Abstract:
A semiconductor device includes a semiconductor substrate of a first conductive type, a collector layer formed on the semiconductor substrate and made of a first semiconductor being of the first conductive type and having a higher resistance than that of the semiconductor substrate, an intrinsic base region having a junction surface with the collector layer and made of a second semiconductor of a second conductive type, and an emitter region having a junction surface with the intrinsic base region and made of a third semiconductor of the first conductive type. A periphery of the intrinsic base region is surrounded by an insulating region extending from the collector layer to the semiconductor substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The disclosure of Japanese Patent Application No. 2003-333388 filed on Sep. 25, 2003 including specification, drawings and claims is incorporated herein by reference in its entity.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to bipolar transistors and more particularly relates to a semiconductor device including a bipolar transistor having a substrate itself serving as an electrode.  
         [0003]     Semiconductor devices have a configuration in which many active devices can be integrated on a semiconductor substrate. However, in a high frequency device for use in a cellular phone, a device is required to have high performance and a small variation in electrical characteristics and, therefore, a discrete semiconductor element such as an elemental bipolar transistor is used in many cases. Specifically, in a VCO (voltage-controlled oscillator) device required to have a breakdown voltage of 5 V or more at a power supply voltage of 3 V, a discrete semiconductor element is mainly used.  
         [0004]     As for a bipolar transistor and a semiconductor device including the bipolar transistor, configurations and fabrication methods are disclosed in Japanese Laid-Open Publication No. 3-235334, Japanese Laid-Open Publication No. 3-110852 and the like.  
         [0005]     Hereinafter, a known npn-type high-frequency bipolar transistor, i.e., a VCO device and the like used for a cellular phone, which is required to have a breakdown voltage of 5 V or more, will be described. To ensure a breakdown voltage of 5 V or more, in general, a high resistance epitaxial layer is formed on a substrate serving as a low resistance collector electrode. The high resistance epitaxial layer has to have a thickness of 0.4 μm or more.  
         [0006]     Specifically, the known npn-type high-frequency bipolar transistor includes an n-type semiconductor substrate formed of a silicon single crystal having a resistivity of, for example, 0.01 Ωcm or less and serving as a collector electrode, a collector layer formed of an n − -type epitaxial layer having a thickness of 0.4 μm to 2.0 μm and a resistivity of, for example, 0.5 Ωcm to 5.0 Ωcm on the semiconductor layer, an intrinsic base region selectively formed in an upper portion of the collector layer and an emitter region selectively formed in an upper portion of the intrinsic base region.  
         [0007]     Furthermore, an emitter electrode made of polysilicon is formed on the emitter region and an external base electrode is formed on the intrinsic base region.  
         [0008]     Under the external base electrode, a field insulating film having a thickness of 600 nm is formed so as to electrically insulate the collector layer from an external base lead electrode and reduce a capacitance therebetween.  
         [0009]     In an npn transistor for high-frequency signal processing, negative feedback due to a feedback capacitance between collector bases causes the problem of deterioration of high-frequency characteristics such as reduction in a cut-off frequency (f T ). The feedback capacitance is the sum of a junction capacitance due to a pn junction between collector bases and an interconnect capacitance between interconnects with an insulating film interposed therebetween. To reduce the junction capacitance, reduction in a junction area by size reduction is effective. Moreover, to reduce the interconnect capacitance, it is effective to increase the thickness of the insulating film.  
         [0010]     Thus, as described in Japanese Laid-Open Publication No. 3-235334, reduction in the feedback capacitance is intended by forming a field oxide film of LOCOS and the like directly under the external lead electrode while reducing the respective sizes of the emitter region and the base region.  
         [0011]     However, in the known semiconductor device, when the power supply voltage, i.e., a collector voltage becomes high, a depletion layer extends in the lateral direction (i.e., in the parallel direction to a substrate surface) in the collector layer. Because of this, as shown in  FIG. 11 , as the power supply voltage is increased, the junction capacitance is gradually reduced, so that the feedback capacitance is also reduced. When the feedback capacitance is reduced, the impedance of a bipolar transistor is changed. Therefore, when a device in which a transistor whose impedance is variable is formed is used, high frequency characteristics such as a cut-off frequency are deteriorated.  
