Abstract:
A semiconductor integrated circuit device includes a silicon substrate having a first region and a second region identical in conductivity to said first region and having a lower dopant concentration than said first region, a second MOS transistor formed on a main surface of said second region and configuring a radio frequency switch circuit switching on/off an input and output of a radio frequency signal, and a first MOS transistor formed on a main surface of said first region and configuring a radio frequency circuit other than said radio frequency switch circuit. There can be provided a high performance, highly reliable semiconductor integrated circuit with an RF switch circuit provided on a silicon substrate by SOPing.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to semiconductor integrated circuits and particularly to silicon semiconductor integrated circuits having a radio frequency (RF) circuit processing an RF signal.  
           [0003]    2. Description of the Background Art  
           [0004]    In recent years, as mobile phones have widely been used and wireless LANs have practically been used, semiconductor integrated circuits that are used in such electronics have been noted and RF semiconductor devices have been noted in particular. To provide electronics with high performance and miniaturize and produce it inexpensively, it is essential that an RF semiconductor device serving as a main component thereof provide high performance have a small size and be produced inexpensively.  
           [0005]    Conventionally, group III-V compound semiconductor having high electron mobility such as GaAs has been a main stream of material for a substrate that is used in an RF semiconductor device. Group III-V compound semiconductor, however, is much more expensive than silicon semiconductor typically used as a material for substrates of semiconductor devices and has been an obstacle to inexpensively producing RF semiconductor devices.  
           [0006]    The recent rapid advance in silicon MOS transistor microfabrication technology has now allowed silicon MOS transistors to have a small gate length less than 0.2 μm. Such silicon MOS transistors allow significantly improved transconductance Gm and have now achieved characteristics applicable as gigahertz RF semiconductor devices.  
           [0007]    If a silicon MOS transistor can be used to fabricate an RF semiconductor device, a significant cost reduction can be achieved and it can also be expected that a baseband portion or any other similar logic circuit portion conventionally fabricated using silicon MOS process techniques is provided in the form of a single chip, and by System On Chip (SOP) a reduction in cost and that in area for mounting can also be achieved. Thus there is a demand for rapidly developing an RF semiconductor device using a silicon substrate and having more satisfactory characteristics.  
           [0008]    As has been described above, a silicon RF semiconductor device has RF characteristics having attained a level sufficiently applicable as an RF semiconductor device. However, it has several disadvantages in SOPing with an RF switch circuit switching on/off an input and output of an RF signal (hereinafter an RF signal processing circuit other than the RF switch circuit will be referred to as a “specific RF circuit”). In particular, if it is used in a radio frequency range of no less than the 5 GHz band, the RF switch circuit&#39;s insertion loss is disadvantageously increased and SOPing can hardly be implemented.  
           [0009]    In RF semiconductor devices that are used in mobile phones, wireless LANs and the like, as aforementioned, an RF switch circuit is a significantly important circuit. As shown in FIG. 18, typically a switch circuit  140  is configured by a transmission and reception switch circuit using a single pole double throw (SPDT) switch  140 ′. In reception, the switch receives an RF signal from an antenna  141  and transfers the signal to a reception portion&#39;s low noise amplifier (an RF low noise amplification circuit)  150 . In transmission, the switch receives an RF signal from a power amplifier  150 ′ and transmits the signal to antenna  141 .  
           [0010]    Fabricating SPDT switch  140 ′ using an MOS transistor  130  allows SOPing with another, specific RF circuit. A possible, simplest configuration of the SPDT switch is shown in FIG. 19 by way of example. Furthermore, FIG. 20 is an equivalent circuit diagram of the FIG. 19 SPDT switch with a transmission side ON and a reception side OFF. In this SPDT switch an insertion loss increases for the following reason:  
           [0011]    In FIG. 20, C d  represents an MOS transistor&#39;s source/drain junction capacitance. Through source/drain junction capacitance C d  a substrate resistance R si  is connected to a circuit. Improving the MOS transistor&#39;s ON characteristics entails reducing an ON resistance R ON  and a relatively large gate width of approximately 100 μm to 400 μm is accordingly used. As such, source/drain junction capacitance C d  would assume a relatively large value. Furthermore, the source/drain junction capacitance C d  impedance |z| is represented by 1/(2π×f×C d ) and it decreases for a gigahertz RF frequency range, since frequency f assumes a large value. Consequently, an RF signal flows to substrate resistance R si  and an RF signal to be transmitted by a switch would have a loss in transmission, i.e., an insertion loss is introduced. As such, for an RF range (of no less than the 5 GHz band in particular) an insertion loss is significantly increased and a satisfactory switch circuit can hardly be fabricated.  
