition ratio CR of the semiconductor layer. Thus, the conditional expressions (3) to (5) are equivalent to the following conditional expressions (6) to (8) using the band gap: 
         BGac 1 =BGng=BGpg&lt;BGcl 1  (6)
 
         BGac 1 ≤BGpg&lt;BGcl 1  (7)
 
         BGac 1 ≤BGng&lt;BGcl 1  (8)
 
     where BGpg represents the band gap of the third semiconductor layer (P-type gate layer)  1020 , BGng represents the band gap of the second semiconductor layer (N-type gate layer)  1030 , BGac 1  represents the band gap of the active layer  1041 , and BGcl 1  represents the band gap of the electron cladding layer  1042 . 
     In  FIG. 3 , the following numerical examples are shown as the Al composition ratios: 
         CRac 1 =CRng=CRpg≈ 0.15 
         CRcl 1≈0.40
 
     However, the Al composition ratios are not limited to the example of  FIG. 3 . 
     The semiconductor materials forming the light-emitting thyristor  10  are, for example, InP (indium-phosphorous)-based semiconductor materials, AlGaAs-based semiconductor materials, AlInGaP (aluminum-indium-gallium-phosphorous)-based semiconductor materials, or the like. 
     In a case where the light-emitting thyristor  10  is formed with AlGaAs-based semiconductor materials, each semiconductor layer can be configured as below, for example. The cathode layer  1011  of the fourth semiconductor layer  1010  is famed with an N-type Al 0.25 Ga 0.75 As layer, and the hole cladding layer  1012  of the fourth semiconductor layer  1010  is famed with an N-type Al 0.4 Ga 0.6 As layer. The third semiconductor layer (P-type gate layer)  1020  is famed with a P-type Al 0.15 Ga 0.85 As layer, and the second semiconductor layer (N-type gate layer)  1030  is famed with an N-type Al 0.15 Ga 0.85 As layer. Further, in the first semiconductor layer  1040 , the active layer  1041  is famed with a P-type Al 0.15 Ga 0.85 As layer, the electron cladding layer  1042  is formed with a P-type Al 0.4 Ga 0.6 As layer, and the anode layer  1043  is formed with a P-type Al 0.25 Ga 0.75 As layer. 
     In a case where AlGaAs is expressed as Al y Ga 1-y As (0≤y≤1), y is the Al composition ratio. The Al composition ratio CRcl 1  of the electron cladding layer  1042  is desired to be within a range from 0.2 to 1.0. The electron cladding layer  1042  whose Al composition ratio CRcl 1  is 1.0 is an AlAs layer since the composition ratio of Ga is 0. 
     Further, the Al composition ratio CRac 1  of the active layer  1041  is desired to be within a range from 0.14 to 0.18, and the Al composition ratios CRng and CRpg of the second semiconductor layer (N-type gate layer)  1030  and the third semiconductor layer (P-type gate layer)  1020  are desired to be within a range from 0.14 to 0.3. 
     The gate electrode  51 G and the anode electrode  61 A can be formed with metal capable of forming an ohmic contact with P-type AlGaAs, such as Ti (titanium), Pt (platinum), Au (gold), Ni (nickel) or Zn (zinc), alloy of these metals, a laminated structure of these metals or alloys, or the like. The cathode electrode  41 K can be formed with metal capable of forming an ohmic contact with N-type AlGaAs, such as Au, Ge (germanium), Ni or Pt, alloy of these metals, a laminated structure of these metals or alloys, or the like. 
     The insulation film  71  can be formed with an inorganic insulation film such as a SiN film (silicon nitride film) or a SiO 2  film (silicon dioxide film), or an organic insulation film such as a polyimide film. 
     (1-2) Operation 
     In the semiconductor device  1000 , the drive IC part supplies gate current from the gate electrode  51 G to the cathode electrode  41 K, and thereby the light-emitting thyristor  10  is brought into a lighted state (light emission state), i.e., an on state. Further, the drive IC part lets current higher than or equal to a holding current flow between the anode electrode  61 A and the cathode electrode  41 K, and thereby the lighted state is maintained. In the first embodiment, the light emission from the light-emitting thyristor  10  is mainly caused by the recombination of a hole in the active layer  1041  and an electron moving from the second semiconductor layer (N-type gate layer)  1030  into the active layer  1041 . Light generated by the recombination passes through the electron cladding layer  1042  and the anode layer  1043  and then exits upward (upward in  FIG. 2 ) from the top surface of the anode layer  1043 . 
     When the light-emitting thyristor  10  is in the lighted state, the recombination of an electron and a hole occurs also in the third semiconductor layer (P-type gate layer)  1020  and the second semiconductor layer (N-type gate layer)  1030 . However, carrier mobility in the active layer  1041  is lower than that in the P-type and N-type gate layers since the impurity concentration IMac 1  of the active layer  1041  is set higher than the impurity concentrations IMpg and IMng of the third semiconductor layer (P-type gate layer)  1020  and the second semiconductor layer (N-type gate layer)  1030  as indicated by the aforementioned conditional expressions (1) and (2). Thus, in the active layer  1041 , the recombination occurs at an occurrence probability higher than occurrence probabilities of the recombination in the third semiconductor layer (P-type gate layer)  1020  and the second semiconductor layer (N-type gate layer)  1030 . Namely, if the impurity concentration IMac 1  of the active layer  1041  is set higher than the impurity concentrations IMpg and IMng of the third semiconductor layer (P-type gate layer)  1020  and the second semiconductor layer (N-type gate layer)  1030 , the concentration of carries (holes in  FIG. 2 ) in the active layer  1041  increases, and accordingly, the occurrence probability of the recombination of a hole and an electron increases. 
     Further, in a case where the band gap BGcl 1  of the electron cladding layer  1042  is wider than the band gaps BGng and BGpg of the second semiconductor layer (N-type gate layer)  1030  and the third semiconductor layer (P-type gate layer)  1020  as indicated by the aforementioned conditional expression (6) or conditional expressions (7) and (8), electrons that have moved from the second semiconductor layer (N-type gate layer)  1030  to the active layer  1041  are received by the electron cladding layer  1042 , by which the amount of electrons leaking from the electron cladding layer  1042  to the anode layer  1043  is reduced. Namely, since the electron cladding layer  1042  with the wide band gap has the function as a barrier layer limiting the passage of electrons, the leakage of the electrons to the anode layer  1043  that have moved from the second semiconductor layer (N-type gate layer)  1030  to the active layer  1041  is reduced. This will be referred to as an “electron confinement effect”. Accordingly, the decrease in the amount of electrons in the active layer  1041  is inhibited and the occurrence probability of the recombination of a hole and an electron in the active layer  1041  increases. 
     (1-3) Effect 
     As described above, in the semiconductor device  1000 , the electron confinement effect in the active layer  1041  is achieved by the electron cladding layer  1042  satisfying BGac 1 &lt;BGcl 1  as indicated by the aforementioned conditional expression (6) or conditional expressions (7) and (8). With this electron confinement effect, the probability of the recombination of an electron heading from the cathode layer  1011  towards the anode layer  1043  with a hole in the active layer  1041  increases and the luminous efficiency rises, and accordingly, the amount of light emission increases. 
     Further, in the semiconductor device  1000 , the impurity concentration IMpg of the third semiconductor layer (P-type gate layer)  1020  and the impurity concentration IMng of the second semiconductor layer (N-type gate layer)  1030  are set low and the impurity concentration IMac 1  of the active layer  1041  is set high as indicated by the conditional expressions (1) and (2). Thus, the carrier mobility in the third semiconductor layer (P-type gate layer)  1020  and the second semiconductor layer (N-type gate layer)  1030  gets high and the occurrence probability of the recombination in the third semiconductor layer (P-type gate layer)  1020  and the second semiconductor layer (N-type gate layer)  1030  gets low. Meanwhile, the carrier mobility in the active layer  1041  gets low and the occurrence probability of the recombination in the active layer  1041  gets high. Therefore, the occurrence probability of the recombination of a hole and an electron in the active layer  1041  increases and the luminous efficiency rises, and accordingly, the amount of light emission increases. 
     Furthermore, in the semiconductor device  1000 , the active layer  1041  is provided in an upper part (i.e., on a side farther from the substrate part  101 ) of the light-emitting thyristor  10  as a semiconductor laminated structure. Since the light generated in the active layer  1041  is extracted in the upward direction in  FIG. 2  as above, absorption of the light generated in the active layer  1041  is reduced and light extraction efficiency is increased, and accordingly, the amount of light emission increases. 
     Ad described above, according to the semiconductor device  1000  and the light-emitting device chip  100  in the first embodiment, the amount of light emission increases in comparison with the conventional gate light emission type light-emitting thyristors. 
     (1-4) First Modification of First Embodiment 
       FIG. 4  is a schematic cross-sectional view showing the structure of a semiconductor device  1100  of a first modification of the first embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C). The semiconductor device  1100  differs from the semiconductor device  1000  shown in  FIG. 2  in that a second semiconductor layer (N-type gate layer)  1130  is formed in a large region similar to a third semiconductor layer (P-type gate layer)  1120  (i.e., a large region including a formation region of a gate electrode  51 G) and the gate electrode  51 G is formed on the second semiconductor layer (N-type gate layer)  1130 . Except this feature, the semiconductor device  1100  and a light-emitting device chip  110  in  FIG. 4  are the same as the semiconductor device  1000  and the light-emitting device chip  100  in  FIG. 2 . 
     A light-emitting thyristor  11  of the semiconductor device  1100  in  FIG. 4  includes a P-type first semiconductor layer  1140 , the N-type second semiconductor layer (N-type gate layer)  1130 , the P-type third semiconductor layer (P-type gate layer)  1120 , and an N-type fourth semiconductor layer  1110 . The first semiconductor layer  1140  includes an anode layer  1143  as a first layer, an electron cladding layer (barrier layer)  1142  as a second layer, and an active layer  1141  as a third layer. The fourth semiconductor layer  1110  includes a cathode layer  1111  and a hole cladding layer  1112 . The first to fourth semiconductor layers  1140 ,  1130 ,  1120  and  1110  of the light-emitting thyristor  11  in  FIG. 4  are formed with the same semiconductor materials as the first to fourth semiconductor layers  1040 ,  1030 ,  1020  and  1010  of the light-emitting thyristor  10  in  FIG. 2 . Thus, the light-emitting thyristor  11  in  FIG. 4  satisfies the aforementioned conditional expressions (1) to (8) similarly to the light-emitting thyristor  10  in  FIG. 2 . Accordingly, in the semiconductor device  1100  and the light-emitting device chip  110  in  FIG. 4 , the amount of light emission increases due to the rise in the luminous efficiency similarly to the case of the semiconductor device  1000  and the light-emitting device chip  100  in  FIG. 2 . 
     (1-5) Second Modification of First Embodiment 
       FIG. 5  is a schematic cross-sectional view showing the structure of a semiconductor device  1200  of a second modification of the first embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C).  FIG. 6  is a diagram showing an example of the impurity concentration IM and the Al composition ratio CR of each semiconductor layer in a light-emitting thyristor  12  of the semiconductor device  1200  in  FIG. 5 . The semiconductor device  1200  differs from the semiconductor device  1000  shown in  FIG. 2  in that the Al composition ratio CRng of a second semiconductor layer (N-type gate layer)  1230  and the Al composition ratio CRpg of a third semiconductor layer (P-type gate layer)  1220  are higher than the Al composition ratio CRac 1  of an active layer  1241 . Except this feature, the semiconductor device  1200  and a light-emitting device chip  120  in  FIG. 5  are the same as the semiconductor device  1000  and the light-emitting device chip  100  in  FIG. 2 . 
     The light-emitting thyristor  12  of the semiconductor device  1200  in  FIG. 5  includes a P-type first semiconductor layer  1240 , the N-type second semiconductor layer (N-type gate layer)  1230 , the P-type third semiconductor layer (P-type gate layer)  1220 , and an N-type fourth semiconductor layer  1210 . The first semiconductor layer  1240  includes an anode layer  1243  as a first layer, an electron cladding layer (barrier layer)  1242  as a second layer, and the active layer  1241  as a third layer. The fourth semiconductor layer  1210  includes a cathode layer  1211  and a hole cladding layer  1212 . The first and fourth semiconductor layers  1240  and  1210  of the light-emitting thyristor  12  in  FIG. 5  are formed with the same semiconductor materials as the first and fourth semiconductor layers  1040  and  1010  of the light-emitting thyristor  10  in  FIG. 2 . The second and third semiconductor layers  1230  and  1220  in  FIG. 5  are the same as the second and third semiconductor layers  1030  and  1020  of the light-emitting thyristor  10  in  FIG. 2  except for the Al composition ratios. 
     The light-emitting thyristor  12  of the semiconductor device  1200  in  FIG. 5  satisfies the aforementioned conditional expressions (1) and (2). 
     Further, the light-emitting thyristor  12  of the semiconductor device  1200  in  FIG. 5  satisfies the following conditional expression (3a): 
         CRac 1 &lt;CRng=CRpg&lt;CRcl 1  (3a)
 
