1686 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 59, NO. 6, JUNE 2012 In-Pixel Source Follower Transistor RTS Noise Behavior Under Ionizing Radiation in CMOS Image Sensors Philippe Martin-Gonthier, Member, IEEE, Vincent Goiffon, Member, IEEE, and Pierre Magnan, Member, IEEE Abstract—This paper presents temporal noise measurement results for several total ionizing dose (TID) steps up to 2.19 Mrad of an image sensor designed with a 0.18-μm CMOS image sensor process. The noise measurements are focused on the random telegraph signal (RTS) noise due to the in-pixel source follower transistor of the sensor readout chain inducing noisy pixels. Re- sults show no significant RTS noise degradation up to 300 krad of TID. Beyond this TID step, a limited RTS noise degradation is observed, and for the 2.19-Mrad step, an additional increase of total noise, including thermal, 1/f, and RTS noises, is noted. Noisy pixels have been studied for high TIDs, and three cases have been observed: 1) no change on RTS behavior; 2) creation of RTS behavior; and 3) modifications of RTS behavior. Index Terms—Active-pixel sensor, CMOS image sensors (CISs), correlated double sampling (CDS), ionizing radiation, low-fre- quency noise (LFN), noisy pixels, random telegraph signal (RTS) noise. I. I NTRODUCTION N OWADAYS, CMOS image sensors (CISs) are extensively considered in commercial, scientific, and space appli- cations [1]–[3]. The use of CIS processes has significantly enhanced their performances such as dark current (DC) and quantum efficiency [4]. In addition, the use of aggressive technologies and small in-pixel MOS transistors (gate area < 1 μm 2 ) allows pixel photosensitive area improvement, leading to an increase of MOS transistor low-frequency noise (LFN) and particularly random telegraph signal (RTS) noise. The use of correlated double sampling (CDS) circuits and its associated readout mode allows elimination of photodiode reset noise which is usually the major noise contributor. At the same time, it reveals noisy pixels, coming from the in-pixel source follower (SF) transistor RTS noise, which becomes an issue for the low light sensitivity applications [3], [5]. LFN in large-area devices, showing a 1/f power spectral density, is well characterized by the use of appropriate models, known as McWhorter model [6], dealing with carrier number fluctuation; Hooge model [7], dealing with mobility fluctuation; or the unified model [8], dealing with carrier number fluctuation inducing mobility fluctuation. Manuscript received November 16, 2011; revised February 3, 2012; accepted February 21, 2012. Date of publication March 12, 2012; date of current version May 23, 2012. The review of this paper was arranged by Editor J. R. Tower. The authors are with the CIMI Integrated Image Sensor Laboratory, Institut Supérieur de l’Aéronautique et de l’Espace, Université de Toulouse, 31055 Toulouse, France (e-mail: philippe.martin-gonthier@isae.fr). Digital Object Identifier 10.1109/TED.2012.2189115 Fig. 1. Mechanism of carrier trapping/detrapping at Si/SiO 2 interface in small MOS transistor devices. Fig. 2. RTS noise example coming from one defect at Si/SiO2 interface of a small MOSFET. (a) Measurements. (b) Model. For small MOS transistor devices (gate area < 1 μm 2 ), carrier number becomes small, and carriers in the transistor channel are captured and released by interface and near-oxide traps in contact with the channel, caused by individual interface defects at Si/SiO 2 interface, as shown in Fig. 1. The impact of trapping/detrapping events shows discrete drain current fluctu- ations [9]. Fig. 2(a) shows the measurements of this current fluctuation caused by one defect at Si/SiO2 interface for a small test MOS transistor. A two-level RTS noise appears. As can be seen in Fig. 2(b), three parameters can describe a two-level RTS noise: τ e , the average carrier emission time; τ c , the average carrier capture time; and ΔI D , the drain current RTS amplitude depending on trap features [10]. The following equations depict these parameters, where ΔE B is the trap energy level, ΔE CT is the difference between energy levels of conduction band and trap, σ 0 is the trap capture cross section, x T is the distance between the trap and Si/SiO2 interface, t OX is the gate oxide thickness, T is the temperature, k is the Boltzmann constant, q is the elementary charge, I D is the MOSFET drain current, g m is the MOSFET transconductance, W and L are the MOSFET dimensions, C ox 0018-9383/$31.00 © 2012 IEEE