       SUMMARY OF THE INVENTION  
       [0012]     It is therefore an object of the present invention to solve the above-described known problems and prevent fluctuation of a feedback capacitance between collector bases in a semiconductor device even when a power supply voltage fluctuates.  
         [0013]     To achieve the object, according to the present invention, a semiconductor device using a low resistance semiconductor substrate as a collector electrode and including a collector layer and an intrinsic base region formed on the semiconductor substrate is formed to have a configuration in which an insulation for insulating the periphery of the intrinsic base region having a junction surface with the collector layer is provided to extend from the collector layer to the semiconductor substrate.  
         [0014]     Note that in Japanese Laid-Open Publication No. 3-110852, a bipolar transistor using an SOI (silicon on insulator) substrate in which an insulating region is formed in the semiconductor substrate itself for the purpose of reduction in a parasitic capacitance is described and fluctuation of a feedback capacitance can be suppressed as in the present invention. However, the SOI substrate is different from a regular silicon substrate and very special and costs are increased. Therefore, the SOI substrate is not suitable to practical use for fabrication of a bipolar transistor.  
         [0015]     Specifically, a first bipolar transistor according to the present invention is characterized in that the transistor includes: a semiconductor substrate of a first conductive type; a collector layer formed on the semiconductor substrate and made of a first semiconductor being of the first conductive type and having a higher resistance than that of the semiconductor substrate; an intrinsic base region having a junction surface with the collector layer and made of a second semiconductor of a second conductive type; and an emitter region having a junction surface with the intrinsic base region and made of a third semiconductor of the first conductive type, and a periphery of the intrinsic base region is surrounded by an insulating region extending from the collector layer to the semiconductor substrate.  
         [0016]     In the first semiconductor device, the periphery of the intrinsic base region is surrounded by the insulating region extending from the collector layer to the semiconductor substrate, so that a depletion layer is not expanded in the lateral direction in the collector layer. Accordingly, a junction capacitance between collector bases becomes invariable, and even when the power supply voltage (i.e., a collector voltage) fluctuates, the width of the depletion layer is not changed. Therefore, a semiconductor device in which a feedback capacitance is stabilized and high-frequency characteristics are not deteriorated can be obtained in a simple manner. Moreover, an absolute value for the feedback capacitance itself can be reduced, compared to a known semiconductor device.  
         [0017]     In the first semiconductor device, it is preferable that the intrinsic base region is divided into a plurality of regions and the insulating region is formed for each of the plurality of regions so that a lower portion of the insulating region reaches the semiconductor substrate in each of the plurality of regions into which intrinsic base regions has been divided.  
         [0018]     Thus, breakdown voltage characteristics can be improved while reducing the size. Moreover, the lower portion of the insulating region divided into a plurality of regions each said region reaches the semiconductor substrate in each region, so that the absolute value for the feedback capacitance can be reduced more.  
         [0019]     According to a second semiconductor device according to the present invention is characterized in that the semiconductor device includes: a semiconductor substrate of a first conductive type; a collector layer formed on the semiconductor substrate and made of a first semiconductor being of the first conductive type and having a higher resistance than that of the semiconductor substrate; an intrinsic base region having a junction surface with the collector layer, made of a second semiconductor of a second conductive type and divided into a plurality of regions; a plurality of emitter regions each having a junction surface with an associated one of the regions into which the intrinsic base region is divided and made of a third semiconductor of the first conductive type, and an outer periphery of the plurality of intrinsic base regions is surrounded by an insulating region extending from the collector layer to the semiconductor substrate.  
