Abstract:
A plurality of semiconductor devices are disposed in a line on the surface of a supporting substrate. Each semiconductor device is adapted to generate an electric signal depending on the intensity of incident light. Adjacent semiconductor devices are optically coupled by an interconnecting optical waveguide so that light can pass through the semiconductor device one by one in a direction from a first stage closest to an input end to a last stage. An electric signal transmission line is formed of a pair of conductors connected to the semiconductor devices so that the electric signal generated by the semiconductor devices can propagate. One conductor of the pair of conductors of the electric signal transmission line is formed so as to extend in the air above the supporting substrate between adjacent semiconductor devices.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This invention is based on and claims priority of Japanese patent application 2001-364665, filed on Nov. 29, 2001, the whole contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a semiconductor light receiving device, and more particularly, to a semiconductor light receiving device for use in a broadband optical communication system. Advances in the Internet have produced a need, becoming increasingly greater, for a high-speed optical communication system. In optical communication systems, an information transmission rate higher than 40 Gbits/s is required. To meet this requirement, a semiconductor light receiving device capable of operating at a sufficiently high speed is needed. 
   2. Description of the Related Art 
     FIG. 10A  is a perspective view of a semiconductor light receiving device according to a first conventional technique disclosed in Japanese Unexamined Patent Application Publication No. 2001-127333, and  FIG. 10B  is a cross-sectional view thereof. A tapered optical waveguide  501  is formed on the surface of a semi-insulating InP substrate  500 . A pin photodiode  502 , which is buried in an InP region, is coupled with an output end of the tapered optical waveguide  501 . The pin photodiode  502  is connected to an n-side electrode  505  and a p-side electrode  506 . The thickness of the tapered optical waveguide  501  gradually increases in a direction from its input end toward its output end. 
   An optical signal inputted into the tapered optical waveguide  501  through its input end travels along the tapered optical waveguide  501  to the photodiode  502 . When the photodiode  502  receives the optical signal, the photodiode  502  converts the input optical signal into an electric signal. The resultant electric signal is outputted to the electrodes  505  and  506 . 
   This semiconductor light receiving device using the tapered optical waveguide has as high response performance as capable of operating at 40 GHz. Using this semiconductor light receiving device, an apparatus having high-efficiency performance regardless of polarization has been achieved (N. Yasuda, et at. CPT2001 Technical Digest, (2001), pp. 105). 
     FIG. 11A  is a plan view of a traveling-wave light receiving device according to a second conventional technique disclosed in U.S. Pat. No. 5,270,532 and also in a paper by Kirk S. Giboney published in IEEE Trans. on Microwave Theory and Techniques, Vol. 45, No. 8 (1997), pp. 1310-1319.  FIGS. 11B and 11C  are cross-sectional views taken along one-dot-chain-line B 11 —B 11  and one-dot-chain line C 11 —C 11 , respectively, of FIG.  11 A. 
   A multilayer structure comprising an n-type semiconductor layer  520  located at the bottom, a light receiving layer  511  made of an intrinsic semiconductor located in a middle layer, and a p-type semiconductor layer  521  located at the top is disposed on the surface of a semi-insulating semiconductor substrate  510 . This multilayer structure extends along a single straight line. A center electrode  512  is disposed on the surface of the p-type semiconductor layer  521 . A ground electrode  513  formed on the surface of the semiconductor substrate  510  is connected to the n-type semiconductor layer  520 . 
   The multilayer structure consisting of the three layers, that is, the n-type semiconductor layer  520 , the light receiving layer  511 , and the p-type semiconductor layer  521 , forms an optical waveguide-type light receiving element. The ground electrode  513  and the center electrode  512  form an electric signal transmission line extending in parallel with the optical waveguide-type light receiving element. An optical signal is inputted to the light receiving layer  511  through its input end. A ground pad  523  is connected to the output end of the ground electrode  513 . An output pad  524  is connected to the output end of the center electrode  512 . 
   An optical signal is inputted into an optical waveguide formed by the light receiving layer  511  through its input end, and the optical signal propagates inside the optical waveguide. The propagation of the optical signal causes an electric signal to be generated between the n-type semiconductor layer  520  and the p-type semiconductor layer  521 , and the generated electric signal propagates along the electric signal transmission line consisting of the ground electrode  513  and the center electrode  512 . A high quantum efficiency can be achieved over a wide band by matching the propagation velocity of the optical signal with the propagation velocity of the electric signal. 
     FIG. 12A  is a perspective view of a velocity-matched traveling-wave light receiving device according to a third conventional technique disclosed in a paper by M. S. Islam et al. published in Microwave Photonics Technical Digest 2000, Oxford, UK, pp. 217, a paper by T. Chau et al. published in IEEE Photonics Technology Letters, Vol. 12, No. 8 (2000), pp. 1055-1057, and in U.S. Pat. No. 5,572,014.  FIG. 12B  is a side view thereof, and  FIG. 12C  is a cross-sectional view taken along a plane vertical to a light propagation direction. 
   An optical waveguide  531  is formed on a semi-insulating semiconductor substrate  530 . On the upper surface of the optical waveguide  531 , a plurality of photodiodes  532 , spaced apart from each other, are disposed along the light propagation direction. Each photodiode  532  is coupled, in an evanescent coupling, with the optical waveguide  531 . An electrically conductive film  533  is disposed at one side of the optical waveguide  531  and an electrically conductive film  534  is disposed at the opposite side. The electrically conductive films  533  and  534  form an electric signal transmission line. One electrode of each photodiode  532  is connected to the electrically conductive film  533 , and the other electrode is connected to the electrically conductive film  534 . 