           [0012]    An SOPed RF silicon semiconductor device needs to employ a substrate formed of silicon providing a small resistance (of approximately 10 mΩ to 10 Ω) to prevent latch-up. As such, in an RF switch circuit, with a large source/drain junction capacitance C d , as described above, the silicon substrate&#39;s resistance R si  contributes to a significant loss. As such, SOPing with another specific RF circuit has significantly been difficult.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention contemplates a semiconductor integrated circuit that can also provide high performance and high reliability when SOPing is employed to provide an RF switch circuit on a silicon substrate to switch on/off an input and output of an RF signal.  
           [0014]    In accordance with the present invention a semiconductor integrated circuit includes a silicon substrate and first and second MOS transistors formed on the silicon substrate. The silicon substrate has a first region and a second region identical in conductivity to the first region and having a lower dopant concentration than the first region. The second MOS transistor is formed on a main surface of the second region and configures an RF switch circuit switching on/off an input and output of an RF signal. The first MOS transistor is formed on a main surface of the first region and configures a specific RF circuit other than the RF switch circuit.  
           [0015]    Thus the first MOS transistor that is biased can provide a depletion layer larger in width, as seen in the direction of the thickness of the silicon substrate, than the second MOS transistor that is biased can. As such, the second MOS transistor can significantly be smaller in source/drain junction capacitance than the first MOS transistor. As a result, the first MOS transistor can be used to configure the specific RF circuit, which requires small-current-leakage characteristics, and the second MOS transistor can be used to configure the RF switch circuit, which essentially requires reduced source/drain junction capacitance, to provide a semiconductor integrated circuit with the specific RF circuit and the RF switch circuit arranged together on a single silicon substrate and having satisfactory characteristics. Note that they can discretely be fabricated in a conventional MOS process fabrication process simply by using an additional photomask.  
           [0016]    The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    In the drawings:  
         [0018]    [0018]FIG. 1 is a cross section of a silicon semiconductor integrated circuit of a first embodiment of the present invention including an RF switch circuit and another specific RF circuit;  
         [0019]    [0019]FIG. 2 represents a lateral, one-dimensional dopant concentration profile traversing source and drain diffusion layers of an n type MOS transistor of the RF switch circuit of the silicon semiconductor integrated circuit of the first embodiment;  
         [0020]    FIGS.  3 - 8  show a process for fabricating the silicon semiconductor integrated circuit of the first embodiment;  
         [0021]    [0021]FIG. 9 represents a result of a simulation of a longitudinal, one-dimensional dopant concentration profile traversing the source or drain diffusion layer of the n type MOS transistor of the RF switch circuit or that of an n type MOS transistor of the specific RF circuit in the silicon semiconductor integrated circuit of the first embodiment;  
         [0022]    [0022]FIG. 10 represents a result of a simulation of a profile of an electric field in a silicon substrate of the silicon semiconductor integrated circuit of the first embodiment that is obtained when the RF switch circuit and the specific RF circuit have their n type MOS transistors biased;  
         [0023]    [0023]FIG. 11 represents a result of an estimation of discrete source/drain passage characteristics of an MOS transistor of the RF switch circuit in the silicon semiconductor integrated circuit of the first embodiment by S parameter RF characteristics estimation;  
         [0024]    [0024]FIG. 12 is a Smith chart of S parameter characteristics S 11  of the MOS transistor of the RF switch circuit in the silicon semiconductor integrated circuit of the first embodiment;  
         [0025]    [0025]FIG. 13 represents a result of a measurement of a breakdown voltage between source and drain diffusion layers of the MOS transistor of the RF switch circuit in the silicon semiconductor integrated circuit of the first embodiment;  
         [0026]    [0026]FIG. 14 is a cross section of an RF switch circuit of a silicon semiconductor integrated circuit of a second embodiment of the present invention;  
         [0027]    [0027]FIG. 15 is a cross section of an RF switch circuit of a silicon semiconductor integrated circuit of a third embodiment of the present invention;  
         [0028]    [0028]FIG. 16 is a circuit diagram showing one example of a configuration of an RF switch circuit in an exemplary application of the present invention;  
         [0029]    [0029]FIG. 17 shows another example of the configuration of the RF switch circuit in the exemplary application of the present invention;  
         [0030]    [0030]FIG. 18 shows a circuit configuration of an SPDT switch;  
         [0031]    [0031]FIG. 19 is a circuit diagram of an SPDT switch using an MOS transistor to configure its circuit; and  
         [0032]    [0032]FIG. 20 is an equivalent circuit diagram of the SPDT switch shown in FIG. 19. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Hereinafter the present invention in embodiments will be described with reference to the drawings.  