     Alternatively, the light-emitting thyristor  12  satisfies the following conditional expression (6a) equivalent to the conditional expression (3a): 
         BGac 1 &lt;BGng=BGpg&lt;BGcl 1  (6a)
 
     However, CRng=CRpg in the expression (3a) does not necessarily have to be satisfied. The light-emitting thyristor  12  may also be famed so as to satisfy the following conditional expressions (4a) and (5a) instead of the conditional expression (3a): 
         CRac 1 &lt;CRpg&lt;CRcl 1  (4a)
 
         CRac 1 &lt;CRng&lt;CRcl 1  (5a)
 
     Alternatively, the light-emitting thyristor  12  may also be famed so as to satisfy the following conditional expressions (7a) and (8a) equivalent to the conditional expressions (4a) and (5a): 
         BGac 1 &lt;BGpg&lt;BGcl 1  (7a)
 
         BGac 1 &lt;BGng&lt;BGcl 1  (8a)
 
     Since the semiconductor device  1200  and the light-emitting device chip  120  in  FIG. 6  satisfy the conditional expressions (1), (2) and (6a) or the conditional expressions (1), (2), (7a) and (8a), the amount of light emission increases due to the rise in the luminous efficiency similarly to the case of the semiconductor device  1000  and the light-emitting device chip  100  in  FIG. 2 . 
     (1-6) Third Modification of First Embodiment 
       FIG. 7  is a schematic cross-sectional view showing the structure of a semiconductor device  1300  of a third modification of the first embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C).  FIG. 8  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor  13  of the semiconductor device  1300  in  FIG. 7 . While cases where the first conductivity type is the P type and the second conductivity type is the N type have been described in the examples of  FIG. 2  and  FIG. 4 , a case where the first conductivity type is the N type and the second conductivity type is the P type will be described in the example of  FIG. 7 . Namely, the example of  FIG. 7  is an example obtained by changing the P type and the N type in the example of  FIG. 4  respectively to the N type and the P type. 
     The light-emitting thyristor  13  of the semiconductor device  1300  includes an N-type first semiconductor layer  1340 , a P-type second semiconductor layer (P-type gate layer)  1330 , an N-type third semiconductor layer (N-type gate layer)  1320 , and a P-type fourth semiconductor layer  1310 . The semiconductor device  1300  includes a cathode electrode  61 K as a first electrode electrically connected with the first semiconductor layer  1340 , a gate electrode  51 G as a second electrode electrically connected with the second semiconductor layer (P-type gate layer)  1330 , and an anode electrode  41 A as a third electrode electrically connected with the fourth semiconductor layer  1310 . The anode electrode  41 A is connected with anode wiring  42 A. 
     As shown in  FIG. 7 , the N-type first semiconductor layer  1340  includes a cathode layer  1343  as a first layer electrically connected with the cathode electrode  61 K, a hole cladding layer (barrier layer)  1342  as a second layer arranged adjacent to the cathode layer  1343 , and an active layer  1341  as a third layer arranged adjacent to the hole cladding layer  1342 . The P-type fourth semiconductor layer  1310  includes an anode layer  1311  and an electron cladding layer  1312  arranged adjacent to the anode layer  1311 . 
     The light-emitting thyristor  13  of the semiconductor device  1300  in  FIG. 7  satisfies the aforementioned conditional expressions (1) and (2). 
     Further, the light-emitting thyristor  13  satisfies the aforementioned conditional expression (3). Alternatively, the light-emitting thyristor  13  satisfies the conditional expression (6) equivalent to the conditional expression (3). 
     However, the light-emitting thyristor  13  may also be formed so as to satisfy the aforementioned conditional expressions (4) and (5) instead of the conditional expression (3). 
     Alternatively, the light-emitting thyristor  13  may also be famed so as to satisfy the aforementioned conditional expressions (7) and (8) equivalent to the conditional expressions (4) and (5). 
     In the semiconductor device  1300 , by the hole cladding layer  1342  satisfying the conditional expression BGac 1 &lt;BGcl 1  as indicated by the aforementioned conditional expression (6) or conditional expressions (7) and (8), the movement of holes in the active layer  1341  towards the cathode layer  1343  is limited and the holes are confined in the active layer  1341 . With such a hole confinement effect, the probability of the recombination of an electron heading from the cathode layer  1343  towards the anode layer  1311  with a hole in the active layer  1341  increases and the luminous efficiency rises, and accordingly, the amount of light emission increases. 
     Further, in the semiconductor device  1300 , the impurity concentration IMng of the third semiconductor layer (N-type gate layer)  1320  and the impurity concentration IMpg of the second semiconductor layer (P-type gate layer)  1330  are set low and the impurity concentration IMac 1  of the active layer  1341  is set high as indicated by the aforementioned conditional expressions (1) and (2). Thus, the carrier mobility in the third semiconductor layer (N-type gate layer)  1320  and the second semiconductor layer (P-type gate layer)  1330  gets high and the occurrence probability of the recombination in the third semiconductor layer (N-type gate layer)  1320  and the second semiconductor layer (P-type gate layer)  1330  gets low. Meanwhile, the carrier mobility in the active layer  1341  gets low and the occurrence probability of the recombination in the active layer  1341  gets high. Therefore, the occurrence probability of the recombination of an electron and a hole in the active layer  1341  increases and the luminous efficiency rises, and accordingly, the amount of light emission increases. 
     Furthermore, in the semiconductor device  1300 , the active layer  1341  is provided in an upper part (i.e., on a side farther from the substrate part  101 ) of the light-emitting thyristor  13  as a semiconductor laminated structure and the light generated in the active layer  1341  is extracted in the upward direction in  FIG. 7 , by which the absorption of the light generated in the active layer  1041  is reduced and the light extraction efficiency is increased. 
     As described above, according to the semiconductor device  1300  and a light-emitting device chip  130  in this modification, the amount of light emission increases due to the rise in the luminous efficiency in comparison with the conventional gate light emission type light-emitting thyristors. 
     (1-7) Fourth Modification of First Embodiment 
       FIG. 9  is a schematic cross-sectional view showing the structure of a semiconductor device  1400  of a fourth modification of the first embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C). The semiconductor device  1400  differs from the semiconductor device  1300  shown in  FIG. 7  in that a second semiconductor layer (P-type gate layer)  1430  is formed in a region smaller than a third semiconductor layer (N-type gate layer)  1420  and a gate electrode  51 G is formed on the third semiconductor layer (N-type gate layer)  1420 . Except this feature, the semiconductor device  1400  and a light-emitting device chip  140  in  FIG. 9  are the same as the semiconductor device  1300  and the light-emitting device chip  130  in  FIG. 7 . 
     A light-emitting thyristor  14  of the semiconductor device  1400  includes an N-type first semiconductor layer  1440 , the P-type second semiconductor layer (P-type gate layer)  1430 , the N-type third semiconductor layer (N-type gate layer)  1420 , and a P-type fourth semiconductor layer  1410 . The semiconductor device  1400  includes a cathode electrode  61 K electrically connected with the first semiconductor layer  1440 , the gate electrode  51 G electrically connected with the third semiconductor layer (N-type gate layer)  1420 , and an anode electrode  41 A electrically connected with the fourth semiconductor layer  1410 . 
     As shown in  FIG. 9 , the N-type first semiconductor layer  1440  includes a cathode layer  1443  as a first layer electrically connected with the cathode electrode  61 K, a hole cladding layer (barrier layer)  1442  as a second layer arranged adjacent to the cathode layer  1443 , and an active layer  1441  as a third layer arranged adjacent to the hole cladding layer  1442 . The P-type fourth semiconductor layer  1410  includes an anode layer  1411  and an electron cladding layer  1412 . 
     The constituent materials of the light-emitting thyristor  14  of the semiconductor device  1400  in  FIG. 9  are the same as those of the aforementioned light-emitting thyristor  13  of the semiconductor device  1300  in  FIG. 7 . Thus, according to the semiconductor device  1400  and the light-emitting device chip  140  in  FIG. 9 , the amount of light emission increases due to the rise in the luminous efficiency for the same reason as in the light-emitting thyristor  13  of the semiconductor device  1300  in  FIG. 7 . 
     (2) Second Embodiment 
     (2-1) Configuration 
       FIG. 10  is a schematic cross-sectional view showing the structure of a semiconductor device  2000  in the second embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C). In the first embodiment ( FIG. 2 ), the description was given of an example in which the first semiconductor layer  1040  including the active layer  1041  is arranged on a side farther from the substrate part  101  than the fourth semiconductor layer  1010 . In contrast, the description in the second embodiment will be given of an example in which a first semiconductor layer  2010  including an active layer  2013  is arranged on a side closer to the substrate part  101  than a fourth semiconductor layer  2040 . 
     As shown in  FIG. 10 , a light-emitting thyristor  20  includes the first semiconductor layer  2010  of a first conductivity type, a second semiconductor layer (P-type gate layer)  2020  of a second conductivity type different from the first conductivity type arranged adjacent to the first semiconductor layer  2010 , a third semiconductor layer (N-type gate layer)  2030  of the first conductivity type arranged adjacent to the second semiconductor layer  2020 , and the fourth semiconductor layer  2040  of the second conductivity type arranged adjacent to the third semiconductor layer  2030 . 
     In the semiconductor device  2000  in  FIG. 10 , the first semiconductor layer  2010  of the first conductivity type is an N-type semiconductor layer, the second semiconductor layer  2020  of the second conductivity type is a P-type gate layer, the third semiconductor layer  2030  of the first conductivity type is an N-type gate layer, and the fourth semiconductor layer  2040  of the second conductivity type is a P-type semiconductor layer. 
     Further, the semiconductor device  2000  in  FIG. 10  includes a cathode electrode  41 K as a first electrode electrically connected with the first semiconductor layer  2010 , a gate electrode  51 G as a second electrode electrically connected with the second semiconductor layer (P-type gate layer)  2020 , and an anode electrode  61 A as a third electrode electrically connected with the fourth semiconductor layer  2040 . 
     As shown in  FIG. 10 , the N-type first semiconductor layer  2010  includes a cathode layer  2011  as a first layer electrically connected with the cathode electrode  41 K, a hole cladding layer (barrier layer)  2012  as a second layer arranged adjacent to the cathode layer  2011 , and an active layer  2013  as a third layer arranged adjacent to the hole cladding layer  2012 . 
     As shown in  FIG. 10 , the P-type fourth semiconductor layer  2040  includes an anode layer  2042  electrically connected with the anode electrode  61 A and an electron cladding layer  2041  arranged between the anode layer  2042  and the third semiconductor layer  2030 . 
       FIG. 11  is a diagram showing an example of the impurity concentration IM (cm −3 ) and the Al composition ratio CR of each semiconductor layer in the light-emitting thyristor  20  of the semiconductor device  2000 . 
     Let IMng represent the impurity concentration of the third semiconductor layer (N-type gate layer)  2030 , IMpg represent the impurity concentration of the second semiconductor layer (P-type gate layer)  2020 , and IMac 2  represent the impurity concentration of the active layer  2013  as the third layer of the first semiconductor layer  2010 , the light-emitting thyristor  20  satisfies the following conditional expressions (9) and (10): 
         IMpg&lt;IMac 2  (9)
 
