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By using a Monte Carlo simulator, the static and dynamic characteristics of 50-nm-gate AlInAs-GaInAs δ-doped high-electron mobility transistors (HEMTs) are investigated. The Monte Carlo model includes some important effects that are indispensable when trying to reproduce the real behavior of the devices, such as degeneracy, presence of surface charges, T-shape of the gate, presence of dielectrics, and contact resistances. Among the large quantity of design parameters that enter the fabrication of the devices, we have studied the influence on their performance of two important factors: the doping level of the δ-doped layer, and the width of the devices. We have confirmed that the value of the δ-doping must be increased to avoid the reduction of the drain current due to the depletion of the channel by the surface potential. However, a higher δ-doping has the drawback that the frequency performance of the HEMTs is deteriorated, and its value must be carefully chosen depending on the system requirements in terms of delivered power and frequency of operation. The reduction of the device width has been also checked to improve the cutoff frequencies of the HEMTs, with a lower limit imposed by the degradation provoked by the offset extrinsic capacitances.