Abstract: A pin diode, having a p-layer, an n-layer, and an i-layer sandwiched by the p-layer and the n-layer, is constituted by a quantum-wave interference unit with a plurality of pairs of a first layer W and a second layer B. 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 odd number one fourth of quantum-wave wavelength of carriers in each of the first layer W and the second layer B. A &dgr; layer sharply varying in band gap energy from the first and second layers is formed at every interface 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. A plurality of quantum-wave interference units are formed sandwiching carrier accumulation layers in series. Then, the I-V characteristic of the diode indicates that, for values of an applied backward voltage, a backward electric current can flow rapidly.