         IMng&lt;IMac 2  (10)
 
     In the example of  FIG. 11 , the following numerical examples are shown as the impurity concentrations: 
         IMac 2≈1×10 18 (cm −3 )
 
         IMpg≈ 5×10 17 (cm −3 )
 
         IMng≈ 2×10 17 (cm −3 )
 
     However, the impurity concentrations are not limited to the example of  FIG. 11 . 
     Further, let CRng represent the Al composition ratio of the third semiconductor layer (N-type gate layer)  2030 , CRpg represent the Al composition ratio of the second semiconductor layer (P-type gate layer)  2020 , CRac 2  represent the Al composition ratio of the active layer  2013 , and CRcl 2  represent the Al composition ratio of the hole cladding layer  2012 , the light-emitting thyristor  20  satisfies the following conditional expression (11): 
         CRac 2 =CRng=CRpg&lt;CRcl 2  (11)
 
     However, CRac2=CRng=CRpg in the expression (11) does not necessarily have to be satisfied. The light-emitting thyristor  20  may also be formed so as to satisfy the following conditional expressions (12) and (13) instead of the conditional expression (11): 
         CRac 2 ≤CRpg&lt;CRcl 2  (12)
 
         CRac 2 ≤CRng&lt;CRcl 2  (13)
 
     The Al composition ratio CR of each semiconductor layer of the light-emitting thyristor  20  corresponds to the band gap BG of each semiconductor layer. Thus, the conditional expressions (11) to (13) are equivalent to the following conditional expressions (14) to (16) using the band gap: 
         BGac 2 =BGng=BGpg&lt;BGcl 2  (14)
 
         BGac 2 ≤BGpg&lt;BGcl 2  (15)
 
         BGac 2≤ BGng&lt;BGcl 2  (16)
 
     where BGpg represents the band gap of the second semiconductor layer (P-type gate layer)  2020 , BGng represents the band gap of the third semiconductor layer (N-type gate layer)  2030 , BGac 2  represents the band gap of the active layer  2013 , and BGcl 2  represents the band gap of the hole cladding layer  2012 . 
     In  FIG. 11 , the following numerical examples are shown as the Al composition ratios: 
         CRac 2 =CRng=CRpg≈ 0.15 
         CRcl 2≈0.40
 
