Patent Publication Number: US-2023139615-A1

Title: Optical Semiconductor Chip

Description:
BACKGROUND ART 
     The present invention relates to an optical semiconductor chip used in an optical transmitter or the like. The capacity of optical communication is increasing from backbone networks to access system networks. Larger capacities and higher performance are also required for optical transmitters, and a high-speed optical modulator is required as one key technology. An electro absorption (EA) optical modulator using an electro absorption effect of a semiconductor is one typical high-speed optical modulator that uses certain light from a laser light source. 
       FIG.  1    shows diagrams of a configuration of an EA optical modulator chip in the conventional art.  FIG.  1 ( a )  is a top view of a part of an EA optical modulator chip  10  in which an EA optical modulator unit is formed, and  FIG.  1 ( b )  is a cross-sectional view of a substrate taken along the line Ib-Ib. First, referring to the cross-sectional view of  FIG.  1 ( b ) , an EA optical modulator has a waveguide structure in which a light absorption layer  15  and a p-type semiconductor layer  14  are formed on an n-type semiconductor substrate  16 . Both sides of an optical waveguide are embedded by an insulation layer  13 . Light from a laser unit (not shown in  FIG.  1   ) is modulated by applying a baseband signal (hereinafter referred to as a modulation signal) to a modulation electrode  12  formed on the p-type semiconductor layer  14 . Referring to  FIG.  1 ( a ) , in order to input a modulation signal via wire bonding, bumps or the like, the electrode is extended from the modulation electrode  12  substantially perpendicular to the optical waveguide, and an electrode pad  11  is formed. The electrode pad  11  has a size of about 40 to 100 μm square and has a structure directly connected to the modulation electrode  12 . 
     The portion under the electrode pad  11  is embedded with a material  17  having a lower dielectric constant than that of a semiconductor insulation layer  15 , and thereby the parasitic capacitance of the electrode pad  11  is reduced. When the parasitic capacitance of the electrode pad  11  receiving a modulation signal is reduced, deterioration of frequency response characteristics and reflection characteristics is prevented in a high frequency range exceeding 10 GHz. 
     The above EA optical modulator chip is formed on an optical semiconductor substrate having a function of conversion between light and electricity, and although not limited, in many cases, a chip is realized by disconnecting a plurality of circuits formed on a wafer-shaped optical semiconductor substrate. The EA optical modulator chip shown in  FIG.  1    is also called an optical semiconductor chip. 
       FIG.  2    shows diagrams of a configuration of an EA optical modulator subassembly according to an external modulation method.  FIG.  2 ( a )  shows a top view of a subassembly  20 , and  FIG.  2 ( b )  shows a cross-sectional view of the subassembly  20  taken along the line IIb-IIb. Referring to  FIG.  2 ( b ) , in the subassembly  20 , a radio frequency (RF) wiring board  22  and an optical semiconductor chip  10  are mounted on a subcarrier  21 . The RF wiring board  22  and the optical semiconductor chip  10  are connected by a terminator integrated chip  23  via gold bumps  24   a  and  24   b.    
     Referring to  FIG.  2 ( a ) , the optical semiconductor chip  10  includes a distributed feedback (DFB) laser  18  and an optical waveguide, and the above  FIG.  1    shows a part of the optical semiconductor chip  10  in which the modulation electrode  12  of the optical waveguide is formed. The RF wiring board  22  includes an RF pattern  25  receiving a modulation signal, and similarly, an RF pattern is formed in the terminator integrated chip  23 . In the terminator integrated chip  23 , a resistor  26  for terminating a modulation signal is formed. The RF pattern  25  is electrically connected to the electrode pad  11  of the optical semiconductor chip  10  via the gold bump  24   a , the terminator integrated chip  23 , and the gold bump  24   b.    
