A novel device structure for organic light-emitting field-effect transistors has been developed. The devices comprise bilayer-crystal organic semiconductors of a p-type and an n-type. The pn-junction can readily be formed by successively laminating two crystals on top of a gate insulator. This structure enables the efficient injection and transport of electrons and holes, leading to their effective recombination. As a result, bright emissions are attained. The devices are operated by AC gate voltages. Gate-voltage phase-resolved drain-current and emission-intensity measurements enable us to study the relationship between the emissions and carrier transport. The maximum external quantum efficiency reaches 0.045%.