before describing the preferred embodiment of the present invention , the gist of the present invention will be described for a better understanding of the present invention . the present inventor carried out a simulation , in which the thermal conduction paths are examined in a memory module board by thermal analysis , in order to find the structure for suppressing the temperature rise of a memory module board . as a result of the simulation , the following was found . the heat generated in the memory module board was radiated from the surface of the memory module board to the ambient air and simultaneously radiated through a socket toward the mount board . the amount of heat reached to the mount board was equal to about 45 % of the whole heat radiation . based on the result of this simulation , the present inventor derived an idea that the temperature rise of the memory module board can be effectively suppressed by efficiently radiating the heat toward the mount board from the memory module board . the present inventor further discussed specifically the structure to efficiently radiate the heat from the memory module board to the mount board , and decided to provide a heat radiation guide plate which contacts each of the socket and the mount board . the memory module board is mounted on the socket via a plug . in this case , in addition to the conventional heat radiation paths , heat generated by the memory module board is transferred to the heat radiation guide plate from the combination of plug and socket , and further to the mount board therefrom . thus , the heat is efficiently radiated from the memory module board . in addition , radiation of heat from the socket to the mount board was efficiently carried out by setting thermal conductivity of the heat radiation guide plate at higher than 1 w / m · k furthermore , the surface portion of the mount board that the socket or heat radiation guide plate contacts is configured by a heat radiation layer having a thermal conductivity of 1 w / m · k or higher , in place of the conventional layer having a thermal conductivity of less than 1 w / m · k in this case , the heat transferred to the mount board through the combination of plug and socket or the heat radiation guide plate spreads quickly in the in - plane directions of the mount board through the heat radiation layer , and is radiated to the mount board having a larger thermal capacity through the heat radiation layer . thus , the heat generated in the memory module board can be efficiently radiated to the mount board . alternatively , a similar advantage can be obtained by providing a heat radiation sheet having a thermal conductivity of 1 w / m · k or higher on the mount board , which is kept in contact with the socket or heat radiation guide plate . in particular , the heat can be radiated more efficiently from the memory module if the heat radiation guide plate , heat radiation layer , or heat radiation sheet is made of iron , aluminum , copper or a material having a thermal conductivity of 50 w / m · k or higher , such as pgs . now , the present invention will be described below in more detail with reference to the accompanying drawings , based on the embodiments of the present invention . fig1 is a perspective view showing the structure of a semiconductor device according to a first embodiment of the present invention . fig2 is a top plan view showing the semiconductor device shown in fig1 , from which memory module boards are removed . the semiconductor device , generally designated by numeral 100 , has a mount board 10 , an elongate socket 20 provided on the mount board 10 , and memory module boards 30 mounted on the socket 20 . the memory module board 30 is a card - like board having a rectangular shape . a plurality of memory devices 31 are provided on both surfaces of the memory module board 30 , and an elongate plug is provided on an edge portion of the memory module board 30 . the plug has a plurality of plug terminals arranged in a row on both sides of the memory module board 30 and near the edge of the memory module board 30 . the socket 20 has a plurality of socket terminals arranged in two rows and each corresponding to one of the plug terminals of the plug . the mount board 10 is configured as a multilayer printed circuit board , and has a structure in which a plurality of insulating layers 11 of fr4 ( flame retardant type 4 ) are layered one on another . the socket in an area underlying the socket 20 except for the center of the socket 20 , a heat radiation layer 13 made of copper is formed in place of the uppermost insulating layer 11 a . copper heat radiation guide plates 40 a and 40 b are provided in contact with the surface of the heat radiation layer 13 and the side surfaces of the socket 20 . fr4 and copper have a heat conductivity of about 0 . 3 w / m · k and 385 w / m · k , respectively . fig3 a shows a part of a cross - section taken along the line a - a shown in fig1 . in the mount board 10 , interconnection patterns made of copper are formed between adjacent two insulating layers 11 and on top and bottom surfaces of the mount board 10 . a number of through - holes ( not shown ) each penetrating the mount board 10 and a number of via holes ( not shown ) each penetrating one or a plurality of insulating layers 11 are formed in the mount board 10 . interconnection plugs are formed inside these through - holes and via holes , thereby connecting together different interconnection layers . in the uppermost insulating layer 11 a , the area underlying the center of the socket 20 configures an interconnection area 12 . in the interconnection area 12 , a plurality of via holes each penetrating one or a plurality of insulating layers 11 including