On-chip leakage current variation measurement using external-reference-free current-to-time conversion for densely placed MOSFETs

Abstract

Abstract This paper proposes a time-domain leakage current measurement circuit that uses an external-reference-free current-to-time conversion. Our proposed current-to-time converter (CTC) utilizes a dual inverter-based conversion to provide a stable reference. We share the CTC among the devices under test (DUTs) for accurate characterization of variation. Our CTC, along with a tree-based switch structure, allows us to densely place and route the DUT transistors using a cell-based design flow similar to a digital circuit design. We demonstrate our circuit by measuring subthreshold leakage currents of 256 minimum sized nMOSFETs in a 65 nm bulk low-power CMOS process. We could successfully extract the variations in subthreshold coefficient and subthreshold leakage current at different temperatures. Our circuit is also robust to supply voltage fluctuation making the circuit suitable for accurate characterization of MOSFET parameters for subthreshold operation.