     However, the Al composition ratios are not limited to the example of  FIG. 11 . 
     In a case where the light-emitting thyristor  20  is formed with AlGaAs-based semiconductor materials, each semiconductor layer can be configured as below. The anode layer  2042  of the fourth semiconductor layer  2040  is famed with a P-type Al 0.25 Ga 0.75 As layer, and the electron cladding layer  2041  of the fourth semiconductor layer  2040  is famed with a P-type Al 0.4 Ga 0.6 As layer. The third semiconductor layer (N-type gate layer)  2030  is formed with an N-type Al 0.15 Ga 0.85 As layer, and the second semiconductor layer (P-type gate layer)  2020  is formed with a P-type Al 0.15 Ga 0.85 As layer. In the first semiconductor layer  2010 , the active layer  2013  is formed with an N-type Al 0.15 Ga 0.85 As layer, the hole cladding layer  2012  is formed with an N-type Al 0.4 Ga 0.6 As layer, and the cathode layer  2011  is formed with an N-type Al 0.25 Ga 0.75 As layer. 
     The Al composition ratio CRcl 2  of the hole cladding layer  2012  is desired to be within a range from 0.2 to 1.0. 
     Further, the Al composition ratio CRac 2  of the active layer  2013  is desired to be within a range from 0.14 to 0.18, and the Al composition ratios CRpg and CRng of the second semiconductor layer (P-type gate layer)  2020  and the third semiconductor layer (N-type gate layer)  2030  are desired to be within a range from 0.14 to 0.3. 
     The reason for setting the band gaps BGpg and BGng at small values, by setting the Al composition ratios CRpg and CRng of the second semiconductor layer (P-type gate layer)  2020  and the third semiconductor layer (N-type gate layer)  2030  at small values, and lowering the impurity concentrations IMpg and IMng is to increase the carrier mobility in the second semiconductor layer (P-type gate layer)  2020  and the third semiconductor layer (N-type gate layer)  2030  and thereby lower the occurrence probability of the recombination of an electron and a hole in the second semiconductor layer (P-type gate layer)  2020  and the third semiconductor layer (N-type gate layer)  2030 . 
     The reason for setting the band gap BGac 2  at a small value, by setting the Al composition ratio CRac 2  of the active layer  2013  at a small value, and raising the impurity concentration IMac 2  is to increase the occurrence probability of the recombination of an electron and a hole in the active layer  2013 . 
     Further, the reason for providing the hole cladding layer  2012  of the high Al composition ratio CRcl 2  and the wide band gap BGcl 2  between the active layer  2013  and the cathode layer  2011  is to make the hole cladding layer  2012  work as a barrier layer against holes heading from the anode layer  2042  towards the cathode layer  2011  and thereby increase the occurrence probability of the recombination of a hole and an electron in the active layer  2013 . 
     (2-2) Operation 
     In the semiconductor device  2000 , the drive IC part supplies the gate current from the gate electrode  51 G to the cathode electrode  41 K, and thereby the light-emitting thyristor  20  is brought into the lighted state (light emission state), i.e., the on state. Further, the drive IC part lets current higher than or equal to the holding current flow between the anode electrode  61 A and the cathode electrode  41 K, and thereby the lighted state is maintained. The light emission from the light-emitting thyristor  20  is mainly caused by the recombination of an electron in the active layer  2013  and a hole moving from the second semiconductor layer (P-type gate layer)  2020  into the active layer  2013 . Light generated by the recombination travels upward in  FIG. 10  and exits from the top surface of the anode layer  2042  or the like. 
     When the light-emitting thyristor  20  is in the lighted state, the recombination of a hole and an electron occurs also in the third semiconductor layer (N-type gate layer)  2030  and the second semiconductor layer (P-type gate layer)  2020 . However, the carrier mobility in the active layer  2013  is low since the impurity concentration IMac 2  of the active layer  2013  is set higher than the impurity concentrations IMng and IMpg of the third semiconductor layer (N-type gate layer)  2030  and the second semiconductor layer (P-type gate layer)  2020  as indicated by the aforementioned conditional expressions (9) and (10). Thus, in the active layer  2013 , the recombination occurs at an occurrence probability higher than occurrence probabilities of the recombination in the third semiconductor layer (N-type gate layer)  2030  and the second semiconductor layer (P-type gate layer)  2020 . Namely, if the impurity concentration IMac 2  of the active layer  2013  is set higher than the impurity concentrations IMpg and IMng of the second semiconductor layer (P-type gate layer)  2020  and the third semiconductor layer (N-type gate layer)  2030 , the concentration of carries (electrons) in the active layer  2013  increases, and thus the occurrence probability of the recombination of a hole and an electron increases and the luminous efficiency rises. Accordingly, the amount of light emission increases. 
     Further, in a case where the band gap BGcl 2  of the hole cladding layer  2012  is wider than the band gaps BGpg and BGng of the second semiconductor layer (P-type gate layer)  2020  and the third semiconductor layer (N-type gate layer)  2030  as indicated by the aforementioned conditional expression (14) or conditional expressions (15) and (16), holes that have moved from the second semiconductor layer (P-type gate layer)  2020  to the active layer  2013  are received by the hole cladding layer  2012 , by which the amount of holes leaking from the hole cladding layer  2012  to the cathode layer  2011  is reduced. Namely, since the hole cladding layer  2012  satisfying the aforementioned conditional expressions (15) and (16) has the function as a barrier layer limiting the passage of carriers, the leakage of the holes as carries to the cathode layer  2011  that have moved from the second semiconductor layer (P-type gate layer)  2020  to the active layer  2013  is reduced. Accordingly, the amount of carriers in the active layer  2013  hardly decreases and the occurrence probability of the recombination in the active layer  2013  increases, and thus the amount of light emission increases due to the rise in the luminous efficiency. 
     (2-3) Effect 
     As described above, in the semiconductor device  2000 , the effect of limiting the movement of holes in the active layer  2013  is achieved by the hole cladding layer  2012  satisfying the conditional expression BGac 2 &lt;BGcl 2  as indicated by the aforementioned conditional expression (14) or conditional expressions (15) and (16). With this effect, the probability of the recombination of an electron heading from the cathode layer  2011  towards the anode layer  2042  with a hole in the active layer  2013  increases and the luminous efficiency rises, and accordingly, the amount of light emission increases. 
     Further, in the semiconductor device  2000 , the impurity concentration IMng of the third semiconductor layer (N-type gate layer)  2030  and the impurity concentration IMpg of the second semiconductor layer (P-type gate layer)  2020  are set low and the impurity concentration IMac 2  of the active layer  2013  is set high as indicated by the conditional expressions (9) and (10). Thus, the carrier mobility in the third semiconductor layer (N-type gate layer)  2030  and the second semiconductor layer (P-type gate layer)  2020  gets high and the recombination in the third semiconductor layer (N-type gate layer)  2030  and the second semiconductor layer (P-type gate layer)  2020  is inhibited. Meanwhile, the carrier mobility in the active layer  2013  gets low and the recombination in the active layer  2013  increases. Therefore, the occurrence probability of the recombination of a hole and an electron in the active layer  2013  increases and the luminous efficiency rises, and accordingly, the amount of light emission increases. 
     Furthermore, in the semiconductor device  2000 , the area of the active layer  2013  is larger than the area of the active layer in the first embodiment, and thus the density of electric current flowing into the light-emitting thyristor  20  does not increase excessively. Accordingly, the luminous efficiency of the light-emitting thyristor  20  can be increased and the amount of emitted light increases. 
     As described above, according to the semiconductor device  2000  and a light-emitting device chip  200  in the second embodiment, the amount of light emission increases due to the rise in the luminous efficiency in comparison with the conventional gate light emission type light-emitting thyristors. 
     (2-4) First Modification of Second Embodiment 
       FIG. 12  is a schematic cross-sectional view showing the structure of a semiconductor device  2100  of a first modification of the second embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C). The semiconductor device  2100  differs from the semiconductor device  2000  shown in  FIG. 10  in that a third semiconductor layer (N-type gate layer)  2130  is formed in a large region similar to a second semiconductor layer (P-type gate layer)  2120  (i.e., a large region including a formation region of a gate electrode  51 G) and the gate electrode  51 G is famed on the third semiconductor layer (N-type gate layer)  2130 . Except this feature, the semiconductor device  2100  and a light-emitting device chip  210  in  FIG. 12  are the same as the semiconductor device  2000  and the light-emitting device chip  200  in  FIG. 10 . 
     A light-emitting thyristor  21  of the semiconductor device  2100  in  FIG. 12  includes an N-type first semiconductor layer  2110 , the P-type second semiconductor layer (P-type gate layer)  2120 , the N-type third semiconductor layer (N-type gate layer)  2130 , and a P-type fourth semiconductor layer  2140 . The first semiconductor layer  2110  includes a cathode layer  2111  as a first layer, a hole cladding layer (barrier layer)  2112  as a second layer, and an active layer  2113  as a third layer. The fourth semiconductor layer  2140  includes an anode layer  2142  and a hole cladding layer  2141 . The first to fourth semiconductor layers  2110 ,  2120 ,  2130  and  2140  of the light-emitting thyristor  21  in  FIG. 12  are formed with the same semiconductor materials as the first to fourth semiconductor layers  2010 ,  2020 ,  2030  and  2040  of the light-emitting thyristor  20  in  FIG. 10 . Thus, the light-emitting thyristor  21  in  FIG. 12  satisfies the aforementioned conditional expressions (9) to (16). Accordingly, in the semiconductor device  2100  and the light-emitting device chip  210  in  FIG. 12 , the amount of light emission increases due to the rise in the luminous efficiency for the same reason as in the semiconductor device  2000  and the light-emitting device chip  200  in  FIG. 10 . 
     (2-5) Second Modification of Second Embodiment 
       FIG. 13  is a schematic cross-sectional view showing the structure of a semiconductor device  2200  of a second modification of the second embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C).  FIG. 14  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor  22  of the semiconductor device  2200  in  FIG. 13 . The semiconductor device  2200  differs from the semiconductor device  2100  shown in  FIG. 12  in that the Al composition ratio CRng of a third semiconductor layer (N-type gate layer)  2230  and the Al composition ratio CRpg of a second semiconductor layer (P-type gate layer)  2220  are higher than the Al composition ratio CRac 2  of an active layer  2213 . Except this feature, the semiconductor device  2200  and a light-emitting device chip  220  in  FIG. 13  are the same as the semiconductor device  2100  and the light-emitting device chip  210  in  FIG. 12 . 
     The light-emitting thyristor  22  of the semiconductor device  2200  in  FIG. 13  includes an N-type first semiconductor layer  2210 , the P-type second semiconductor layer (P-type gate layer)  2220 , the N-type third semiconductor layer (N-type gate layer)  2230 , and a P-type fourth semiconductor layer  2240 . The first semiconductor layer  2210  includes a cathode layer  2211  as a first layer, a hole cladding layer (barrier layer)  2212  as a second layer, and the active layer  2213  as a third layer. The fourth semiconductor layer  2240  includes an anode layer  2242  and an electron cladding layer  2241 . The first and fourth semiconductor layers  2210  and  2240  of the light-emitting thyristor  22  in  FIG. 13  are famed with the same semiconductor materials as the first and fourth semiconductor layers  2110  and  2140  of the light-emitting thyristor  21  in  FIG. 12 . The second and third semiconductor layers  2220  and  2230  in  FIG. 13  are the same as the second and third semiconductor layers  2120  and  2130  of the light-emitting thyristor  21  in  FIG. 12  except for the Al composition ratios. 
     Thus, the light-emitting thyristor  22  of the semiconductor device  2200  in  FIG. 13  satisfies the aforementioned conditional expressions (9) and (10). 
     Further, the light-emitting thyristor  22  of the semiconductor device  2200  in  FIG. 13  satisfies the following conditional expression (11a): 
         CRac 2 &lt;CRng=CRpg&lt;CRcl 2  (11a)
 
     Alternatively, the light-emitting thyristor  22  satisfies the following conditional expression (12a) equivalent to the conditional expression (11a): 
         BGac 2 &lt;BGng=BGpg&lt;BGcl 2  (12a)
 
