the invention will be more fully understood by reference to the following examples . fig7 is a sectional view of a variable capacity device 100 in which a quantum - wave interference layer is formed in an i - layer . the variable capacity device 100 has a substrate 10 made of gallium arsenide ( gaas ). a gaas buffer layer 12 of n - type conduction , having a thickness generally of 0 . 3 μm and an electron concentration of 2 × 10 18 / cm 3 , is formed on the substrate 10 . an n - ga 0 . 51 in 0 . 49 p contact layer 14 of n - type conduction , having a thickness generally of 0 . 13 μm and electron concentration of 2 × 10 18 / cm 3 , is formed on the buffer layer 12 . an n - al 0 . 51 in 0 . 49 p n - layer 16 of n - type conduction , having a thickness generally of 0 . 2 μm and an electron concentration of 1 × 10 18 / cm 3 , is formed on the contact layer 14 . a non - doped i - layer 18 is formed on the n - layer 16 . a al 0 . 51 in 0 . 49 p n - layer 20 of n - type conduction , having a thickness generally of 0 . 2 μm and an electron concentration of 1 × 10 18 / cm 3 , is formed on the i - layer 18 . a n - ga 0 . 51 in 0 . 49 p second contact layer 22 of n - type conduction , having a thickness generally of 0 . 13 μm and an electron concentration of 2 × 10 18 / cm 3 , is formed on the n - layer 20 . an n - gaas first contact layer 24 of n - type conduction , having a thickness generally of 0 . 06 μm and an electron concentration of 2 × 10 18 / cm 3 , is formed on the second contact layer 22 . an electrode layer 26 made of gold and germanium ( au / ge ), having a thickness generally of 0 . 2 μm , is formed so as to cover the entire back of the substrate 10 . another electrode layer 28 made of au / ge , having a thickness generally of 0 . 2 μm , is formed on some portion of the first contact layer 24 . a quantum - wave interference unit a 1 having a multi - quantum layer structure with 10 pairs of a non - doped ga 0 . 51 in 0 . 49 p first layer w and a non - doped al 0 . 51 in 0 . 49 p second layer b is formed in the i - layer 18 . a 2 , . . . , a 4 are formed like a 1 , and 4 quantum - wave interference units in total are formed in the i - layer 18 . fig8 a to 8 c show band structures of the quantum - wave interference layer a 1 in detail . the first layer w has a thickness of 5 nm , and the second layer b has a thickness of 7 nm . a non - doped δ layer made of al 0 . 33 ga 0 . 33 in 0 . 33 p is formed at each interface between the first layer w and the second layer b . non - doped al 0 . 51 in 0 . 49 p carrier accumulation layers c 1 to c 3 , having a thickness of 177 nm , is formed between any quantum - wave interference units a i and a i + 1 , respectively , as shown in fig7 . thicknesses of the first layer w and the second layer are determined according to eqs . 1 and 2 , respectively , on condition that no external voltage is applied thereto . the second layers b which contact to the n - layer 20 and the p - layer 16 have a thickness of 0 . 05 μm , respectively . the substrate 10 has a diameter of 2 . 0 inches and the normal direction of its main surface is offset toward the [ 011 ] axis by 15 degree from the ( 100 ) plane . the voltage - variable capacity device 100 was manufactured by gas source molecular beam epitaxial deposition ( gs - mbe ) which is an epitaxial growth method under extremely high vacuum condition . gs - mbe is different from a conventional mbe which supplies group iii and v elements both from said state sources . in gs - mbe , group iii elements such as indium ( in ), gallium ( ga ), and aluminum ( al ) are supplied from a solid source and group v elements such as arsenic ( as ) and phosphorous ( p ) are supplied by heat decomposition of gas material such as ash 3 and ph 3 . fig6 a shows an electric potential gradient occurring in the i - layer 18 as an insulation layer on the condition that an external voltage is applied to the interface between the n - layer 20 and the n - layer 16 of the variable capacity device 100 . although the δ layer is not shown in fig6 a to 6 c , it is present at each interface between the first layer w and the second layer b , as shown in fig8 a to 8 c . each quantum - wave interference units a 1 to a 4 reflects electrons when the external voltage is in the range of 0 to v 1 as shown in fig6 a . at this stage , electrons which exists in the i - layer 18 are not conducted in the i - layer and are confined in each carrier accumulation layers c 1 to c 3 , respectively . as a result , an effective thickness of a depletion layer is smaller , and an electrostatic capacity between the