   An optical signal propagating through the optical waveguide  531  causes the photodiodes  532  to generate an electric signal. The generated electric signal propagates through the electric signal transmission line consisting of the electrically conductive films  533  and  534 . In this light receiving device, the propagation velocity of the optical signal is matched with the propagation velocity of the electric signal so as to achieve high performance to respond a signal at a very high frequency such as several ten GHz. 
   In the first conventional technique shown in  FIGS. 10A and 10B , it is required that the capacitance of the photodiode  502  should be as small as possible to achieve a high speed operation. The capacitance can be reduced by reducing the length of the photodiode  502  in the direction in which light propagates. 
     FIG. 13A  shows the dependence of the capacitance on the length of the photodiode  502 . In  FIG. 13A , the horizontal axis represents the length of the photodiode  502  in units of μm, and the vertical axis represents the capacitance in units of fF. Herein, the photodiode  502  has a width of 4 μm. As can be seen, the capacitance decreases with decreasing length of the photodiode  502 . When the length of the photodiode  502  is 3 μm, the capacitance becomes about 15 fF. When an electric circuit at the following stage connected to the photodiode  502  has an input impedance of 50 Ω, the cutoff frequency determined by a CR time constant becomes as high as 300 GHz, and thus a very high-speed operation is possible. 
   However, the reduction in length of the photodiode  502  results in a reduction in the absorption of light. 
     FIG. 13B  shows the dependence of the internal quantum efficiency on the length of the photodiode  502 . In  FIG. 13B , the horizontal axis represents the length of the photodiode  502  in units of μm, and the vertical axis represents the internal quantum efficiency in units of %. As can be seen, the reduction in the length of the photodiode  502  results in a reduction in internal quantum efficiency. Thus, with the first conventional light receiving device, it is difficult to achieve both a high-speed operation and a high efficiency at the same time. 
   In the second conventional light receiving device shown in  FIGS. 11A  to  11 C, a high-speed operation can be achieved regardless of the capacitance between the n-type semiconductor layer  520  and the p-type semiconductor layer  521 . It is desirable that the characteristic impedance of the optical waveguide-type light receiving element be adjusted to 50 Ω so as to match the input impedance of an electric circuit at the following stage connected to this semiconductor light receiving device. In the case where the thickness of the light receiving layer  511  is about 0.2 μm, if the width of the optical waveguide-type light receiving element is adjusted to be about 1 μm, the characteristic impedance of the electric signal transmission line becomes equal to 50 Ω. However, if the width of the optical waveguide-type light receiving element is set to be so small, it becomes difficult to form the center electrode  512  on the upper surface of the optical waveguide-type light receiving element. 
   In the third conventional light receiving device shown in  FIGS. 12A  to  12 C, not only the characteristic impedance of the electric signal transmission line is adjusted to be 50 Ω, but also other various parameters should be adjusted. Furthermore, in order to match the propagation velocity of the electric signal with the propagation velocity of the optical signal, it is required to adjust the distance of two adjacent photodiode  532  to about 0.15 mm. In the case where 10 photodiodes  532  are disposed to achieve a high quantum efficiency, the length of the light receiving device becomes as great as 1.5 mm or greater. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a small-sized wide-band semiconductor light receiving device having a high quantum efficiency. 
   According to one aspect of the present invention, there is provided a semiconductor light receiving device comprising: a plurality of semiconductor devices, which are disposed in a line on a surface of a supporting substrate and each of which is adapted to generate an electric signal depending on the intensity of incident light; an interconnecting optical waveguide formed such that the plurality of semiconductor devices are coupled by the interconnecting optical waveguide from one semiconductor device to an adjacent one so as to allow light to pass through the semiconductor devices from one semiconductor device to an adjacent one in a direction from the first stage toward the last stage; and an electric signal transmission line formed by a pair of conductors connected to the semiconductor devices, for transmitting an electric signal generated by the semiconductor devices, a first conductor of the pair of conductors extending in the air above the supporting substrate between adjacent semiconductor devices. 
   When an optical signal passes through the semiconductor device from one to another, an electric signal is generated by the semiconductor devices and transmitted along the electric signal transmission line. Forming the first conductor so as to extend in the air makes it possible to adjust the inductance of the first conductor such that the propagation velocity of the electric signal is matched with the propagation velocity of the optical signal and such that impedance matching between the electric signal transmission line and an electric circuit at a following stage is achieved. 
   According to another aspect of the present invention, there is provided a semiconductor light receiving device comprising: a first conductive layer formed of a semiconductor having a first conduction type on the surface of a supporting substrate; a plurality of multilayer structures disposed in a line on the surface of the first conductive layer, each multilayer structure including a multilayer structure comprising a light receiving layer and a second conductive layer formed of a semiconductor having a second conduction type opposite to the first conduction type, each multilayer structure being adapted to generate an electric signal between the first conductive layer and the second conductive layer in response to an optical signal incident on the light receiving layer, an interconnecting optical waveguide disposed on the first conductive layer so as to optically connect light receiving layers of adjacent multilayer structures with each other; and a conductive thin wire disposed so as to connect the second conductive layers of the multilayer structures from one to another thereby allowing the generated electric signal to be transmitted. 