       FIG.  3    is a diagram showing an equivalent circuit model of an EA optical modulator subassembly in the conventional art. An equivalent circuit model  30  corresponds to the entire EA optical modulator subassembly  20  in  FIG.  2   , and is divided into a chip part equivalent circuit  36  corresponding to the optical semiconductor chip  10  in  FIG.  1    and another equivalent circuit part corresponding to the RF wiring board  22  and the terminator integrated chip  23 . The other equivalent circuit is composed of an impedance  32  corresponding to the RF pattern  25 , an impedance  34  corresponding to the terminating resistor  26 , and an inductor component  33  of a gold bump. 
     CITATION LIST 
     Non Patent Literature 
     
         
         [NPL 1] S. Kanazawa et. al., “Equalizer-free transmission of 100-Gbit/s 4-PAM signal generated by flip-chip interconnection EADFB laser module”, Feb. 15, 2017, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 35, NO. 4 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Here, focusing on an equivalent circuit  36  of an optical semiconductor chip  10 , a parasitic capacitance  38  corresponding to an electrode pad  11  has already been considerably reduced by the configuration such as embedding a low-dielectric-constant material  17  shown in  FIG.  1   . For bandwidth characteristics of an optical modulator, a depletion layer capacitance  39  in an equivalent circuit  35  corresponding to a light absorption part  15  has a dominant influence in an equivalent circuit  37  of an optical semiconductor modulator unit in the optical semiconductor chip  10 . 
     When an optical modulator having a higher speed and a wider bandwidth is realized, there is a new problem that frequency response characteristics and reflection characteristics are limited by a depletion layer capacitance of a light absorption part. The present invention has been made in view of such problems, and an object of the present invention is to realize an optical modulator having a higher speed and a wider bandwidth. 
     Means for Solving the Problem 
     In order to achieve such an object, one embodiment of the present invention provides an optical semiconductor chip, including: a laser light source; an optical modulator being optically connected to the laser light source and having an optical waveguide structure having a first type semiconductor base layer, a light absorption layer and a second type semiconductor layer arranged in this order; an electrode pad receiving a modulation signal; a modulation electrode being formed on the second type semiconductor layer; and a high frequency line connecting the electrode pad and the modulation electrode and providing inductance in series with respect to a depletion layer capacitance of the optical waveguide. The optical semiconductor chip can operate as an electro absorption optical modulator. 
     Effects of the Invention 
     As described above, the present invention realizes an optical modulator having a higher speed and a wider bandwidth. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    shows diagrams of a configuration of an electro absorption (EA) optical modulator chip in the conventional art. 
         FIG.  2    shows diagrams of a configuration of an EA optical modulator subassembly according to an external modulation method. 
         FIG.  3    is a diagram showing an equivalent circuit model of an EA optical modulator subassembly in the conventional art. 
         FIG.  4    shows diagrams of a configuration of an optical semiconductor chip of the present disclosure. 
         FIG.  5    is a diagram showing an equivalent circuit model of an EA optical modulator subassembly of the present disclosure. 
         FIG.  6    shows other diagrams of a frequency response of the EA optical modulator subassembly of the present disclosure. 
         FIG.  7    shows diagrams of a configuration of an optical semiconductor chip of Example 2 according to the present disclosure. 
         FIG.  8    shows diagrams of a configuration of a subassembly including the optical semiconductor chip of Example 2. 
         FIG.  9    is a diagram showing an equivalent circuit model of an EA optical modulator subassembly of Example 2. 
         FIG.  10    shows other diagrams of frequency response characteristics of the EA optical modulator subassembly of Example 2. 
         FIG.  11    is a diagram illustrating the effect of widening a bandwidth of an optical modulator by a serial inductor. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An optical semiconductor chip of the present disclosure includes a high frequency line between an electrode pad receiving a modulation signal and a modulation electrode on an optical waveguide having a light absorption layer. A depletion layer capacitance is canceled by an inductor component of a high frequency line. When a portion directly below the high frequency line is embedded with a low-dielectric-constant material or is made hollow, the parasitic capacitance is further reduced. The high frequency line may have a zigzag (meander) shape as well as a linear shape. The electrode pad on the optical semiconductor chip can be connected to other substrates including RF lines via bumps or wire bonding. In the following description, since the optical semiconductor chip operates as an EA modulator, the two terms “optical semiconductor chip” and “EA optical modulator chip” are used interchangeably. 
