An electron trap state has been found to appear in InGaAsN/GaAs quantum wells (QWs) as a result of composition fluctuation. Evidence of composition fluctuation is shown by photoluminescence spectra which display an additional low-energy emission due to local regions of N-rich clusters. Enhancing composition fluctuation leads to undulated InGaAsN/GaAs interface with dot-like islands. The capacitance-voltage profiling shows an additional peak following the QW electron ground state peak, suggesting the presence of an electron trap state below the QW electron ground state. The emission properties of this trap state are similar to those of quantum dots immersed in a well. Enhancing composition fluctuation can cause an energy downward shift and spectral broadening of this electron trap state, suggesting an increase in island size and fluctuations of the size and composition. Thus, it is deduced that the electron trap state originated from the N-rich clusters in the well behaving like quantum dots. Upon modulation, the electrons are thermally activated from this trap state to the QW electron ground state and subsequently emitted to the GaAs bottom electrode.