   The three layers, that is, the first conductive layer, the light receiving layer, and the second conductive layer, form a photodiode. An electric signal generated by the photodiode is transmitted through the conductive thin wire. 
   According to still another aspect of the present invention, there is provided a semiconductor light receiving device comprising: a plurality of semiconductor devices, which are disposed in a line on a surface of a supporting substrate and each of which is adapted to generate an electric signal depending on the intensity of incident light; an interconnecting optical waveguide formed such that the plurality of semiconductor devices are coupled by the interconnecting optical waveguide from one semiconductor device to an adjacent one so as to allow light to pass through the semiconductor devices from one semiconductor device to an adjacent one in a direction from the first stage toward the last stage, the interconnecting optical waveguide being butt-coupled with the semiconductor devices; and an electric signal transmission line formed by a pair of conductors connected to the semiconductor devices, for transmitting an electric signal generated by the semiconductor devices. 
   The butt-coupled structure used herein allows an increase in the optical coupling efficiency. 
   The semiconductor devices are connected from one to another using the conductive thin wire having inductance selected such that the transmission line formed by the conductive thin wire has a characteristic impedance equal to 50 Ω, thereby achieving impedance matching with the electric circuit at the following stage. The propagation velocity of the electric signal propagating through the transmission line is matched with the propagation velocity of the optical signal propagating through the semiconductor devices disposed in a line from one to another, thereby achieving a high optical-to-electric conversion efficiency even at very high frequencies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a cross-sectional view of a semiconductor light receiving device according to a first embodiment, and  FIG. 1B  is a plan view thereof; 
       FIG. 2  is a cross-sectional view of an optical-to-electric converter of the semiconductor light receiving device according to the first embodiment; 
       FIG. 3  is an equivalent circuit of the optical-to-electric converter of the semiconductor light receiving device according to the first embodiment; 
       FIG. 4  is a graph showing the internal quantum efficiency of an optical-to-electrical converter; 
       FIGS. 5A  to  5 N are cross-sectional views and plan views showing a method of producing the semiconductor light receiving device according to the first embodiment; 
       FIG. 6  is a crosssectional view of an optical-to-electric converter of a semiconductor light receiving device according to a second embodiment, 
       FIG. 7  is a cross-sectional view of an optical-to-electric converter of a semiconductor light receiving device according to a third embodiment; 
       FIG. 8  is a plan view of an optical-to-electric converter of a semiconductor light receiving device according to a fourth embodiment; 
       FIG. 9  is a cross-sectional view of a semiconductor light receiving device according to a fifth embodiment; 
       FIG. 10A  is a perspective view of a semiconductor light receiving device according to a first conventional technique, and  FIG. 10B  is a cross-sectional view thereof; 
       FIG. 11A  is a plan view of a semiconductor light receiving device according to a second conventional technique, and  FIGS. 11B and 11C  are cross-sectional views thereof; 
       FIG. 12A  is a perspective view of a semiconductor light receiving device according to a third conventional technique,  FIG. 12B  is a side view thereof, and  FIG. 12C  is a cross-sectional view thereof; and 
       FIG. 13A  is a graph showing the dependence of the capacitance of a photodiode on its length, and  FIG. 13B  is a graph showing the dependence of the internal quantum efficiency of a photodiode on its length. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1A and 1B  are a cross-sectional view and a plan view of a semiconductor light receiving device according to a first embodiment of the present invention, wherein the cross-sectional view shown in  FIG. 1A  is taken along a line A 1 —A 1  of FIG.  1 B. 
   As shown in  FIG. 1A , an n-type layer  2  made of n + -type InP with a thickness of about 2 μm is formed in a partial area of the surface of a semiconductor substrate  1  made of semi-insulating InP. In  FIG. 1B , ⅔ of the surface, on the left-hand side, of the semiconductor substrate  1  is covered with the n-type layer  2 . 
   On the n-type layer  2 , a first-stage multilayer structure  10 A, a second-stage multilayer structure  10 B, and a third-stage multilayer structure  10 C are disposed in a line from left to right in FIG.  1 . One photodiode is formed by the n-type layer  2  and one of multilayer structures  10 A to  10 C. Thus, hereinafter, each of the multilayer structures  10 A to  10 C will be called a photodiode. Each photodiode  10 A to  10 C has a three-layer structure including a light receiving layer, a p-type layer, and a cap layer, as will be described in detail later with reference to FIG.  2 . 
   A tapered optical waveguide  11 A extends from the first-stage photodiode  10 A to left in FIG.  1 B. The thickness of the tapered optical waveguide  11 A gradually increases toward the first-stage photodiode  10 A. An interconnecting optical waveguide  11 B is disposed between the first-stage photodiode  10 A and the second-stage photodiode  10 B, and an interconnecting optical waveguide  11 C is disposed between the second-stage photodiode  10 B and the third-stage photodiode  10 C. The tapered optical waveguide  11 A and the interconnecting optical waveguides  11 B and  11 C are made of undoped InGaAsP. 
   A clad layer  12  formed of undoped InP is disposed on the tapered optical waveguide  11 A and the interconnecting optical waveguides  11 B and  11 C. The tapered optical waveguide  11 A, the light receiving layer of the first-stage photodiode  10 A, the interconnecting optical waveguide  11 B, the light receiving layer of the second-stage photodiode  10 B, the interconnecting optical waveguide  11 C, and the light receiving layer of the third-stage photodiode  10 C are butt-coupled from one to another so as to form an optical waveguide. 