     However, CRng=CRpg in the expression (11a) does not necessarily have to be satisfied. The light-emitting thyristor  22  may also be famed so as to satisfy the following conditional expressions (13a) and (14a) instead of the conditional expression (11a): 
         CRac 2 &lt;CRpg&lt;CRcl 2  (13a)
 
         CRac 2 &lt;CRng&lt;CRcl 2  (14a)
 
     Alternatively, the light-emitting thyristor  22  may also be formed so as to satisfy the following conditional expressions (15a) and (16a) equivalent to the conditional expressions (13a) and ( 14   a ): 
         BGac 2 &lt;BGpg&lt;BGcl 2  (15a)
 
         BGac 2 &lt;BGng&lt;BGcl 2  (16a)
 
     Since the semiconductor device  2200  and the light-emitting device chip  220  in  FIG. 13  satisfy the conditional expressions (9), (10) and (12a) or the conditional expressions (9), (10), (15a) and (16a), the amount of light emission increases due to the rise in the luminous efficiency for the same reason as in the semiconductor device  2000  and the light-emitting device chip  200  in  FIG. 10 . 
     (2-6) Third Modification of Second Embodiment 
       FIG. 15  is a schematic cross-sectional view showing the structure of a semiconductor device  2300  of a third modification of the second embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C).  FIG. 16  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor  23  of the semiconductor device  2300  in  FIG. 15 . While cases where the first conductivity type is the N type and the second conductivity type is the P type have been described in the examples of  FIG. 10  and  FIG. 12 , a case where the first conductivity type is the P type and the second conductivity type is the N type will be described in the example of  FIG. 15 . Namely, the example of  FIG. 15  is an example obtained by changing the N type and the P type in the example of  FIG. 12  respectively to the P type and the N type. 
     The light-emitting thyristor  23  of the semiconductor device  2300  includes a P-type first semiconductor layer  2310 , an N-type second semiconductor layer (N-type gate layer)  2320 , a P-type third semiconductor layer (P-type gate layer)  2330 , and an N-type fourth semiconductor layer  2340 . The semiconductor device  2300  includes a cathode electrode  61 K as a first electrode electrically connected with the fourth semiconductor layer  2340 , a gate electrode  51 G as a second electrode electrically connected with the third semiconductor layer (P-type gate layer)  2330 , and an anode electrode  41 A as a third electrode electrically connected with the first semiconductor layer  2310 . 
     As shown in  FIG. 15 , the P-type first semiconductor layer  2310  includes an anode layer  2311  as a first layer electrically connected with the anode electrode  41 A, an electron cladding layer (barrier layer)  2312  as a second layer arranged adjacent to the anode layer  2311 , and an active layer  2313  as a third layer arranged adjacent to the electron cladding layer  2312 . The N-type fourth semiconductor layer  2340  includes a cathode layer  2342  and a hole cladding layer  2341  arranged adjacent to the cathode layer  2342 . 
     The light-emitting thyristor  23  of the semiconductor device  2300  in  FIG. 15  satisfies the conditional expressions (9) and (10). 
     Further, the light-emitting thyristor  23  satisfies the aforementioned conditional expression (11). Alternatively, the light-emitting thyristor  23  satisfies the conditional expression (14) equivalent to the conditional expression (11). 
     However, CRng=CRpg in the conditional expression (11) does not necessarily have to be satisfied. The light-emitting thyristor  23  may also be formed so as to satisfy the conditional expressions (12) and (13) instead of the conditional expression (11). Alternatively, the light-emitting thyristor  23  may also be formed so as to satisfy the conditional expressions (15) and (16) equivalent to the conditional expressions (12) and (13). 
     In the semiconductor device  2300 , by the electron cladding layer  2312  satisfying the conditional expression BGac2&lt;BGcl 2  as indicated by the aforementioned conditional expression (14) or conditional expressions (15) and (16), the movement of electrons in the active layer  2313  towards the anode layer  2311  is limited and the electrons are confined in the active layer  2313 . With this electron confinement, the probability of the recombination of an electron heading from the cathode layer  2342  towards the anode layer  2311  with a hole in the active layer  2313  increases and the luminous efficiency rises, and accordingly, the amount of light emission increases. 
     Further, in the semiconductor device  2300 , the impurity concentration IMpg of the third semiconductor layer (P-type gate layer)  2330  and the impurity concentration IMng of the second semiconductor layer (N-type gate layer)  2320  are set low and the impurity concentration IMac 2  of the active layer  2313  is set high as indicated by the conditional expressions (9) and (10). Thus, the carrier mobility in the third semiconductor layer (P-type gate layer)  2330  and the second semiconductor layer (N-type gate layer)  2320  gets high and the recombination in the third semiconductor layer (P-type gate layer)  2330  and the second semiconductor layer (N-type gate layer)  2320  is inhibited. Meanwhile, the carrier mobility in the active layer  2313  gets low and the recombination in the active layer  2313  increases. Therefore, the occurrence probability of the recombination of an electron and a hole in the active layer  2313  increases and the luminous efficiency rises, and accordingly, the amount of light emission increases. 
     As described above, according to the semiconductor device  2300  and a light-emitting device chip  230  in this modification, the amount of light emission increases due to the rise in the luminous efficiency in comparison with the conventional gate light emission type light-emitting thyristors. 
     (2-7) Fourth Modification of Second Embodiment 
       FIG. 17  is a schematic cross-sectional view showing the structure of a semiconductor device  2400  of a fourth modification of the second embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C). The semiconductor device  2400  differs from the semiconductor device  2300  shown in  FIG. 15  in that a third semiconductor layer (P-type gate layer)  2430  is formed in a region smaller than a second semiconductor layer (N-type gate layer)  2420  and the gate electrode  51 G is formed on the second semiconductor layer (N-type gate layer)  2420 . Except this feature, the semiconductor device  2400  and a light-emitting device chip  240  in  FIG. 17  are the same as the semiconductor device  2300  and the light-emitting device chip  230  in  FIG. 15 . 
     A light-emitting thyristor  24  of the semiconductor device  2400  includes a P-type first semiconductor layer  2410 , the N-type second semiconductor layer (N-type gate layer)  2420 , the P-type third semiconductor layer (P-type gate layer)  2430 , and an N-type fourth semiconductor layer  2440 . The semiconductor device  2400  includes an anode electrode  41 A electrically connected with the first semiconductor layer  2410 , a gate electrode  51 G electrically connected with the second semiconductor layer (N-type gate layer)  2420 , and a cathode electrode  61 K electrically connected with the fourth semiconductor layer  2440 . 
     As shown in  FIG. 17 , the P-type first semiconductor layer  2410  includes an anode layer  2411  as a first layer electrically connected with the anode electrode  41 A, an electron cladding layer (barrier layer)  2412  as a second layer, and an active layer  2413  as a third layer. The N-type fourth semiconductor layer  2440  includes a cathode layer  2442  and a hole cladding layer  2441 . 
     The constituent materials of the light-emitting thyristor  24  of the semiconductor device  2400  in  FIG. 17  are the same as those of the aforementioned light-emitting thyristor  23  of the semiconductor device  2300  in  FIG. 15 . Thus, according to the semiconductor device  2400  and the light-emitting device chip  240  in  FIG. 17 , the amount of light emission increases due to the rise in the luminous efficiency for the same reason as in the light-emitting thyristor  23  of the semiconductor device  2300  in  FIG. 15 . 
     (3) Third Embodiment 
     (3-1) Configuration 
       FIG. 18  is a schematic cross-sectional view showing the structure of a semiconductor device  3000  in the third embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C). In the third embodiment, a description will be given of a semiconductor device and a light-emitting device chip having a structure in which the active layer and the electron cladding layer (or the hole cladding layer) in the first embodiment are combined with the active layer and the hole cladding layer (or the electron cladding layer) in the second embodiment. 
     As shown in  FIG. 18 , a light-emitting thyristor  30  of the semiconductor device  3000  includes a first semiconductor layer  3040  of a first conductivity type, a second semiconductor layer  3030  of a second conductivity type arranged adjacent to the first semiconductor layer  3040 , a third semiconductor layer  3020  of the first conductivity type arranged adjacent to the second semiconductor layer  3030 , and a fourth semiconductor layer  3010  of the second conductivity type arranged adjacent to the third semiconductor layer  3020 . 
     In the semiconductor device  3000  in  FIG. 18 , the first semiconductor layer  3040  of the first conductivity type is a P-type semiconductor layer, the second semiconductor layer  3030  of the second conductivity type is an N-type gate layer, the third semiconductor layer  3020  of the first conductivity type is a P-type gate layer, and the fourth semiconductor layer  3010  of the second conductivity type is an N-type semiconductor layer. 
     Further, the semiconductor device  3000  in  FIG. 18  includes an anode electrode  61 A electrically connected with the first semiconductor layer  3040 , a gate electrode  51 G electrically connected with the third semiconductor layer (P-type gate layer)  3020 , and a cathode electrode  41 K electrically connected with the fourth semiconductor layer  3010 . 
     As shown in  FIG. 18 , the P-type first semiconductor layer  3040  includes an anode layer  3043  as a first layer electrically connected with the anode electrode  61 A, an electron cladding layer (barrier layer)  3042  as a second layer arranged adjacent to the anode layer  3043 , and an active layer  3041  as a third layer arranged adjacent to the electron cladding layer  3042 . Namely, the first to third semiconductor layers  3040 ,  3030  and  3020  of the light-emitting thyristor  30  have a structure similar to that of the first to third semiconductor layers  1040 ,  1030  and  1020  of the light-emitting thyristor  10  of the semiconductor device  1000  described with reference to  FIG. 2  and  FIG. 3 . 
