InAlAs/InGaAs high electron mobility transistors (HEMTs) are a type of field effect transistor that can achieve extremely high high-frequency gain owing to quantum effects operating in the channel layer. HEMTs are important components for devices that involve millimeter waves and high-speed optical transmission systems. However, InAlAs/InGaAs HEMTs have a frequency dispersion that depends on carrier recombination within the device. This is an InGaAs-specific phenomenon, which degrades the device's high-frequency gain performance. In this study, we investigated the direct current (DC) and radio frequency (RF) characteristics of InAlAs/InGaAs HEMTs by varying the operating temperature from liquid nitrogen temperature to 125 °C. The DC characteristics showed an increase of the device transconductance (Gm) at low temperature. Reducing the operating temperature from 125 °C to liquid nitrogen temperature increased Gm to 60 mS. High-frequency gain was also confirmed in the RF characteristics. The current gain cutoff frequency was 50.2 GHz at room temperature, and 66.8 GHz at liquid nitrogen temperature, representing an increase of 33.1%. Analyzing the high-frequency characteristics showed that the high-frequency gain increase at low temperature was related to the temperature dependence of the parasitic capacitance.