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The utilization of microfluidic "lab-on-a-chip" devices in fundamental medical research, drug discovery and clinical diagnostics has rapidly increased in the past decade. Lab-on-a-chip devices process small volumes of analytes and reagents through on-chip microfluidic signal processing circuits. This paper discusses the implementation of a basic microfluidic circuit block, the concentration digital-to-analog converter (or C-DAC) which produces discretized chemical concentrations in a constant stream of solvent. The chemical concentration is controlled by a time-varying digital word; hence C-DACs are suitable for on-chip generation of arbitrary chemical signals. A 4-bit continuous-flow C-DAC was fabricated in two-level PDMS technology and tested. Several chemical waveforms (sawtooth, cosine, and ramp) were generated at flow rates of 2 microL min(-1) and frequencies of 0.6-4 mHz. The frequency cut off of this C-DAC was approximately 500 mHz.

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Formation of I1 Shockley stacking faults by recombination-enhanced defect glide in 4HSiC p-i-n diodes subject to high forward current stress is studied in diodes on both c-oriented and aoriented substrates. The forward voltage increases during stressing for both orientations, accompanied by nucleation and expansion of faults visible in electroluminescence (EL) imaging. Low temperature photoluminescence (PL) measurements on degraded diodes of both orientations reveal the same set of exciton peaks, confirming that the electronic structure of the faults is the same in both cases. The spectroscopic data are compared to self-consistent solutions of the Schrödinger and Poisson equations including polarization charge. Dislocations nucleating the faults are bright in EL images but dark in electron beam-induced current (EBIC) imaging, confirming that they are sites of enhanced radiative recombination.