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    Introduction :

    If a Light falls on a semiconductor material, the charge carriers  (Electron Holes Pair are generated).They are produced either by band to band transitions(intrinsic) or by transmission involving forbidden energy levels(extrinsic )as shown in figure .

    process of intrinsic and extrinsic  photoexcitation



     Emission is of two types:


    types of Emission


    Fluorescence (Direct or fast process): :

    In direct or fast emission, the emission of photons stop within approximately 10-8 s after the excitation is turned off. Such a fast process is referred as fluorescence. As we know 10-8 s is the lofe time of an atom in excited state it means after this time excited atom returns to ground state.


    Phosphorescence (Slow Process) :

    if the emission occurs even after the excitation has ceased, the process is known as Phosphorescence.


    excutation and recombination mechanism in photolumninescence with trapping leve