         [0020]     In the second semiconductor device, the outer periphery of the plurality of intrinsic base regions is surrounded by the insulating region extending from the collector layer to the semiconductor substrate, so that a depletion layer is not expanded in the lateral direction in the collector layer. Accordingly, a junction capacitance between collector bases becomes invariable, and even when the power supply voltage (i.e., a collector voltage) fluctuates, the width of the depletion layer is not changed. Therefore, a semiconductor device in which a feedback capacitance is stabilized and high-frequency characteristics are not deteriorated can be obtained in a simple manner. Furthermore, even though the intrinsic base region is divided into a plurality of regions in the collector layer, only ones of the plurality of intrinsic base regions located in the outer side are surrounded by the insulating region and thus an insulating trench structure in which an inner wall face of the insulating region is oxidized and then polysilicon is filled with a trench can be used. As a result, even with the collector layer having a relatively large thickness, a device in which change in the feedback capacitance is small can be achieved.  
         [0021]     In the first or second semiconductor device, it is preferable that the semiconductor substrate has a resistivity of 1×10 −2  Ωcm or less and the collector layer has a resistivity of 0.5 Ωcm or more and a thickness of 0.4 μm or more.  
         [0022]     Moreover, in the first or second semiconductor device, it is preferable that the intrinsic base layer is formed on the collector layer by epitaxial growth.  
         [0023]     A method for fabricating a semiconductor device according to the present invention is characterized in that the method includes: a) forming on a semiconductor substrate of a first conductive type a collector layer made of a first semiconductor being of the first conductive type and having a higher resistance than that of the semiconductor substrate by epitaxial growth; b) forming an insulating region in the collector layer so as to surround an intrinsic region; c) forming on the intrinsic region of the collector layer an intrinsic base region made of a second semiconductor of a second conductive type; and d) forming on the intrinsic base region an emitter region made of a third semiconductor of the first conductivity, and in the step b), the insulating region is formed so as to surround a periphery of the intrinsic base region and extend from the collector layer to the semiconductor substrate.  
         [0024]     According to the inventive method for fabricating a semiconductor device, an insulating region is formed in a periphery of an intrinsic base region so as to extend from a collector layer to a semiconductor substrate. Thus, a depletion layer is not expand in the lateral direction in the collector layer. Accordingly, a junction capacitance between collector bases becomes invariable, and even when the power supply voltage (i.e., a collector voltage) fluctuates, the width of the depletion layer is not changed. Therefore, a semiconductor device in which a feedback capacitance is stabilized and high-frequency characteristics are not deteriorated can be obtained in a simple manner. Moreover, an absolute value for the feedback capacitance itself can be reduced, compared to a known semiconductor device.  
         [0025]     In the inventive method for fabricating a semiconductor device, it is preferable that in the step b), the insulating region is provided plural in number on an upper surface of the collector layer and the step b) of forming the plurality of insulating regions includes forming a plurality of first insulating regions each having a bottom portion located in an upper portion of the collection layer, and forming a second insulating region so that the second insulating region passes through an associated one of the plurality of first insulating regions located in a more outer side than the intrinsic base region and extends from the collector layer to the semiconductor substrate. Thus, the second semiconductor device of the present invention can be obtained.  
         [0026]     Moreover, in the inventive method for fabricating a semiconductor device, it is preferable that in the step c), the intrinsic base region is formed on the collector layer by epitaxial growth. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is a cross-sectional view illustrating a semiconductor device according to a first embodiment of the present invention.  
         [0028]      FIG. 2  is a graph showing the dependency of a feedback capacitance on a power supply voltage in the semiconductor device of the first embodiment of the present invention.  
         [0029]      FIG. 3  is a cross-sectional view illustrating a semiconductor device according to a modified example of the first embodiment of the present invention.  
         [0030]      FIGS. 4A through 4E  are cross-sectional views illustrating respective steps for fabricating the semiconductor device of the modified example of the first embodiment of the present invention.  
         [0031]      FIGS. 5A through 5D  are cross-sectional views illustrating respective steps for fabricating the semiconductor device of the modified example of the first embodiment of the present invention.  
         [0032]      FIG. 6  is a cross-sectional view illustrating a semiconductor device according to a second embodiment of the present invention.  
         [0033]      FIGS. 7A through 7D  are cross-sectional views illustrating respective steps for fabricating the semiconductor device of the second embodiment of the present invention.  
