Patent Publication Number: US-8981343-B2

Title: Semiconductor device and receiver

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-056232, filed on Mar. 13, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a semiconductor device and a receiver. 
     BACKGROUND 
     For example, in a receiver that receives a weak radio wave of a millimeter wave band, a terahertz wave band or a like wave band, a low noise amplifier, a detector and so forth are required. A Schottky diode is used for the detector. 
     However, where the Schottky diode is used, it is difficult to obtain a sufficient detection characteristic in the proximity of a bias of 0 V. 
     Therefore, the inventor of the present invention has proposed to use a backward diode wherein a p-type GaAsSb layer and an n-type InGaAs layer are pn-joined in place of the Schottky diode in order to improve the detection characteristic. 
     It is to be noted that a technique is available wherein, in order to efficiently confine injection carriers in an active layer so that electrons and holes are re-coupled in a semiconductor light emitting device such as a semiconductor laser, a multiple superlattice structure is provided to increase the height of an energy barrier against electrons or holes. Further, a technique is available wherein, in a semiconductor light emitting device such as a semiconductor laser, a multiple quantum well structure portion or a multiple quantum barrier structure portion for controlling the advancement of minority carriers into a cladding layer is provided between the cladding layer and an active layer. 
     SUMMARY 
     According to an aspect of the embodiment, a semiconductor device includes a p-type semiconductor layer, an n-type semiconductor layer, a pn junction portion at which the p-type semiconductor layer and the n-type semiconductor layer are joined to each other, and a multiple quantum barrier structure or a multiple quantum well structure that is provided in at least one of the p-type semiconductor layer and the n-type semiconductor layer and functions as a barrier against at least one of electrons and holes upon biasing in a forward direction, wherein, upon biasing in a reverse direction, a portion that allows band-to-band tunneling of electrons is formed at the pn junction portion. 
     According to another aspect of the embodiment, a receiver includes an amplifier, and a detector connected to the amplifier, wherein the detector is the semiconductor device described above. 
     According to a further aspect of the embodiment, a receiver includes a mixer circuit, wherein the mixer circuit includes the semiconductor device described above. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  are schematic views illustrating an energy band structure of a semiconductor device (backward diode) of a first embodiment, wherein  FIG. 1A  illustrates a balanced state in which a voltage is not applied,  FIG. 1B  illustrates a reverse bias state in which a voltage is applied in the reverse direction and  FIG. 1C  illustrates a forward bias state in which a voltage is applied in the forward direction; 
         FIG. 2  is a schematic view illustrating a configuration of a receiver of the first embodiment; 
         FIG. 3  is a schematic view illustrating a principle of an MQB structure provided in the semiconductor device (backward diode) of the first embodiment; 
         FIGS. 4A and 4B  are schematic views illustrating a principle and a working effect of the MQB structure provided in the semiconductor device (backward diode) of the first embodiment; 
         FIG. 5  is a schematic view illustrating an energy band structure in a flat band state in the semiconductor device (backward diode) of the first embodiment; 
         FIG. 6  is a schematic view illustrating an energy band structure in a flat band state in a semiconductor device (backward diode) of a modification to the first embodiment; 
         FIGS. 7A and 7B  are schematic views illustrating an energy band structure in a flat band state where an MQB or an MQW is provided in a semiconductor laser including an active layer between a p-type semiconductor layer and an n-type semiconductor layer; 
         FIGS. 8A and 8B  are schematic views illustrating an effect where doping is carried out for barrier layers that configure the MQB structure provided in the semiconductor device (backward diode) of the first embodiment, wherein  FIG. 8A  illustrates an energy band structure where doping is not carried out for the barrier layers and  FIG. 8B  illustrates an energy band structure where doping is carried out for the barrier layers; 
         FIG. 9  is a schematic view depicting an energy band structure and illustrating an effect where doping is not carried out for barrier layers which configure the MQB structure provided in the semiconductor device (backward diode) of the first embodiment; 
         FIGS. 10A and 10B  are schematic sectional views illustrating a fabrication method for the semiconductor device (backward diode) of the first embodiment; 
         FIGS. 11A and 11B  are schematic sectional views illustrating the fabrication method for the semiconductor device (backward diode) of the first embodiment; 
         FIGS. 12A and 12B  are schematic sectional views illustrating the fabrication method for and a configuration of the semiconductor device (backward diode) of the first embodiment; 
         FIG. 13  is a schematic view illustrating an effect by the semiconductor device (backward diode) of the first embodiment; 
         FIGS. 14A to 14C  are schematic views illustrating an energy band structure of the semiconductor device (backward diode) of the modification to the first embodiment, wherein  FIG. 14A  illustrates a balanced state in which a voltage is not applied,  FIG. 14B  illustrates a reverse bias state in which a voltage is applied in the reverse direction and  FIG. 14C  illustrates a forward bias state in which a voltage is applied in the forward direction; 
         FIGS. 15A and 15B  are schematic sectional views illustrating a fabrication method for the semiconductor device (backward diode) of the modification to the first embodiment; 
         FIGS. 16A and 16B  are schematic sectional views illustrating the fabrication method for the semiconductor device (backward diode) of the modification to the first embodiment; 
         FIGS. 17A and 17B  are schematic sectional views illustrating the fabrication method for and a configuration of the semiconductor device (backward diode) of the modification to the first embodiment; 
         FIGS. 18A to 18C  are schematic views illustrating an energy band structure of a semiconductor device (backward diode) of another modification to the first embodiment, wherein  FIG. 18A  illustrates a balanced state in which a voltage is not applied,  FIG. 18B  illustrates a reverse bias state in which a voltage is applied in the reverse direction and  FIG. 18C  illustrates a forward bias state in which a voltage is applied in the forward direction; 
         FIGS. 19A and 19B  are schematic sectional views illustrating a fabrication method for the semiconductor device (backward diode) of another modification to the first embodiment; 
         FIGS. 20A and 20B  are schematic sectional views illustrating the fabrication method for the semiconductor device (backward diode) of another modification to the first embodiment; 
         FIGS. 21A and 21B  are schematic sectional views illustrating the fabrication method for and a configuration of the semiconductor device (backward diode) of another modification to the first embodiment; 
         FIGS. 22A and 22B  are schematic views illustrating an energy band structure of the semiconductor device (backward diode) of a different modification to the first embodiment; 
         FIG. 23  is a schematic sectional view illustrating a configuration of the semiconductor device (backward diode) of the different modification to the first embodiment; 
         FIG. 24  is a schematic view illustrating an energy band structure of a semiconductor device (backward diode) of a further modification to the first embodiment and illustrating a balanced state in which a voltage is not applied; 
         FIG. 25  is a schematic sectional view illustrating a configuration of the semiconductor device (backward diode) of the further modification to the first embodiment; 
         FIG. 26  is a schematic view illustrating an energy band structure of a semiconductor device (backward diode) of a still further modification to the first embodiment and depicting a balanced state in which a voltage is not applied; 
         FIG. 27  is a schematic sectional view illustrating a configuration of the semiconductor device (backward diode) of the still further modification to the first embodiment; 
         FIG. 28  is a schematic view illustrating a configuration of a receiver of a modification to the first embodiment; 
         FIG. 29  is a schematic view illustrating a configuration of a mixer circuit provided in the receiver of the modification to the first embodiment; 
         FIG. 30  is a schematic view illustrating an energy band structure in a flat band state in a semiconductor device (backward diode) of a second embodiment; 
         FIG. 31  is a schematic view illustrating an energy band structure in a flat band state in a semiconductor device (backward diode) of a modification to the second embodiment; 
         FIG. 32  is a schematic view illustrating an energy band structure in a flat band state where an MQB is provided in a semiconductor laser including an active layer between a p-type semiconductor layer and an n-type semiconductor layer; 
         FIGS. 33A to 33C  are schematic views illustrating an energy band structure of the semiconductor device (backward diode) of the second embodiment, wherein  FIG. 33A  illustrates a balanced state in which a voltage is not applied,  FIG. 33B  illustrates a reverse bias state in which a voltage is applied in the reverse direction and  FIG. 33C  illustrates a forward bias state in which a voltage is applied in the forward direction; 
         FIGS. 34A to 34C  are schematic views illustrating an energy band structure of a semiconductor device (backward diode) of a modification to the second embodiment, wherein  FIG. 34A  illustrates a balanced state in which a voltage is not applied,  FIG. 34B  illustrates a reverse bias state in which a voltage is applied in the reverse direction and  FIG. 34C  illustrates a forward bias state in which a voltage is applied in the forward direction; 
         FIGS. 35A to 35C  are schematic views illustrating an energy band structure of a semiconductor device (backward diode) of another modification to the second embodiment, wherein  FIG. 35A  illustrates a balanced state in which a voltage is not applied,  FIG. 35B  illustrates a reverse bias state in which a voltage is applied in the reverse direction and  FIG. 35C  illustrates a forward bias state in which a voltage is applied in the forward direction; 
         FIGS. 36A to 36C  are schematic views illustrating an energy band structure of a semiconductor device (backward diode) of a further modification to the second embodiment, wherein  FIG. 36A  illustrates a balanced state in which a voltage is not applied,  FIG. 36B  illustrates a reverse bias state in which a voltage is applied in the reverse direction and  FIG. 36C  illustrates a forward bias state in which a voltage is applied in the forward direction; 
         FIGS. 37A to 37C  are schematic views illustrating an energy band structure of a semiconductor device (backward diode) of a still further modification to the second embodiment, wherein  FIG. 37A  illustrates a balanced state in which a voltage is not applied,  FIG. 37B  illustrates a reverse bias state in which a voltage is applied in the reverse direction and  FIG. 37C  illustrates a forward bias state in which a voltage is applied in the forward direction; 
         FIGS. 38A to 38C  are schematic views illustrating an energy band structure of a semiconductor device (backward diode) of a yet further modification to the second embodiment, wherein  FIG. 38A  illustrates a balanced state in which a voltage is not applied,  FIG. 38B  illustrates a reverse bias state in which a voltage is applied in the reverse direction and  FIG. 38C  illustrates a forward bias state in which a voltage is applied in the forward direction; 
         FIG. 39  is a schematic view illustrating an energy band structure of a semiconductor device (backward diode) of a modification to the first and second embodiments and depicting a balanced state in which a voltage is not applied; and 
         FIGS. 40A to 40C  are schematic views illustrating a subject of a conventional backward diode, wherein  FIG. 40A  illustrates an energy band structure,  FIG. 40B  illustrates an I-V characteristic and  FIG. 40C  is a view illustrating a characteristic where current is logarithm-plotted. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     However, in the backward diode described above, since the barrier against electrons and holes is low upon biasing in a forward direction as illustrated in  FIG. 40A , leakage current flows. 