       FIG.  4    shows diagrams of a configuration of the optical semiconductor chip of the present disclosure, that is, the EA optical modulator chip. Similar to  FIG.  1    showing the configuration in the conventional art,  FIG.  4 ( a )  shows a top view of a part of an optical semiconductor chip  40  in which an EA optical modulator unit is formed, and  FIG.  4 ( b )  shows a cross-sectional view of the substrate taken along the line IVb-IVb. Referring to the cross-sectional view of  FIG.  4 ( b ) , the EA optical modulator has an optical waveguide structure in which a light absorption layer  45  and a p-type semiconductor layer  44  are formed on an n-type semiconductor substrate  46 . Both sides of the optical waveguide are embedded by an insulation layer  43 . Light from a laser unit (not shown in  FIG.  4   ) is modulated by applying a modulation signal to a modulation electrode  42  formed on the p-type semiconductor layer  44 . The above optical waveguide structure has the same configuration as the EA optical modulator chip in the conventional art in  FIG.  1   . The difference from the EA optical modulator chip in the conventional art is the configuration between an electrode pad  41  and the modulation electrode  42 . 
     Referring to  FIG.  4 ( a ) , a high frequency line  48  is provided between the electrode pad  41  receiving a modulation signal via wire bonding, bumps or the like, and the modulation electrode  42 . The high frequency line  48  is a high-frequency-transmission line having an inductive impedance up to a predetermined length at a frequency corresponding to a modulation symbol rate. That is, the high frequency line  48  operates as an inductor. 
     A portion directly below the electrode pad  41  and the high frequency line  48  is embedded with a low-dielectric-constant material  47 . Thereby, the capacitive impedance component in the electrode pad  41  and the high frequency line  48  can be made as small as possible. Instead of embedding the entire portion directly below the electrode pad  41  and the high frequency line  48  with the low-dielectric-constant material  47 , only a portion directly below the electrode pad  41  may be embedded with the low-dielectric-constant material  47 , and a portion directly below the high frequency line  48  can be made hollow. After an electrode is formed on the entire top surface of the chip, by selectively etching and removing only a portion of the low-dielectric-constant material  47  under the high frequency line  48 , and leaving the high frequency line  48  in a bridge shape, such a hollow portion can be formed. The low-dielectric-constant material  47  or the hollow portion can further reduce the capacitive impedance component of the electrode pad  41  and the high frequency line  48 . 
     Therefore, the optical semiconductor chip of the present disclosure including a laser light source, an optical modulator being optically connected to the laser light source and having an optical waveguide structure in which the first type semiconductor base layer  46 , the light absorption layer  45  and the second type semiconductor layer  44  are arranged in this order, the electrode pad  41  receiving a modulation signal, the modulation electrode  42  formed on the second type semiconductor layer, and the high frequency line  48  which connects the electrode pad and the modulation electrode and provides inductance in series with respect to a depletion layer capacitance of the optical waveguide can be implemented. 
     Here, the first type and the second type correspond to the type of doping of each part of the above optical waveguide. In the example of  FIG.  4   , the first type may be an n-type semiconductor, and the second type may be a p-type semiconductor. These types can be reversed. The semiconductor base layer corresponds to a semiconductor substrate on which an optical modulator chip is formed. 
       FIG.  5    is a diagram showing an equivalent circuit model of a subassembly including the optical semiconductor chip of the present disclosure. In the optical semiconductor chip  40  of the present disclosure shown in  FIG.  4   , a configuration between the electrode pad  41  and the modulation electrode  42  is different from that of an optical semiconductor chip  10  in the conventional art. Therefore, the optical semiconductor chip  40  can be mounted in the same configuration as the subassembly shown in  FIG.  2   . That is, the optical semiconductor chip  10  in the conventional art in  FIG.  2    is replaced with the optical semiconductor chip  40  with the configuration in  FIG.  4   , and can be mounted as a subassembly as shown in  FIG.  2    together with an RF wiring board  22  on a subcarrier  21  and a terminator integrated chip  23 . 