         [0034]      FIGS. 8A through 8D  are cross-sectional views illustrating respective steps for fabricating the semiconductor device of the second embodiment of the present invention.  
         [0035]      FIGS. 9A and 9B  are cross-sectional views illustrating respective steps for fabricating the semiconductor device of the second embodiment of the present invention.  
         [0036]      FIG. 10  is a cross-sectional view illustrating a semiconductor device according to a modified example of the second embodiment of the present invention.  
         [0037]      FIG. 11  is a graph showing the dependency of a feedback capacitance on a power supply voltage in a known bipolar transistor having a substrate serving as an electrode. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0038]     A first embodiment of the present invention will be described with reference to the accompanying drawings.  
         [0039]      FIG. 1  is a cross-sectional view illustrating a bipolar transistor, i.e., a semiconductor device according to a first embodiment of the present invention.  
         [0040]     As shown in  FIG. 1 , an n-type collector layer (high resistance collector layer)  102  made of silicon having a relatively high resistivity of, for example, 0.5 Ωcm to 5.0 Ωcm and epitaxially grown to a thickness of 0.4 μm to 0.2 μm is formed on a low resistance n-type semiconductor substrate  101  made of a silicon single crystal having a resistivity of, for example, 0.01 Ωcm or less and serving as a support substrate.  
         [0041]     In the collector layer  102 , a plurality of insulating trenches  103  each having a width of about 0.8 μm are formed so as to divide the collector layer  102  into a plurality of intrinsic regions. In respective upper portions of the regions of the collector layer  102  surrounded by the insulating trenches  103 , a plurality of p-type intrinsic base regions  104  having a resistivity of, for example, 0.01 Ωcm to 0.1 Ωcm are formed, respectively. In respective center portions of the intrinsic base regions  104 , a plurality of emitter regions  105  are formed, respectively. Then, a plurality of emitter electrodes  106  made of polysilicon are formed on the emitter regions  105 , respectively.  
         [0042]     A plurality of external base electrodes  107  made of polysilicon are formed so that each of the external base electrodes  107  is located on a side portion of each of the intrinsic base regions  104  in the collector layer  102  and one of the insulating trenches  103  located adjacent to the side portion.  
         [0043]     Moreover, a protection insulating film  108  made of, for example, oxide silicon is formed over the collector layer  102  as well as the emitter electrodes  106  and the external base electrodes  107 .  
         [0044]     In the protection insulating film  108 , at least a contact hole  108   a  is formed for each of the base electrodes  106  and the emitter base electrodes  107  so that each of the base electrodes  106  or each of the emitter base electrodes  107  is exposed through the contact hole  108   a.  In each contact hole  108   a,  a base interconnect  109  electrically connected to an associated one of the external base electrodes  107  or an emitter interconnect  110  electrically connected to an associated one of the emitter electrodes  106  is formed.  
         [0045]     As a feature of the first embodiment, each of the insulating trenches  103  for insulating each of the external base electrode  107  and the low resistance n-type semiconductor substrate  101  serving as a collector electrode or the high resistance n-type collector layer  102  epitaxially grown on the n-type semiconductor substrate  101  is formed so as to pass through the collector layer  102  and have a lower end portion reaching to the n-type semiconductor substrate  101 . Thus, a capacitance between the collector electrode (i.e., the n-type semiconductor substrate  101 ) and each of the external base electrodes  107  is reduced and expansion of a depletion layer in the lateral direction (i.e., in the parallel direction to the principal surface of the substrate) in the collector layer  102  can be prevented. Accordingly, a junction capacitance between collector bases becomes invariable, so that even when a power supply voltage (i.e., a collector voltage) fluctuates, the width of the depletion layer is not changed. Therefore, a semiconductor device in which a feedback capacitance is stabilized and high-frequency characteristics are not deteriorated can be achieved at low costs. Moreover, an absolute value for the feedback capacitance itself can be made smaller than that of the known semiconductor device.  
