Prabhat Agarwal

RF figures-of-merit for process optimization

Today, transistor y-parameters are routinely being measured for the determination of the current-gain cut-off frequency f/sub T/ and the maximum oscillation frequency f/sub max/. In this paper, it is shown that a much wider use of y-parameter measurements can be made for the RF characterization of transistors. A method is presented to determine the small-signal behavior of actual RF circuit-blocks from the measurements of the y-parameters of the individual circuit components. This is applied to define additional RF figures-of-merit for basic building blocks of analogue and digital RF circuits. No equivalent transistor circuit or compact-model parameters are needed, which is important for giving quick feedback to process developers. This approach is illustrated on three basic RF circuit blocks using bipolar transistors.

A new trench bipolar transistor for RF applications

A new vertical trench SiGe heterojunction bipolar transistor (HBT) is proposed that improves the tradeoff between the cutoff frequency (f/sub T/) and the off-state collector-base breakdown voltage (BV/sub cbo/). Extensive device simulations show that a record f/sub T//spl middot/BV/sub cbo/ product of about 2375 GHz/spl middot/V can be obtained for an HBT having a trench field plate connected to the emitter and a linearly graded doping profile in the collector drift region, while about 700 GHz/spl middot/V can be obtained for a standard optimized HBT. This large improvement is explained mainly by the suppression of the base-widening effect.

The influence of parasitic resistances on the f/sub T/-optimisation of high-speed SiGe-HBTs

We study the influence of parasitic series resistances on the cut-off frequency of high-speed SiGe hetero-junction bipolar transistors. Due to coupling of the parasitic resistances with the internal collector-base capacitance, significant extra delay time is introduced. This extra delay will cause saturation, or even a decrease of f/sub T/ at higher collector doping levels. In addition, we study the optimisation of an n-cap emitter profile, which is only possible when the collector delay is reduced to a minimum, and the series resistances are properly included.