     As shown in  FIG. 18 , the N-type fourth semiconductor layer  3010  includes a cathode layer  3011  as a fourth layer electrically connected with the cathode electrode  41 K, a hole cladding layer (barrier layer)  3012  as a fifth layer arranged adjacent to the cathode layer  3011 , and an active layer  3013  as a sixth layer arranged adjacent to the hole cladding layer  3012 . Namely, the second to fourth semiconductor layers  3030 ,  3020  and  3010  of the light-emitting thyristor  30  have a structure similar to that of the third to first semiconductor layers  2030 ,  2020  and  2010  of the light-emitting thyristor  20  of the semiconductor device  2000  described with reference to  FIG. 10  and  FIG. 11 . 
     The third embodiment has a combined structure as a combination of the first embodiment and the second embodiment, in which a P-type active layer is introduced into a P-type emitter of a conventional gate light emission type light-emitting thyristor and an N-type active layer is introduced into an N-type emitter of a conventional gate light emission type light-emitting thyristor. The active layer  3013  is an N-type Al 0.15 Ga 0.85 As layer, for example, and the active layer  3041  is a P-type Al 0.15 Ga 0.85 As layer, for example. 
       FIG. 19  is a diagram showing an example of the impurity concentration IM (cm −3 ) and the Al composition ratio CR of each semiconductor layer in the light-emitting thyristor  30  of the semiconductor device  3000 . 
     As is understandable from comparison between  FIG. 19  and  FIG. 3  (the first embodiment), the first to third semiconductor layers  3040 ,  3030  and  3020  of the light-emitting thyristor  30  satisfy the conditional expressions (1) to (8) explained with reference to  FIG. 2  and  FIG. 3  (the first embodiment). 
     Further, as is understandable from comparison between  FIG. 19  and  FIG. 11  (the second embodiment), the second to fourth semiconductor layers  3030 ,  3020  and  3010  of the light-emitting thyristor  30  satisfy the conditional expressions (9) to (16) explained with reference to  FIG. 10  and  FIG. 11  (the second embodiment). 
     (3-2) Operation 
     In the third embodiment, the first to third semiconductor layers  3040 ,  3030  and  3020  of the light-emitting thyristor  30  operate similarly to the first to third semiconductor layers  1040 ,  1030  and  1020  of the light-emitting thyristor  10  of the semiconductor device  1000  described with reference to  FIG. 2  and  FIG. 3  (the first embodiment). 
     Further, the second to fourth semiconductor layers  3030 ,  3020  and  3010  of the light-emitting thyristor  30  operate similarly to the third to first semiconductor layers  2030 ,  2020  and  2010  of the light-emitting thyristor  20  of the semiconductor device  2000  described with reference to  FIG. 10  and  FIG. 11  (the second embodiment). 
     (3-3) Effect 
     According to the third embodiment, the amount of light emission increases due to the rise in the luminous efficiency for the reasons described in the first and second embodiment. 
     Further, the amount of light emission increases further since light as a combination of light generated in the active layer  3041  and light generated in the active layer  3013  exits as outgoing light from a large region including the top surface of the anode layer  3043 . 
     (3-4) First Modification of Third Embodiment 
       FIG. 20  is a schematic cross-sectional view showing the structure of a semiconductor device  3100  of a first modification of the third embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C). The semiconductor device  3100  differs from the semiconductor device  3000  shown in  FIG. 18  in that a second semiconductor layer (N-type gate layer)  3130  is formed in a large region similar to a third semiconductor layer (P-type gate layer)  3120  (i.e., a large region including a formation region of a gate electrode  51 G) and the gate electrode  51 G is formed on the second semiconductor layer (N-type gate layer)  3130 . Except this feature, the semiconductor device  3100  and a light-emitting device chip  310  in  FIG. 20  are the same as the semiconductor device  3000  and the light-emitting device chip  300  in  FIG. 18 . 
     A light-emitting thyristor  31  of the semiconductor device  3100  in  FIG. 20  includes a P-type first semiconductor layer  3140 , the N-type second semiconductor layer (N-type gate layer)  3130 , the P-type third semiconductor layer (P-type gate layer)  3120 , and an N-type fourth semiconductor layer  3110 . The first semiconductor layer  3140  includes an anode layer  3143  as a first layer, an electron cladding layer (barrier layer)  3142  as a second layer, and an active layer  3141  as a third layer. The fourth semiconductor layer  3110  includes a cathode layer  3111  as a fourth layer, a hole cladding layer (barrier layer)  3112  as a fifth layer, and an active layer  3113  as a sixth layer. 
     The first to third semiconductor layers  3140 ,  3130  and  3120  of the light-emitting thyristor  31  have a structure similar to that of the first to third semiconductor layers  1140 ,  1130  and  1120  of the light-emitting thyristor  11  of the semiconductor device  1100  described with reference to  FIG. 4  (the first modification of the first embodiment) and operate similarly to the first to third semiconductor layers  1140 ,  1130  and  1120  of the light-emitting thyristor  11  of the semiconductor device  1100  described with reference to  FIG. 4 . 
     The second to fourth semiconductor layers  3130 ,  3120  and  3110  of the light-emitting thyristor  31  have a structure similar to that of the third to first semiconductor layers  2130 ,  2120  and  2110  of the light-emitting thyristor  21  of the semiconductor device  2100  described with reference to  FIG. 12  (the first modification of the second embodiment) and operate similarly to the third to first semiconductor layers  2130 ,  2120  and  2110  of the light-emitting thyristor  21  of the semiconductor device  2100  described with reference to  FIG. 12 . 
     Thus, according to the semiconductor device  3100  and the light-emitting device chip  310  in  FIG. 20 , the amount of light emission increases due to the rise in the luminous efficiency for the reasons described in the first modification of the first embodiment and the first modification of the second embodiment. 
     Further, the amount of light emission increases further since light as a combination of light generated in the active layer  3141  and light generated in the active layer  3113  exits as outgoing light from a large region including the top surface of the anode layer  3143 . 
     (3-5) Second Modification of Third Embodiment 
       FIG. 21  is a schematic cross-sectional view showing the structure of a semiconductor device  3200  of a second modification of the third embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C).  FIG. 22  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor  32  of the semiconductor device  3200  in  FIG. 21 . The semiconductor device  3200  differs from the semiconductor device  3100  shown in  FIG. 20  in that the Al composition ratio CRng of a second semiconductor layer (N-type gate layer)  3230  and the Al composition ratio CRpg of a third semiconductor layer (P-type gate layer)  3220  are higher than the Al composition ratio CRac 1  of an active layer (third layer)  3241  and the Al composition ratio CRac 2  of an active layer (sixth layer)  3213 . Except this feature, the semiconductor device  3200  in  FIG. 21  is the same as the semiconductor device  3100  in  FIG. 20 . 
     The light-emitting thyristor  32  of the semiconductor device  3200  in  FIG. 21  includes a P-type first semiconductor layer  3240 , the N-type second semiconductor layer (N-type gate layer)  3230 , the P-type third semiconductor layer (P-type gate layer)  3220 , and an N-type fourth semiconductor layer  3210 . The first semiconductor layer  3240  includes an anode layer  3243  as a first layer, an electron cladding layer (barrier layer)  3242  as a second layer, and the active layer  3241  as a third layer. The fourth semiconductor layer  3210  includes a cathode layer  3211  as a fourth layer, a hole cladding layer (barrier layer)  3212  as a fifth layer, and the active layer  3213  as a sixth layer. 
     The first to third semiconductor layers  3240 ,  3230  and  3220  of the light-emitting thyristor  32  have a structure similar to that of the first to third semiconductor layers  1240 ,  1230  and  1220  of the light-emitting thyristor  12  of the semiconductor device  1200  described with reference to  FIG. 5  (the second modification of the first embodiment) and operate similarly to the first to third semiconductor layers  1240 ,  1230  and  1220  of the light-emitting thyristor  12  of the semiconductor device  1200  described with reference to  FIG. 5 . 
     The second to fourth semiconductor layers  3230 ,  3220  and  3210  of the light-emitting thyristor  32  have a structure similar to that of the third to first semiconductor layers  2230 ,  2220  and  2210  of the light-emitting thyristor  22  of the semiconductor device  2200  described with reference to  FIG. 13  (the second modification of the second embodiment) and operate similarly to the third to first semiconductor layers  2230 ,  2220  and  2210  of the light-emitting thyristor  22  of the semiconductor device  2200  described with reference to  FIG. 13 . 
     Thus, according to the semiconductor device  3200  and a light-emitting device chip  320  in  FIG. 21 , the amount of light emission increases due to the rise in the luminous efficiency for the reasons described in the first and second embodiments. 
     Further, the amount of light emission increases further since light as a combination of light generated in the active layer  3241  and light generated in the active layer  3213  exits as outgoing light from a large region including the top surface of the anode layer  3243 . 
     (3-6) Third Modification of Third Embodiment 
       FIG. 23  is a schematic cross-sectional view showing the structure of a semiconductor device  3300  of a third modification of the third embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C).  FIG. 24  is a diagram showing an example of the impurity concentration and the Al composition ratio of each semiconductor layer in a light-emitting thyristor  33  of the semiconductor device  3300  in  FIG. 23 . While cases where the first conductivity type is the P type and the second conductivity type is the N type have been described in the examples of  FIG. 18  and  FIG. 20 , a case where the first conductivity type is the N type and the second conductivity type is the P type will be described in the example of  FIG. 23  and  FIG. 24 . Namely, the example of  FIG. 23  and  FIG. 24  is an example obtained by changing the P type and the N type in the example of  FIG. 20  respectively to the N type and the P type. 