         [0046]      FIG. 2  is a graph showing the dependency of a feedback capacitance on a power supply voltage in the semiconductor device of the first embodiment. As shown in  FIG. 2 , compared to the dependency of a feedback capacitance on a power supply voltage in the known semiconductor device shown in  FIG. 11 , the reduction in the feedback capacitance is small. Accordingly, in the first embodiment, the impedance of the semiconductor device is not largely changed, so that deterioration of high-frequency characteristics can be prevented.  
         [0047]     Note that to make a breakdown voltage between the collector bases to be 5 V or more, the thickness of the high resistance collector layer  102  has to be set at 0.4 μm or more.  
       Modified Example of First Embodiment  
       [0048]     Hereinafter, a modified example of the first embodiment of the present invention will be described with reference to the accompanying drawings.  
         [0049]      FIG. 3  is a cross-sectional view illustrating a semiconductor device according to the modified example of the first embodiment of the present invention. In  FIG. 3 , each member also shown in  FIG. 1  is identified by the same reference numeral and therefore the description thereof will be omitted.  
         [0050]     As shown in  FIG. 3 , in this modified embodiment, an intrinsic base region  204  is formed on a collector layer  102  by selective epitaxial growth.  
         [0051]     Hereinafter, a method for fabricating the semiconductor device of this modified example configured in the above-described manner will be described with reference to  FIGS. 4A through 4E  and  FIGS. 5A through 5D .  
         [0052]     First, as shown in  FIG. 4A , on an n-type semiconductor substrate  101  made of a silicon (Si) single crystal doped with arsenic (As) so as to have a resistivity of about 0.01 Ωm, a collector layer  102  made of n-type silicon doped with phosphorous (P) so as to have a resistivity of 1 Ωcm is epitaxially grown to a thickness of about 0.4 μm by, for example, chemical vapor deposition (CVD).  
         [0053]     Next, as shown in  FIG. 4B , a resist pattern (not shown) having an opening corresponding to an insulating trench forming region in the collector layer  102  is formed by lithography and dry-etching using an etchant containing chlorine (Cl 2 ) as a main component is performed using the resist pattern as a mask, thereby forming a plurality of trenches each having a depth of about 0.45 μm and reaching an n-type semiconductor substrate  101 . Then, bottom and side surfaces of each of the trenches are thermally oxidized to a thickness of 10 nm. Thereafter, silicon oxide is deposited over the collector layer  102  to a thickness of 0.8 μm by CVD to fill each of the trenches. Subsequently, silicon oxide which has been deposited on the collector layer  102  and is unnecessary is removed by planarization using chemical mechanical polishing (CMP). Thus, a plurality of insulating trenches  103  are formed in the collector layer  102 .  
         [0054]     Next, as shown in  FIG. 4C , a surface oxide film in an intrinsic base region forming portion of the collector layer  102  is removed and then selective epitaxial growth in which the intrinsic base region forming portion is doped with boron (B) is performed to form a p-type intrinsic base region  204 .  
         [0055]     Next, as shown in  FIG. 4D , a first underlying insulating film  210  made of silicon oxide is formed by CVD and then part of the first underlying insulating film  210  located on both of side portions of the intrinsic base regions  204  is removed by selectively etching the part using lithography and etching.  
         [0056]     Next, as shown in  FIG. 4E , a polysilicon film for forming an external base electrode is grown over the first underlying insulating film  210  and the intrinsic base region  204  by CVD and boron (B) ions are implanted into the polysilicon film. Thereafter, implanted boron ions are thermally diffused. Subsequently, selective etching is performed to the thermally treated polysilicon film using lithography and dry-etching to pattern the p-type polysilicon film, thereby forming a plurality of external base electrodes  107 .  
         [0057]     Next, as shown in  FIG. 5A , a second underlying insulating film  211  made of silicon oxide is grown over the first underlying insulating film  210  and the external base electrodes  107  by CVD. Subsequently, part of the second underlying insulating film  211  located on a center portion of the intrinsic base region  204  is removed by lithography and etching, so that the intrinsic base region  204  is exposed.  