   The upper surfaces and the side faces of the tapered optical waveguide  11 A and the interconnecting optical waveguides  11 B and  11 C are covered with a clad layer  15  formed of a Fe-doped semi-insulating InP. A contact layer  16  having a three-layer structure of Au/Zn/Au is formed on the upper surface of each photodiode  10 A to  10 C. The contact layers  16  on the photodiodes  10 A to  10 C are connected from one to another by a conductive thin wire  20 . An end of the conductive thin wire  20  is connected to a pad  21  formed on the surface of the semiconductor substrate  1 . 
   As shown in  FIG. 1B. A  contact layer  30  are formed in areas, where the n-type layer  2  is exposed, at both sides (upper and lower sides in  FIG. 1B ) of the line of optical-to-electric converter formed by the photodiodes  10 A to  10 C. The contact layer  30  has a two-layer structure formed by a AuGe layer and a Au layer and is ohomically connected to the n-type layer  2 . A coplanar electrode  31  formed on the surface of the semiconductor substrate  1  is connected to the contact layer  30 . 
     FIG. 2  is a cross-sectional view illustrating the details of the optical-to-electric converter shown in FIG.  1 A. Each of the photodiodes  10 A to  10 C has a three-layer structure including a light receiving layer  3  formed of undoped InGaAs with a thickness of 0.06 to 0.3 μm, a p-type layer  4  formed of p-type InP with a thickness of about 2 μm, and a cap layer  5  formed of p-type InGaAs with a thickness of 0.05 μm, disposed in this order. A contact layer  16  is formed on the p-type layer  5  of each photodiode  10 A to  10 C. 
   Conductive elements  19 A to  19 C are disposed on the respective contact layers  16  of the first to third-stage photodiodes  10 A to  10 C such that conductive elements  19 A to  19 C are ohmically connected to the respective contact layers  16 . The conductive element  19 A at the first stage and the conductive element  19 B at the second stage are connected to each other by a conductive thin wire  20 A. The conductive element  19 B at the second stage and the conductive element  19 C at the third stage are connected to each other by a conductive thin wire  20 B. The conductive element  19 C at the third stage and the pad  21  are connected to each other by a conductive thin wire  20 C. The conductive thin wires  20 A to  20 C are formed so as to extend in the air above the semiconductor substrate  1 . The conductive thin wires  20 A to  20 C and the n-type layer  2  form an electric signal transmission line. 
   From the tapered optical waveguide  11 A, an optical signal is input to the light receiving layer  3  of the first-stage photodiode  10 A. After passing through the light receiving layer  3  of the first-stage photodiode  10 A, the optical signal further passes through the interconnecting optical waveguide  11 B, the light receiving layer  3  of the second-stage photodiode  10 B, the interconnecting optical waveguide  11 C, and the light receiving layer  3  of the third-stage photodiode  10 C, from one to another. When the optical signal passes through the light receiving layers  3  of the photodiodes  10 A to  10 C, part of the optical signal is converted into an electric signal. 
   The electric signal generated by the first-stage photodiode  10 A propagates through the conductive thin wire  11 A and joins with the electric signals generated by the photodiodes  10 B and  10 C at the second and third stages. The resultant electric signal further propagates until reaching the pad  21 . By matching the propagation velocity of the optical signal traveling along the interconnecting optical waveguides  11 B and  11 C with the propagation velocity of the electric signal traveling along the conductive thin wires  11 A and  11 B, a high conversion efficiency can be achieved. 
     FIG. 3  is an equivalent circuit of the semiconductor light receiving device according to the first embodiment. The thee photodiodes  10 A to  10 C are represented by capacitors C, the conductive thin wires  11 A to  11 C are represented by inductors L, and the n-type layer  2  is represented by a single interconnection line. In general, an electric circuit  40  having an input impedance of 50 Ω at a following stage is connected between the pads  21  and  31 . To connect such an electric circuit, it is desirable that the electric signal transmission line formed by the capacitors C and the inductors L be designed to have a characteristic impedance of 50Ω. 
   This electric signal transmission line can be regarded as a transmission line formed by cascading several unit segments (three unit segments, in this first embodiment) each consisting of one photodiode and one conductive thin wire. The length of the unit segment is at most 100 μm. Therefore, if the frequency of the electric signal being propagated is assumed to be 80 to 160 GHz, each the unit segment can be regarded as a lumped-constant circuit. 
   The characteristic impedance of a transmission line is discussed below. If the characteristic impedance of the transmission line shown in  FIG. 3  is denoted by Z, and the angular frequency of the electric signal is denoted by ω, then the characteristic impedance Z is given by
 
 Z=iωL /2+( L/C−ω   2   L   2 /4) 1/2   (1)
 
Within the operating frequency region, the second term in an expression of a square root of the real part of equation (1) can be neglected. Therefore, the characteristic impedance can be approximated as (L/C) 1/2 . Therefore, if
 
( L/C ) 1/2 =50  (2)
 
is satisfied, attenuation and reflection of the high-frequency electric signal are suppressed, and the electric signal is efficiently transmitted to the electric circuit at the following stage.
 
   The matching between the propagation velocity of the optical signal and the propagation velocity of the electric signal is discussed below. 