     The light-emitting thyristor  33  of the semiconductor device  3300  in  FIG. 23  includes an N-type first semiconductor layer  3340 , a P-type second semiconductor layer (P-type gate layer)  3330 , an N-type third semiconductor layer (N-type gate layer)  3320 , and a P-type fourth semiconductor layer  3310 . The semiconductor device  3300  in  FIG. 23  includes a cathode electrode  61 K electrically connected with the first semiconductor layer  3340 , a gate electrode  51 G electrically connected with the second semiconductor layer (P-type gate layer)  3330 , and an anode electrode  41 A electrically connected with the fourth semiconductor layer  3310 . 
     As shown in  FIG. 23 , the N-type first semiconductor layer  3340  includes a cathode layer  3343  as a first layer electrically connected with the cathode electrode  61 K, a hole cladding layer (barrier layer)  3342  as a second layer arranged adjacent to the cathode layer  3343 , and an active layer  3341  as a third layer arranged adjacent to the hole cladding layer  3342 . Namely, the first to third semiconductor layers  3340 ,  3330  and  3320  of the light-emitting thyristor  33  have a structure similar to that of the first to third semiconductor layers  1340 ,  1330  and  1320  of the light-emitting thyristor  13  of the semiconductor device  1300  described with reference to  FIG. 7  and  FIG. 8  (the third modification of the first embodiment) and operate similarly to the first to third semiconductor layers  1340 ,  1330  and  1320  of the light-emitting thyristor  13  of the semiconductor device  1300  described with reference to  FIG. 7  and  FIG. 8 . 
     The P-type fourth semiconductor layer  3310  includes an anode layer  3311  as a fourth layer electrically connected with the anode electrode  41 A, an electron cladding layer (barrier layer)  3312  as a fifth layer arranged adjacent to the anode layer  3311 , and an active layer  3313  as a sixth layer arranged adjacent to the electron cladding layer  3312 . Namely, the second to fourth semiconductor layers  3330 ,  3320  and  3310  of the light-emitting thyristor  33  have a structure similar to that of the third to first semiconductor layers  2330 ,  2320  and  2310  of the light-emitting thyristor  23  of the semiconductor device  2300  described with reference to  FIG. 15  and  FIG. 16  (the third modification of the second embodiment) and operate similarly to the third to first semiconductor layers  2330 ,  2320  and  2310  of the light-emitting thyristor  23  of the semiconductor device  2300  described with reference to  FIG. 15  and  FIG. 16 . 
     Thus, according to the semiconductor device  3300  and a light-emitting device chip  330  in  FIG. 23 , the amount of light emission increases due to the rise in the luminous efficiency for the reasons described in the first and second embodiments. 
     Further, the amount of light emission increases further since light as a combination of light generated in the active layer  3341  and light generated in the active layer  3313  exits as outgoing light from a large region including the top surface of the cathode layer  3343 . 
     (3-7) Fourth Modification of Third Embodiment 
       FIG. 25  is a schematic cross-sectional view showing the structure of a semiconductor device  3400  of a fourth modification of the third embodiment (i.e., cross-sectional structure corresponding to the sections of  FIG. 1  along the line A-B-C). The semiconductor device  3400  differs from the semiconductor device  3300  shown in  FIG. 23  in that a second semiconductor layer (P-type gate layer)  3430  is formed in a region smaller than a third semiconductor layer (N-type gate layer)  3420  and a gate electrode  51 G is formed on the third semiconductor layer (N-type gate layer)  3420 . Except this feature, the semiconductor device  3400  and a light-emitting device chip  340  in  FIG. 25  are the same as the semiconductor device  3300  and the light-emitting device chip  330  in  FIG. 23 . 
     A light-emitting thyristor  34  of the semiconductor device  3400  includes an N-type first semiconductor layer  3440 , the P-type second semiconductor layer (P-type gate layer)  3430 , the N-type third semiconductor layer (N-type gate layer)  3420 , and a P-type fourth semiconductor layer  3410 . The semiconductor device  3400  includes a cathode electrode  61 K electrically connected with the first semiconductor layer  3440 , the gate electrode  51 G electrically connected with the third semiconductor layer (N-type gate layer)  3420 , and an anode electrode  41 A electrically connected with the fourth semiconductor layer  3410 . 
     As shown in  FIG. 25 , the N-type first semiconductor layer  3440  includes a cathode layer  3443  as a first layer electrically connected with the cathode electrode  61 K, a hole cladding layer (barrier layer)  3442  as a second layer arranged adjacent to the cathode layer  3443 , and an active layer  3441  as a third layer arranged adjacent to the hole cladding layer  3442 . The P-type fourth semiconductor layer  3410  includes an anode layer  3411  as a fourth layer electrically connected with the anode electrode  41 A, an electron cladding layer (barrier layer)  3412  as a fifth layer arranged adjacent to the anode layer  3411 , and an active layer  3413  as a sixth layer arranged adjacent to the electron cladding layer  3412 . 
     Namely, the first to third semiconductor layers  3440 ,  3430  and  3420  of the light-emitting thyristor  34  have a structure similar to that of the first to third semiconductor layers  1440 ,  1430  and  1420  of the light-emitting thyristor  14  of the semiconductor device  1400  described with reference to  FIG. 9  (the fourth modification of the first embodiment) and operate similarly to the first to third semiconductor layers  1440 ,  1430  and  1420  of the light-emitting thyristor  14  of the semiconductor device  1400  described with reference to  FIG. 9 . 
     Further, the second to fourth semiconductor layers  3430 ,  3420  and  3410  of the light-emitting thyristor  34  have a structure similar to that of the third to first semiconductor layers  2430 ,  2420  and  2410  of the light-emitting thyristor  24  of the semiconductor device  2400  described with reference to  FIG. 17  (the fourth modification of the second embodiment) and operate similarly to the third to first semiconductor layers  2430 ,  2420  and  2410  of the light-emitting thyristor  24  of the semiconductor device  2400  described with reference to  FIG. 17 . 
     Thus, according to the semiconductor device  3400  and the light-emitting device chip  340  in  FIG. 25 , the amount of light emission increases due to the rise in the luminous efficiency for the reasons described in the first and second embodiments. 
     Further, the amount of light emission increases further since light as a combination of light generated in the active layer  3441  and light generated in the active layer  3413  exits as outgoing light from a large region including the top surface of the cathode layer  3443 . 
     (4) Fourth Embodiment 
       FIG. 26  is a schematic perspective view showing the structure of a substrate unit  400  as a principal part of an optical print head in the fourth embodiment. As shown in  FIG. 26 , the substrate unit  400  includes a printed wiring board  401  and a plurality of light-emitting device chips  404  arranged like an array. The plurality of light-emitting device chips  404  are fixed on the printed wiring board  401  by using a thermosetting resin or the like. The light-emitting device chip  404  is the light-emitting device chip described in one of the first to third embodiments and their modifications (e.g., the light-emitting device chip  100  shown in  FIG. 1  and  FIG. 2 ). The external connection pad  104  of each light-emitting device chip  404  and a connection pad  402  of the printed wiring board  401  are electrically connected to each other by a bonding wire  403 . The printed wiring board  401  may be equipped with various types of wiring patterns, electronic components, connectors and so on. The shape of the light-emitting device chip  404  is not limited to that shown in  FIG. 26 . 
       FIG. 27  is a schematic cross-sectional view showing the structure of the optical print head  500  in the fourth embodiment. The optical print head  500  is an exposure device of an electrophotographic printer as an image forming device. As shown in  FIG. 27 , the optical print head  500  includes a base member  501 , the substrate unit  400  including the printed wiring board  401  used as a mounting substrate as a COB (Chip On Board) substrate, a lens array  504  including a plurality of upright equal-magnification imaging lenses, a lens holder  505 , and clampers  506  as spring members. The base member  501  is a member for fixing the printed wiring board  401 , and side faces of the base member  501  are provided with opening parts  503  to be used for fixing the printed wiring board  401  and the lens holder  505  to the base member  501  by use of the clampers  506 . The lens holder  505  is formed by injection molding of organic polymeric material or the like, for example. The lens array  504  is a set of optical lenses imaging light emitted from the light-emitting device chips  404  of the substrate unit  400  on a photosensitive drum as an image bearing body. The lens holder  505  holds the lens array  504  at a prescribed position with respect to the base member  501 . The clampers  506  clamp and hold components via the opening parts  503  of the base member  501  and opening parts of the lens holder  505 . 
     In the optical print head  500 , some of the light-emitting thyristors of the light-emitting device chips  404  (e.g., the light-emitting thyristors  10  in  FIG. 1 ) emit light according to print data, and the light is imaged on the uniformly charged photosensitive drum by the lens array  504 . By this process, an electrostatic latent image is formed on the photosensitive drum, and thereafter, an image made of a developing agent is famed (printed) on a print medium (sheet) by a development process, a transfer process and a fixation process. 
     As described above, the optical print head  500  in the fourth embodiment includes the light-emitting device chips  404  according to one of the first to third embodiments and their modifications, and thus the amount (intensity) of the light applied to the photosensitive drum can be increased. Consequently, adjustment of the amount (intensity) of the light applied to the photosensitive drum is facilitated and improvement of print quality (e.g., printing with high-quality gradation expression) becomes possible. 
     (5) Fifth Embodiment 
