Transient thermal characterization by thermal imaging in our earlier work demonstrated its ability to extract key information about the thermal response of transistors under operating conditions. We investigate a non-equilibrium response to a short time pulse for very high frequency devices, e.g. hetero-junction bipolar transistors (HBTs). Characterizing high speed thermal signals from devices with very small thermal mass has been an ongoing challenge, especially for the microwave and communication industries. A collaboration with a high time-precision pulsed I-V equipment with further advanced high speed thermoreflectance imaging technology enables a measurement of temperature response down to a sub microsecond pulse. Due to the short time pulse, the thermal response wave-form in rise and decay times shows differences from that expected for a typical thermal response, which is more symmetric in rise and decay. Modeling of the thermal diffusion considering the thermal boundaries helps to explain this phenomenon.