   The time T 1  needed for the optical signal to propagate through the interconnecting optical waveguide  11 B or  11 C shown in  FIG. 2  is given by
 
 T   1 =( n   eff   /C   o )( L   pin   +L   gap )  (3)
 
where n eff  denotes the effective refractive index of the interconnecting optical waveguides  11 B and  11 C, c o  denotes the velocity of light in a vacuum, L pin  denotes the length of each photodiode  10 A to  10 C, and L gap  denotes the space between two adjacent photodiodes.
 
   On the other hand, the time T 2  needed for the generated electric signal to propagate through the conductive thin wire  11 A or  11 B is given by
 
 T   2 =( L·C ) 1/2   (4)
 
   In order to match the velocity of the optical signal and the velocity of the electric signal with each other, it is required that the times T 1  and T 2  should be equal to each other. That is, the following equation should be satisfied.
 
( L·C ) 1/2 =( n   eff   /c   o )( L   pin   +L   gap )  (5)
 
   In equation (5), the effective refractive index n eff  is equal to 3.1704. If the width of each of the conductive thin wires  11 A and  11 B is selected to be 7 μm and the length L 1  thereof is selected to be 47 μm, the inductance L of the unit segment becomes 0.0345 nH. Furthermore, if the width of each of the photodiodes  10 A to  10 C is selected to be 4 μm, the length L pin  thereof to 3 μm, and the thickness of the light receiving layer  3  to 0.15 μm, then the capacitance C of the unit segment becomes 0.015 fF. Thus, in this case, equation (2) is satisfied. Under the above conditions, if the space L gap  between adjacent photodiodes is set to 55 μm, then equation (5) is satisfied. 
   The light detection sensitivity is discussed below. In the first embodiment, as described above, the plurality of photodiodes are connected by the interconnecting optical waveguide such that the optical signal passes through the photodiodes from one to another, thereby allowing part of light remaining without being absorbed by a photodiode to be effectively fed to a photodiode at a following stage. This makes it possible to improve the internal quantum efficiency. 
   The coupling loss between the interconnecting optical waveguide  11 B and the photodiode  10 A can be minimized by designing the interconnecting optical waveguide  11 B and the photodiode  10 A so as to maximize the overlap integral between the electric field distributing in the interconnecting optical waveguide  11 B in width and thickness directions and the electric field distributing in the photodiode  10 A in width and thickness directions. For example, when the wavelength of the optical signal is 1.55 μm, if the thickness of the interconnecting optical waveguide  11 B is set to be 0.25 μm, the thickness of the light receiving layer  3  of the photodiode  10 A is set to be 0.15 μm, and the interconnecting optical waveguide  11 B and the photodiode  10 A are butt-coupled, a coupling efficiency as high as about 98% can be achieved. Similarly, high coupling efficiencies can be achieved also for the other coupling interfaces between the interconnecting optical waveguide and the photodiodes and for the coupling interface between the tapered optical waveguide  11 A and the photodiode  10 A. This makes it possible to achieve a high conversion efficiency using a small number of photodiodes. The reduction in the number of photodiodes results in a reduction in the total apparatus size. 
     FIG. 4  shows the dependence of the internal quantum efficiency on the number of photodiodes. In  FIG. 4 , the horizontal axis represents the number of photodiodes, and the vertical axis represents the internal quantum efficiency in units of %. As can be seen, although the internal quantum efficiency is about 40% when one photodiode is used, the internal quantum efficiency becomes as high as 70% if three photodiodes are used. With the structure according to the first embodiment, the calculated total coupling loss between the interconnecting optical waveguide and the photodiodes is as small as about 5%. As described above, the butt-coupling structure makes it possible to achieve a higher internal quantum efficiency using a small number of photodiodes than can be achieved by the evanescent coupling structure. 
   Referring to  FIGS. 5A  to  5 N, a method of producing the semiconductor light receiving device according to the first embodiment is described below. 
   As shown in  FIG. 5A , an n-type layer  2  of n + -type InP with a thickness of about 2 μm, a light receiving layer  3  of undoped InGaAs with a thickness of 0.06 to 0.3 μm, a p-type layer  4  of p-type InP with a thickness of 2 μm, and a cap layer  5  of p-type InGaAs with a thickness of 0.05 μm are formed, in this order, on the surface of a semiconductor substrate  1  made of semi-insulating InP. 
   These films may be formed by means of, for example, metal organic chemical vapor deposition (MOCVD) at a growth temperature of 630° C. and at a pressure of 1.33×10 4  Pa (100 Torr). In this case, phosphine (PH 3 ), arsine (AsH 3 ), monosilane (SiH 4 ), trimethyl indium (TMI), and triethyl gallium (TEG) may be used as source gases. 
   Thereafter, as shown in  FIG. 5B , the three layers from the cap layer  5  to the light receiving layer  3  are patterned using a mask pattern  6  formed of SiO 2  or the like. A multilayer structure composed of the light receiving layer  3 , the p-type layer  4 , and the cap layer  5  is formed in area covered with the mask pattern  6 , and the n-type layer  2  is exposed in areas that are not covered with the mask pattern  6 . 
     FIG. 5C  is a plan view of the mask pattern  6 , wherein  FIG. 5B  is a cross-sectional view taken along line B 5 —B 5  of FIG.  5 C. The mask pattern  6  is formed so as to cover, in  FIG. 1B , areas at both sides of a part of the tapered optical waveguide  11 A whose distance from the photodiode  10 A is equal to or smaller than D 1  and area at both sides of a band-shaped area extending from the end of the tapered optical waveguide  11 A in a direction toward the photodiode  10 A. Furthermore, the mask pattern  6  also covers areas corresponding to the photodiodes  10 A to  10 C. The width G 1  of a band-shaped area that corresponds to the tapered optical waveguide  11 A and that is not covered with the mask pattern  6  is set to be slightly greater than the width of the tapered optical waveguide  11 A to be produced. 
