The thermal dynamics of multifinger AlGaN/GaN high electron mobility transistors (HEMTs) with varying gate pitch was investigated using time-resolved Raman thermography. An identical temperature rise was measured in all gate fingers within the first similar to 500 ns duration of electrical pulses. Gate finger temperature differences only emerge after 500 ns, due to the onset of thermal crosstalk. This thermal crosstalk onset delay is attributed to the finite rate of heat diffusion between gate fingers. Reducing the device gate pitch was found to increase the magnitude of transient thermal crosstalk. Implications of each gate finger within a multifinger HEMT having a different transient temperature are discussed in the context of device characteristics. The experimental results are compared to time-domain three-dimensional finite difference heat diffusion simulations. (C) 2010 Elsevier Ltd. All rights reserved.