This thesis focuses on the electrical and noise characterization of High Electron Mobility Transistors (HEMTs) employed as Low Noise Amplifiers (LNAs) in the read-out electronic chain of qubits in quantum computers. Low-frequency (1/f) noise properties of such devices at cryogenic temperatures, in particular, are not well understood and are the main topic of this work. While such LNAs typically operate in the gigahertz regime, where 1/f noise is negligible for standard amplifier applications, 1/f noise has been shown to impact qubit readout performance even at high frequencies. Therefore, it is essential to understand the 1/f noise behavior of LNAs, to map it to physical mechanisms, and to propose ways to reduce it through new device designs.