   Thereafter, as shown in  FIG. 5D , an optical waveguide layer  11  of undoped InGaAsP and a clad layer  12  of undoped InP are selectively grown by means of MOCVD on the surface of the n-type layer  2 . The optical waveguide layer  11  and the clad layer  12  are not grown on the mask pattern  6 . The film growth may be performed, for example, at a growth temperature of 630° C. and at a pressure of 1.33×10 4  Pa (100 Torr), using phosphine, arsine, monosilane, trimethyl indium, and triethyl gallium as source gases. By forming the mask pattern  6  so as to have a shape such as that shown in  FIG. 5C , it becomes possible to form the optical waveguide layer  11  such that the thickness thereof gradually decreases in a direction from the leftmost multilayer structure to left in FIG.  5 D. After completion of the selective growth, the mask pattern  6  is removed. 
   Thereafter, as shown in  FIG. 5E , a mask pattern  7  of SiN is formed on the clad layer  12 . 
     FIG. 5F  is a plan view of the mask pattern  7 . The tapered optical waveguide  11 A shown in  FIG. 1B and a  band-shaped area extending from the tapered optical waveguide  11 A is covered by the mask pattern  7 . 
   The clad layer  12 , the optical waveguide layer  11 , and the three layers from the cap layer  5  to the light receiving layer  3  are etched using the mask pattern  7  as an etching mask. The three layers may be etched by means of dry etching using an inductively coupled plasma of SiC 4  and Ar. 
   The area covered with the mask pattern  7  remains without being etched and thus, in this area, the tapered optical waveguide  11 A, the photodiodes  10 A to  10 C, and the interconnecting optical waveguides  11 B and  11 C are formed. In the areas on both sides of this area covered with the mask pattern  7 , the n-type layer  2  is exposed. 
   Thereafter, as shown in  FIG. 5G , a clad layer  15  of Fe-doped semi-insulating InP is grown by means of MOCVD over the entire surface of the substrate. The concentration of Fe doped in the clad layer  15  is set to be 5×10 16  cm −3 . The growth of the clad layer  15  may be performed, for example, at a growth temperature of 630° C. and at a pressure of 1.33×10 4  Pa (100 Torr), using phosphine, trimethyl indium, ferrocene, and chloromethane as source gases. 
   Thereafter, as shown in  FIG. 5H , the clad film  15  in an area, on the same side as the optical-to-electric converter, adjacent to the boundary between the photodiode  10 A and the tapered optical waveguide  11 A is dry-etched using an inductively coupled plasma such that the clad film  15  is partially etched in its thickness direction. A mixture of SiC 4  and Ar may be used as an etching gas. 
   Thereafter, as shown in  FIG. 5I , the clad film  15  in an area, on the same side as the photodiode  10 A, adjacent to the boundary between the photodiode  10 A and the tapered optical waveguide  11 A so that the upper surface of the photodiodes  10 A to  10 C are exposed. In this etching process, a chlorine-based etchant may be used. A contact layer  16  consisting of three layers of Au/Zn/Au is then formed by means of a lift-off method on the surface of the cap layer  5  of the exposed photodiodes  10 A to  10 C. 
   Thereafter, as shown in  FIG. 5J , a mask pattern  17  of SiN is formed on the surface of the substrate so as to cover an area, on the same side as the tapered optical waveguide  11 A, adjacent to the boundary between the tapered optical waveguide  11 A and the first-stage photodiode  10 A shown in FIG.  1 B and an area including the optical-to-electric converter area in which the photodiodes  10 A to  10 C and the interconnecting optical waveguides  11 B and  11 C are disposed. 
   Thereafter, as shown in  FIG. 5K , the clad layer  12  and the optical waveguide layer  11  are etched using the mask pattern  17  as an etching mask such that the clad film  15  in areas at both sides of the optical-to-electric converter area including the photodiodes  10 A and  10 C shown in  FIG. 1B  (areas on the upper and lower sides of  FIG. 1B ) is etched and thus the n-type layer  2  is exposed in those areas. 
   Thereafter, as shown in  FIG. 5L , the n-type layer  2  in an area to the right of the right end of the etched clad layer  12  and optical waveguide layer  11  is etched so that the semiconductor substrate  1  is exposed in this area. The n-type layer  2  in areas at both sides (areas on the upper and lower sides of  FIG. 1B ) of the optical-to-electric converter area including the photodiodes  10 A to  10 C shown in  FIG. 1B  remains without being etched. 
   Thereafter, as shown in  FIG. 5M , the mask pattern  17  is removed. The contact layer  30  shown in  FIG. 1B  is then formed by means of a lift-off technique. The contact layer has a two-layer structure composed of a AuGe layer and a Au layer disposed in this order. The coplanar electrode  31  is then formed by means of the lift-off technique. The coplanar electrode  31  has a three-layer structure composed of a Ti layer, a Pt layer, and a Au layer disposed in this order. 
   Thereafter, as shown in  FIGS. 1A and 2 , the conductive thin wire  20  and the pad  21  are formed. The conductive thin wire  20  and the pad  21  may be formed by means of, for example, a method described in paragraphs 33 to 35 with reference to FIG. 7 of Japanese Unexamined Patent Application Publication No. 2001-127333. This method is briefly described below. 