electrodes 26 and 28 is larger . when the applied voltage v between the electrodes 26 and 28 become larger , a kinetic energy of electrons become larger and quantum - wavelength become smaller . that becomes not to satisfy the condition in which each quantum - well interference layers a 1 to a 4 reflects electrons . accordingly , electrons are conducted and a electron concentration of each carrier accumulation layers c 1 to c 3 is decreased . because each carrier accumulation layers c 1 to c 3 becomes a depletion layer , which means that equivalently the thickness of the depletion layer in the i - layer 18 becomes larger , the capacity of the variable capacity device 100 decreases . thus by applying a larger applied voltage v , the capacity can be decreased . capacity value with respect to an alternating current when a voltage v was applied as a bias voltage was measured by varying the respective value of the applied voltage v . fig9 shows a result when the alternating currents are fixed at 100 khz and 1 mhz , respectively . a characteristics x shows a curve when the alternating current is 1 mhz . when the applied voltage v is over 0 . 6 v , the capacity value of x suddenly drops . a characteristics y shows a curve when the alternating current is 100 khz . a peak of capacity value of y is obtained when the applied voltage v is 0 . 7 v . and a characteristics z shows a curve of a voltage - variable capacity device in the prior art . as shown in fig9 it is obvious from the characteristics x and y that a voltage - variation rate of capacity in the region over 0 . 8 v is improved compared with that of the prior art . and it is understood that a dynamic range of variable capacity is wider than that of the prior art . alternatively , as shown in fig4 in place of a quantum - wave interference layer in the i - layer , plurality of partial quantum - wave interference layers can be constructed with serial pairs of t 1 to t j to obtain the characteristics of a voltage - capacity . in the embodiments , a δ layer is formed in the device 100 . the δ layer improves a reflectivity at the interface between the first layer w and the second layer b . alternatively , because the reflectivity can be improved by a maltipath reflection , the δ layer is not necessarily needed . in the embodiments , four quantum - wave interference units a 1 to a 4 are connected in series , with a carrier accumulation layer c lying between each of the quantum - wave interference units . alternatively , one quantum - wave interference layer can be made in an i - layer . in example 1 , the quantum - wave interference layer was formed to have a multi - layer structure including ga 0 . 51 in 0 . 49 p and al 0 . 51 in 0 . 49 p . alternatively , the quantum - wave interference layer can be made of two combinations of quaternary compounds such as al x ga y in 1 − x − y p , selecting different arbitrary composition ratio within the range of 0 ≦ x ≦ 1 , 0 ≦ y ≦ 1 , and 0 ≦ x + y ≦ 1 . further alternatively , the quantum - wave interference layer can be made of arbitrary group iii - v compound semiconductor , group ii - vi compound semiconductor , si and ge , and semiconductors of other hetero - material . in the embodiment , an i - layer is a non - doped layer . alternatively , a small amount of donor impurity or acceptor impurity can be doped with the i - layer . and also pip structure can be used in which i - layer has a quantum - wave interference layer with respect to holes . fig1 is a sectional view of a variable capacity device 200 in which a quantum - wave interference layer is formed in a n − - type layer . the variable capacity has a substrate 110 made of gallium arsenide ( gaas ). a gaas buffer layer 112 of n - type conduction , having a thickness generally of 0 . 3 μm and an electron concentration of 2 × 10 18 / cm 3 , is formed on the substrate 110 . an n - ga 0 . 51 in 0 . 49 p contact layer 114 of n - type conduction , having a thickness generally of 0 . 13 μm and an electron concentration of 2 × 10 18 / cm 3 , is formed on the buffer layer 112 . an n - al 0 . 51 in 0 . 49 p n - layer 116 of n - type conduction , having a thickness generally of 0 . 2 μm and an electron concentration of 1 × 10 18 / cm 3 , is formed on the contact layer 114 . an n − layer 118 , having an electron concentration which becomes smaller in accordance with approaching to a p - layer 120 , is formed . the p - al 0 . 51 in 0 . 49 p p - layer 120 of p - type conduction , having a thickness generally of 0 . 2 μm and a hole concentration of 1 × 10 18 / cm 3 , is formed on the n − layer 118 . a p - ga 0 . 