     (5-1) Configuration 
       FIG. 28  is a schematic cross-sectional view showing the structure of an image forming device  600  in the fifth embodiment of the present invention. The image forming device  600  is an electrophotographic color printer, for example. The image forming device  600  includes optical print heads  611 Y,  611 M,  611 C and  611 K as exposure devices, each of which is the optical print head  500  described in the fourth embodiment. 
     As shown in  FIG. 28 , the image forming device  600  includes, as principal components, image formation sections  610 Y,  610 M,  610 C and  610 K for forming developing agent images (toner images) on a print medium  626  such as a sheet of paper by an electrophotographic method, a medium supply section  620  for supplying the print medium  626  to the image formation sections  610 Y,  610 M,  610 C and  610 K, a conveyance section  630  for conveying the print medium  626 , transfer rollers  640  as transfer sections arranged respectively corresponding to the image formation sections  610 Y,  610 M,  610 C and  610 K, a fixation device  650  for fixing the toner image transferred onto the print medium  626 , and an ejection roller pair  625  as a medium ejection section for ejecting the print medium  626  after passing through the fixation device  650  to the outside. Incidentally, the number of image formation sections of the image forming device  600  may also be three or less or five or more. Further, the image forming device  600  can also be a monochrome printer, in which the number of image formation sections is one, as long as the image forming device  600  is a device forming an image on a print medium  626  by means of the electrophotographic process. 
     As shown in  FIG. 28 , the medium supply section  620  includes a sheet cassette  621 , a hopping roller  622  for drawing out the print media  626  loaded in the sheet cassette  621  sheet by sheet, a registration roller  623  for conveying the print medium  626  drawn out of the sheet cassette  621 , and a roller pair  624  for conveying the print medium  626 . 
     The image formation sections  610 Y,  610 M,  610 C and  610 K respectively form a yellow (Y) toner image, a magenta (M) toner image, a cyan (C) toner image and a black (K) toner image on the print medium  626 . The image formation sections  610 Y,  610 M,  610 C and  610 K are arranged side by side along a medium conveyance path from an upstream side to a downstream side (i.e., from right to left) in a medium conveyance direction (horizontal direction in  FIG. 28 ). The image formation sections  610 Y,  610 M,  610 C and  610 K respectively include image formation units  612 Y,  612 M,  612 C and  612 K for their colors formed so as to be detachable. The image formation units  612 Y,  612 M,  612 C and  612 K arranged in series are provided respectively corresponding to the colors of the image formation sections  610 Y,  610 M,  610 C and  610 K. The image formation unit  612 Y forms an image with a yellow toner, the image formation unit  612 M forms an image with a magenta toner, the image formation unit  612 C forms an image with a cyan toner, and the image formation unit  612 K forms an image with a black toner. The image formation units  612 Y,  612 M,  612 C and  612 K have the same structure as each other except for the difference in the toner color. 
     The image formation sections  610 Y,  610 M,  610 C and  610 K respectively include the optical print heads  611 Y,  611 M,  611 C and  611 K as exposure devices for their colors. 
     Each of the image formation units  612 Y,  612 M,  612 C and  612 K includes a photosensitive drum  613  as a rotatably supported image bearing body, a charging roller  614  as a charging member for uniformly charging the surface of the photosensitive drum  613 , and a development device  615  for forming a toner image corresponding to an electrostatic latent image by supplying the toner to the surface of the photosensitive drum  613  after the electrostatic latent image is formed on the surface of the photosensitive drum  613  by the exposure by the optical print head  611 Y,  611 M,  611 C,  611 K. 
     The development device  615  includes a toner storage section as a developing agent storage section forming a developing agent storage space for storing the toner, a development roller  616  as a developing agent bearing body for supplying the toner to the surface of the photosensitive drum  613 , a supply roller  617  for supplying the toner stored in the toner storage section to the development roller  616 , and a development blade  618  as a toner regulation member for regulating the thickness of a toner layer on the surface of the development roller  616 . 
     The exposure by each of the optical print heads  611 Y,  611 M,  611 C and  611 K is performed on the uniformly charged surface of the photosensitive drum  613  based on image data for the printing. Each of the optical print heads  611 Y,  611 M,  611 C and  611 K includes a light-emitting device array in which a plurality of light-emitting thyristors as a plurality of light-emitting devices are arranged in an axis line direction of the photosensitive drum  613 . 
     As shown in  FIG. 28 , the conveyance section  630  includes a conveyance belt (transfer belt)  633  electrostatically attracting and conveying the print medium  626 , a drive roller  631  rotated by a drive section and driving the conveyance belt  633 , a tension roller (driven roller)  632  forming a pair with the drive roller  631  and stretching the conveyance belt  633 . 
     As shown in  FIG. 28 , the transfer roller  640  is arranged so as to face the photosensitive drum  613  of each image formation unit  612 Y,  612 M,  612 C,  612 K across the conveyance belt  633 . By the transfer rollers  640 , the developing agent images (toner images) formed on the surfaces of the photosensitive drums  613  of the image formation units  612 Y,  612 M,  612 C and  612 K are successively transferred to the top surface of the print medium  626  conveyed along the medium conveyance path in the direction of the arrow, by which a color image as a stack of a plurality of toner images is formed. Each image formation unit  612 Y,  612 M,  612 C,  612 K includes a cleaning device  619  for removing the toner remaining on the photosensitive drum  613  after the image developed on the photosensitive drum  613  (toner image) is transferred to the print medium  626 . 
     The fixation device  650  includes a pair of rollers  651  and  652  pressed against each other. The roller  651  is a heat roller including a built-in heater, while the roller  652  is a pressure roller pressed against the roller  651 . The print medium  626  with the unfixed toner images passes between the pair of rollers  651  and  652  of the fixation device  650 . At the time of passage, the unfixed toner images are heated and pressed and thereby fixed on the print medium  626 . 
     (5-2) Operation 
     First, the print medium  626  in the sheet cassette  621  is drawn out by the hopping roller  622  and is sent to the registration roller  623 . Subsequently, the print medium  626  is sent from the registration roller  623  to the conveyance belt  633  via the roller pair  624  and is conveyed to the image formation units  612 Y,  612 M,  612 C and  612 K with the traveling of the conveyance belt  633 . In the image formation units  612 Y,  612 M,  612 C,  612 K, the surface of the photosensitive drum  613  is charged by the charging roller  614  and is exposed by the optical print heads  611 Y,  611 M,  611 C,  611 K, by which an electrostatic latent image is formed. The toner formed into a thin layer on the development roller  616  electrostatically adheres to the electrostatic latent image, by which the toner image of each color is formed. The toner images of the colors are transferred onto the print medium  626  by the transfer rollers  640 , by which the color toner image is formed on the print medium  626 . After the transfer, the toner remaining on the photosensitive drum  613  is removed by the cleaning device  619 . The print medium  626  with the color toner image formed thereon is sent to the fixation device  650 . In the fixation device  650 , the color toner image is fixed on the print medium  626 , by which a color image is formed. The print medium  626  with the color image formed thereon is ejected by the ejection roller pair  625  to a sheet stacker. 
     (5-3) Effect 
     As described above, in the image forming device  600  in the fifth embodiment, the optical print head  500  in the fourth embodiment is provided as each optical print head  611 Y,  611 M,  611 C,  611 K as the exposure device. Thus, according to the image forming device  600  in the fifth embodiment, the quality of printed images can be improved. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of following claims. 
     DESCRIPTION OF REFERENCE CHARACTERS 
       10 - 14 ,  20 - 24 ,  30 - 34 : light-emitting thyristor,  41 K,  61 K: cathode electrode  41 K,  41 A,  61 A: anode electrode,  51 G: gate electrode,  71 : insulation film,  100 ,  110 ,  120 ,  130 ,  140 ,  200 ,  210 ,  220 ,  230 ,  240 ,  300 ,  310 ,  320 ,  330 ,  340 : light-emitting device chip,  101 : substrate part,  102 : substrate,  103 : planarization layer,  400 : substrate unit,  500 : optical print head,  600 : image forming device,  1000 ,  1100 ,  1200 ,  1300 ,  1400 ,  2000 ,  2100 ,  2200 ,  2300 ,  2400 ,  3000 ,  3100 ,  3200 ,  3300 ,  3400 : semiconductor device,  1040 ,  1140 ,  1240 ,  1340 ,  1440 ,  2010 ,  2110 ,  2210 ,  2310 ,  2410 ,  3040 ,  3140 ,  3240 ,  3340 ,  3440 : first semiconductor layer,  1030 ,  1130 ,  1230 ,  1330 ,  1430 ,  2020 ,  2120 ,  2220 ,  2320 ,  2420 ,  3030 ,  3230 ,  3330 ,  3430 : second semiconductor layer,  1020 ,  1120 ,  1220 ,  1320 ,  1420 ,  2030 ,  2130 ,  2230 ,  2330 ,  2430 ,  3020 ,  3120 ,  3220 ,  3320 ,  3420 : third semiconductor layer,  1010 ,  1110 ,  1210 ,  1310 ,  1410 ,  2040 ,  2140 ,  2240 ,  2340 ,  3010 ,  3110 ,  3210 ,  3310 ,  3410 : fourth semiconductor layer,  1043 ,  1143 ,  1243 ,  2311 ,  2411 ,  3043 ,  3143 ,  3243 : anode layer (first layer),  1343 ,  1443 ,  2011 ,  2111 ,  2211 ,  3343 ,  3443 : cathode layer (first layer),  1042 ,  1142 ,  1242 ,  2312 ,  2412 ,  3042 ,  3142 ,  3242 : electron cladding layer (second layer),  1342 ,  1442 ,  2012 ,  2112 ,  2212 ,  3342 ,  3442 : hole cladding layer (second layer),  1041 ,  1141 ,  1241 ,  1341 ,  1441 ,  2013 ,  2113 ,  2213 ,  2313 ,  2413 ,  3041 ,  3141 ,  3241 ,  3341 ,  3441 : active layer (third layer),  3011 ,  3111 ,  3211 : cathode layer (fourth layer),  3311 ,  3411 : anode layer (fourth layer),  3012 ,  3112 ,  3212 : hole cladding layer (fifth layer),  3312 ,  3412 : electron cladding layer (fifth layer),  3013 ,  3113 ,  3213 ,  3313 ,  3413 : active layer (sixth layer).