     Here, in the current-voltage characteristic (I-V characteristic) where current is linear-plotted, it looks at a glance that leakage current does not flow upon biasing in the forward direction as indicated by an arrow mark in  FIG. 40B . However, if current is logarithm-plotted, then a leakage component of current is noticed as indicated by an arrow mark in  FIG. 40C . Consequently, the value of γ (curvature coefficient) that has an influence on a detection characteristic of a diode is less likely to become high. It is to be noted that the curvature coefficient γ is defined by the following expression (1):
 
γ= d   2   I/dv   2   /dI/dv   (1)
 
     Therefore, if it is tried to detect a weak radio wave, for example, of a millimeter wave band or a terahertz wave band using the backward diode described above as a detector, then a satisfactory detection characteristic is not obtained. 
     It is to be noted that, while the subject is described taking the backward diode wherein a p-type GaAsSb layer and an n-type InGaAs layer are pn-joined as an example, the subject is not limited to this, and also other backward diodes have a similar subject. 
     Therefore, it is desired to suppress leakage current upon biasing in the forward direction of the backward diode. Further, it is desired to improve the detection characteristic of a detector for which the backward diode is used and hence improve a characteristic of a receiver. 
     In the following, a semiconductor device and a receiver according to embodiments are described with reference to the drawings. 
     First Embodiment 
     First, a semiconductor device and a receiver according to a first embodiment are described with reference to  FIGS. 1A to 13 . 
     As depicted in  FIG. 2 , the receiver according to the present embodiment is a receiver  1  that receives a weak radio wave, for example, of a millimeter wave band or a terahertz wave band and includes a monolithic microwave integrated circuit (MMIC)  5  in which a low noise amplifier (LNA)  2 , a detector  3  and an inductor  4  are integrated. It is to be noted that the low noise amplifier  2  is sometimes referred to also as amplifier. 
     Here, for example, a high electron mobility transistor (HEMT) is used for the low noise amplifier  2  and a backward diode hereinafter described is used for the detector  3 . The HEMT and the backward diode connected to the HEMT are configured as a semiconductor device formed from a compound semiconductor (here, a GaAsSb-based semiconductor) on a semiconductor substrate. In particular, the semiconductor device according to the present embodiment includes a backward diode used as the detector  3  in the receiver  1  that receives a weak radio wave (high-frequency signal), for example, of a millimeter wave band or a terahertz wave band. It is to be noted that the detector is hereinafter referred to sometime as high-sensitivity detector. 
     Further, an antenna  6  is connected to an input terminal of the MMIC  5 , namely, to an input terminal of the low noise amplifier  2 . Here, the antenna  6  is connected to the gate electrode of the HEMT. Further, an output terminal of the low noise amplifier  2 , namely, the drain electrode of the HEMT, is connected to one of terminals of the detector  3 , namely, to one of electrodes (to the n-side electrode) of the backward diode and one of terminals of the inductor  4 . Further, the other one of the terminals of the detector  3 , namely, the other one of the electrodes (the p-side electrode) of the backward diode, is grounded. Further, the other one of the terminals of the inductor  4  is connected to an output terminal of the MMIC  5 . A weak radio wave of a millimeter wave band or a like band received by the antenna  6  is amplified by the low noise amplifier  2  and then converted into a detection signal V det  that is a DC voltage by the detector  3  and the inductor  4 . The detection signal V det  is outputted from the output terminal of the MMIC  5 . As the detection signal V det , a potential difference ΔV of several hundred mV is outputted. The detection sensitivity (detection characteristic) of the backward diode has an influence on the detection performance. 
     Incidentally, as depicted in  FIG. 12B , the semiconductor device according to the present embodiment includes a backward diode  12  wherein a p-type GaAsSb layer (p-type semiconductor)  10  and an n-type InGaAs layer (n-type semiconductor)  11  are pn-joined. In particular, the semiconductor device includes a pn junction portion  13  at which the p-type GaAsSb layer  10  and the n-type InGaAs layer  11  are joined to each other. 
     In particular, the p-type GaAsSb layer  10  that configures the backward diode  12  is configured from a p + -GaAs 0.51 Sb 0.49  layer having a band gap Eg of approximately 0.78 eV and a doping concentration of 2×10 19  cm −3 . It is to be noted that the p-type semiconductor layer is not limited to this, and a p-InGaAsSb layer (Eg&lt;0.78 eV) that has a band gap Eg smaller than that of the layer just described or a like layer may be used. Or conversely, a GaAs x Sb 1-x  (x&gt;0.51) layer (Eg&gt;0.78 eV) that has a band gap Eg greater than that of the layer described above or a like layer may be used. 
     Further, the n-type InGaAs layer  11  that configures the backward diode  12  is configured from an n-In 0.53 Ga 0.47 As layer that has a band gap Eg of approximately 0.74 eV and a doping concentration of 5×10 15  cm −3 . It is to be noted that the n-type semiconductor layer is not limited to this, and an n-In x Ga 1-x As (x&gt;0.53) layer (Eg&lt;0.74 eV) that has a band gap Eg smaller than that of the layer just described, an n-InAlGaAs layer (Eg&gt;0.74 eV) that has a band gap Eg greater than that of the layer described above or a like layer may be used. 
     While the backward diode  12  in the present embodiment is a pn-junction diode wherein the p-type semiconductor layer  10  and the n-type semiconductor layer  11  whose materials are different from each other are hetero-joined (hereto-junction), there is a condition for the band junction. In particular, as depicted in  FIG. 5 , the diode has a hetero junction of the type II wherein, in a flat band state, the energy at a lower end of the conduction band of the n-type semiconductor layer  11  is lower than that at a lower end of the conduction band of the p-type semiconductor layer  10  and the energy at an upper end of the valence band of the n-type semiconductor layer  11  is lower than that at an upper end of the valence band of the p-type semiconductor layer  10 , and besides the energy at the lower end of the conduction band of the n-type semiconductor layer  11  is higher than that at the upper end of the valence band of the p-type semiconductor layer  10 . By using such a backward diode  12  as described above for the detector  3 , the detection sensitivity of a weak radio wave, for example, of a millimeter wave band or a like band can be improved significantly in comparison with that in an alternative case in which a Schottky diode is used for the detector  3 . It is to be noted that the flat band state signifies a state in which a voltage is applied so that a curved portion of the energy band becomes flat, and is referred to also as flat band condition. 
     Particularly, in the present embodiment, as depicted in  FIGS. 5 and 1A  to  1 C, multiple quantum barrier (MQB; Multi Quantum Barrier) structures  14  and  15  are provided in both of the p-type GaAsSb layer  10  and the n-type InGaAs layer  11  of such a backward diode  12  as described above, respectively, so that re-coupling of electrons and holes may be prevented. In particular, as MQB structures, the p-side MQB structure  14  disposed in the p-type GaAsSb layer  10  and the n-side MQB structure  15  disposed in the n-type InGaAs layer  11  are provided. It is to be noted that, in  FIGS. 1A to 1C , reference character Ef indicates a Fermi level. 
     Consequently, as depicted in  FIG. 1C , upon biasing in the forward direction, electrons are reflected by the p-side MQB structure  14  provided in the p-type GaAsSb layer  10  and holes are reflected by the n-side MQB structure  15  provided in the n-type InGaAs layer  11  so that leakage current is suppressed. In particular, upon biasing in the forward direction, the p-side MQB structure  14  provided in the p-type GaAsSb layer  10  functions as a barrier (energy barrier) against electrons of the conduction band and the n-side MQB structure  15  provided in the n-type InGaAs layer  11  functions as a barrier (energy barrier) against holes of the valence band so that leakage current upon biasing in the forward direction is suppressed. 
     Particularly, since the MQB structures  14  and  15  in the present embodiment are provided in both of the p-type GaAsSb layer  10  and the n-type InGaAs layer  11 , respectively, a current suppression effect upon biasing in the forward direction is higher than that where the MQB structure  14  ( 15 ) is provided in one of the p-type GaAsSb layer  10  and the n-type InGaAs layer  11  as in a modification hereinafter described. 
     Since the leakage current upon biasing in the forward direction is suppressed in such a manner as described above, the value of γ that indicates a nonlinearity of the diode increases, and, for example, where the semiconductor device in the present embodiment is used for a detector for detecting a weak radio wave, for example, of a millimeter wave band or a terahertz wave band, the detection performance in zero biasing is particularly enhanced and a satisfactory detection characteristic can be obtained. 
