In this work, we present a detailed high frequency characterization of conventional and optimized Gunn-diodes containing a graded gap injector. It is shown that the Al-concentration in the hot electron injector influences the occupation of the Gamma- and L-valleys and hence the electron drift velocity in the active Gunn layer. Quantitative values for the occupation of the L-valley can be extracted from the S-parameter measurements. (C) 2004 Elsevier Ltd. All rights reserved.