         [0034]    First Embodiment  
         [0035]    The present invention in a first embodiment provides a silicon semiconductor integrated circuit with an RF switch circuit and another, specific RF circuit processing an RF signal that are SOPed on a single silicon substrate. Furthermore as the specific RF circuit a logic circuit such as a baseband portion may be included.  
         [0036]    Configuration  
         [0037]    Reference will initially be made to FIG. 1 to describe a configuration of the silicon semiconductor integrated circuit of the present embodiment. The silicon semiconductor integrated circuit in the present embodiment employs an MOS transistor as a semiconductor device used in the specific RF circuit and it also employs an MOS transistor in the RF switch circuit.  
         [0038]    Of the above MOS transistors, a p type MOS transistor  20  for the specific RF circuit includes a p type source diffusion layer  22  and a p type drain diffusion layer  23  on a main surface of a silicon substrate  1 . Source and drain diffusion layers  22  and  23  include lightly doped drain (LDD) diffusion layers  24  and  25 , respectively, adjacent thereto. Furthermore between LDD diffusion layers  24  and  25  a channel doping layer  26  is formed for adjusting a threshold voltage Vth. Furthermore LDD diffusion layers  24  and  25  are surrounded by n +  type pocket diffusion layers  27  and  28  formed by shallow ion injection. V th  adjusting channel doping layer  26  and n +  type pocket diffusion layers  27  and  28  are provided to improve the MOS transistor&#39;s basic characteristics, such as ON current characteristics and threshold voltage V th , to sufficiently eliminate source-drain current leakage.  
         [0039]    Furthermore in a substrate region underlying p type MOS transistor  20  of the specific RF circuit a highly doped n well layer  21  is formed to extend in the direction of the depth of silicon substrate  1  from a pn junction interface corresponding to a bottom plane of source and drain diffusion layers  22  and  23  and also to a boundary between layers  22  and  23  and the substrate region. Furthermore, although not shown in the figure, a plurality of highly doped n type layers such as an n type punchthrough stopper layer, an n type isolation layer and an n type buried layer are formed, as required, in the substrate region underlying the pn junction interface. This highly doped n type layers are essential to currently used MOS transistors for example to prevent latch-up and reduce current leakage.  
         [0040]    The specific RF circuit and the RF switch circuit include n type MOS transistors  10  and  30 , respectively, having n type source diffusion layers  12  and  32 , respectively, and n type drain diffusion layers  13  and  33 , respectively on a main surface of silicon substrate  1 . Source and drain diffusion layers  12  and  13  are adjacent to LDD diffusion layers  14  and  15 , respectively, and source and drain diffusion layers  32  and  33  are adjacent to LDD diffusion layers  34  and  35 , respectively. Furthermore between LDD diffusion layers  14  and  15  and between LDD diffusion layers  34  and  35  channel doping layers  16  and  36  are formed, respectively, for adjusting threshold voltage V th . Furthermore, LDD diffusion layers  14  and  15 , and  34  and  35  are surrounded by p +  type pocket diffusion layers  17  and  18 , and  37  and  38 , respectively, formed by shallow ion injection.  
