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In this paper, the effect of electron irradiation fluence on direct current (DC) and radio frequency (RF) of InP-based high electron mobility transistors (HEMTs) was investigated comprehensively. The devices were exposed to a 1 MeV electron beam with varied irradiation fluences from 1 &#215; 10¹⁴ cm^&#8722;2, 1 &#215; 10¹⁵ cm^&#8722;2, to 1 &#215; 10¹⁶ cm^&#8722;2. Both the channel current and transconductance dramatically decreased as the irradiation fluence rose up to 1 &#215; 10¹⁶ cm^&#8722;2, whereas the specific channel on-resistance (<i>R</i>_on) exhibited an apparent increasing trend. These changes could be responsible for the reduction of mobility in the channel by the irradiation-induced trap charges. However, the kink effect became weaker with the increase of the electron fluence. Additionally, the current gain cut-off frequency (<i>f</i>_T) and maximum oscillation frequency (<i>f</i>_max) demonstrated a slightly downward trend as the irradiation fluence rose up to 1 &#215; 10¹⁶ cm^&#8722;2. The degradation of frequency properties was mainly due to the increase of gate-drain capacitance (C_GD) and the ratio of gate-drain capacitance and gate-source capacitance (C_GD/C_GS). Moreover, the increase of <i>R</i>_on may be another important factor for <i>f</i>_max reduction.