In describing high-field effects on electric conductivity and on carrier transport in pn junctions, different approaches and approximations have been used as the basis for the theoretical representation of high-field effects. In this review the basic principles of various models describing high-field effects are presented. The main emphasis is given to carrier generation-recombination in amorphous silicon pn junctions in which quantum tunnelling contributes a great deal to carrier transitions between band states and the traps, resulting in strongly enhanced generation-recombination rates. A model describing trap-assisted tunnelling carrier capture and emission in amorphous silicon is given a thorough presentation. Carrier capture at traps is regarded as a thermally assisted tunnelling transition combined by Poole-Frenkel barrier-lowering. The incident carriers which take part in the tunnelling capture process are majority carriers in the low field regions at the edges of the pn junction depletion region. Carriers, penetrating or crossing the barrier, are captured by thermalising to the ground level of traps. In the inverse process of carrier emission, the carriers are thermally lifted from traps, tunnelling subsequently to band states. Summing up tunnelling carrier transitions and pure thermal carrier capture-emission transitions gives expressions for the nonequilibrium occupancy function of traps. The generation-recombination rate within a-Si pn junctions is then expressed by virtue of the occupancy function and the continuous distribution of states in the gap of amorphous silicon.