         [0041]    With reference to FIG. 2, n type MOS transistors  10  and  30 , with the aforementioned LDD diffusion layers  14  and  15 , and  34  and  35 , threshold voltage V th  adjusting channel doping layers  16  and  36 , and furthermore p +  type pocket diffusion layers  17  and  18 , and  37  and  38 , have a channel region having an uneven dopant concentration. Furthermore, the channel region&#39;s dopant concentration, as seen in a direction parallel to a main surface of silicon substrate  1 , increases in a vicinity of a portion of source and drain diffusion layers  12  and  13 , and  32  and  33 , i.e., LDD diffusion layers  14  and  15 , and  34  and  35 . V th  adjusting channel doping layers  16  and  36 , and n +  type pocket diffusion layers  17  and  18 , and  37  and  38  are formed to enhance the MOS transistors&#39; basic characteristics (such as ON current characteristics and threshold voltage Vth) and sufficiently eliminate source-drain current leakage. Note that p type MOS transistor  20  also has a channel region having a similar, uneven dopant concentration profile, although different in conductivity.  
         [0042]    Furthermore in a substrate region  1   a  underlying p type MOS transistor  10  of the specific RF circuit a highly doped p well layer  11  is formed to extend in the direction of the depth of silicon substrate  1  from a pn junction interface corresponding to a bottom plane of source and drain diffusion layers  12  and  13  and also to a boundary between layers  12  and  13  and substrate region  1   a . Furthermore, although not shown in the figure, a plurality of highly doped p type layers such as a p type punchthrough stopper layer and a p type isolation layer are formed, as required, in substrate region  1   a  underlying the pn junction interface. These highly doped p type layers are essential to currently used MOS transistors for example to prevent latch-up and reduce current leakage.  
         [0043]    In contrast, n type MOS transistor  30  of the RF switch circuit overlies a substrate region  1   b  free of any highly doped, p type well layer extending from a pn junction interface in the direction of the depth of silicon substrate  1 . Furthermore, a p type isolation layer, a p type punchthrough stopper layer and any other similar, highly doped p type layer are also absent. As such, substrate region  1   b  has the initial dopant concentration of silicon substrate  1 . As such, the dopant concentration of substrate region  1   b  immediately underlying an interface between source and drain diffusion layers  32  and  33  and substrate region  1   b  of n type MOS transistor  30  of the RF switch circuit, is lower than that of substrate region  1   a  immediately underlying an interface between source and drain diffusion layers  12  and  13  and substrate region  1   a  of n type MOS transistor  10  of the specific RF circuit.  
         [0044]    Fabrication  
         [0045]    Reference will now be made to FIGS.  3 - 8  to describe a process for fabricating the silicon semiconductor integrated circuit of the present embodiment. The silicon semiconductor integrated circuit in the present embodiment is fabricated in accordance with a basic MOS transistor process flow. More specifically, as shown in FIG. 3, silicon substrate  1  initially has a main surface provided with an element isolation film  2 . Then, as shown in FIG. 4, ion injection is provided for forming p type MOS transistor  20 . Then, as shown in FIGS.  5 - 7 , ion injection is provided for forming n type MOS transistors  10  and  30 . Furthermore, as shown in FIG. 8, a gate electrode  4  has a side wall provided with a sidewall  5  to complete a semiconductor integrated circuit configured as shown in FIG. 1.  
         [0046]    Note, however, that to fabricate the semiconductor integrated circuit of the present embodiment, it is necessary to use an additional photomask to selectively, differently configure a substrate region underlying the n type MOS transistor of the specific RF circuit and that underlying the n type MOS transistor of the RF switch circuit. More specifically, n type MOS transistor  30  of the RF switch circuit overlies substrate region  1   b  free of any highly doped, p type layer. To achieve this, in forming a plurality of highly doped, p type layers of n type MOS transistor  10  of the specific RF circuit such as p well layer  11 , a p type punchthrough stopper layer and a p type isolation layer a region of silicon substrate  1  that is to be served for n type MOS transistor  30  of the RF switch circuit is covered with photoresist  7  (see FIG. 5). A highly doped, p type layer is thus formed selectively only under n type MOS transistor  10  of the specific RF circuit, while the layer is not formed under n type MOS transistor  30  of the RF switch circuit.  