     Here, the principle of the MQB structure is described with reference to  FIGS. 3 ,  4 A and  4 B. 
     As depicted in  FIG. 3 , a multilayer structure is configured by alternately stacking a barrier layer that has the thickness with which a carrier can tunnel and a well layer. At this time, if also the well layer is formed thin, then the existence probability of carriers is quantized and a quantum level is formed. Then, the thickness of the barrier layers of the MQB structure is made different thereamong so as to gradually change thereby to gradually change the level of the quantum level. Consequently, an MQB structure is produced through which, as viewed generally, carries can not pass and by which even carriers having energy higher than the barrier height caused by a band gap difference between the well layer and the barrier layer are reflected by the Bragg reflection. A condition of the thickness of the semiconductor layer that configures the well layer and the barrier layer at this time, namely, a reflection condition of carriers by the semiconductor layer that configures the well layer and the barrier layer, can be represented by the expressions (2) and (3) given below. 
     In the expressions, L 1  and L 2  indicate the thickness of the well layer and the thickness of the barrier layer, respectively. Further, m and n are integers. Further, m 1 * and m 2 * indicate an effective mass of carriers in the semiconductor that configures the well layer and an effective mass of carriers in the semiconductor that configures the barrier layer, respectively. Further, E indicates a vacuum level. Further, h indicates the Planck constant (conversion Planck constant). Further, U 0  indicates the barrier height caused by the band gap difference between the well layer and the barrier layer. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     By using such an MQB structure as described above, as depicted in  FIGS. 4A and 4B , where a bulk barrier layer having a band gap same as that of the barrier layer that has a band gap greater than that of the well layer is used, a barrier (qφ MQB ) having a height greater than the height (qφ b ) of the hetero junction barrier generated by the band gap difference can be implemented. 
     In the present embodiment, as depicted in  FIG. 12B , the p-side MQB structure  14  provided in the p-type GaAsSb layer  10  is configured by providing a plurality of p-type AlGaSb layers  16  in the p-type GaAsSb layer  10 . In particular, the p-side MQB structure  14  is configured by alternately stacking the p-type GaAsSb layer  10  and the p-type AlGaSb layer  16  in the proximity of the pn junction portion  13  of the p-type GaAsSb layer  10 . 
     In particular, the p-side MQB structure  14  is configured by alternately stacking the p + -GaAs 0.51 Sb 0.49  layers  10  having the thickness of approximately 5 nm and the p + -Al 0.2 Ga 0.8 Sb layers  16  that have thicknesses that successively and gradually decrease thereamong like approximately 3 nm, approximately 3 nm, approximately 2.5 nm, approximately 2.5 nm, approximately 2 nm, approximately 2 nm, approximately 1.5 nm and approximately 1.5 nm. Here, the p + -GaAs 0.51 Sb 0.49  layers  10  have a band gap Eg of approximately 0.78 eV and a doping concentration of 1×10 19  cm −3 . Meanwhile, the p + -Al 0.2 Ga 0.8 Sb layers  16  have a band gap Eg of approximately 0.95 eV and a doping concentration of 1×10 19  cm −3 . 
     It is to be noted that such p-type AlGaSb layers  16  as described above are each referred to sometimes as barrier layer, p-side barrier layer or p-type barrier layer. Meanwhile, each p-type GaAsSb layer  10  sandwiched by the p-type AlGaSb layers  16  is referred to sometimes as well layer, p-side well layer or p-type well layer. Further, while the p + -GaAs 0.51 Sb 0.49  layer  10  is used as the well layer here, the well layer is not limited to this and a p-InGaAsSb layer (Eg&lt;0.78 eV) that has a band gap Eg smaller than that of the layer  10  or the like may be used. In this instance, a p + -GaAs 0.51 Sb 0.49  layer may be used as the p-type semiconductor layer and a p-InGaAsSb layer (Eg&lt;0.78 eV) may be used as the well layer of the p-side MQB structure  14 . Further, a p-InGaAsSb layer (Eg&lt;0.78 eV) may be used for both of the p-type semiconductor layer and the well layers of the p-side MQB structure  14 . In this manner, a p-GaAsSb layer, a p-InGaAsSb layer or a like layer may be used for the well layer. Further, while the p + -Al 0.2 Ga 0.8 Sb layer  16  is used for the barrier layer, the barrier is not limited to this and a p-Al x Ga 1-x Sb (x&gt;0) layer (Eg&gt;0.78 eV) that has a band gap Eg greater than that of the well layer, namely, a band gap Eg greater than approximately 0.78 eV, may be used. It is to be noted that, when increasing the band gap of the well layer, by using, for example, AlAsSb or the like, the band gap of the barrier layer may be greater than that of the well layer. Further, while the thicknesses of the p + -Al 0.2 Ga 0.8 Sb layers  16  are made different from each other, the p + -Al 0.2 Ga 0.8 Sb layers  16  may have a fixed thickness. Further, while the p + -Al 0.2 Ga 0.8 Sb layers  16  have thicknesses that are made different thereamong so as to gradually decrease, the layers  16  may otherwise have thicknesses that are made different thereamong so as to gradually increase. 
     Further, the p-side barrier layers  16  that configure the p-side MQB structure  14  may be non-doped. However, it is preferable to apply a p-type barrier layer doped in the p type to the p-side barrier layers  16 . Consequently, such a situation that a barrier against electrons upon reverse bias is generated because the upper end of the valence band of the plural barrier layers  16  that configure the p-side MQB structure  14  comes to the lower side with respect to the upper end of the valence band of the p-type semiconductor layer  10  can be suppressed as depicted in  FIGS. 8A and 8B . As a result, the influence of the barrier can be suppressed. However, as hereinafter described, it is necessary to form a portion that allow band-to-band tunneling of electrons at the pn junction portion  13  upon biasing in the reverse direction from one p-type AlGaAs layer  16  included in the p-side MQB structure  14  and one n-type InP layer  17  included in the n-side MQB structure  15  as depicted in  FIG. 1B , thereby obtain a backward diode characteristic. It is to be noted that the band-to-band tunneling is hereinafter referred to sometimes as inter-band tunneling. 
     On the other hand, as depicted in  FIG. 12B , the n-side MQB structure  15  provided in the n-type InGaAs layer  11  is configured by providing a plurality of n-type InP layers  17  in the n-type InGaAs layer  11 . In particular, the n-side MQB structure  15  is configured by alternately stacking the n-type InGaAs layers  11  and the n-type InP layers  17  in the proximity of the pn junction portion  13  of the n-type InGaAs layer  11 . 
     In particular, the n-side MQB structure  15  is formed by alternately stacking the n-In 0.53 Ga 0.47 As layers  11  having the thickness of approximately 5 nm and the n-type InP layers  17  having thicknesses that are made different thereamong so as to gradually increase in order like approximately 1.5 nm, approximately 1.5 nm, approximately 2 nm, approximately 2 nm, approximately 2.5 nm, approximately 2.5 nm, approximately 3 nm and approximately 3 nm. Here, the n-In 0.53 Ga 0.47 As layers  11  have a band gap Eg of approximately 0.74 eV and a doping concentration of 5×10 18  cm −3 . Further, such n-type InP layers  17  have a band gap Eg of approximately 1.35 eV and a doping concentration of 5×10 18  cm −3 . 
     It is to be noted that the n-type InP layer  17  is referred to sometime as barrier layer, n-side barrier layer or n-type barrier layer. Further, the n-type InGaAs layer  11  sandwiched by the n-type InP layers  17  is referred to sometime as well layer, n-side well layer or n-type well layer. Further, while the n-In 0.53 Ga 0.47 As layer  11  is used as the well layer here, the well layer is not limited to this and an n-In x Ga 1-x As (x&gt;0.53) layer (Eg&lt;0.74 eV), an n-InAlGaAs layer (Eg&gt;0.74 eV) or a like layer that have a band gap Eg smaller than that of the layer  11  may be used. In this instance, an n-In 0.53 Ga 0.47 As layer is used as the n-type semiconductor layer and an n-In x Ga 1-x As (x&gt;0.53) layer (Eg&lt;0.74 eV) or an n-InAlGaAs layer (Eg&gt;0.74 eV) may be used as the well layer of the n-side MQB structure  15 . Further, an n-In x Ga 1-x As (x&gt;0.53) layer (Eg&lt;0.74 eV) or an n-InAlGaAs layer (Eg&gt;0.74 eV) may be used for both of the n-type semiconductor layer and the well layers of the n-side MQB structure  15 . In this manner, an n-InGaAs layer, an n-InAlGaAs layer or a like layer may be used for the well layer. Further, while the n-type InP layer  17  is used as the barrier layer, the barrier layer is not limited to this, and an n-In x Ga 1-x As (x&gt;0.53) layer, an n-In x Al 1-x As (x&gt;0.7), an n-InAlGaAs layer (Eg&gt;0.74 eV) or a like layer that have a band gap greater than that of the well layer, namely, a band gap Eg greater than approximately 0.74 eV, may be used. In particular, an n-InP layer, an n-InGaAs layer, an n-InAlAs layer, an n-InAlGaAs layer or a like layer may be used for the barrier layer. However, where the band gap of the well layer is set smaller, the band gap of the barrier layer may greater than that of the well layer. Further, while the n-type InP layers  17  have different thicknesses, a fixed thickness may be used Further, while the n-type InP layers  17  have gradually increasing thicknesses thereamong, the n-type InP layers  17  may have gradually decreasing thicknesses thereamong. 