     In an equivalent circuit  50  shown in  FIG.  5   , an impedance  52  corresponding to an RF pattern  25 , an impedance  54  corresponding to a terminating resistor  26 , and an inductor component  53  of a gold bump are the same as those of the equivalent circuit in  FIG.  3   . Only an equivalent circuit  56  of the chip part corresponding to the optical semiconductor chip  40  is different from the equivalent circuit of the EA optical modulator subassembly in the conventional art in  FIG.  3   . That is, in the equivalent circuit  56  of the chip part corresponding to the semiconductor chip  40 , an inductance  55  corresponding to the high frequency line  48  is included in series between a parasitic capacitance  58  of the electrode pad  41  and an equivalent circuit  57  of the modulator unit corresponding to the optical waveguide. A depletion layer capacitance  59  in the equivalent circuit  57  of the modulator unit and the inductance  55  are connected in series. Therefore, the decrease in impedance of a capacitive impedance  59  in a high frequency range due to the depletion layer capacitance can be canceled by the increase in impedance of the inductance  55  of the high frequency line  48 . Thereby, the reactance component when the chip is viewed from the modulation input side can be made substantially constant, and it is possible to improve frequency response characteristics and reflection characteristics of modulation characteristics. Hereinafter, examples will be described in more detail. 
     Example 1 
     Referring to  FIG.  4    again, as an example, in the optical semiconductor chip  40  of the present disclosure, InP, GaAs or Si can be used for a substrate. The length of the semiconductor EA optical modulator was 75 μm, the signal line width of the high frequency line  48  was 10 μm, the line length was 150 μm, the thickness of the low-dielectric-constant material  47  was 5 μm, and the specific dielectric constant was 2.3. The electrode pad had a circular shape with a diameter of 60 μm. As the example of the low-dielectric-constant material  47 , an organic material such as a polyimide can be used. 
     In order to compare frequency response characteristics and reflection characteristics of the optical modulator unit of the optical semiconductor chip  40  of the present disclosure shown in  FIG.  4    with those in the conventional art, the optical semiconductor chip  40  was mounted in the form of the subassembly shown in  FIG.  2   , and the performance of the EA optical modulator was checked. For comparison, a subassembly using an EA optical modulator chip  10  shown in  FIG.  1    having the same structure as the optical semiconductor chip  40  except that the high frequency line  48  was not provided was formed. In a subassembly  20  shown in  FIG.  2   , for the RF wiring board  22  and the terminator integrated chip  23 , a ceramic material such as aluminum nitride or a quartz plate can be used. For the subcarrier  21 , an aluminum nitride substrate or the like can be used. 
     The optical waveguide of the optical semiconductor chip  40  had a configuration in which the light absorption layer  45  composed of a quantum well structure of InGaAsP and the p-type semiconductor layer  44  composed of p-type InP were sequentially formed on the n-type InP substrate  46 . The configuration of each part of these optical waveguides is an example, and in order to produce a laser and an optical waveguide of the EA optical modulator as described above, various configurations can be formed using InP, GaAs, or Si for a substrate. In addition, in the configuration of the optical semiconductor chip  40  in  FIG.  4   , the substrate side was an n-type semiconductor, but the substrate side can also be a p-type semiconductor. In the electrode pad  41 , both a DC bias voltage (−0.5 to −2.5 V) and a modulation signal (0.5 to 2.0 Vpp) were superimposed and applied to the modulation electrode  42  via the RF wiring board  22  from the outside. 
     The implementation form in which the optical semiconductor chip shown in  FIG.  2    was mounted is called a subassembly because the subassembly  20  in  FIG.  2    was additionally mounted on a final device such as an optical transmitter and an optical transmission device. The subassembly  20  had an intermediate functional block form for realizing an optical transmitter including an EA optical modulator according to an external modulation method. The subassembly can be simply referred to as an assembly, a module or the like. 