51 in 0 . 49 p second contact layer 122 of p - type conduction , having a thickness generally of 0 . 13 μm and a hole concentration of 2 × 10 18 / cm 3 , is formed on the p - layer 120 . a p - gaas first contact layer 124 of p - type conduction , having a thickness generally of 0 . 06 μm and a hole concentration of 2 × 10 18 / cm 3 , is formed on the second contact layer 122 . an electrode layer 126 made of gold and germanium ( au / ge ), having a thickness generally of 0 . 2 μm , is formed so as to cover the entire back of the substrate 110 . another electrode layer 128 made of gold and zinc ( au / zn ), having a thickness generally of 0 . 2 μm , is formed on some portion of the first p - type contact layer 124 . the n − layer 118 has an electron concentration of 3 × 10 18 / cm 3 . a quantum - wave interference layer g 1 having a multi - quantum layer structure with 12 pairs of a ga 0 . 51 in 0 . 49 p first layer w and an al 0 . 51 in 0 . 49 p second layer b is formed in the n − layer 118 . g 2 , . . . g 5 are formed like g 1 , and 5 quantum - wave interference units in total are formed in the n − layer 118 . fig8 a to 8 c shows a structure of the quantum - wave interference layer g 1 in detail . the first layer w has a thickness of 5 nm , and the second layer b has a thickness of 7 nm . a non - doped δ layer made of al 0 . 33 ga 0 . 33 in 0 . 33 p , having a thickness of 1 . 3 nm , is formed at each interface between the first layer w and the second layer b . a non - doped al 0 . 51 in 0 . 49 p carrier accumulation layers c 1 to c 4 , having a thickness of 14 nm , is formed between any quantum - wave interference units g 1 and g i + 1 , respectively , as shown in fig1 . a thickness of the first layer w and that of the second layer b are determined according to eqs . 3 and 4 , respectively , on condition that no external voltage is applied thereto . the second layers b which contact to the p - layer 120 and the n - layer 116 have a thickness of 0 . 05 μm , respectively . the substrate 110 has a diameter of 2 . 0 inches and the normal direction of its main surface is offset toward the [ 011 ] axis by 15 degree from the ( 100 ) plane . the variable capacity device 200 was manufactured by gas source molecular beam epitaxial deposition ( gs - mbe ) which is an epitaxial growth method under extremely high vacuum condition . gs - mbe is different from a conventional mbe which supplies group iii and v elements both from solid state sources . in gs - mbe , group iii elements such as indium ( in ), gallium ( ga ), and aluminum ( al ) are supplied from a solid source and group v elements such as arsenic ( as ) and phosphorus ( p ) are supplied by heat decomposition of gas material such as ash 3 and ph 3 . alternatively , metal organic chemical vapor deposition ( mocvd ) can be applied in place of gs - mbe . fig1 a shows an electric potential gradient occurring in the n − - layer 118 because of a higher resistivity compared with the p - layer 120 or n - layer 116 on the condition that a backward voltage is applied to the interface between the p - layer 120 and the n - layer 116 of the variable capacity device 200 . although the δ layer is not shown in fig1 a to 11 c , it is present at each interface between the first layer w and the second layer b , as shown in fig8 a to 8 c . although only two carrier accumulation layers c 1 and c 2 are shown in fig1 a to 11 c , four exist from c 1 to c 4 . although only five pairs of layers in each of quantum - wave interference units are shown in fig1 a to 11 c , they exist in twelve pairs . each quantum - wave interference units g 1 to g 5 reflects electrons when an external voltage is in the range of 0 to v 1 as shown in fig1 a . at this stage , electrons which exists in the n − - layer 118 are not conducted in the n − - layer 118 and are confined in each carrier accumulation layers c 1 to c 4 , respectively . as a result , an effective thickness of a depletion layer is smaller , and an electrostatic capacity between the electrodes 126 and 128 is larger . when the applied voltage v between the electrodes 126 and 128 become larger , a kinetic energy of electrons become larger and quantum - wavelength become smaller . that becomes not to satisfy the condition in which each quantum - well interference layers g 1 to g 5 reflects electrons . accordingly , electrons are conducted and a electron concentration of each carrier accumulation layers c 1 to c 4 is decreased . because each carrier accumulation layers c 1 to c 4 becomes a depletion layer , which means that equivalently the thickness of the depletion