Gallium nitride based high electron mobility transistors offer high carrier density combined with high electron mobility and enable operation at high frequencies, voltages and temperatures. However, they are generally working in depletion mode, requiring a negative bias to switch them off. As this is disadvantageous for circuit design, power consumption and safety of operation, enhancement mode devices would be favourable. This operation mode can be reached by shifting the threshold voltage towards more positive values by implantation of negatively charged fluorine ions. As these ions were reported to form deep level trap states in the material, we investigated the influence of the implantation and subsequent anneal steps on current collapse and threshold voltage. We found that anneal steps of over 550 °C are effective in reducing the current collapse up to factor 4, but also shift the threshold voltage again to negative values, thus eliminating enhancement mode operation. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)