       FIG.  6    shows diagrams of frequency response characteristics and reflection characteristics of the subassembly including the optical semiconductor chip of the present disclosure.  FIG.  6 ( a )  shows frequency characteristics of electrical/optical response (E/O response), and  FIG.  6 ( b )  shows the reflection (return loss) of an input electrical signal in dB. Both can be acquired by an optical component analyzer or the like. In  FIG.  6 ( a ) , the 3 dB bandwidth of the subassembly using the EA optical modulator chip in the conventional art was 58.3 GHz. On the other hand, when the optical semiconductor chip in  FIG.  4    was used, the bandwidth was 61.6 GHz, and the bandwidth was improved by about 3 GHz. In addition, regarding reflection characteristics in  FIG.  6 ( b ) , in the case of the configuration in the conventional art, the reflection loss (reflection level) increased up to a maximum of about −7 dB in a range up to 70 GHz. On the other hand, in the case of the configuration using the optical semiconductor chip in  FIG.  4   , the reflection loss was reduced to a maximum of about −9 dB. Based on the drawings in  FIG.  6   , it was confirmed that, according to the configuration including a high frequency line between the electrode pad receiving a modulation signal and the modulation electrode on the optical waveguide having a light absorption layer, frequency response characteristics and reflection characteristics of the EA optical modulator were improved. 
     In the next example of the optical semiconductor chip, an example in which a meander structure was used as a high frequency line in place of a linear high-frequency waveguide will be shown. 
     Example 2 
       FIG.  7    shows diagrams of a configuration of an optical semiconductor chip of Example 2 according to the present disclosure.  FIG.  7 ( a )  is a top view of a part of an optical semiconductor chip  70  in which an EA optical modulator unit was formed, and  FIG.  7 ( b )  is a cross-sectional view of the substrate taken along the line VIIb-VIIb. Referring to the cross-sectional view of  FIG.  7 ( b ) , the optical semiconductor chip  70  had an optical waveguide structure in which a light absorption layer  75  and a p-type semiconductor layer  74  were formed on an n-type semiconductor substrate  76 . Both sides of the optical waveguide were embedded by an insulation layer  73 . They were embedded by an insulation layer so that an electrode pad  71  was higher than a modulation electrode  72 . Light from a laser unit (not shown in  FIG.  7   ) (for example, DFB laser) was modulated with a modulation signal applied to the modulation electrode  72  formed on the p-type semiconductor layer  74 . The above optical waveguide structure of the optical semiconductor chip  70  was the same as the structure of the optical semiconductor chip  40  in  FIG.  4    except for the configuration of the insulation layer  73  on both sides. The major difference from the semiconductor chip  40  in  FIG.  4    was a configuration between the electrode pad  71  and the modulation electrode  72 . 
     In this example, the electrode pad  71  and the modulation electrode  72  were connected by a zigzag-shaped meander wiring  77 . In the optical semiconductor chip  70  in  FIG.  7   , the length of the EA optical modulator, that is, the modulation electrode  72 , was 75 μm, and the high frequency line was the meander wiring  77  which is a 0.06 nH inductor. The thickness of the electrode pad  71  and the insulation layer  73  under the meander wiring  77  was set to 10 μm, which was higher than the top surface of the optical waveguide constituting the optical modulator, and the parasitic capacitance generated from the electrode pad  71  or the like was reduced. In Example 1, an example of so-called flip chip mounting using gold bumps is shown, but this example had a configuration in which a wire bonding was connected to a region  78  in the subassembly, and a modulation signal was input to the electrode pad  71 . 
       FIG.  8    shows diagrams of a configuration of a subassembly including the EA optical modulator chip of Example 2.  FIG.  8 ( a )  shows a top view of a subassembly  80 , and  FIG.  8 ( b )  shows a cross-sectional view of the subassembly  80  taken along the line VIIIb-VIIIb. Referring to  FIG.  2 ( b ) , in the subassembly  20 , an RF wiring board  82 , an optical semiconductor chip  70 - 1  and a terminator integrated chip  83  were mounted on a subcarrier  81 . The RF wiring board  82 , the optical semiconductor chip  70 - 1 , and the terminator integrated chip  83  were connected by wires  84 - 1  to  84 - 3 , respectively. The optical semiconductor chip  70 - 1  included, for example, a DFB laser  18  and an optical waveguide, and the above EA optical modulator chip  70  shown in  FIG.  7    is shown by a dotted rectangular region  70  as a part of the optical semiconductor chip  70 - 1 . 