         [0047]    Result of Simulation  
         [0048]    Reference will now be made to FIGS. 9 and 10 to describe a result of a simulation of electrical characteristics of the silicon semiconductor integrated circuit configured as described above. As has been described above, the n type MOS transistor of the RF switch circuit overlies a substrate region free of any highly doped, p type layer, and, as is apparent from FIG. 9, for the n type MOS transistor of the RF switch circuit a p type dopant concentration immediately under a pn junction interface is significantly smaller than for the n type MOS transistor of the specific RF circuit. More specifically, the n type MOS transistor of the specific RF circuit overlies a p type punchthrough stopper layer, a p type isolation layer, a p well layer or the like and under the pn junction interface a dopant concentration of no less than 10 17  cm −3  is provided, whereas for the n type MOS transistor of the RF switch circuit a dopant concentration of approximately 10 15  cm −3  is provided. This presumably contributes to a larger width, as seen in the direction of the depth of the silicon substrate, of a depletion layer introduced at the pn junction of the n type MOS transistor of the RF switch circuit that is biased than the n type MOS transistor of the specific RF circuit that is biased.  
         [0049]    Furthermore, in FIG. 10, as a region having a high electric field may be considered a region of a depletion layer, presumably for the n type MOS transistor of the specific RF circuit a depletion layer in the vicinity of the pn junction interface would be introduced with a significantly small width, whereas for the n type MOS transistor of the RF switch circuit a depletion layer would extend much deeper than the pn junction interface. More specifically, in a semiconductor integrated circuit designed to have a dopant concentration profile as shown in FIG. 9, presumably for the n type MOS transistor of the specific RF circuit a depletion layer would extend only to a depth of approximately 0.1 μm as measured from the pn junction interface, whereas for the n type MOS transistor of the RF switch circuit a depletion layer would extend as deep as approximately 1 μm.  
         [0050]    Function and Effect  
         [0051]    A silicon semiconductor integrated circuit configured as described above can provide a larger width of a depletion layer extending downward from a pn junction of a MOS transistor of an RF switch circuit that is biased than an n type MOS transistor of another, specific RF circuit that is biased. As such, the former transistor&#39;s source/drain junction capacitance Cd can significantly be reduced and accordingly also in an RF range its impedance can sufficiently be increased. This can eliminate a loss of an RF signal that is attributed to a small source/drain junction capacitance Cd of the MOS transistor of the RF switch circuit. An RF semiconductor device with a satisfactory switch function can thus be provided.  
         [0052]    Furthermore, as a p type dopant concentration immediately under a pn junction interface of the source and drain is reduced, a resistance R si  of a silicon substrate in a grounding path connected to capacitance C d  is also increased and an impedance corresponding to C d  and R si  together serving as a grounding path that are added together is increased, and a loss of an RF signal in the MOS transistor of the RF switch circuit is further reduced. A high-performance, RF silicon semiconductor device that has not conventionally been implemented for an RF range such as no less than 5 GHz, can thus be provided.  
         [0053]    Note that providing an uneven, lateral, one-dimension dopant concentration laterally traversing source and drain diffusion layers of the n type MOS transistor of the RF switch circuit to form an effectively heavily doped, p type region in a portion laterally adjacent to a pn junction provided by a p +  pocket diffusion layer, ensures preventing a depletion layer from laterally extending and also preventing current leakage attributed to puncthrough.  
         [0054]    Prototype  
         [0055]    A prototype device was fabricated and had its electrical characteristics measured, as represented in FIGS. 11 and 12. FIG. 11 represents a result of an estimation of a discrete, n type MOS transistor&#39;s source and drain passage characteristics by S parameter RF characteristics estimation. Note that FIG. 11 also represents a result of a measurement of an n type MOS transistor of an RF switch circuit conventionally configured for reference. As shown in FIG. 11, it can be understood that the n type MOS transistor of the RF switch circuit conventionally configured provides a significantly increased insertion loss for an RF range, whereas that of the RF switch circuit in accordance with the present invention provides a hardly increased insertion loss for the RF range.  
