A SiGe heterojunction bipolar transistor (HBT) power unit-cell for S-Band high efficiency linear power amplifier with an open collector adaptive bias linearizer was designed and fabricated in this paper. The unit-cell was investigated in both electrical and thermal performances. and 34 unit-cells were then combined as a power device for output stage. An adaptive linearizer for the output stage was constructed with an open collector HBT bias circuit and improved the power gain (G(p)), output 1 dB compressed power (OP1 (dB)), power added efficiency (PAE), and output third-order intermodulation point PIN when compared to those of traditional adaptive bias circuit. The measured power amplifier achieved a G(p) of 13 (dB), an OP1 (dB) of 22 dBm with a PAE of 28.6%, and an OIP3 of 31.4 dBm. Compared to traditional adaptive bias technique, the proposed linear power amplifier improved the output power of 2 dB, PAE (at P-1 (dB)) of 8.6% and OIP3 of 3.4 dB. The fabricated die size, including pads, is less than 1.44 mm(2). (c) 2007 Elsevier Ltd. All rights reserved.