TID Induced small signal model variation in CMOS and SiGe BiCMOS

Abstract

The thesis presents data, and fitting equations, that model the degradation of linear small signal parameters for SiGe HBT and bulk CMOS devices due to TID irradiation damage. The 0.13 µm SiGe HBT was fabricated with a base of 4.5 µm in common emitter configuration with the substrate tied to emitter and connected to ground. The device was exposed to electron radiation using the Sr 90 source. Linear measurements were done in the frequency range of 1 GHz to 110 GHz before and after incremental radiation doses. The measured S-parameters were de-embedded using SHORT and OPEN calibration standards to remove the effect of parasitics from pads, vias and feedlines, and shift the calibration plane to the terminals of the device. Small signal parameters and noise parameters were extracted from the de-embedded S-parameter pre-and post-radiation. The major model variations due to total ionizing dose (TID) were found to be an increase in junction resistances and capacitances, as well as a reduction in transconductance. These result in a decrease in the unilateral gain (U), ft and fmax, as reported in prior literature. Exponential regression and curve fitting techniques were employed to derive model equations for the line of best fit for each of the small signal parameter. The coefficient of determination was calculated to ascertain the accuracy of the established equations. Through the coefficient of determination, it was observed that all the derived equations were reliable and could be used to predict the performance of a transistor at a given radiation dose. Performance degradation up to 10 Mrad dose was predicted using extrapolation from small-signal model parameters. Noise modelling was also conducted using measured S-parameters at incremental radiation doses. The modelled noise figure was compared to the simulated noise figure from the model in the process design kit (PDK), with good agreement observed.

Four 0.35 µm bulk CMOS devices were fabricated with different gate widths in common source configuration with the substrate tied to the source and connected to ground. The devices were exposed to electron radiation. Linear measurement was done pre- and post- irradiation in the frequency range of 1 GHz to 50 GHz with a vector network analyser (VNA). The measured S-parameters were de-embedded using THRU, REFLECT and LINE calibration standards. Small signal parameters were extracted from the de-embedded S-parameters before and at incremental radiation dose. The major model variations due to total ionizing dose exposure were increases in the gate resistance (Rg), gate drain capacitance (Cgd) and gate source capacitance (Cgs), with a reduction in transconductances (gm and gds). This caused S11 and S22 to become more resistive as d was increased, with a decrease in the unilateral gain, fT and fmax. A curve fitting tool was used to derive equations for the curves of best fit for each of the small signal parameters. Coefficient of determination values were calculated to evaluate the accuracy of the curves of best fit. Good fitting was observed. The application of the data in predictive modelling of radiation damage was demonstrated.

This study represents the first effort in published literature to model TID-induced changes to small-signal CMOS and SiGe BiCMOS models at mm-wave frequencies, and the results support previously published data on degradation of S-parameters and other performance metrics.