Effects of the subthreshold slope of transistor switching on the sensitivity of the square law detector composed of a high electron mobility transistor (HEMT) to detect terahertz waves have been investigated. Theoretical analysis starting from the unified charge control model of channel carriers has been carried out, which comes down to mathematical formulae suggesting that the subthreshold slope significantly affects the output voltage and the sensitivity of the square law detector. Square law detectors have been fabricated using HEMT with a MOS structured gate on glass substrate to detect 1.0 THz waves. HEMT was composed of InGaAs/InAs/InGaAs double hetero-structured channel. Detection tests using the MOS-HEMT square law detector have clearly shown that the output voltage increases with the subthreshold slope.