     The RF wiring board  82  included an RF pattern  85  receiving a modulation signal, and in the terminator integrated chip  83 , a resistor  86  for terminating a modulation signal was formed. The RF pattern  85 , and the electrode pad  71  of the optical semiconductor chip  70 - 1  were connected by the wire  84 - 1 . In addition, the electrode pad  71 , and one end of the resistor  86  of the terminator integrated chip  23  were connected by the wire  84 - 2 , and the other end of the resistor  86  was grounded by the wire  84 - 3 . 
       FIG.  9    is a diagram showing an equivalent circuit model of the subassembly in  FIG.  8    including the EA optical modulator chip of Example 2. An equivalent circuit  90  was different from the equivalent circuit  50  of the subassembly of Example 1 shown in  FIG.  5    in that the high frequency line  77  become a meander, and was connected by the wires  84 - 1  to  84 - 3  in place of gold bumps. A part except for an equivalent circuit  96  of the chip part corresponding to the optical semiconductor chip  70 - 1  included an equivalent circuit  92  corresponding to the RF wiring board  82 , an equivalent circuit  94  corresponding to the terminator integrated chip  83 , and inductors  100 ,  101 , and  102  corresponding to the three wires  84 - 1  to  84 - 3 . The equivalent circuit  96  of the chip part included a parasitic capacitance  98 , an inductor  93  corresponding to a meander, and an equivalent circuit  97  corresponding to the optical waveguide of the optical modulator unit. The equivalent circuit  97  included a depletion layer capacitance  99 , and limited frequency response characteristics and reflection characteristics. 
       FIG.  10    shows diagrams of frequency response characteristics and reflection characteristics of the subassembly including the EA optical modulator chip of Example 2.  FIG.  10 ( a )  shows frequency characteristics of E/O response, and  FIG.  10 ( b )  shows the reflection of an input electrical signal in dB. Frequency characteristics of the subassembly when the EA optical modulator chip  10  in the conventional art in  FIG.  1    was mounted using wire bonding as in  FIG.  8    are also shown. In  FIG.  10 ( a ) , the 3 dB bandwidth of the subassembly using the EA optical modulator chip in the conventional art was 53.2 GHz. On the other hand, when the optical semiconductor chip of Example 2 in  FIG.  7    was used, the bandwidth was 55 GHz, and the bandwidth was improved by about 1.8 GHz. In addition, for reflection characteristics in  FIG.  10 ( b ) , in the case of the configuration in the conventional art, the reflection loss (reflection level) increased up to a maximum of about −5 dB in a range up to 70 GHz. On the other hand, in the case of the configuration using the optical semiconductor chip in  FIG.  7   , the reflection loss was reduced to a maximum of about −6 dB. Based on the drawings in  FIG.  10   , it was confirmed that, according to the configuration including a high frequency line by a meander between the electrode pad receiving a modulation signal and the modulation electrode on the optical waveguide having a light absorption layer, frequency response characteristics and reflection characteristics of the EA optical modulator were improved. In addition, it was effective in reducing the parasitic capacitance by forming the insulation layer so that the insulation layer directly below the electrode pad and the high frequency line was made thick. 
     Here, it has been confirmed that the amount of improvement in the 3 dB bandwidth when an inductance component is added in series depends on the length of the laser unit (modulation electrode). The results of E/O response shown in  FIG.  11    show an example when the EA length (the length of the modulation electrode) was 75 μm. For example, when the EA length was shorter, the optimal inductance value changed to a smaller value. The configuration from the electrode pad to the modulation electrode in the EA modulator chip in the conventional art was for reducing unnecessary inductance and the capacitance as much as possible and securing an electrode structure sufficient for wire bonding. Therefore, the distance between the electrode pad and the modulation electrode was short as possible and reduced to a maximum of 30 μm or less. It should be noted that the EA modulator chip of the present disclosure differed from that in the conventional art in that the high frequency line operated as an inductor (inductive reactance) added in series to the modulator unit when viewed from the modulation input side. 