         [0056]    [0056]FIG. 12 represents S parameter characteristics S 11  of the n type MOS transistor of the RF switch circuit as conventional and S parameter characteristics S 11  of the n type MOS transistor of the RF switch circuit in accordance with the present invention, as represented in a Smith chart. It can be understood that the n type MOS transistor of the RF switch circuit as conventional provides an arc moving on the chart, starting at the chart&#39;s center (50 Ω) and proceeding round in a right downward direction as frequency increases. This indicates that a large source/drain junction capacitance component exists. In contrast, the n type MOS transistor of the RF switch circuit in the present invention does not provide a movement in an arc starting at the chart&#39;s center (50 Ω) and proceeding round in the right downward direction. It hardly provides a movement away from the chart&#39;s center as frequency increases. That is, it can be understood that junction capacitance is significantly reduced.  
         [0057]    It can thus be understood that the n type MOS transistor of the RF switch circuit in the present invention can exhibit significantly satisfactory RF characteristics. However, its source and drain diffusion layer&#39;s bottom surface or a pn junction interface overlies a low p type dopant concentration, and punchthrough characteristics between the source and drain diffusion layers of the MOS transistor may be impaired. FIG. 13 represents a result of a measurement of breakdown voltage between source-drain diffusion layers of the n type MOS transistor of the RF switch circuit of the present embodiment. As can be apparent from the figure, the configuration in the present embodiment provides worse breakdown voltage characteristics than a conventional configuration. More specifically, for a most important power supply voltage (of 1.8V for this MOS transistor) a current leakage in a device used as a switch and having a gate width of approximately 200 μm has a maximal value of approximately 300 nA, which is no less than 100 times a value (of approximately 2 nA at its maximum) of an MOS transistor of another, specific RF circuit.  
         [0058]    For typical logic circuits, an MOS transistor with such a large current leakage cannot be used, since a logic circuit uses a large number of MOS transistors in total, and in total an enormous current would be consumed and reduced power consumption cannot be achieved. An RF switch circuit, as will be described hereinafter, employs as few as two or four devices having a gate width of approximately 200 μm, and a maximal current leakage of 200 nA per MOS transistor is sufficiently acceptable. As such, forming an RF switch circuit of the MOS transistor structured as above, as described in the present embodiment, is not disadvantageous. On the contrary, it can be understood that an effect obtained therefrom is more enormous.  
         [0059]    Second Embodiment  
         [0060]    Reference will now be made to FIG. 14 to describe a second embodiment of the present invention. In the present embodiment a semiconductor integrated circuit is configured similarly as described in the first embodiment, although an isolation film  2   a  adjacent to an active region of n type MOS transistor  30  of an RF switch circuit has a width larger than a minimal design rule (a design rule applied in another, specific RF circuit). This can prevent an isolation film from having a reduced device isolating capability attributed to absence of ion injection for isolation and hence prevent current leakage between adjacent devices from increasing. A circuit using an n type MOS transistor free of ion injection for isolation, as assumed in the present invention, includes an RF switch circuit. The circuit employs no more than several MOS transistors. As such, if an isolating oxide film is increased in width, it does not result in a significantly increased chip area or affect the cost.  
         [0061]    Third Embodiment  
         [0062]    Reference will now be made to FIG. 15 to describe a third embodiment of the present invention. In the present embodiment a semiconductor integrated circuit is configured similarly as described in the first embodiment, although isolation film  2  adjacent to an active region of n type MOS transistor  30  of an RF switch circuit overlies a low doped diffusion layer  9 . Thus, the dopant concentration under isolation film  2  adjacent to n type MOS transistor  30  of the RF switch circuit is lower than that under an isolation film adjacent to an active region of an n type MOS transistor of another, specific RF circuit. This can prevent an isolation film from having a reduced device isolating capability attributed to absence of ion injection for isolation in a substrate region, i.e., prevent current leakage between adjacent devices from increasing. Furthermore, the present embodiment allows a chip area to be smaller than the second embodiment does.  
