One of the main objectives of modern Microelectronics is the fabrication of devices with increased cutoff frequency and decreased level of noise. At this moment, the best devices for high-frequency, low-noise behavior are High electron mobility transistors (HEMTs) based on InGaAs and InAs channels. In this work, a complete analysis of ultra-short-gate HEMTs has been carried out by using a semiclassical Monte Carlo simulator, paying special attention to the noise performance. The validity of the model has been checked through the comparison of the simulated results with static, dynamic and noise measurements in real HEMTs. In order to reproduce the experimental results, we have included in the model some important real effects such as degeneracy, surface charges, presence of dielectrics and contact parasitics. The cryogenic performance of the HEMTs has also been analyzed. The influence of the parasitic resistances, width of the devices, value of the \(δ \)-doping and recess length has been analyzed when scaling down the gate length of the transistors to 50 nm aiming at achieving higher cutoff frequencies and better noise performance. The important effect of the impact ionization mechanisms and the consequent kink effect on the noise in both InGaAs and InAs based HEMTs have also been studied. Finally the advantages of the use of a double gate topology are quantified.