We have investigated the cryogenic stability of two-finger InP HEMTs aimed for Ka-band ultra-low noise amplifiers (LNAs). Unlike two-finger transistors with a large gate-width above 2 χ 50 μm, the transistors with a small gate-width exhibit unstable cryogenic behavior. The instability is suppressed by adding a source air-bridge. The stabilizing effect of the air-bridge is demonstrated both on device and circuit level. A three-stage 2440 GHz monolithic microwave integrated circuit (MMIC) LNA using a stabilized 100-nm HEMT technology is presented. The amplifier achieves a record noise temperature of 7 K at 25.6 GHz with an average noise of 10.6 K across the whole band at an ambient temperature of 5.5 K. The amplifier gain is 29 dB ± 0.6 dB exhibiting very stable and repeatable operation. To our knowledge, this amplifier presents the lowest noise temperature reported so far for InP cryogenic LNAs covering the Ka-band.