         [0063]    Exemplary Application  
         [0064]    In the present exemplary application the MOS transistor of the RF switch circuit as described in any of the first to third embodiments is used to fabricate an SPDT switch as shown in FIG. 16. N type MOS transistor  30  that is used preferably has a gate width of approximately 10 μm to 1 mm and desirably has a gate length in accordance with a minimal design rule to reduce ON resistance R on . Forming each device&#39;s connection line and each terminal&#39;s connection line of a transmission line exhibiting a 50 Ω characteristics impedance for an operating frequency, allows more preferable characteristics.  
         [0065]    As such, a p type dopant concentration under a pn junction of source/drain of the n type MOS transistor forming the RF switch circuit, is reduced. A pn junction depletion layer significantly extends and a source/drain junction capacitance significantly decreases. This can significantly reduce a loss of an RF signal passing through the source/drain junction capacitance of the transistor of the RF switch circuit that is attributed to the silicon substrate&#39;s resistance and significantly reduce insertion loss, one of the most important RF characteristics of the RF switch circuit.  
         [0066]    Furthermore the MOS transistor of the RF switch circuit as described in any of the first to third embodiments is used to fabricate an SPDT switch as shown in FIG. 17. N type MOS transistor  30  that is used preferably has a gate width of approximately 10 μm to 1 mm and desirably has a gate length in accordance with a minimal design rule to reduce ON resistance R on , as described above. Forming each device&#39;s connection line and each terminal&#39;s connection line of a transmission line exhibiting a 50 Ω characteristics impedance for an operating frequency, allows more preferable characteristics.  
         [0067]    As such, a p type dopant concentration under a pn junction of source/drain of the n type MOS transistor forming the RF switch circuit, is reduced. A pn junction depletion layer significantly extends and a source/drain junction capacitance significantly decreases. This can significantly reduce a loss of an RF signal passing through the source/drain junction capacitance of the transistor of the RF switch circuit that is attributed to the silicon substrate&#39;s resistance and significantly reduce insertion loss, one of the most important RF characteristics of the RF switch circuit.  
         [0068]    Essentially, a series-parallel SPDT switch having an effect improving isolation characteristics is preferably used as a switch circuit. In effect, however, it has been inapplicable, since series-parallel type has conventionally used a larger number of MOS transistors than series type and further increased an increased insertion loss attributed to a source/drain junction capacitance. The present invention can eliminate the increased insertion loss attributed to a source/drain junction capacitance. Accordingly, a series-parallel SPDT switch allowing isolation characteristics to be improved can be used and an SPDT switch having satisfactory RF characteristics can be provided.  
         [0069]    In the above description an RF switch circuit has employed an MOS transistor overlying a substrate region free of injection for isolation to have a low concentration. However, it is not limited thereto. The present invention lies in providing a lower dopant concentration in a substrate region of an MOS transistor of an RF switch circuit than in an MOS transistor of another, specific RF circuit, and it is not limited to any particular level of concentration. As such, for example, the MOS transistor of the RF switch circuit may have its well layer, isolation layer, punchthrough stopper layer and the like formed with a smaller dosage than the MOS transistor of the specific RF circuit to achieve a reduced concentration of a substrate region of the MOS transistor of the RF switch circuit.  
         [0070]    Furthermore, while in the first embodiment a gate oxide film of the MOS transistor of the specific RF circuit and that of the MOS transistor of the RF switch circuit have simultaneously been formed by way of example, they may be provided separately. More specifically, these gate oxide films may be different in thickness. Desirably, however, they are simultaneously provided to simplify the fabrication process.  
         [0071]    Furthermore, desirably, the MOS transistor of the specific RF circuit and that of the RF switch circuit have their respective gates with their respective lengths each set to be a dimension in accordance with a minimal design rule to allow the MOS transistors to have the most excellent gate characteristics.  
         [0072]    Note that in general, semiconductor integrated circuits of this type often employ a p type silicon substrate and the RF switch circuits often employ an n type MOS transistor. Partially, however, a p type MOS transistor can be used and the present invention is also applicable thereto.  
         [0073]    Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.