Effect of ionising radiation on the characteristics of a MOSFET zyxwv S.Dasgupta zyxwvutsrqpo and P.Chakrabarti Abstract: The effect of radiation-induced changes on the characteristics of an n-channel MOSFET has been investigated theoretically. A one-dimensional semi-numerical model of the device has been developed whch can estimate fairly accurate characteristics of the device under unirradiated and irradiated conditions. The effect of ionising radiation on the channel voltage and electric field profde in the channel has been estimated numerically for the first time. The present model enables one to determine the zyxwvutsrqp ZD-VD and transfer characteristics of the device by considering the field dependent mobility of the surface channel in the irradiated condition. The model presented here can be used as a basic tool for analysing MOS transistors exposed to a nuclear environment. 1 Introduction Metal-oxide-semiconductor (MOS) technology is widely used in space applications for its zyxwvuts hgh integration, high per- formance and low static power consumption capabilities. MOS circuits are, however, sensitive and are susceptible to degradation under the influence of ionising radiation and have been an active subject of research for more than two decades [l-131. The study of response of these devices to ionising radiation is of interest for two major reasons. First, instruments used in measuring radiation dose and dose rates that have been designed with these components must respond correctly, and therefore a knowledge of the response to ionising radiation is required for proper design of the circuits. Likewise, similar knowledge is desired for space application [l 11 where electronic circuitry must func- tion in a radiation environment. Ionising radiation causes a change in the threshold voltage, current drives, leakage CUT- rent and mobility of the surface channel of the MOS tran- sistor depending on the total dose received by the device [6]. A MOS transistor is the basic component that is used in the electronic instrumentation parts of spacecraft and nuclear reactors. The major effect of ionising radiation in MOS devices is the generation of electrowhole pairs (EHPs) in the oxide layer because of the high energy of the incident radiation zyxwvutsrqp [6]. T h s leads to long term effects due to trapping of these EHPs in the bulk traps in the oxide, pro- ducing a sMt in the flatband voltage zyxwvutsrq (AVFB) and threshold voltage (AVT) [3]. Another effect is the production of inter- face trapped charges (Nit) at the Si-SiO, interface. These interface states also cause an additional shift in the flatband voltage and threshold voltage [6]. Simulation and modelling of radiation effects in the vari- ous MOS structures can provide valuable insight into the mechanism of radiation damage and degradation if the 0 IEE, 2 0 ZEE Proceedizgs zyxwvutsrqponm online no. 2oooO203 DOf 10.1O49/ipcds:2oooO203 Paper fmt received 6th July 1998 and in revised form 15th February 1999 The authom zyxwvutsrqpo are with the Department of Electronics Engineering Institute of Technology, Banaras Bndu University, Varanasi 221 005, India device is in the nuclear environment. Simulation results can also be used as a guideline for desipng MOS devices and components for use in the radiation environment. Conven- tional circuit simulators such as SPICE [lo] cannot be directly used to study the effect of nuclear radiation on MOS devices and circuits. However, these simulation pro- grams can be modified suitably to model the characteristics of the device in the presence of nuclear radiation [13]. Such models can only provide changes in the global characteris- tics of the device. To have a thorough knowledge about the performance of the device in the nuclear environment it is necessary to model the various physical mechanisms that account for the changes in various parameters and charac- teristics of the device in the irradiated condition, It is there- fore necessary to develop rigorous physics based device simulation programs separately to examine theoretically the effect of radiation on the individual device that can be used for the purpose of circuit analysis. In this paper the effect of nuclear radiation of varying dose on the electrical characteristics of the MOSFET has been studied. The variation of the channel voltage and elec- tric field in the channel has been estimated numerically in the presence of nuclear radiation for the first time. The knowledge of the field profile in the channel enables one to compute the drain current by considering the field depend- ent mobility. This feature of the present model will be espe- cially attractive for modelling short channel MOSFETs which are the basic elements of VLSI circuits. 2 Model The structure under consideration is an n-channel MOSFET with a moderately large gate length. The effect of nuclear radiation on the characteristics of the MOSFET has been modelled on the basis of one-dimensional anal- ysis, which is fairly accurate for long channel devices. The model is applicable to unirradiated devices and can predict the device characteristics for radiation doses up to 500 h a d . The ionising radiation has been assumed to be inci- dent in the vertical y-direction and the source to drain cur- rent flows in the horizontal x-direction. It is understood that the radiation exposure of MOSFET results in a build- up of positive trap charges in the oxide layer and an 133 IEE Proc.-Circuits Devices Syst., Val. 147, No. 2, April 2000