     As described above, when the inductor component of the high frequency line was provided in series between the electrode pad in the EA optical modulator chip and the modulation electrode on the optical waveguide functioning as the optical modulator, the depletion layer capacitance of the optical waveguide was canceled. The line width of the inductor component is not limited as long as it has a line width that provides a predetermined characteristic impedance at, for example, a frequency of the modulation signal (for example, the symbol frequency). In addition, the line length may be in a range in which the impedance of the high frequency line becomes an inductive reactance. In order to reduce the parasitic capacitance in the optical waveguide and in order to add inductance in series, the width of the high frequency lines  48  and  77  is preferably narrow, but there is a problem of the physical strength of the electrode in a width of microns or less. Therefore, a range of the diameter of the electrode pad  71  or less and 1 μm or more is preferable. 
     In the EA optical modulator chip of the present disclosure, in the equivalent circuits  56  and  96  corresponding to the optical semiconductor chip, the inductances  55  and  93  were included in series between the parasitic capacitances  58  and  98  of the electrode pads  41  and  71  and the equivalent circuits  57  and  97  of the modulator unit corresponding to the optical waveguide. The depletion layer capacitances  59  and  99  and the inductances  55  and  93  in the equivalent circuits  57  and  97  of the modulator unit were connected in series. Therefore, the reactance component was canceled, and deterioration of high frequency characteristics due to the capacitive impedances  59  and  99  of the depletion layer capacitance can be reduced. The effect of serial inductance can be confirmed from the following simulation. 
       FIG.  11    is a diagram illustrating the effect of widening a bandwidth of a modulator by a serial inductor. In  FIG.  11   , when the EA optical modulator chip having a high frequency line by a meander shown in the drawing was used to constitute a subassembly by a flip chip as shown in  FIG.  2   , the effect of the inductor on the frequency characteristics of E/O response was simulated. The dotted line 0 pH corresponded to the structure in the conventional art not including the high frequency line. It can be understood that, when the inductance was around 60 to 100 pH, the 3 dB bandwidth was the maximum, and when the inductance was around 170 pH, bandwidth characteristics were the same as those in the structure in the conventional art. In addition, it can be understood that, when the inductance increased to 200 pH, adversely, the bandwidth deteriorated as compared with when no high frequency line was applied. Therefore, in an optical modulator composed of an InP substrate, the inductance added by the high frequency line was preferably about 170 pH (0.17 nH) or less. It was confirmed from the simulation that the inductance component added by the high frequency line was selected according to the depletion layer capacitance of the optical waveguide corresponding to the configuration of the optical modulator, and the modulation bandwidth of the EA optical modulator can be maximized. 
     As described above in detail, according to the configuration of the optical semiconductor chip of the present disclosure, it is possible to realize an optical modulator having a higher speed and a wider bandwidth. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be generally used in an optical communication system. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  40 ,  70  Optical semiconductor chip 
           11 ,  41 ,  71  Electrode pad 
           12 ,  42 ,  72  Modulation electrode 
           13 ,  43 ,  73  Insulation layer 
           14 ,  44 ,  74   p -type semiconductor layer 
           15 ,  45 ,  75  Light absorption layer (active layer) 
           16 ,  46 ,  76   n -type semiconductor substrate 
           17 ,  47  low-dielectric-constant material 
           20 ,  80  Subassembly 
           21 ,  81  Subcarrier 
           22 ,  82  RF wiring board 
           23 ,  83  Terminator integrated chip 
           38 ,  58 ,  98  Parasitic capacitance 
           39 ,  59 ,  99  Depletion layer capacitance 
           48 ,  77  High frequency line 
           49  Hollow portion