   First, a resist pattern is formed on the surface of a substrate such that an opening corresponding to the pad  21  is formed in the resist pattern. An underlying metal layer is then evaporated such that the surface of the resist pattern and the area inside the opening are covered with the evaporated metal layer. The underlying metal layer has a two-layer structure composed of a AuZn layer and a Au layer. The underlying metal layer is then coated with a second resist pattern such that the underlying metal layer is covered with the second resist pattern except for the area corresponding to the pad  21 . Au is then plated using the underlying metal layer as a plating electrode thereby forming the pad  21  in the opening. The first-layer resist pattern and the second-layer resist patterns are then removed such that the pad  21  remains. 
   Thereafter, as shown in  FIG. 5N , a first-layer resist film  50  is formed on the substrate. Openings corresponding to the contact layers  16  and the pad  21  are formed in the resist film  50 . An underlying metal layer  51  is then evaporated such that the surface of the resist film  50  and the area in the openings are covered with the underlying metal layer  51 . The underlying metal layer  51  has a two-layer structure composed of a AuZn layer and a Au layer. 
   A second-layer resist film  53  is then formed on the underlying metal layer  51 . Openings corresponding to the conductive elements  19 A to  19 C and the pad  21  are formed in the resist film  53 . Au is then plated using the underlying metal layer  51  as a plating electrode so that Au is embedded in the openings thereby forming the conductive elements  19 A to  19 C. Via this plating process, a Au film  54  with the same thickness as the conductive elements  19 A to  19 C is also formed on the pad  21 . 
   Thereafter, a second-layer underlying metal layer  55  is evaporated on the resist film  53 . A thirdlayer resist film  57  is then formed on the underlying metal layer  55 . An opening corresponding to the conductive thin wire  20  is formed in the resist film  57 . Au is then plated using the second-layer underlying metal layer  55  as a plating electrode so as to form the conductive thin wire  20 . 
   The third-layer resist film  57  is then removed. The second-layer resist film  53  is then removed together with the second-layer underlying metal layer  55  on the second-layer resist film  53 . Furthermore, the first-layer resist film  50  is removed together with the first-layer underlying metal layer  51  on the first-layer resist film  50 . The conductive elements  19 A to  19 C and the conductive thin wire  20  remain without being removed. 
   Now, referring to  FIGS. 6  to  8 , semiconductor light receiving devices according to second to fourth embodiments are described. In the above-described semiconductor light receiving device according to the first embodiment, a greatest photocurrent passes through the first-stage photodiode  10 A, and the photocurrents passing through photodiodes decrease in a direction toward the last stage. However, if a large photocurrent flows through a small area of a light receiving layer, a space-charge effect can cause a reduction in the response speed. In the second to fourth embodiments described below, an excess photocurrent that can cause such a problem is prevented. 
     FIG. 6  is a cross-sectional view of an optical-to-electric converter of a semiconductor light receiving device according to a second embodiment. The thicknesses of light receiving layers  3 A to  3 C of photodiodes  10 A to  10 C at the first to third stages are respectively set to 0.1 μm, 0.15 μm, and 0.2 μm. That is, the light receiving layer thickness decreases in a direction from the last stage to the first stage. This means that an optical signal traveling through the light receiving layers encounters a gradual reduction in thickness of the light receiving layer. The reduction in the thickness of the light receiving layer results in a reduction in refinement of light and thus a reduction in absorption of light. 
   As in the first embodiment, the width of each photodiode  10 A to  10 C is set to 4 μm and the length L pin  is set to 3 μm. The space L gap1  between the first-stage photodiode  10 A and the second-stage photodiode  10 B is set to 64 μm, and the space L gap2  between the second-stage photodiode  10 B and the third-stage photodiode  10 C is set to 47 μm. The length L 21  of the conductive thin wire  20 A connecting the first-stage photodiode  10 A and the second-stage photodiode  10 B with each other is set to 54 μm, and the length L 22  of the conductive thin wire  20 B connecting the second-stage photodiode  10 B and the third-stage photodiode  10 C with each other is set to 47 μm, The width and the thickness of the conductive thin wires  20 A and  20 B are set to values equal to those employed in the first embodiment. 
   With varying thickness of the light receiving layer  3 , the capacitance C in the equivalent circuit shown in  FIG. 3  varies. The length of the conductive thin wire  20  and the space between adjacent photodiodes  10  are adjusted so that equations (2) and (5) are satisfied regardless of the variation in capacitance C. 
   Because the thickness of the light receiving layer is smallest at the first stage and increases toward the last stage, the absorption of light is averaged, and thus an excess photocurrent is prevented from flowing through some photodiode. 
   A method of producing the semiconductor light receiving device according to the second embodiment is described below. In the process of producing the semiconductor light receiving device according to the first embodiment described earlier, the light receiving layer  3  is grown over the entire surface until the thickness becomes equal to the thickness of the light receiving layer  3 A of the first-stage photodiode  10 A, which is the smallest in thickness of all photodiodes. Thereafter, an area in which the first-stage photodiode  10 A is to be formed is covered with a mask formed of SiO 2  or the like, and a light receiving layer is further grown by a thickness equal to the difference between the thickness of the light receiving layer  3 A of the first-stage photodiode  10 A and the thickness of the light receiving layer  3 B of the second-stage photodiode  10 B. Thereafter, the area in which the firststage photodiode  10 A is to be formed and an area in which the second-stage photodiode  10 B is to be formed are covered with a mask, and a light receiving layer is further grown by a thickness equal to the difference between the thickness of the light receiving layer  3 B of the second-stage photodiode  10 B and the thickness of the light receiving layer  3 C of the third-stage photodiode  10 C. Steps after that are similar to those of the process of producing the semiconductor light receiving device according to the first embodiment. 