         [0058]     Next, as shown in  FIG. 5B , an n-type polysilicon film doped with phosphorous (P) for forming an emitter electrode is grown over the second underlying insulating film  211  and the intrinsic base region  204  by CVD. Subsequently, implanted phosphorous ions are diffused in the intrinsic base region  204  by thermal treatment such as rapid thermal annealing (RTA), thereby forming an emitter region  105  in an upper portion of the intrinsic base region  204 . Thereafter, the polysilicon film is selectively etched by lithography and dry-etching to perform patterning to the n-type polysilicon film, thereby forming each emitter electrode  106 .  
         [0059]     Next, as shown in  FIG. 5C , a protection insulating film  108  is deposited by CVD so as to cover the second underlying insulating film  211  and each of the emitter electrodes  106 .  
         [0060]     As shown in  FIG. 5D , lithography and dry-etching are performed to the protection insulating film  108  to form a plurality of contact holes  108   a,  so that each of the external base electrodes  107  and the emitter electrodes  106  is exposed through an associated one of the contact holes  108   a.  Subsequently, an interconnect layer made of metal is formed on the protection insulating film  108  by vapor deposition or the like so as to fill each of the contact holes  108   a.  Thereafter, patterning is performed using selectively etching the interconnect layer so as to form a base interconnect  109  connected to the each of the base electrode  107  and an emitter interconnect  110  connected to each of the emitter electrodes  106 .  
         [0061]     Note that if the intrinsic base region  204  is formed in an upper portion of the collector layer  102  by ion implantation, instead of epitaxial growth, the semiconductor device of the first embodiment can be obtained.  
       Second Embodiment  
       [0062]     Hereinafter, a second embodiment of the present invention will be described with reference to the accompanying drawings.  
         [0063]      FIG. 6  is a cross-sectional view illustrating a bipolar transistor, i.e., a semiconductor device according to the second embodiment of the present invention.  
         [0064]     As shown in  FIG. 6 , an n-type collector layer (high resistance collector layer)  302  made of silicon having a relatively high resistivity of, for example, 0.5 Ωcm to 5.0 Ωcm and epitaxially grown to a thickness of 0.4 μm to 0.2 μm is formed on a low resistance n-type semiconductor substrate  301  made of a silicon single crystal having a resistivity of, for example, 0.01 Ωm or less and serving as a support substrate.  
         [0065]     In an upper portion of the collector layer  302 , a plurality of insulating oxide films  303  each having a width of about 0.8 μm and a depth of 0.04 μm to 0.4 μm are formed so as to divide the collector layer  302  into a plurality of intrinsic regions. In respective upper portions of the regions of the collector layer  302  surrounded by the insulating oxide films  303 , a plurality of p-type intrinsic base regions  304  having a resistivity of, for example, 0.01 Ωcm to 0.1 Ωcm are formed, respectively. In respective center portions of the intrinsic base regions  304 , a plurality of emitter regions  305  are formed, respectively. Then, a plurality of emitter electrodes  306  made of polysilicon are formed on the emitter regions  305 , respectively.  
         [0066]     A plurality of external base electrodes  307  made of polysilicon are formed so that each of the external base electrodes  307  is located on a side portion of an associated one of the intrinsic base regions  304  in the collector layer  302  and one of the insulating oxide films  303  located adjacent to the side portion.  
         [0067]     Moreover, a protection insulating film  308  made of, for example, oxide silicon is formed over the collector layer  302  as well as the emitter electrodes  306  and the external base electrodes  307 .  
         [0068]     In the protection insulating film  308 , at least a contact hole  308   a  is formed for each of the base electrodes  306  and the emitter base electrodes  307  so that each of the base electrodes  306  or each of the emitter base electrodes  307  is exposed through the contact hole  308 a. In each contact hole  308 a, a base interconnect  309  electrically connected to an associated one of the external base electrodes  307  or an emitter interconnect  310  electrically connected to an associated one of the emitter electrodes  306  is formed.  
