Search

The response of floating gate (FG) devices to 60Co $\gamma $ -rays under switched bias conditions is studied by real-time monitoring of the threshold voltage evolution with accumulated dose. Samples were fabricated in a 1.5- $\mu \text{m}$ CMOS process, suitable for dosimetry applications. A physics-based numerical model of total ionizing dose (TID) effects in FG MOS devices is developed, taking into consideration the dominant microscopic processes leading to charge accumulation/neutralization across the structure. The main parameters of the model are the capture and neutralization rates in both oxides (tunnel and interpoly), and the initial FG charge. A dataset of experimental measurements is used to extract those parameters. Then, the model is applied to predict the response of later experiments. In one case, the device is irradiated under different switching bias voltages, yielding a mean absolute error of 1.8%. With the same set of parameters, but using another device, the model is able to predict with an error of 2.1%, an exposition performed with a $V_{t}$ -shift range twice the one in the first experiment. The predictive power of the model in a wide range of experimental conditions relies on the physics principles behind it. Combined experiments plus simulations allow to distinguish how charges in each layer of the structure evolve and contribute to the overall response. As an application of the model, cycled measurement technique for dosimetry is investigated. [ABSTRACT FROM AUTHOR]

We propose the use of a CMOS differential circuit with inherent amplification to enhance the performance of n-channel field oxide MOSFETs as ionizing radiation dosimeters. These new dosimeters are aimed to be used in low dose applications such as X-ray diagnosis. The circuit is presented and described, and a discrete-level prototype was tested as regards sensitivity, temperature variations compensation and signal-to-noise ratio at different operation conditions. Results show that, comparing to a single MOSFET dosimeter, on chip amplification is possible along with temperature induced error attenuation. The highest sensitivity measured with respect to \gamma radiation was 0.4 V/rad. The circuit successfully measured the dose delivered in an X-ray image diagnosis environment with a sensitivity of approximately 0.5 V/rad. [ABSTRACT FROM PUBLISHER]

This paper presents a differential MOSFET sensor reading technique based on the bias controlled cycled measurement. The circuit was implemented, and tested with gamma radiation from a 60-cobalt source. Temperature rejection performance was assessed during the exposure in real-time measurements. The results show that in comparison with a single MOSFET dosimeter the thermal drift is 20 times smaller and the radiation sensitivity is approximately 10% higher. The switched biasing allows to extend the measurement range beyond MOSFET’s threshold voltage saturation. [ABSTRACT FROM AUTHOR]

This paper presents a new technique to build MOS dosimeters using unmodified standard CMOS processes. The devices are n-channel MOS transistors built with the regular Field Oxide as a thick radiation-sensitive gate. The devices were fabricated in two different commercial 0.6~\mu m CMOS processes, gate oxide thicknesses of \sim 600 nm and \sim 400 nm. Responsivities up to 4.4 mV/rad with positive bias, and 1.7 mV/rad with zero gate bias were obtained in the thicker oxides. The effect of charge trapped in the oxide and interface states on the shift in the threshold voltage are analyzed. [ABSTRACT FROM PUBLISHER]

Through the injection of a Fowler-Nordheim tunnel current or the inversion of oxide fields during irradiation (Radiation-Induced Charge Neutralization), the oxide charge trapped in thick-oxide (300 nm) commercial RADFETs, often called QOT could be erased. Novel trapped-hole and interface characteristics were observed after treatments of this type at high doses. With both erasure techniques, it was possible only to neutralize a fraction of the oxide trapped charge. A non negligible amount of charge and border traps is deemed here to be “intractable”. That adjective an a symbol, QIN, are introduced for the first time in this paper. Later sections discuss the possible impact of these results. The conclusion for dosimetry is that a “reusable RADFET” dosimeter, working up to an unprecedented dose before wearing out, may be a practical possibility. [ABSTRACT FROM AUTHOR]