the uppermost insulating layer 11 a are formed . a plurality of electrodes provided at the bottom of the socket 20 and extending from the respective socket terminals are connected to the interconnection plugs formed inside the via holes and through - holes . the socket 20 has a u - shaped cross - section . a plurality of socket terminals 21 are provided on both inner side surfaces of the u - shaped socket 20 . on the memory module board 30 , a plurality of plug terminals ( not shown ) are formed on both side surfaces of the board at a lower portion thereof . these plug terminals are coupled to the socket terminals 21 of the socket by insertion of the plug into the socket 21 . the heat radiation guide plates 40 a and 40 b have a height substantially equal to the height of the socket 20 . a grease ( not shown ) is applied between side surfaces of the socket 20 and the heat radiation guide plates 40 a and 40 b , to tighten the contact between the side surfaces of the socket 20 and the heat radiation guide plates 40 a and 40 b . thus , efficiency of thermal conduction between the socket 20 and the heat radiation guide plates 40 a and 40 b is improved . fig3 b shows a longitudinal - section taken along the direction b - b shown in fig1 and fig3 a the heat radiation guide plates 40 a and 40 b have substantially the same dimensions as the side surface of the socket 20 in the lengthwise direction and in the height direction . the heat radiation guide plates 40 a and 40 b are provided adjacent to the socket 20 . the heat radiation layer 13 has such a planar shape that is large enough to encompass the whole bottom surface of the socket 20 and the heat radiation guide plates 40 a and 40 b , and yet does not obstruct connection of the mount board 10 . in manufacture of the semiconductor device 100 , at first , the heat radiation layer 13 made of copper is formed around the interconnection area 12 prior to formation of the uppermost insulating layer 11 a , during the process for forming the mount board 10 . after forming the uppermost insulating layer 11 a , the socket 20 is provided on the mount board 10 . thereafter , the heat radiation guide plates 40 a and 40 b each are formed so as to contact a side surface of the socket 20 and the upper surface of the heat radiation layer 13 . in operation of the semiconductor device 100 , wherein electric power is supplied to the semiconductor device 100 to activate the memory module board 30 , most of the heat generated in the memory module board 30 is transferred to the heat radiation guide plates 40 a and 40 b through the socket 20 , in addition to the conventional heat radiation paths . the heat is then transferred to the heat radiation layer 13 from the heat radiation guide plates 40 a and 40 b . alternatively , the heat is directly transferred to the heat radiation layer 13 from the socket 20 . the heat transferred to the heat radiation layer 13 quickly spreads in the in - plane directions of the mount board 10 , and is thus quickly radiated toward the mount board 10 having a larger thermal capacity . accordingly , in comparison with the conventional semiconductor device , the heat is efficiently radiated from the memory module board 30 , whereby temperature rise of the memory module board 30 can be effectively suppressed . in the present embodiment , the heat radiation layer 13 is made of copper . however , the heat radiation layer 13 may be made of another material having a thermal conductivity of 1 w / m · k or higher . for example , the heat radiation layer 13 may preferably be made of a pgs ( pyrolytic graphite sheet ). the pgs has a thermal conductivity of 600 to 800 w / m · k or so in the direction in which the crystal surface thereof extends , and a thermal conductivity of 15 w / m · k or so in the thickness direction of the crystal surface . therefore , if the heat radiation layer 13 is made of pgs , the heat can be efficiently radiated by arranging the crystal surface of the pgs in parallel with the extending direction of the heat radiation layer 13 . in addition , in the present embodiment , the heat radiation guide plates 40 a and 40 b are made of copper . however , the heat radiation guide plates 40 a and 40 b may be made of another material . as a preferable example , the heat radiation guide plates 40 a and 40 b may be made of aluminum having a thermal conductivity of 201 w / m · k , iron having a thermal conductivity of 80 w / m · k further , the heat radiation layer 13 or heat radiation guide plates 40 a and 40 b may be made of insulating material having a high thermal conductivity . examples of the insulating material having a high thermal conductivity include silicon nitride , aluminum nitride , and the like . fig4 a is a perspective view showing the structure of a semiconductor device according to a first modification of the first embodiment . fig4 b shows a longitudinal - section taken along the line b - b shown in fig4 a . in the semiconductor device 101 , heat radiation fins 41 are provided on a line extending from the longitudinal direction of the socket 20 and on the surface of the heat radiation layer 13 . that is , the heat radiation fins 41 are additionally provided in the structure of the semiconductor device 100 shown in fig1 . according to the semiconductor device 101 of the present modification , the heat radiation fins 41 , provided on the heat radiation layer 13 , assist radiation of the heat generated in the heat radiation layer 13 for the mount board 10 toward the ambient air . accordingly , in comparison with the semiconductor device 100 shown in fig1 , the temperature rise of the memory module board 30 can be more effectively suppressed . a heat radiation fan may be additionally provided on the