         [0069]     As a feature of the second embodiment, the insulating oxide films  303  for electrically insulating the epitaxially grown high resistance n-type collector layer  302  from each of the external base electrodes  307  reduces a capacitance between the low resistance n-type semiconductor substrate  301  serving as a collector electrode and each of the external base electrode  307 . Furthermore, in each one of the insulating oxide films  303  located in an outer side, an insulating trench  312  having a depth of 0.4 μm to 3 μm is formed so as to pass through a center portion of the insulating oxide film  303  and reach an n-type semiconductor substrate  301 . With the insulting trenches  312  formed in each one of the insulating oxide films  303  located in an outer side, a capacitance between the n-type semiconductor substrate  301  and each of the external base electrodes  307  is reduced and expansion of a depletion layer in the lateral direction (i.e., in the parallel direction to the principal surface of the substrate) in the collector layer  302  can be prevented, so that a feedback capacitance is stabilized. As a result, a semiconductor device in which high-frequency characteristics are not deteriorated can be achieved at low costs. Moreover, an absolute value for the feedback capacitance itself can be reduced, compared to that of the known semiconductor device.  
         [0070]     Hereinafter, a method for fabricating the semiconductor device of this modified example configured in the above-described manner will be described with reference to  FIGS. 7A through 7D ,  FIGS. 8A through 8D  and  FIGS. 9A and 9B .  
         [0071]     First, as shown in  FIG. 7A , on an n-type semiconductor substrate  301  made of a silicon (Si) single crystal doped with arsenic (As) so as to have a resistivity of about 0.01 Ωm, a collector layer  302  made of n-type silicon which has been doped with phosphorous (P) so as to have a resistivity of 1 Ωcm is epitaxially grown to a thickness of about 0.8 μm by, for example, chemical vapor deposition (CVD).  
         [0072]     Next, as shown in  FIG. 7B , a resist pattern (not shown) having an opening corresponding to an insulating oxide film forming region in the collector layer  302  is formed by lithography, and dry-etching using an etchant containing chlorine (Cl 2 ) as a main component is performed using the resist pattern as a mask, thereby forming a plurality of first trenches each having a depth of about 0.45 μm. Then, bottom and side surfaces of each of the first trenches are thermally oxidized to a thickness of 10 nm. Thereafter, silicon oxide is deposited over the collector layer  302  to a thickness of 0.8 μm by CVD to fill each of the first trenches. Subsequently, silicon oxide which has been deposited on the collector layer  302  and is unnecessary is removed by planarization using chemical mechanical polishing (CMP). Thus, a plurality of insulating oxide films  303  are formed in an upper portion of the collector layer  302 .  
         [0073]     Next, as shown in  FIG. 7C , a resist pattern (not shown) having an opening corresponding to a center portion of each one of the plurality of insulating oxide films  303  located in an outer side is formed by lithography, and dry-etching is performed using the resist pattern as a mask, thereby forming a plurality of second trenches each having a depth of about 2 μm and reach an n-type semiconductor substrate  301 . Then, bottom and side surfaces of each of the second trenches are thermally oxidized to a thickness of 10 nm. Thereafter, polysilicon is deposited over the collector layer  302  to a thickness of 3 μm by low pressure CVD to fill each of the second trenches. Subsequently, polysilicon which has been deposited on the collector layer  302  and is unnecessary is removed by planarization using etch back. Thus, a plurality of insulating trenches  312  are formed in an upper portion of the collector layer  302 .  
         [0074]     As has been described, in the second embodiment, even when the collector layer  302  has a relatively large thickness, the insulating trenches  312  each of which reaches the n-type semiconductor substrate  301  for preventing expansion of the depletion layer generated in the lateral direction in the collector layer  302  are formed in only ones of the insulating oxide films  303  located in outer sides. Therefore, the insulating trenches  312  can be formed in a more simple manner than in the first embodiment.  
         [0075]     Next, as shown in  FIG. 7D , boron ions are selectively implanted into part of the collector layer  302  located an inner side than the insulating trenches  312 , thereby forming a p-type intrinsic base region  304  having a smaller depth than that of insulating oxide films  303 .  