     FIG. 7  is a cross-sectional view of an optical-to-electric converter of a semiconductor light receiving device according to a third embodiment. The lengths L pin1  to L pin3  of the photodiodes  10 A to  10 C at the first to third stages are respectively set to 1.5 μm, 2.0 μm, and 3.0 μm. That is, the length of the photodiode increases in a direction in which an optical signal propagates. This means that the length of the light receiving layer is smallest at the first stage and increases in a direction toward the last stage. The absorption of light decreases with decreasing length of the light receiving layer. 
   The width of each photodiode  10 A to  10 C is set to 4 μm as in the first embodiment, and the thickness of each light receiving layer  3 A to  3 C is set to 0.15 μm. The space L gap1  between the first-stage photodiode  10 A and the second-stage photodiode  10 B is set to 23 μm, and the space L gap2  between the second-stage photodiode  10 B and the third-stage photodiode  10 C is set to 41 μm. 
   The conductive thin wire  20 A connecting the first-stage photodiode  10 A and the second-stage photodiode  10 B with each other is formed so as to rise up by a height of H 31  at points where the conductive thin wire  20 A joins with the photodiodes  10 A and  10 B. The longitudinal length L 31  of the conductive thin wire  20 A is set to be 20 μm, and the vertical length H 31  of the rising portions is set to be 5 μm. Thus, the total length of the conductive thin wire  20 A is 30 μm. The length L 32  of the conductive thin wire  20 B connecting the second-stage photodiode  10 B and the third-stage photodiode  10 C with each other is set to be 39 μm. The width and the thickness of each of conductive thin wires  20 A and  20 B are the same as those in the first embodiment. 
   With varying length of the light receiving layer  3 , the capacitance C in the equivalent circuit shown in  FIG. 3  varies. The length of the conductive thin wire  20  and the space between adjacent photodiodes  10  are adjusted so that equations (2) and (5) are satisfied regardless of the variation in capacitance C. 
   Because the length of the light receiving layer is smallest at the first stage and increases toward the last stage, the absorption of light is averaged, and thus an excess photocurrent is prevented from flowing through some photodiode. 
   A method of producing the semiconductor light receiving device according to the second embodiment is described below. Parts other than the conductive thin wire are produced via steps that are similar to those of the method of producing the semiconductor light receiving device according to the first embodiment described above. The conductive thin wire  20 A rising up at joining points with the photodiodes  10 A and  10 B may be produced by disposing, before performing a Au plating process, an additional resist pattern with a thickness of H 31  in an area where the conductive thin wire  20 A is to be formed. 
     FIG. 8  is a plan view of an opticaltoelectric converter of a semiconductor light receiving device according to the embodiment. The width W 1  of the first-stage photodiode  10 A is set to 3 μm, the width W 2  of the second-stage photodiode  10 B to 5 μm, and the width W 3  of the third-stage photodiode  10 C to 10 μm. The space L gap1  between the first-stage photodiode  10 A and the second-stage photodiode  10 B is set to 15 μm, and the space L gap2  between the second-stage photodiode  10 B and the third-stage photodiode  10 C is set to 20 μm. 
   The width of the interconnecting optical waveguide  11 B and  11 C connecting adjacent photodiodes with each other gradually increases in the direction toward the last stage so as to achieve matching with the width of the photodiodes  10 A to  10 C. The other parts are formed in a similar manner as in the semiconductor light receiving device according to the first embodiment. 
   Because the width of the light receiving layer is smallest at the first stage and increases toward the last stage, the absorption of light is averaged, and thus an excess photocurrent is prevented from flowing through some photodiode. 
   Referring to  FIG. 9 , a semiconductor light receiving device according to a fifth embodiment is described below. 
     FIG. 9  is a cross-sectional view of the semiconductor light receiving device according to the fifth embodiment. In the first embodiment described earlier, as shown in  FIG. 1A , the optical waveguides  11 A to  11 C and the light receiving layer  3  are directly formed on the n-type layer  2 . In this fifth embodiment, unlike the first embodiment, a lower optical waveguide  14  is formed on the n-type layer  2 , and the optical waveguides  11 A to  11 C and the light receiving layer  3  are formed on the lower optical waveguide  14 . The other parts are formed in a similar manner as in the first embodiment The lower optical waveguide  14  may be formed of n-type InP. 
   Use of the lower optical waveguide  14  allows a reduction in loss at a butt-coupling point between the optical waveguide and the light receiving layer. 
   In the first to fifth embodiments described above, three photodiodes  10 A to  10 C are used. Alternatively, two photodiodes or four or more photodiodes may be used. Furthermore, although in the first to fifth embodiments described above, the conductive thin wire  20  is formed so as to extend in the air, the conductive thin wire  20  may be formed so as to extend on the surface of an insulating film formed on the substrate. 
   The present invention has been described above with reference to specific embodiments. Note that the present invention is not limited to those specific embodiments. It should be obvious to those skilled in the art that various changes, modifications, and improvements may be made without departing from the spirit and scope of the present invention.