     Further, the n-side barrier layers  17  that configure the n-side MQB structure  15  may be non-doped. However, it is preferable to use n-type barrier layers doped in the n type as the n-side barrier layers  17 . Consequently, it can be suppressed that a barrier against electrons upon biasing in the reverse direction is generated because the lower end of the conduction band of the plural barrier layers  17  that configure the n-side MQB structure  15  comes to the upper side with respect to the lower end of the conduction band of the n-type semiconductor layer  11 . Consequently, the influence of the barrier can be reduced. However, as hereinafter described, it is necessary to form a portion that allows band-to-band tunneling of electrons at the pn junction portion  13  upon biasing in the reverse direction from one p-type AlAsSb layer  16  included in the p-side MQB structure  14  and one n-type InP layer  17  included in the n-side MQB structure  15  as depicted in  FIG. 1B , thereby obtain a backward diode characteristic. 
     Where the MQB structures  14  and  15  are provided, the p-type GaAsSb layer  10  and the n-type InGaAs layer  11  are joined to each other at the pn junction portion  13  in such a manner as to sandwich therebetween one p-side AlGaSb layer (p-side barrier layer)  16  included in the p-side MQB structure  14  and one n-type InP layer (n-side barrier layer)  17  included in the n-side MQB structure  15  as depicted in  FIGS. 1A to 1C . In this instance, a junction plane between one p-type AlGaSb layer  16  and one n-type InP layer  17  functions as a pn junction plane (pn junction interface). Then, the portion that allows band-to-band tunneling of electrons is formed at the pn junction portion  13  upon biasing in the reverse direction from the one p-type AlGaSb layer  16  and the one n-type InP layer  17 . 
     Here, it is preferable to set the distance in a horizontal direction from the p side to the n side of the portion formed from the one p-type AlGaSb layer  16  and the one n-type InP layer  17  to 100 {acute over (Å)} or less, namely, to 10 nm or less, in order that band-to-band tunneling of electrons is allowed. Here, the band is radially curved by doping so that the distance in the horizontal direction is set to 10 nm or less. Consequently, band-to-band tunneling of electrons in the valence band of the p-type semiconductor layers  10  and  16  occurs at the portion upon biasing in the reverse direction, and consequently, band-to-band tunneling current flows. 
     In this instance, the present backward diode includes, on opposite sides of the pn junction plane, the MQB structures  14  and  15  having such a band structure that band-to-band tunneling of electros is allowed upon biasing in the reverse direction. Here, the p-side MQB structure  14 , namely, the p-type GaAsSb layer  10  on which the p-side MQB structure  14  is provided, and the n-side MQB structure  15  is provided, namely, the n-type InGaAs layer  11  on which the n-side MQB structure  15 , individually have a type II band structure as depicted in  FIG. 5 , by which band-to-band tunneling is likely to occur. In particular, where the MQB structure or the MQW structure is provided in a device that includes an active layer between the p-type semiconductor layer and the n-type semiconductor layer such as a semiconductor laser, the device comes to have a type I band structure as depicted in  FIGS. 7A and 7B . However, in the present embodiment, since the device has a type II band structure as depicted in  FIG. 5 , the band structure which is likely to allow band-to-band tunneling can be implemented. 
     It is to be noted that, while, in the present embodiment, in a flat band state, the energy at the lower end of the conduction band of the n-type semiconductor layer  11  is higher than that at the upper end of the valence band of the p-type semiconductor layer  10  as depicted in  FIG. 5 , the energy condition is not limited to this. For example, in a flat band state, the energy at the lower end of the conduction band of the n-type semiconductor layer  11  may be set lower than that at the upper end of the valence band of the p-type semiconductor layer  10  as depicted in  FIG. 6 . This can be implemented by changing the materials and the compositions of the semiconductor materials that configure the p-type semiconductor layer  10  and the n-type semiconductor layer  11 . 
     In this instance, the portion that allows band-to-band tunneling of electros is formed at the pn junction portion  13  upon biasing in the reverse direction from an effective barrier formed by the p-side MQB structure  14  and an effective barrier formed by the n-side MQB structure  15  as indicated by dotted lines in  FIG. 6 . Where the effective barrier formed on the valence band side of the p-type semiconductor layer  10  by the p-side MQB structure  14  and the effective barrier formed on the conduction band side of the n-type semiconductor layer  11  by the n-side MQB structure  15  are positively utilized in this manner, it is preferable to configure the barrier layers  16  and  17  that configure the MQB structures  14  and  15  as non-doped barrier layers. Consequently, a lower junction capacitance can be implemented, and high-speed operation of the device can be implemented. In this instance, the upper end of the valence band of the barrier layer  16  that configures the p-side MQB structure  14  comes to the lower side with respect to the upper end of the valence band of the p-type semiconductor layer  10  as depicted in  FIG. 9  to make the effective barrier higher. Besides, the lower end of the conduction band of the barrier layer  17  that configures the n-side MQB structure  15  comes to the upper side with respect to the lower end of the conduction band of the n-type semiconductor layer  11  to make the effective barrier higher. Therefore, even if the band structure is different by a great amount between the p-type semiconductor layer  10  and the n-type semiconductor layer  11 , namely, even if the energy difference between the lower end of the conduction band of the p-type semiconductor layer  10  and the lower end of the conduction band of the n-type semiconductor layer  11  is great and the energy difference between the upper end of the valence band of the p-type semiconductor layer  10  and the upper end of the valence band of the n-type semiconductor layer  11  is great, the backward diode characteristic can be obtained. It is to be noted that the barrier layers  16  and  17  that configure the MQB structures  14  and  15  may be doped. In this manner, the composition of the semiconductor material for configuring the p-type semiconductor layer  10 , n-type semiconductor layer  11 , p-side MQB structure  14  and n-side MQB structure  15  and whether or not doping is to be applied may be determined in order that the backward diode characteristic may be obtained. 
     Now, a fabrication method for the semiconductor device according to the present embodiment is described with reference to  FIGS. 10A to 12B . 
     First, as depicted in  FIG. 10A , an i-InAlAs buffer layer  21 , an n-InGaAs ohmic contact layer  22 , an n-InP etching stopping layer  23 , an n-InGaAs layer  11  as an n-type semiconductor layer, n-type InP layers  17  and n-InGaAs layers  11  that configure the n-MQB structure  15 , p-AlGaSb layers  16  and p-GaAsSb layers  10  that configure the p-MQB structure  14 , and a p-GaAsSb layer  10  as a p-type semiconductor layer are successively formed, for example, on a semi-insulating InP substrate  20 , for example, by an MOCVD method. It is to be noted that the uppermost side p-GaAsSb layer  10  is referred to sometimes as ohmic contact layer. 
     Here, the i-InAlAs buffer layer  21  has a thickness of approximately 300 nm. Further, the n-InGaAs ohmic contact layer  22  is an n + -In 0.53 Ga 0.47 As layer having a doping concentration of approximately 1×10 19  cm −3  and has a thickness of approximately 200 nm. Further, the n-InP etching stopping layer  23  has a doping concentration of approximately 1×10 18  cm −3  and a thickness of approximately 5 nm. Further, the lowermost side n-InGaAs layer  11  is an n-In 0.53 Ga 0.47 As layer having a doping concentration of 5×10 18  cm −3  and has a thickness of 50 nm. 
     Meanwhile, the n-MQB structure  15  is formed by alternately and repetitively stacking the n-InP layers  17  and the n-InGaAs layers  11 . Here, each of the n-InP layers  17  has a doping concentration of 5×10 −18  cm −3 . Meanwhile, each of the n-InGaAs layers  11  is an n-In 0.53 Ga 0.47 As layer and has a doping concentration of 5×10 −18  cm −3 . Further, the thickness of the n-InGaAs layers  11  is set fixed while the thickness of the n-InP layers  17  is made different thereamong. Here, the thickness of each n-InGaAs layer  11  is set to approximately 5 nm, and the thickness t 1  of the n-InP layers  17  is made different thereamong so as to gradually increase in order like approximately 1.5 nm, approximately 1.5 nm, approximately 2 nm, approximately 2 nm, approximately 2.5 nm, approximately 2.5, approximately 3 nm and approximately 3 nm. It is to be noted here that, while the thickness of the n-InP layers  17  is made different thereamong, otherwise it may be set fixed. Further, while the thickness of the n-InP layers  17  is made different thereamong so as to gradually increase, otherwise it may be made different thereamong so as to gradually decrease. 
     Further, the p-MQB structure  14  is formed by alternately and repetitively stacking the p-AlGaSb layers  16  and the p-GaAsSb layers  10 . Here, each of the p-AlGaSb layers  16  is a p + -Al 0.2 Ga 0.8 Sb layer having a doping concentration of 1×10 19  cm −3 . Further, each of the p-GaAsSb layers  10  is a p + -GaAs 0.51 Sb 0.49  layer having a doping concentration of 1×10 19  cm −3 . Further, the thickness of the p-GaAsSb layers  10  is set fixed while the thickness of the p-AlGaSb layers  16  is made different thereamong. Here, the thickness of the p-GaAsSb layers  10  is set to approximately 5 nm, and the thickness t 2  of the p-AlGaSb layers  16  is made different thereamong so as to gradually decrease in order like approximately 3 nm, approximately 3 nm, approximately 2.5 nm, approximately 2.5 nm, approximately 2 nm, approximately 2 nm, approximately 1.5 nm and approximately 1.5 nm. It is to be noted here that, while the thickness of the p-AlGaSb layers  16  is made different thereamong, otherwise it may be set fixed. Further, while the thickness of the p-AlGaSb layers  16  is made different thereamong so as to gradually decrease, it may otherwise be made different thereamong so as to gradually increase. 
     Further, the uppermost side p-GaAsSb layer  10  is a p + -GaAs 0.51 Sb 0.49  layer that has a doping concentration of 2×10 19  cm −3  and has a thickness of approximately 50 nm. 
