Harnessing Unipolar Threshold Switches for Enhanced Rectification

Md. Mazharul Islam, Shamiul Alam, Garrett S. Rose, Aly E. Fathy, Sumeet Kumar Gupta, Ahmedullah Aziz

Abstract: Phase transition materials (PTMs) have drawn significant attention in recent years due to their abrupt threshold switching characteristics and hysteretic behavior. Augmentation of the PTM with a transistor has been shown to provide enhanced selectivity (as high as ~107 for Ag/HfO2/Pt) leading to unique circuit-level advantages. Previously, a unipolar PTM, Ag-HfO2-Pt, was reported as a replacement for diodes due to its polarity-dependent high selectivity and hysteretic properties. It was shown to achieve ~50% higher-DC output compared to a diode-based design in a Cockcroft-Walton multiplier circuit. In this article, we take a deeper dive into this design. We augment two different PTMs (unipolar Ag-HfO2-Pt and bipolar VO2) with diode-connected MOSFETs to retain the benefits of hysteretic rectification. Our proposed hysteretic diodes (Hyperdiodes) exhibit a low-forward voltage drop owing to their volatile hysteretic characteristics. However, augmenting a hysteretic PTM with a transistor brings an additional stability concern due to their complex interplay. Hence, we perform a comprehensive stability analysis for a range of threshold voltages (−0.2 V< $V_{\mathrm { th}}$ < 0.8 V) and transistor sizes to ensure operational stability and to choose the most optimum design parameters. We then test a standalone Ag-HfO2-Pt and an Ag-HfO2-Pt-based hyperdiode in two different types of voltage multipliers and report ~500 and ~20 times lower-settling time, respectively. As the PTM possesses additional sources of variation, it is crucial to examine the performance benefits of the structure through an extensive variation analysis. We perform $3 {\sigma }$ Monte-Carlo variation analysis for a Cockcroft-Walton multiplier considering the nonidealities in the host transistor and the PTM. We observe that, hyperdiode-based design achieves ~20% higher-output voltage compared with the conventional designs within a fixed timeframe ( $200~\boldsymbol {\mu }$ s).