A CMOS dB-linear variable gain amplifier (VGA) with a novel I/Q tuning loop for dc-offset cancellation is presented. The CMOS dB-linear VGA provides a variable gain of 60 dB while maintaining its 3-dB bandwidth greater than 2.5 MHz. A novel exponential circuit is proposed to obtain the dB-linear gain control characteristics. Nonideal effects on dB linearity are analyzed and the methods for improvement are suggested. A varying-bandwidth LPF is employed to achieve fast settling. The chip is fabricated in a 0.35-m CMOS technology and the measurement results demonstrate the good dB linearity of the proposed VGA and show that the tuning loop can effectively remove dc offset and suppress I/Q mismatch effects simultaneously.

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In this letter, the contrast reversal effect in scanning-capacitance-microscopy (SCM) dopant concentration extraction is investigated both theoretically and experimentally. The shift of the turning point in the nonmonotonic response of peak dC∕dV signal versus dopant concentration to higher dopant concentrations is explained by the difference of the capture/emission time constant of the interface states and the series resistance of the semiconductor sample. This is verified by comparing the experimental SCM measurements with the simulated peak dC∕dV profile on a p-type multiple dopant step sample. The contrast reversal effect, which affects the accuracy of dopant concentration extraction using the SCM peak dC∕dV signal, can be minimized by using an overlying oxide with good interfacial quality and a semiconductor sample of low series resistance.

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Abstract-Scanning capacitance microscopy (SCM) is a dopant profile extraction tool with nanometer spatial resolution. While it is based on the high-frequency MOS capacitor theory, there are crucial second-order effects which make the extraction of dopant profile from SCM data a challenging task. Due to the small size of the SCM probe, the trapped charges in the interface traps at the oxide-silicon dioxide interface surrounding the probe significantly affect the measured SCM data through the fringing electric field created by the trapped charges. In this paper, we present numerical simulation results to investigate the nature of SCM dC/dV data in the presence of interface traps. The simulation takes into consideration the traps' response to the ac signal used to measure dC/dV as well as the fringing field of the trapped charge surrounding the probe tip. In this paper, we present an error estimation of experimental SCM dopant concentration extraction when the interface traps and fringing field are ignored. The trap distribution in a typical SCM sample is also investigated.