     Then, a diode mesa region is defined, for example, by photoresist, and, as depicted in  FIG. 10B , the uppermost side p-GaAsSb layer  10  and the p-MQB structure  14  are etched, for example, by mixture liquid of phosphoric acid and hydrogen peroxide water, and then the n-MQB structure  15  including P is removed, for example, by CH 4 -based dry etching. Then, the lowermost side n-InGaAs layer  11  is removed using mixture liquid of phosphoric acid and hydrogen peroxide water again and then the etching is stopped at the n-InP etching stopping layer  23 . Then, the n-InP etching stopping layer  23  is etched as depicted in  FIG. 11A , for example, by hydrochloric acid. This etching is stopped at the n-InGaAs ohmic contact layer  22 . The resist is removed here. 
     Then, an element isolation region is defined by resist  24  as depicted in  FIG. 11B , for example, using photolithography. Then, the n-InGaAs ohmic contact layer  22  is etched, for example, by mixture liquid of phosphoric acid and hydrogen peroxide water until the i-InAlAs buffer layer  21  is exposed. Thereafter, the resist  24  is removed. 
     Then, an electrode region is defined by resist  25  as depicted in  FIG. 12A , for example, using photolithography. Then, for example, Ti (approximately 10 nm thick)/Pt (approximately 30 nm thick)/Au (approximately 300 nm thick) is vapor-deposited as depicted in  FIG. 12B , and an upper electrode  26  and a lower electrode  27  of the diode are formed at the same time by a lift off method. The backward diode  12  provided in the semiconductor device of the present embodiment is fabricated in this manner. 
     Accordingly, with the semiconductor device according to the present embodiment, there is an advantage that leakage current upon biasing in the forward direction of the backward diode  12  can be suppressed. Further, there is another advantage that the detection characteristic of the detector  3  using the backward diode  12  and hence the characteristic of the receiver  1  can be improved. 
     Here, if the characteristic where current of the backward diode in the embodiment described above is logarithm-plotted is viewed, then it is recognized that the leakage component of current is suppressed as indicated by an arrow mark in  FIG. 13 . Particularly, it is recognized that current flowing at a portion indicated by reference character X in  FIG. 13  is suppressed. Consequently, since the value of the curvature coefficient γ calculated in accordance with the expression (1) given hereinabove increases, a satisfactory detection characteristic is obtained where the backward diode in the embodiment described above is used for the detector to detect a weak radio wave of, for example, a millimeter wave band, a terahertz wave band or a like band. 
     It is to be noted that, while, in the embodiment described above, one p-type AlGaSb layer (p-side barrier layer)  16  included in the p-side MQB structure  14  and one n-type InP layer (n-side barrier layer)  17  included in the n-side MQB structure  15  are joined to each other, the junction is not limited to this. For example, one p-type GaAsSb layer (p-side well layer) included in the p-side MQB structure and one n-type InGaAs layer (n-side well layer) included in the n-side MQB structure may be joined to each other. Or, for example, one p-type AlGaSb layer (p-side barrier layer) included in the p-side MQB structure and one n-type InGaAs layer (n-side well layer) included in the n-side MQB structure may be joined to each other. Or else, for example, one p-GaAsSb layer (p-side well layer) included in the p-side MQB structure and one n-type InP layer (n-side barrier layer) included in the n-side MQB structure may be joined to each other. 
     Further, while, in the embodiment described above, the MQB structures  14  and  15  are provided in both of the p-type GaAsSb layer (p-type semiconductor layer)  10  and the n-type InGaAs layer (n-type semiconductor layer)  11 , the arrangement of the MQB structures is not limited to this. In particular, the MQB structure may be provided in at least one of the p-type GaAsSb layer (p-type semiconductor layer)  10  and the n-type InGaAs layer (n-type semiconductor layer)  11 . In particular, as the MQB structure, only the p-side MQB structure  14  disposed in the p-type GaAsSb layer  10  may be provided as depicted in  FIGS. 14A to 14C , or only the n-side MQB structure  15  disposed in the n-type InGaAs layer  11  may be provided as depicted in  FIGS. 18A to 18C . 
     For example, where the MQB structure  14  is provided only in the p-type GaAsSb layer  10 , at the pn junction portion  13 , the p-type GaAsSb layer  10  and the n-type InGaAs layer  11  are joined to each other in such a manner as to sandwich therebetween one p-type AlGaSb layer (barrier layer)  16  included in the MQB structure  14  as depicted in  FIGS. 14A to 14C . In this instance, a junction plane between one p-type AlGaSb layer  16  and n-type InGaAs layer  11  serves as the pn junction plane. Then, from the one p-type AlGaSb layer  16  and the n-type InGaAs layer  11 , a portion that allows band-to-band tunneling of electrons is formed at the pn junction portion  13  upon biasing in the reverse direction. Here, it is preferable to set the distance in a horizontal direction of the portion formed from the one p-type AlGaSb layer  16  and the n-type InGaAs layer  11  to 10 {acute over (Å)} or less, namely, to 10 nm or less, so that band-to-band tunneling of electrons is allowed. Consequently, upon biasing in the reverse direction, band-to-band tunneling of electrons on the valence band of the p-type semiconductor layers  10  and  16  occurs at the portion and band-to-band tunneling current flows. 
     In this instance, as depicted in  FIG. 15A , the i-InAlAs buffer layer  21 , n-InGaAs ohmic contact layer  22 , n-InP etching stopping layer  23 , n-InGaAs layer  11  as the n-type semiconductor layer, p-AlGaSb layers  16  and p-GaAsSb layers  10  that configure the p-MQB structure  14 , and p-GaAsSb layer  10  as the p-type semiconductor layer are formed, for example, on the semi-insulating InP substrate  20 , for example, by an MOCVD method. 
     Then, a diode mesa region is defined, for example, by photoresist  30 , and the uppermost side p-GaAsSb layer  10 , p-MQB structure  14  and n-InGaAs layer  11  are etched as depicted in  FIG. 15B , for example, using mixture liquid of phosphoric acid and hydrogen peroxide water. Then, after the n-InGaAs layer  11  is removed, the etching is stopped at the n-InP etching stopping layer  23 . 
     Thereafter, similarly as in the case of the embodiment described above, the n-InP etching stopping layer  23  is etched and the resist  30  is removed as depicted in  FIG. 16A . Then, an element isolation region is defined by resist  31  and the n-InGaAs ohmic contact layer  22  is etched and then the resist  31  is removed as depicted in  FIG. 16B . Then, an electrode region is defined by resist  32  as depicted in  FIG. 17A , and the upper electrode  26  and the lower electrode  27  are formed to fabricate the backward diode  12  as depicted in  FIG. 17B . 
     On the other hand, for example, where the MQB structure  15  is provided only in the n-type InGaAs layer  11 , the p-type GaAsSb layer  10  and the n-type InGaAs layer  11  are joined to each other at the pn junction portion  13  in such a manner as to sandwich therebetween one n-InP layer (barrier layer)  17  included in the MQB structure  15  as depicted in  FIGS. 18A to 18C . In this instance, a junction plane between the p-type GaAsSb layer  10  and one n-InP layer  17  serves as the pn junction plane. Then, from the p-type GaAsSb layer  10  and the one n-InP layer  17 , a portion that allows band-to-band tunneling of electrons is formed at the pn junction portion  13  upon biasing in the reverse direction. Here, it is preferable to set the distance in a horizontal direction of the portion formed from the p-type GaAsSb layer  10  and the one n-InP layer  17  to 100 {acute over (Å)} or less, namely, to 10 nm or less, so that band-to-band tunneling of electrons is allowed. Consequently, upon biasing in the reverse direction, band-to-band tunneling of electrons on the valence band of the p-type semiconductor layer  10  occurs at the portion and band-to-band tunneling current flows. 
     In this instance, the i-InAlAs buffer layer  21 , n-InGaAs ohmic contact layer  22 , n-InP etching stopping layer  23 , n-InGaAs layer  11  as the n-type semiconductor layer, n-InP layers  17  and n-InGaAs layer  11  that configure the n-MQB structure  15  and p-GaAsSb layer  10  as the p-type semiconductor layer are formed, for example, on the semi-insulating InP substrate  20  as depicted in  FIG. 19A , for example, by an MOCVD method. 
     Then, a diode mesa region is defined, for example, by photoresist  33 , and the p-GaAsSb layer  10  is etched as depicted in  FIG. 19B , for example, using mixture liquid of phosphoric acid and hydrogen peroxide water and then the n-MQB structure  15  including P is removed, for example, by CH 4 -based dry etching. Then, after the lowermost n-InGaAs layer  11  is removed using mixture liquid of phosphoric acid and hydrogen peroxide water again, the etching is stopped at the n-InP etching stopping layer  23 . 
     Thereafter, similarly as in the case of the embodiment described above, the n-InP etching stopping layer  23  is etched and the resist  33  is removed as depicted in  FIG. 20A . Then, an element isolation region is defined by resist  34  and the n-InGaAs ohmic contact layer  22  is etched and then the resist  34  is removed as depicted in  FIG. 20B . Then, an electrode region is defined by resist  35  as depicted in  FIG. 21A , and the upper electrode  26  and the lower electrode  27  are formed as depicted in  FIG. 21B  to fabricate the backward diode  12 . 
     Incidentally, while, in the embodiment described above, the p-side MQB structure  14  and the n-side MQB structure  15  that function as the barriers against electrons and holes upon biasing in the forward direction are provided, the structures to be provided are not limited to them. For example, in place of the MQB structures  14  and  15 , multiple quantum well (MQW) structures  40  and  41  that function as the barriers against electrons and holes, respectively, upon biasing in the forward direction may be provided as depicted in a band structure of  FIGS. 22A and 22B . 
