Abstract: The extremely low threshold voltage (VTH) of native MOSFETs (VTH≈0 V @ 300 K) is conducive to the design of cryogenic circuits. Previous research on cryogenic MOSFETs mainly focused on the standard threshold voltage (SVT) and low threshold voltage (LVT) MOSFETs. In this paper, we characterize native MOSFETs within the temperature range from 300 K to 4.2 K. The cryogenic VTH increases up to ∼0.25 V (W/L = 10 µm/10 µm) and the improved subthreshold swing (SS) ≈ 14.30 mV/dec @ 4.2 K. The off-state current (IOFF) and the gate-induced drain leakage (GIDL) effect are ameliorated greatly. The step-up effect caused by the substrate charge and the transconductance peak effect caused by the energy quantization in different subbands are also discussed. Based on the EKV model, we modified the mobility calculation equations and proposed a compact model of large size native MOSFETs suitable for the range of 300 K to 4.2 K. The mobility-related parameters are extracted via a machine learning approach and the temperature dependences of the scattering mechanisms are analyzed. This work is beneficial to both the research on cryogenic MOSFETs modeling and the design of cryogenic CMOS circuits for quantum chips.
Fig: I-V curves of native MOSFETs with W/L= 10/10µm measured (symbol) and calculated (solid line) at various temperatures. (a) Acomparison of the calculation results between this model and the EKV2.6 model at 77K and 4.2K. (b) Measurement and calculation results of the output characteristic at 4.2 K.
[2] Qixu Xie Guoyong Shi; An analytical gm/ID‐based harmonic distortion prediction method for multistage operational amplifiers; Int J Circ Theor Appl. 2021; 1– 27. DOI: 10.1002/cta.3012
Abstract: An analytical stage‐based harmonic distortion (HD) analysis method for multistage operational amplifiers (Op Amps) is developed in this work. This work contributes two fundamental methods that make the analytical HD prediction possible at the circuit level. Firstly, we propose that the traditionally used first order small‐signal transistor quantities gm (transconductance) and go (output conductance) in the gm/ID design methodology for bulk complementary metal‐oxide‐semiconductor (CMOS) technology can be extended to the higher order quantities gm(k) and go(k) (k=1,2,3). With proper normalization, these quantities become neutral to the device dimensions and operation currents, hence can be precharacterized by sweeping simulations and used as lookup tables. Secondly, we further develop analytical nonlinearity expressions for a set of commonly used amplifier stages, represented as the functions of the nonlinearity parameters gm(k) and go(k) of the transistors that form a stage circuit. A combination of these two fundamental methods on hierarchical nonlinearity modeling enables us to apply the existing analytical HD estimation methods for the stage‐form macromodels to predict the circuit‐level HD behavior, overcoming the need of running repeated simulations under device resizing and rebiasing. The proposed harmonic distortion analysis method has been validated by application to real multistage amplifiers, achieving HD prediction results in excellent agreement to fully transistor‐level circuit simulation results but with substantial speedup.
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