In this paper, for the first time, we demonstrate the improvement in power capability and safe operating area of silicon-on-insulator laterally double-diffused MOS (SOI-LDMOS) transistors for power amplifier applications by the introduction of a partial n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> buried layer (PNBL). The power capability of a transistor can be evaluated by P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> /A, which is the maximum power per unit area that can be delivered by the transistor and is an important parameter for power amplifiers. P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> is dependent on the snapback voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sb,QS</sub> ), OFF-state breakdown voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bd,OFF</sub> ), and maximum current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) in the quasi-saturation regime of an LDMOS transistor. Increase of P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> /A by the introduction of a PNBL in the SOI-LDMOS transistors is reported in this paper. The effects of variation of the length, thickness, and doping concentration of the PNBL on V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sb,QS</sub> and P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> /A are analyzed in detail. It is shown that by optimizing the doping and length of the PNBL layer, the maximum power output from the transistor can be made significantly higher than that of a conventional device without PNBL. A procedure to design the optimized structure is also presented.