Reduction Of Base-collector Capacitance Research Articles

Reduction of base-collector capacitance in InP/InGaAs HBT's using a novel double polyimide planarization process

The parasitic base-collector capacitance (C/sub BC/) in InP/InGaAs heterojunction bipolar transistors (HBTs) has been reduced using a novel double polyimide planarization process which avoids damage of the extrinsic base layer. The extrinsic base metal outside the base-collector mesa is placed on the polyimide by polyimide coating and etch-back to the base layer. We obtained f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> of 81 GHz and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> of 103 GHz with a 2×10 μm emitter. Performance comparison between two devices with the same area of 2×2 μm but with different base-collector mesa area showed 56% reduction of C/sub BC/ and 35% increase of f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> .

Reduction of base-collector capacitance by undercutting the collector and subcollector in GaInAs/InP DHBTs

The total base-collector capacitance (C/sub BC/) of GaInAs/InP double heterojunction bipolar transistors (DHBTs) has been reduced by the etching away of the semiconductor layers below the extrinsic base region, resulting in an undercut structure. The reduction was further enhanced by using a novel composite subcollector structure. A 54% reduction of total C/sub BC/ and improvement of microwave characteristics (an increase of 20% in f/sub T/ and 38% in f/sub max/) were observed as a result of the undercut process.