IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 54, NO. 4, AUGUST 2007 1151 Investigations on Photon Energy Response of RadFET Using Monte Carlo Simulations Peter Beck, Member, IEEE, Florian Bock, Helmuth Böck, Marcin Latocha, Robert A. Price, Member, IEEE, Sofia Rollet, and Michael Wind Abstract—We describe investigations of RadFET energy re- sponse simulated with Geant4 and FLUKA2005 Monte Carlo codes. An analysis of energy deposition is carried out for photon irradiation with energies between 35 keV and 2 MeV. The ab- sorbed dose in the silicon dioxide layer (few hundred nanometers) is compared for both transport codes. Index Terms—Basic mechanism, dosimetry, energy deposition, RadFET, radiation detectors, radiation transport, simulation. I. INTRODUCTION M ODELLING of effects in semiconductor components caused by radiation exposure is becoming evermore im- portant. On the one hand radiation hardness testing is expensive in time and costs; therefore a partial replacement by radiation simulations is of major interest. On the other hand modelling assists the development of semiconductor devices, and allows optimizing characteristics either of radiation hardness or ra- diation sensitivity. Several Monte Carlo codes are available for modelling irradiation measurements. For the validation of the models, dosimetry in microstructures plays an important role [1]–[4]. As an example of characterizing radiation effects in semiconductors, we describe investigations of the RadFET energy response calculated with Monte Carlo transport simula- tion codes. The research has been carried out in the framework of the EURADOS working group WG6 initiative and the CONRAD (Coordinated Network for Radiation Dosimetry), WP4: Uncertainty Assessment in Computational Dosimetry [5], where simulation problems are presented to the dosimetry community for the calculation of specified parameters and fur- ther comparison. 1 A RadFET is a metal-oxide semiconductor field-effect transistor (MOSFET) optimized for use as a sensor of ionizing radiation [6]–[8]. The radiation sensitivity of a RadFET is caused by manipulating the production process so Manuscript received October 6, 2006; revised December 12, 2006. This work was supported by the Austria Research Center. P. Beck and S. Rollet are with ARC Seibersdorf Research, 2444 Seibersdorf, Austria (e-mail: peter.beck@arcs.ac.at; sofia.rollet@arcs.ac.at). F. Bock and M. Wind are with ARC Seibersdorf Research, 2444 Seibersdorf, Austria, and also with the Atomic Institute of the Austrian Universities, Wien, Austria (e-mail: florian.bock@arcs.ac.at; michael.wind@arcs.ac.at). H. Böck is with the Vienna University of Technology, Atomic Institute, 1020 Vienna, Austria (e-mail: boeck@ati.ac.at). M. Latocha is with ARC Seibersdorf Research, 2444 Seibersdorf (Austria), and also with the Institute of Nuclear Physics, Polish Academy of Sciences, 31-342 Kraków, Poland (e-mail: marcin.latocha@arcs.ac.at). R. A. Price is with the Department of Radiography, City University, London EC1V OHB, U.K. (e-mail: r.price@city.ac.uk) Digital Object Identifier 10.1109/TNS.2007.902350 1 http://www.eurados.org/conrad/conrad_overview.htm that the device has a thick oxide-layer. Conventional radia- tion-hard MOSFETs have an oxide layer with few nanometers thickness [9]–[11]. The change of the dose response to ionizing radiation can be related to the energy imparted in the gate oxide. This makes the RadFET usable as a dosimeter. The absorbed dose response of the RadFET dosimeter can be calculated by simulation of the energy imparted in the gate oxide layer [12]. The main focus of this investigation is the comparison of Monte Carlo transport simulation of the energy deposition in the gate oxide for photon irradiation with energies between 35 keV and 2 MeV. All simulations performed for the RadFET were done using Geant4.7.1 [13] and FLUKA (version 2005) [14]. The RadFET geometry and material data were taken from [5]. The energy response of the absorbed dose in the silicon dioxide layer is compared for the transport simulation codes. Both models describe the functionality and response of the RadFET in the case of ionizing radiation exposure. In particular, the comparison of the results of the two simulation codes shows different capabilities and possible limitations of these codes because of the fact that the sensitive volume in this example is very small, pointing out the importance of a detailed description of the energy loss fluctuations in thin layers [15]. Further details can be found in [16]. II. MONTE CARLO SIMULATION For the simulations, the geometry of the RadFET can be reduced to the most important parts schematically shown in Fig. 1. The geometry of the RadFET in the proximity of the gate oxide is shown together with the regions around it, modelled as a stack of parallelepipeds, all with the same layer size but different thickness (not in scale). The field oxide is positioned left and right of the gate oxide. Its width and consequently also the lateral dimensions of all the materials below and above it will be varied during our investigations. Simulations are done with and without the top layer which is representing the metal lid of the RadFET. The device is called capped when the metal lid is present; when no lid is used the device is called uncapped. The RadFET simulation geometry is completely surrounded by vacuum, and the source has exactly the same lateral extension as the RadFET. The coordinate system of the irradiation geometry in Fig. 2 is consistent with the coordinate system in Fig. 1, thus the RadFET is irradiated in the direction of the layers above the gate oxide. In this investigation the field oxide thickness is 250 m. For both Monte Carlo simulation codes, the thin gate oxide layer represents a borderline of their validity, both in the energy and thickness domain. 0018-9499/$25.00 © 2007 IEEE