a variable capacity device having an nin , pip , nn − p , np − p , or nip junction whose middle layer is constituted by a quantum - wave interference layer with plural periods of a first layer w and a second layer b as a unit . the second layer b has a wider band gap than the first layer w . each thickness of the first layer w and the second layer b is determined by multiplying by an odd number one fourth of a wavelength of a quantum - wave of carriers in each of the first layer w and the second layer b existing around the lowest energy level of the second layer b . a δ layer , for changing energy band suddenly , is formed at interfaces between the first layer w and the second layer b and has a thickness substantially thinner than the first layer w and the second layer b . plurality of quantum - wave interference units are formed sandwiching carrier accumulation layers in series . then a voltage - variation rate of capacity of the variable capacity device is improved .