layer in the n − - layer 118 becomes larger , the capacity of the variable capacity device 200 decreases . thus by applying a larger applied voltage v , the capacity can be decreased . capacity value with respect to alternating current when a voltage v was applied as a bias voltage was measured by varying the respective value of the applied voltage v . fig1 shows a result when the alternating current is fixed at 100 khz and 1 mhz . a characteristics x shows a curve when the alternating current is 1 mhz . a characteristics y shows a curve when the alternating current is 100 khz . and a characteristics z shows a curve of a voltage - variable capacity device in the prior art . when the applied voltage v is over 1 . 0 v , the decrease of capacity value z is saturated . as shown in fig1 , a dynamic range of variable capacity of the present invention is wider compared with that of the prior art , although a voltage - variation rate of capacity becomes smaller . in this embodiment , a variable capacity device has a structure as same as that of the variable capacity device 200 of fig1 . a thickness of each carrier accumulation layers c 1 to c 4 is duplicated to 8 nm and a measured capacity is shown in fig1 . comparing fig1 with fig1 , a voltage - variation rate of capacity becomes smaller in accordance with a thickness of each carrier accumulation layers c 1 to c 4 being larger . the dynamic range of capacity of both obtained devices are improved to be wider compared with that of the prior art . in the embodiments , a δ layer is formed in the device 200 . the δ layer improves a reflectivity at the interface between the first layer w and the second layer b . alternatively , because the reflectivity can be improved by a maltipath reflection , the δ layer is not necessarily needed . in the embodiments , five quantum - wave interference units g 1 to g 5 are connected in series , with a carrier accumulation layer c lying between each of the quantum - wave interference units . alternatively , one quantum - wave interference layer can be made in an n − layer without forming a carrier accumulation layer c . number of the quantum - wave interference units of a multi - layer structure is arbitrary . in examples 2 and 3 , the quantum - wave interference layer was formed to have a multi - layer structure including ga 0 . 51 in 0 . 49 p and al 0 . 51 in 0 . 49 p . alternatively , the quantum - wave interference layer can be made of two combinations of quaternary compounds such as al x ga y in 1 − x − y p , selecting different arbitrary composition ratio within the range of 0 ≦ x ≦ 1 , 0 ≦ y ≦ 1 , and 0 ≦ x + y ≦ 1 . further alternatively , the quantum - wave interference layer can be made of group iii - v compound semiconductor , arbitrary group ii - vi compound semiconductor , si and ge , and semiconductors of other hetero - material . in the embodiment , a variable capacity having an nn − p junction structure is shown . alternatively , a quantum - wave interference layer , which interferes quantum - waves of holes , can be formed in the p − layer of an np − p junction structure . also alternatively , a quantum - wave interference layer , which interferes quantum - waves of electrons or holes , can be formed in the i - layer of an nip structure . further alternatively , quantum - wave interference units , which interfere quantum - waves of electrons and holes , respectively , can be formed in the i - layer of the nip structure . fig1 shows a conduction band of a variable capacity device , having a pin structure . a quantum - wave interference layer of electrons is formed in an i - layer of the device . according to a backward voltage applied to the device being larger , carriers which exist in carrier accumulation layers decrease , and according to a thickness of a depletion layer being larger , a capacity becomes smaller . while the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments , it is to be understood that the invention is not to be limited to the disclosed embodiments , but on the contrary , the description is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims . the present document claims the benefit of japanese priority documents , filed in japan on feb . 6 , 1998 and feb . 24 , 1998 , the entire contents of which are incorporated herein by reference . obviously , numerous modifications and variations of the present invention are possible in light of the above teachings . it is therefore to be understood that within the scope of the appended claims , the invention may be practiced otherwise than as specifically described herein .