The kink effect in InAlAs/InGaAs/InP composite channel heterojunction field effect transistors (HFET's) was investigated as a function of temperature and optical excitation, Drain source and gate current measurements show that above 325 K the kink effect disappears while the impact ionization process is still present. The kink at low temperatures is suppressed by illumination with photons of energy above 1 eV. These results prove that this parasitic effect is mainly related to the presence of traps in the top layers.