         [0076]     Next, as shown in  FIG. 8A , a first underlying insulating film  410  made of silicon oxide is formed by CVD and then part of the first underlying insulating film  410  located on both of side portions of the intrinsic base regions  304  is removed by selectively etching the part using lithography and etching.  
         [0077]     Next, as shown in  FIG. 8B , a polysilicon film for forming an external base electrode is grown over the first underlying insulating film  410  and the intrinsic base region  304  by CVD, and boron (B) ions are implanted into the polysilicon film. Thereafter, implanted boron ions are thermally diffused. Subsequently, selective etching is performed to the thermally treated polysilicon film using lithography and dry-etching to perform patterning to the p-type polysilicon film, thereby forming a plurality of external base electrodes  307 .  
         [0078]     Next, as shown in  FIG. 8C , a second underlying insulating film  411  made of silicon oxide is grown over the first underlying insulating film  410  and the external base electrodes  307  by CVD. Subsequently, part of the second underlying insulating film  411  located on a center portion of the intrinsic base region  304  is removed by lithography and etching. Thus, the intrinsic base region  304  is exposed.  
         [0079]     Next, as shown in  FIG. 8D , an n-type polysilicon film doped with phosphorous (P) for forming an emitter electrode is grown over the second underlying insulating film  411  and the intrinsic base region  304  by CVD. Subsequently, implanted phosphorous ions are diffused in the intrinsic base region  304  by thermal treatment such as rapid thermal annealing (RTA), thereby forming an emitter region  305  in an upper portion of the intrinsic base region  304 . Thereafter, the polysilicon film is selectively etched by lithography and dry-etching to perform patterning to the n-type polysilicon film, thereby forming a plurality of emitter electrodes  306 .  
         [0080]     Next, as shown in  FIG. 9A , a protection insulating film  308  is deposited by CVD so as to cover the second underlying insulating film  411  and each of the emitter electrodes  306 .  
         [0081]     As shown in  FIG. 9B , lithography and dry-etching are performed to the protection insulating film  308  to form a plurality of contact holes  308   a,  so that each of the external base electrodes  307  and the emitter electrodes  306  is exposed through an associated one of the contact holes  308   a.  Subsequently, an interconnect layer made of metal is formed on the protection insulating film  308  by vapor deposition or the like so as to fill each of the contact holes  308   a.  Thereafter, patterning is performed using selectively etching the interconnect layer so as to form a base interconnect  309  connected to an associated one of the base electrode  307  and an emitter interconnect  310  connected to an associated one of the emitter electrodes  306 .  
       Modified Example of Second Embodiment  
       [0082]     Hereinafter, a modified example of the second embodiment of the present invention will be described with reference to the accompanying drawings.  
         [0083]      FIG. 10  is a cross-sectional view illustrating a semiconductor device according to the modified example of the second embodiment of the present invention. In  FIG. 10 , each member also shown in  FIG. 6  is identified by the same reference numeral and therefore the description thereof will be omitted.  
         [0084]     As shown in  FIG. 10 , in this modified embodiment, an intrinsic base region  404  is formed on part of the collector layer  302  located in the inner side than each of the insulating trenches  312  by selective epitaxial growth. Note that the intrinsic base region  404  formed by epitaxial growth can be formed in the same manner as in  FIG. 4C  of the modified example of the first embodiment.  
         [0085]     Note that in each of the modified examples of the first and second embodiments, description has been made specifically using as an example the npn transistor among bipolar transistors. However, a pnp transistor may be also used.  
         [0086]     Moreover, silicon oxide is used for each of insulating films such as the protection insulating films  108 ,  308  and the like. However, instead of silicon oxide, a material having insulation properties, such as silicon nitride, may be used.  
         [0087]     As has been described, a semiconductor device according to the present invention and a method for fabricating the semiconductor device have the effect of allowing stabilization of a feedback capacity between collector bases and, more specifically, the semiconductor device of the present invention is useful as a semiconductor device or the like including a bipolar transistor in which a substrate itself serves as an electrode.