     For example, the p-side MQW structure  40  provided in the p-type GaAsSb layer  10  may be configured by providing a plurality of p-type GaSb layers  42  in the p-type GaAsSb layers  10  as depicted in  FIG. 23 . In particular, the p-side MQW structure  40  may be configured by alternately stacking the p-type GaAsSb layers  10  and the p-type GaSb layers  42  in the proximity of the pn junction portion  13  of the p-type GaAsSb layer  10 . 
     In particular, the p-side MQW structure  40  is formed by alternately and repetitively stacking the p-type GaAsSb layers  10  and the p-type GaSb layers  42 . Here, each of the p-type GaAsSb layers  10  is a p + -GaAs 0.51 Sb 0.49  layer having a band gap Eg of approximately 0.78 eV and a doping concentration of 1×10 19  cm −3 . Further, each of the p-type GaSb layers  42  is a p + -GaSb layer having a band gap Eg of approximately 0.75 eV and a doping concentration of 1×10 19  cm −3 . Further, the thickness of the p-type GaSb layers  42  is set fixed but the thickness of the p-type GaAsSb layers  10  is mode different thereamong. Here, the thickness of each p-type GaSb layer  42  is set to approximately 5 nm, and the thickness of each p-type GaAsSb layer  10  is set so as to gradually decrease thereamong in order like approximately 3 nm, approximately 3 nm, approximately 2.5 nm, approximately 2.5 nm, approximately 2 nm, approximately 2 nm, approximately 1.5 nm and approximately 1.5 nm. 
     It is to be noted that such p-type GaSb layer  42  is hereinafter referred to sometimes as well layer, p-side well layer or p-type well layer. Further, such p-type GaAsSb layer  10  sandwiched by the p-type GaSb layers  42  is hereinafter referred to sometimes as barrier layer, p-side barrier layer or a p-type barrier layer. Further, while the p-type GaSb layer  42  is used here as the well layer, the well layer is not limited to this, and a p-InGaAsSb layer (Eg&lt;0.78 eV), a p-InGaSb layer (Eg&lt;0.78 eV), a p-AlGaSb layer (Eg&lt;0.78 eV) or a like layer that have a band gap Eg smaller than that of the barrier layer, namely, a band gap Eg smaller than approximately 0.78 eV, may be used. In particular, as the well layer, a p-GaSb layer, a p-InGaAsSb layer, a p-InGaSb layer, a p-AlGaSb layer, a p-InAsSb layer or a like layer may be used. It is to be noted that, as hereinafter described, when increasing the band gap of the barrier layer, the band gap of the well layer may be smaller than that of the barrier layer. Further, while the p + -GaAs 0.51 Sb 0.49  layer  10  is used as the barrier layer, the barrier layer is not limited to this, and a p-InGaAsSb layer (Eg&gt;0.78 eV) that has a band gap Eg greater than that of the layer  10  may be used. In this instance, a p + -GaAs 0.51 Sb 0.49  layer may be used as the p-type semiconductor layer and a p-InGaAsSb layer (Eg&gt;0.78 eV) may be used as the barrier layer of the p-side MQW structure  40 . Further, a p-InGaAsSb layer (Eg&gt;0.78 eV) may be used for both of the p-type semiconductor layer and the barrier layer of the p-side MQW structure  40 . In this manner, a p-GaAsSb layer, a p-InGaAsSb layer or a like layer may be used for the barrier layer. Further, while the thickness of the p-type GaAsSb layers  10  are made different from each other, the p-type GaAsSb layers  10  may have a fixed thickness. Further, while the thickness of the p-type GaAsSb layers  10  is made different thereamong so as to gradually decrease, the thickness may be made different thereamong so as to gradually increase. Further, the thickness of the barrier layers may be set fixed while the thickness of the well layers is made different thereamong. Further, while it is preferable to use a doped p-type well layer for the p-side GaSb layers  42 , a non-doped p-type well layer may be used. 
     Further, the n-side MQW structure  41  provided in the n-type InGaAs layer  11  may be configured by providing a plurality of n-type InGaAs layers  43  that are different in composition from the n-type InGaAs layer  11  on the n-type InGaAs layer  11 . In particular, the n-side MQW structure  41  provided in the n-type InGaAs layer  11  may be configured by providing a plurality of n-type InGaAs layers  43  having a band gap smaller than that of the n-type InGaAs layer  11  on the n-type InGaAs layer  11 . Here, the n-side MQW structure  41  is configured by providing a plurality of n-type In 0.6 Ga 0.4 As layers  43  having a band gap Eg of approximately 0.68 eV on the n-type In 0.53 Ga 0.47 As layer  11  having a band gap Eg of approximately 0.74 eV. In particular, the n-side MQW structure  41  is configured by alternately and repetitively stacking the n-type In 0.53 Ga 0.47 As layers  11  and the n-type In 0.6 Ga 0.4 As layers  43  in the proximity of the pn junction portion  13  of the n-type In 0.53 Ga 0.47 As layer  11 . 
     Here, each of the n-type In 0.53 Ga 0.47 As layers  11  has a doping concentration of 5×10 18  cm −3 . Further, each of the n-type In 0.6 Ga 0.4 As layers  43  has a doping concentration of 5×10 18  cm −3 . Further, the thickness of the n-type In 0.6 Ga 0.4 As layers  43  is set fixed while the thickness of the n-type In 0.53 Ga 0.47 As layers  11  is made different from each other. Here, the thickness of the n-type In 0.6 Ga 0.4 As layers  43  is set to approximately 5 nm, and the thickness of the n-type In 0.53 Ga 0.47 As layers  11  is made different thereamong so as to gradually increase in order like approximately 1.5 nm, approximately 1.5 nm, approximately 2 nm, approximately 2 nm, approximately 2.5 nm, approximately 2.5 nm, approximately 3 nm and approximately 3 nm. 
     It is to be noted that such n-type In 0.6 Ga 0.4 As layer  43  is hereinafter referred to sometimes as well layer, n-side well layer, or n-type well layer. Further, such n-type In 0.53 Ga 0.47 As layer  11  sandwiched by the n-type In 0.6 Ga 0.4 As layers  43  is hereinafter referred to sometimes as barrier layer, n-side barrier layer or n-type barrier layer. Further, while the n-type In 0.6 Ga 0.4 As layer  43  here is used as the well layer, the well layer is not limited to this, and an n-In x Ga 1-x As (x&gt;0.53) layer (Eg&lt;0.74 eV), an n-InAlGaAs layer (Eg&lt;0.74 eV) or a like layer that have a band gap Eg smaller than that of the barrier layer, namely, a band gap Eg smaller than approximately 0.74 eV, may be used. In particular, an n-InGaAs layer, an n-InAlGaAs layer, an n-InAsSb layer or a like layer may be used for the well layer. It is to be noted that, when reducing the band gap of the barrier layer as hereinafter described, the band gap of the well layer may be smaller than that of the barrier layer. Further, while the n-type In 0.53 Ga 0.47 As layer  11  are used as the barrier layer, the barrier layer is not limited to this. In particular, an n-In x Ga 1-x As (x&gt;0.53) layer (Eg&lt;0.74 eV), an n-InAlGaAs layer (Eg&lt;0.74 eV) or a like layer that have a band gap Eg smaller than that of the layer  11  may be used. In this instance, an n-In x Ga 1-x As layer whose band gap is smaller than that of the barrier layer may be used for the well layer. Conversely, an n-In x Ga 1-x As (x&lt;0.53) layer (Eg&gt;0.74 eV), an n-InAlGaAs layer (Eg&gt;0.74 eV) or a like layer that have a band gap Eg greater than that of the layer  11  may be used for the barrier layer. Further, an n-In x Ga 1-x As (x&lt;0.53) layer (Eg&gt;0.74 eV), an n-InAlGaAs layer (Eg&gt;0.74 eV) or a like layer that have a band gap Eg greater than that of the layer  11  may be used for both of the n-type semiconductor layer and the barrier layers of the n-side MQW structure  41 . In this instance, since the band gap of the well layer may be smaller than that of the barrier layer, an n-In x Ga 1-x As layer, an n-InAlGaAs layer or a like layer that have a band gap greater than that of the well layer described above can be used for the well layer. Further, while the thickness of the n-type In 0.53 Ga 0.47 As layers  11  is made different thereamong, it may otherwise be set fixed. Further, while the thickness of the n-type In 0.53 Ga 0.47 As layers  11  is made different thereamong so as to gradually increase, it may otherwise be made different among the layers  11  so as to gradually decrease. Further, the thickness of the barrier layer may be set fixed while the thickness of the well layers is made different thereamong. Further, while it is preferable to use a doped n-type well layer for the n-side well layer  43 , a non-doped n-type well layer may be used. 
     Where the MQW structures  40  and  41  described above are provided, the p-type GaAsSb layer  10  and the n-type InGaAs layer  11  are joined to each other at the pn junction portion  13  in such a manner as to sandwich therebetween one p-type GaSb layer (p-side well layer)  42  included in the p-side MQW structure  40  and one n-type InGaAs layer (n-side well layer)  43  included in the n-side MQW structure  41 . In this instance, a junction plane between the one p-type GaSb layer  42  and the one n-type InGaAs layer  43  serves as a pn junction plane. Then, from the one p-type GaSb layer  42  and the one n-type InGaAs layer  43 , a portion that allows band-to-band tunneling of electrons is formed at the pn junction portion  13  upon biasing in the reverse direction. 
