Optoelectronic properties of light-emitting field-effect transistors (LETs) fabricated on bottom-contact transistor structures using a tetracene film as charge-transport and light-emitting material are investigated. Electroluminescence generation and transistor current are correlated, and the bias dependence of the LETs external quantum efficiency is determined. The device’s performance degrade rapidly upon gate biasing. The effect is attributed to charge trapping, which can be prevented by operating the devices in pulsed mode. A model for the electron injection mechanism in a p-type organic transistor is proposed. On the basis of this model, electrical and optical characteristics, as well as the dependence of the external quantum efficiency on drain- and gate-bias, are well reproduced.