AbstractA monolithic quasi‐millimeter wave power combiner type GaAs high‐power FET is proposed which combines source island viahole PHS structure FET cell and power combiner and divider circuits on the same substrate. Since the individual island source electrode is connected directly to the heat sink electrode via holes through an extremely thin GaAs substrate, the parasitic source inductance per unit gate width has been reduced to 1.0 pH/mm, and the thermal resistance of the device has been reduced by 40 percent compared to conventional structures. Also, the power added efficiency of more than 80 percent has been achieved by the four‐cell combiner of FET cells with gate width 800μm. This shows that this monolithic structure is effective in suppressing the gain reduction due to the phase differences between the parallel gate figures at high frequency. At 28 GHz, the fabricated FET achieves 1 dB compression point power of 1.1 W, linear power gain of 4.0 dB, the power · (frequency)2 product of 860 W · GHz2. Also, such reliability tests as temperature cycling, operation test, and high‐temperature ambient have been carried out, and it has been confirmed that the test device has the basic reliability level suitable for practical quasi‐millimeter wave communication systems.