     Here, it is preferable to set the distance in a horizontal direction of the portion formed from the one p-type GaSb layer  42  and the one n-type InGaAs layer  43  to 100 {acute over (Å)} or less, namely, to 10 nm or less, in order that band-to-band tunneling of electrons is allowed. Here, the distance in a horizontal direction of the portion described above is set to 10 nm or less by radially curving a band by doping. Consequently, upon biasing in the reverse direction, band-to-band tunneling of electrons on the valence band of the p-type semiconductor layers  10  and  42  occurs at the portion and band-to-band tunneling current flows. 
     In this instance, the present backward diode  12  includes, on opposite sides of the pn junction plane, the MQW structures  40  and  41  having a band structure with which band-to-band tunneling of electrons is allowed upon biasing in the reverse direction. Here, the p-side MQW structure  40 , namely, the p-type GaAsSb layer  10  on which the p-side MQW structure  40  is provided, and the n-side MQW structure  41 , namely, the n-type InGaAs layer  11  on which the n-side MQW structure  41  is provided, individually have a type II band structure, with which band-to-band tunneling is likely to occur. 
     Further, similarly as in the case (refer to  FIGS. 14A to 21B ) of the modification to the embodiment described above, the MQW structure  40  or  41  may be provided in at least one of the p-type GaAsSb layer  10  and the n-type InGaAs layer  11 . In particular, as the MQW structure, only the p-side MQW structure  40  may be provided in the p-type GaAsSb layer  10  or only the n-side MQW structure  41  may be provided in the n-type InGaAs layer  11 . 
     For example, where the MQW structure  40  is provided only in the p-type GaAsSb layer  10 , the p-type GaAsSb layer  10  and the n-type InGaAs layer  11  are joined to each other at the pn junction portion  13  in such a manner as to sandwich therebetween one p-type GaSb layer (well layer)  42  included in the MQW structure  40 . In this instance, a joint plane between the one p-type GaSb layer  42  and the n-type InGaAs layer  11  serves as the pn junction plane. Then, from the one p-type GaSb layer  42  and the n-type InGaAs layer  11 , a portion that allows band-to-band tunneling of electrons is formed at the pn junction portion  13  upon biasing in the reverse direction. Here, it is preferable to set the distance in a horizontal direction of the portion formed from the one p-type GaSb layer  42  and the n-type InGaAs layer  11  to 100 {acute over (Å)} or less, namely, to 10 nm or less, in order that band-to-band tunneling of electrons is allowed. Consequently, upon biasing in the reverse direction, band-to-band tunneling of electrons on the valence band of the p-type semiconductor layers  10  and  42  occurs at the portion and band-to-band tunneling current flows. 
     On the other hand, for example, where the MQW structure  41  is provided only in the n-type InGaAs layer  11 , the p-type GaAsSb layer  10  and the n-type InGaAs layer  11  are joined to each other at the pn junction portion  13  in such a manner as to sandwich therebetween one n-type InGaAs layer (well layer)  43  included in the MQW structure  41 . In this instance, a junction plane between the p-type GaAsSb layer  10  and the one n-type InGaAs layer  43  serves as the pn junction plane. Then, from the p-type GaAsSb layer  10  and the one n-type InGaAs layer  43 , a portion that allows band-to-band tunneling of electrons is formed at the pn junction portion  13  upon biasing in the reverse direction. Here, it is preferable to set the distance in a horizontal direction of the portion formed from the p-type GaAsSb layer  10  and the one n-type InGaAs layer  43  to 100 {acute over (Å)} or less, namely, to 10 nm or less, in order that band-to-band tunneling of electrons is allowed. Consequently, upon biasing in the reverse direction, band-to-band tunneling of electrons on the valence band of the p-type semiconductor layer  10  occurs at the portion and band-to-band tunneling current flows. 
     Further, the embodiment described above and the modification described above may be combined such that the MQB structure  14  ( 15 ) is provided in one of the p-type GaAsSb layer  10  and the n-type InGaAs layer  11  while the MQW structure  40  ( 41 ) is provided in the other one of the p-type GaAsSb layer  10  and the n-type InGaAs layer  11 . 
     For example, the MQW structure  40  and the MQB structure  15  may be provided in the p-type GaAsSb layer  10  and the n-type InGaAs layer  11 , respectively, as depicted in  FIG. 24 . In this instance, the MQW structure  40  provided in the p-type GaAsSb layer  10  may be configured by providing a plurality of p-type GaSb layers  42  on the p-type GaAsSb layer  10  as depicted in  FIG. 25 . In particular, the MQW structure  40  may be configured by alternately stacking the p-type GaAsSb layers  10  and the p-type GaSb layers  42  in the proximity of the pn junction portion  13  of the p-type GaAsSb layer  10 . Further, the MQB structure  15  provided in the n-type InGaAs layer  11  may be configured by providing a plurality of n-type InP layers  17  on the n-type InGaAs layer  11 . In other words, the MQB structure  15  provided in the n-type InGaAs layer  11  may be configured by providing a plurality of n-type InP layers  17  on the n-type InGaAs layer  11 . 
     Or, the MQB structure  14  may be provided in the p-type GaAsSb layer  10  while the MQW structure  41  is provided in the n-type InGaAs layer  11 , for example, as depicted in  FIG. 26 . In this instance, the MQB structure  14  provided in the p-type GaAsSb layer  10  may be configured by providing a plurality of p-type AlGaSb layer  16  on the p-type GaAsSb layer  10  as depicted in  FIG. 27 . In other words, the MQB structure  14  may be configured by alternately stacking the p-type GaAsSb layers  10  and the p-type AlGaSb layers  16  in the proximity of the pn junction portion  13  of the p-type GaAsSb layer  10 . Further, the MQW structure  41  provided in the n-type InGaAs layer  11  may be configured by providing a plurality of n-type InGaAs layers  43  on the n-type InGaAs layer  11 . In other words, the MQW structure  41  provided in the n-type InGaAs layer  11  may be configured by providing a plurality of n-type InGaAs layers  43  on the n-type InGaAs layer  11 . 
     In short, the present semiconductor device may be configured such that it includes a p-type semiconductor layer  10 , an n-type semiconductor layer  11 , a pn junction portion  13  at which the p-type semiconductor layer  10  and the n-type semiconductor layer  11  are joined to each other, and an MQB structure  14  ( 15 ) or an MQW structure  40  ( 41 ) that is provided in at least one of the p-type semiconductor layer  10  and the n-type semiconductor layer  11  and functions as a barrier against at least one of electrons and holes upon biasing in a forward direction, wherein, upon biasing in a reverse direction, a portion that allows band-to-band tunneling of electrons is formed at the pn junction portion  13 . 
     Further, while, in the embodiment described above, the backward diode  12  in the embodiment described hereinabove is used as the detector  3  provided in the receiver  1 , the application of the backward diode  12  is not limited to this. For example, it is possible to use the backward diode  12  in the embodiment described hereinabove in a mixer (mixer circuit)  50  provided in the receiver  1  as illustrated in  FIGS. 28 and 29 . In particular, the mixer  50  of the receiver  1  may include the semiconductor device of the embodiment described hereinabove. For example, in the mixer  50  that includes a first diode D 1 , a second diode D 2 , a resistor R 1  and a capacitor C 1  and outputs, when an RF signal and an LO signal are inputted to an input terminal  51  thereof, a difference frequency between the signals is outputted as an intermediate frequency (IF) from an output terminal  52 , the backward diode  12  in the embodiment described hereinabove can be used as the first diode D 1  and the second diode D 2 . Consequently, the conversion loss can be reduced. In other words, the mixer  50  can be implemented with reduced loss. In this instance, the receiver  1  includes, as depicted in  FIG. 28 , for example, an antenna  6 , a low-noise amplifier  2 , a local oscillator  53 , an amplifier  54  and so forth in addition to the mixer  50 . 
     Second Embodiment 
     Now, a semiconductor device according to a second embodiment is described with reference to  FIGS. 30 to 33C . 
     The semiconductor device according to the present embodiment is different from that of the first embodiment (refer to  FIGS. 1A to 1C  and  5 ) described hereinabove in that it includes a barrier layer  60  provided between the p-type semiconductor layer  10  and the n-type semiconductor layer  11  as depicted in  FIG. 30 . 
     In particular, the present semiconductor device includes a backward diode wherein a p-type semiconductor layer (p-type GaAsSb layer)  10  and an n-type semiconductor layer (n-type InGaAs layer)  11  are joined to each other in such a manner as to sandwich the barrier layer  60  therebetween. In other words, the present semiconductor device includes a pn junction portion  13  at which the p-type semiconductor layer  10  and the n-type semiconductor layer  11  are joined to each other in such a manner as to sandwich the barrier layer  60  therebetween. 
     In this instance, if the MQB structures  14  and  15  are provided similarly as in the case of the first embodiment described hereinabove, then the p-side MQB structure  14  provided in the p-type GaAsSb layer  10  and the n-side MQB structure  15  provided in the n-type InGaAs layer  11  are joined to each other at the pn junction portion  13  in such a manner as to sandwich the barrier layer  60  therebetween. In short, the barrier layer  60  is inserted in the pn junction interface between the p-type MQB structure  14  and the n-side MQB structure  15 . 
