In this paper, we present results from the simulation of submicrometer Si/SiGe modulation-doped field-effect transistors (MODFETs) using an electrothermal Monte Carlo method. The relationships between the thermal droop effect observed in the electrothermal I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d </sub> -V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ds</sub> characteristics of the devices and the microscopic properties of electron transport and the temperature profiles are studied. The effects of varying the effective semiconductor die dimensions and the thickness of the SiGe buffers on the electrothermal behavior of the devices are also investigated. A comparison of the electrothermal performance of the simulated Si/SiGe MODFET with that of a GaAs-based HEMT is also carried out