Abstract: The photon-activated monopole domain in a semi-insulating multi-valley photoconductive semiconductor switch and the dipole domain in a Gunn device are compared.The generation mechanism,electric field distribution,electron concentration distribution,growth and evolution of the photon-activated monopole domain are discussed.Compared with the Gunn dipole domain,there is only an accumulation layer of photon-activated electrons in the photon-activated monopole domain,but no layer of positive ions;Because the electric field formed by the photon-activated monopole domain and the photon-activated holes is the opposite of the external electric field,the electric field before the monopole domain is enhanced and the electron concentration at the head of the domain is higher than in other regions;a monopole domain can grow until it becomes a luminous domain because of impact ionization,and a luminous domain will evolve into an avalanche luminous domain if it reaches avalanche conditions.Finally,important experimental phenomena of the switch working in nonlinear mode,such as the ultrafast rising edge,luminous current filaments,and the lock-on state of current,are explained by the model of the photon-activated monopole domain.

Key words: photon-activated monopole domain, dipole domain, Gunn effect, photoconductive semiconductor switch, impact ionization