     Further, a portion that allows band-to-band tunneling of electrons is formed at the pn junction portion  13  upon biasing in the reverse direction at least from the barrier layer  60 . In other words, the barrier layer  60  has a thickness which allows band-to-band tunneling of electrodes so that a portion that allows band-to-band tunneling of electrons is formed at the pn junction portion  13  upon biasing in the reverse direction. For example, the thickness of the barrier layer  60  is preferably set to 100 {acute over (Å)} or less, namely to 10 nm or less, so that band-to-band tunneling can occur. Since the thickness of the barrier layer  60  is small in this manner, a hetero junction having a type II band structure is formed as a whole, and band-to-band tunneling is likely to occur through the band structure. On the other hand, where the MQB structure is provided in a device which includes an active layer between a p-type semiconductor layer and an n-type semiconductor layer like a semiconductor laser, the device comes to have a type I band structure as depicted in  FIG. 32 . In this instance, the actively layer has a band gap smaller than that of the p-type semiconductor layer and the n-type semiconductor layer. In other words, since the confining effect of electrons and holes is enhanced, the energy (Ec) at the lower end of the conduction band of the active layer becomes lower than the energy at the lower end of the conduction band of the p-type semiconductor layer and the energy (Ev) at the upper end of the valance band of the active layer becomes higher than the energy at the upper end of the valence band of the n-type semiconductor layer. 
     It is to be noted that, where a portion that allows band-to-band tunneling is formed at the pn junction portion  13  from the barrier layer  60  and some other layer upon biasing in the reverse direction, the thickness of the barrier layer  60  and the other layer or the like may be adjusted so that the distance of the portion in a horizontal direction may be 100 {acute over (Å)} or less, namely, 10 nm or less. 
     Here, the barrier layer  60  may have, at the lower end of the conduction band thereof, energy between the energy at the lower end of the conduction band of the p-type semiconductor layer  10  and the energy at the lower end of the conduction band of the n-type semiconductor layer  11  and have, at the upper end of the valance band thereof, energy between the energy at the upper end of the valence band of the p-type semiconductor layer  10  and the energy at the upper end of the valence band of the n-type semiconductor layer  11 . 
     It is to be noted that the band gap of the barrier layer  60  may be greater or smaller. However, the barrier layer  60  preferably has a greater band gap. For example, the barrier layer  60  preferably has such a great band gap that it has, at the lower end (Ec) of the conduction band thereof, energy higher than that at the lower end of the conduction band of the p-type semiconductor layer  10  and has, at the upper end of the valance band thereof, energy lower than that at the upper end of the valance band of the n-type semiconductor layer  11 . This is called barrier layer of a wide band gap. It is to be noted that only one of the two conditions described above may be satisfied. In particular, the barrier layer  60  may have a band gap increased such that it has, at the lower end of the conduction band thereof, energy higher than that at the lower end of the conduction band of the p-side barrier layer  16  that configures the p-side MQB structure  14  provided in the p-type semiconductor layer  10 . Or, the barrier layer  60  may have a band gap increased such that it has, at the upper end of the valence band thereof, energy lower than that at the upper end of the valence band of the n-side barrier layer  17  that configures the n-side MQB structure  15  provided in the n-type semiconductor layer  11 . 
     In the present embodiment, the barrier layer  60  is a non-doped InAlAs layer as depicted in  FIGS. 33A to 33C . In particular, the barrier layer  60  is an i-In 0.52 Al 0.48 As layer having a thickness of approximately 5 nm. In this instance, the barrier layer  60  is a barrier layer of a wide band gap that has, at the lower end of the conduction band thereof, energy higher than that at the lower end of the conduction band of the p-type semiconductor layer  10  and has, at the upper end of the valence band thereof, energy lower than that at the upper end of the valence band of the n-type semiconductor layer  11 . Therefore, the barrier layer  60  functions as a barrier that prevents re-coupling of electrons and holes. In other words, the barrier layer  60  functions, upon biasing in the forward direction, as a barrier for electrons in the conduction band and holes in the valence band and functions as a barrier which suppresses leakage current. Further, since the barrier layer  60  is a non-doped layer, it functions also as a barrier that suppresses diffusion of impurities from the semiconductor layer  10  that is doped in the p type on the p side or from the semiconductor layer  11  that is doped in the n type on the n side. 
     It is to be noted that, while an i-In 0.52 Al 0.48 As layer is used as the barrier layer  60  in the present embodiment, the barrier layer  60  is not limited to this. 
     Further, while the barrier layer  60  here is a non-doped layer, the barrier layer  60  is not limited to this but may be doped in the n type or the p type. For example, p-type InAlGaAs, p-type In x Ga 1-x As, p-type AlSb, n-type InAlGaAs, n-type In x Al 1-x As or the like may be used for the barrier layer  60 . In this instance, the doping concentration may be 5×10 18  cm −3 . 
     In this manner, non-doped InAlGaAs, non-doped In x Al 1-x As, non-doped AlSb, p-type InAlGaAs, p-type In x Al 1-x As, p-type AlSb, n-type InAlGaAs, n-type In x Al 1-x As and so forth can be used for the barrier layer  60 . In other words, the barrier layer may include one selected from the group consisting of non-doped InAlGaAs, non-doped InAlAs, non-doped AlAs, non-doped AlSb, p-type InAlGaAs, p-type InAlAs, p-type AlAs, p-type AlSb, n-type InAlGaAs, n-type InAlAs, n-type AlAs and n-type AlSb. 
     It is to be noted that details of the other part are similar to those of the first embodiment described hereinabove, and therefore, overlapping description of the same is omitted herein. 
     Accordingly, with the semiconductor device according to the present embodiment, there is an advantage that leakage current upon biasing in the forward direction of the backward diode  12  can be suppressed. Further, there is another advantage that the detection characteristic of the detector  3  which uses the backward diode  12  and hence the characteristic of the receiver  1  can be improved. 
     It is to be noted that, while, in the embodiment described above, the semiconductor device that includes the barrier layer  60  in addition to the components of the first embodiment (refer to  FIGS. 1A to 1C  and  5 ) described hereinabove is taken as an example, the semiconductor device of the second embodiment is not limited to this, but the barrier layer  60  in the present embodiment may be additionally provided in the modifications ( FIGS. 14A to 14C ,  18 A to  18 C,  22 A,  22 B,  24  and  26 ) to the first embodiment described hereinabove. In short, the barrier layer  60  in the present embodiment can be applied to the semiconductor devices of the first embodiment and the modifications to the first embodiment. 
     For example, the barrier layer  60  in the present embodiment may be additionally provided in a semiconductor device wherein the MQB structure  14  is provided only in the p-type GaAsSb layer  10  as depicted in  FIGS. 34A to 34C . Or, for example, the barrier layer  60  in the present embodiment may be additionally provided in a semiconductor device wherein the MQB structure  15  is provided only in the n-type InGaAs layer  11  as depicted in  FIGS. 35A to 35C . Further, for example, the barrier layer  60  in the present embodiment may be additionally provided in a semiconductor device wherein the MQW structure  40  is provided in the p-type GaAsSb layer  10  and the MQW structure  41  is provided in the n-type InGaAs layer  11  as depicted in  FIGS. 36A to 36C . Furthermore, for example, the barrier layer  60  in the present embodiment may be additionally provided in a semiconductor device wherein the MQW structure  40  is provided only in the p-type GaAsSb layer  10  as depicted in  FIGS. 37A to 37C . Further, for example, the barrier layer  60  in the present embodiment may be provided in a semiconductor device wherein the MQW structure  41  is provided only in the n-type InGaAs layer  11  as depicted in  FIGS. 38A to 38C . 
     [Others] 
     It is to be noted that the present invention is not limited to the configurations described in the foregoing description of the embodiments and the modifications but they can be modified in various manners without departing from the subject matter of the present invention. 
     For example, while, in the embodiments and the modifications described above, the p-type semiconductor layer  10  and the n-type semiconductor layer  11  are made of different materials, the materials are not limited to them. For example, the p-type semiconductor layer  10  and the n-type semiconductor layer  11  may be configured from the same material. In particular, the p-type semiconductor layer  10  and the n-type semiconductor layer  11  may be configured such that they are made of the same material but have different compositions such that they have different conduction types by using different impurities to be doped therein. 
     For example, the p-type semiconductor layer  10  and the n-type semiconductor layer  11  may be configured such that, as depicted in  FIG. 39 , they are formed as a layer including InGaAs (for example, as an In 0.53 Ga 0.47 As layer) and a plurality of barrier layers  16  that configure the MQB structure  14  provided in the p-type semiconductor layer  10  are formed as layers including InGaAs (for example, as p-type In 0.4 Ga 0.6 As layers) that have a band gap greater than that of InGaAs (for example, a p-type In 0.53 Ga 0.47 As layer) that is used for the p-type semiconductor layer  10  while a plurality of barrier layers  17  that configure the MQB structure  15  provided in the n-type semiconductor layer  11  are formed as layers including InP (for example, InP layers). It is to be noted here that the barrier layers  16  that configure the MQB structure  14  provided in the p-type semiconductor layer  10  are not limited to them, but may include InGaAs that has a band gap greater than that of InGaAs (for example, n-type In 0.53 Ga 0.47 As layers) that is used, for example, for the n-type semiconductor layer  11 . Further, while modifications to the first embodiment are described here as examples, the semiconductor devices are not limited to them but can be configured as modifications to the embodiments and the modifications described hereinabove. For example, the MQW structure may be provided in place of the MQB structure. In this instance, a plurality of well layers that configure the MQW structure provided in the p-type semiconductor layer  10  may be formed so as to include InGaAs that have a band gap smaller than that of InGaAs that is used for the p-type semiconductor layer  10 . Further, a plurality of well layers that configure the MQW layer provided in the n-type semiconductor layer  11  may be formed so as to include InGaAs that has a band gap smaller than that of InGaAs that is used for the n-type semiconductor layer  11 . 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relates to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.