mount board 10 . it is to be noted that , if the heat radiation layer 13 in the semiconductor device 100 shown in fig1 is made of a conductive material , interconnection patterns which contact the heat radiation layer 13 cannot be provided without involving a short - circuit failure . therefore , the interconnection area 12 should have a broad area by providing the heat radiation layer 13 apart from the socket 20 , if complicated interconnections are to be provided in the interconnection area 12 . fig5 to 7 show semiconductor devices in which the heat radiation layer 13 is provided apart from the socket 20 , according to second to fourth modifications of the first embodiment . in the semiconductor device 102 shown in fig5 , the interconnection area 12 is provided to encompass the whole bottom surfaces of the socket 20 and the heat radiation guide plates 40 a and 40 b . in the semiconductor device 103 shown in fig6 , a plurality of interconnection areas 12 are provided each to encompass the bottom surface of a corresponding socket 20 and a corresponding pair of heat radiation guide plates 40 a and 40 b . in the semiconductor device 104 shown in fig7 , first heat radiation layers 13 a to 13 c are provided each between adjacent two of the socket 20 , second heat radiation layers 13 d and 13 e are respectively provided outside of the outermost socket 20 , and third heat radiation layers 13 f and 13 g are provided in the vicinity of the edges of the socket 20 arranged therein . the interconnection areas 12 are provided in the space between the heat radiation layers 13 a to 13 g . this configuration provides a simpler structure for the interconnection circuit . the heat radiation layers 13 having a high thermal conductivity and provided near the socket 20 in the semiconductor devices 102 , 103 , and 104 shown in fig5 to 7 effectively radiate the heat generated in the mount board 10 near the socket 20 in the in - plane directions of the mount board 10 . thus , the temperature rise of the memory module board 30 can be effectively suppressed . fig8 a and 8b show a cross - section and a longitudinal section , respectively , of a semiconductor device according to a second embodiment of the present invention . fig8 a shows the structure of the memory module similarly to fig3 a , whereas fig8 b shows the structure of the memory module similarly to fig3 b . in the semiconductor device 105 , the heat radiation layer 13 shown in fig1 is not formed , and the uppermost insulating layer 11 a is formed over the entire surface of the mount board 10 . heat radiation guide plates 40 a and 40 b are provided on the mount board 10 , which is in contact with the side surfaces of a socket 20 . heat radiation sheets 14 are provided on the mount board 10 , which is in contact with the side surfaces of the heat radiation guide plates 40 a and 40 b . the semiconductor device 105 has a structure similar to the structure of the semiconductor device 100 shown in fig1 except for the features described above . according to the semiconductor device 105 of the present embodiment , the heat radiation sheets 14 can be formed easily on the mount board 10 . therefore , the semiconductor device 105 of the present embodiment can be more easily manufactured compared to the semiconductor device 100 of the first embodiment . in addition , the design choice of the circuit interconnections in the mount board 10 is increased since the heat radiation sheets 14 overlies an insulating layer 11 a . the heat radiation sheets 14 may be provided in contact with the side surfaces of the socket 20 , and heat radiation guide plates 40 a and 40 b may be provided on the heat radiation sheets 14 . fig9 shows a longitudinal - section of the memory module board in a semiconductor device according to a modification of the second embodiment , similarly to fig8 b . the semiconductor device 106 of this modification corresponds to the structure of the semiconductor device 105 shown in fig8 provided with heat radiation fins 41 on the surfaces of the heat radiation sheets 14 . the heat radiation fins 41 are arranged on the line extended from the extending direction of the socket 20 . by using the mount board 106 in the present modification , the heat is effectively radiated from of the heat radiation sheets 14 or mount board 10 toward the ambient air through the fins 41 . thus , in comparison with the semiconductor device 105 shown in fig8 , temperature rise of the memory module board 30 can be effectively suppressed . additionally , a heat radiation fan may be provided on the mount board 10 . in the embodiments described above , memory module boards have been described . however , temperature rise of a variety of module boards each provided with one or a plurality of other electronic components can be effectively suppressed by applying a structure similar to those described above . in another electronic component directly mounted on a mount board other than the module boards , the temperature rise of the electronic component can be also effectively suppressed by forming a heat radiation layer as described above or by providing heat radiation guide plates and / or heat radiation sheets . in this case , the heat radiation guide plates are provided in contact with each of the electronic component and the mount board . the present invention has been described above based on preferred embodiments thereof . however , the semiconductor devices according to the present invention are not limited to the structures described in the embodiments . the scope of the present invention should be considered as including those semiconductor devices that would be derived by making various changes and modifications to the structures of the above embodiments .