Journal of Optoelectroncs and Advanced Materals Vol. 7, No., June 5, p. 69-64 FAST ELECTRON IRRAIATION EFFECTS ON MOS TRANSISTOR MICROSCOPIC PARAMETERS EXPERIMENTAL ATA AN THEORETICAL MOELS G. Stoenescu, G. Iacobescu * Faculty of Physcs, Unversty of Craova, Str. A. I. Cuza, 585, Craova, Romana. In ths paper the authors propose a general and a smplfed theoretcal model n order to study the output MOS transstor characterstcs and to compute the mcroscopc (the electron moblty, the Ferm energy) and macroscopc (the uraton voltage and the changng of the dran channel length) parameters, after fast electron rradaton ( MeV, MRd) of the samples. Startng from the epermental data, we used these models and an approprate optmzaton method, for computer smulaton of the crcut characterstcs and for fndng the specfc parameters of three MOSFET transstors wth p-nduced channel (ROS, ROS4, ROS 5), before and after rradaton. (Receved February 7, 5; accepted May 6, 5) Keywords: MOS transstor, Fast electron rradaton, Mcroscopc parameters, Optmzaton. Introducton ue to the large employment of the devces based on MIS (metal-nsulator-semconductor) structures lke MOS transstors, ntegrated MOS type crcuts and charge coupled crcuts n the last decade, t s very usefully to study the physcal and chemcal propertes of these structures under dfferent eternal condtons. MOS based devces may be eposed to onsed rradaton durng the technologcal manufacturng process. The effects of the rradaton on MOS transstors were, for the frst tme, emphassed by Hughes and Grou, n 964 [].It was showed that the most mportant effects of the rradaton on the MOS type crcuts was the accumulaton of the postve charge due to hole ecess produced by the radaton and the accumulaton of the postve charge on nterfaces. Our work s concerned to the study the man characterstcs and the mcroscopc and macroscopc parameters of three MOS type transstors, before and after rradaton wth fast electrons. For ths purpose, we propose a theoretcal background and an optmon method for fttng the epermental data and computng the specfc parameters of the studed devces: the electron moblty, the Ferm energy, the uraton voltage and the changng of the dran channel length.. Epermental We have traced the transfer and output characterstcs for three MOSFET transstors wth p- nduced channel (ROS, ROS4, ROS 5), usng an epermental set-up whch enable a drect recordng of these curves. The electrc crcut contans varable voltage power supples (whch gve V S and V GS nput voltages), an ampere-voltage converter (gvng an output voltage proportonal wth dran current I ), a * Correspondng author: gabrela@topedge.com
6 G. Stoenescu, G. Iacobescu KIPP-ZONEN, B type X-Y recorder and hgh precson dgtal voltmeters of HP 48A and KEITHLEY 6 type (for measure V S and V GS voltages). In order to avod the nfluence of the temperature on the data measurements, the transstors were kept at constant temperature (T= o C) usng an U type thermostat. The connecton between the A-V converter and the transstor (housed n the thermostat) was made by shelded mult-wre cable. The HF oscllatons whch could appear due to the nductance of the connecton cable, between the transstor termnals were wred 47 nf non-nductve capactors. These capactors short-break the transstor termnals at hgh frequences, wthout affectng hs d.c. behavour. In fact, we used two converters one for p-channel transstors and one for n-channel transstors. The A-V converter output voltage s appled to Y nput of the recorder (at an approprate sensblty level). The converson factor (rato) was. V/mA. At the X nput of the recorder we appled V GS or V S voltages for transfer or output characterstc recordng, respectvely. For the output characterstcs recordng the V GS voltage was kept constant, whle varyng V S. The rradaton was performed usng fast electrons ( MeV); the dose was MRd ( 5 Gy) and the rradaton tme, 6 mn. Fgs. - show the epermental characterstcs for ROS, ROS4 and ROS 5 transstors, respectvely, before and after rradaton. (a) (b) Fg.. The transfer (a) and output (b) characterstcs for ROS transstor, before (full lne curves) and after rradaton (dot lne curves). (a) (b) Fg.. The transfer (a) and output (b) characterstcs for ROS 4 transstor, before (full lne curves) and after rradaton (dot lne curves).
Fast electron rradaton effects on MOS transstor mcroscopc parameters epermental data 6 (a) (b) Fg.. The transfer (a) and output (b) characterstcs for ROS 5 transstor, before (full lne curves) and after rradaton (dot lne curves).. Theoretcal models Frst, we use a general model n order to study the ntensty-voltage characterstcs of the transstors. In the frame of ths model, we propose for the dran current the followng general epresson: = S ( V P ) / I = P V + P + S 4, () where V S s the voltage whch appears on the surface of the semconductor, whle a postve voltage s appled on the dran, P (=, 4) are parameters chosen as: wth / P =, () P = V, () GS P =, (4) P =, (5) P =, (6) 4 B n d =, (7) Lw w = e, (8) s N a d = F. (9) We used above the common notatons: V GS the gate voltage, n the electron drft moblty, d and s the delectrc constants of the delectrc and semconductor layer, respectvely, B
6 G. Stoenescu, G. Iacobescu the MOS transstor wdth, w the delectrc thckness, e the electron charge, L the length of the channel between source and dran contact, Na the acceptors concentraton, F the Ferm potental. Substtutng Eqs. ()-(9) n Eq. (), we obtan the well-known [] epresson of the dran ntensty: I = [( + ) / / VGS VS VS VS ]. () The results of the smulaton based on Eq. () for the values of the mcroscopc parameters lead us to the concluson that for analysed MOS transstors the hypothess VS<< F s verfed []. In ths pecular case, we can smplfy the general model of the ntensty-voltage characterstcs by choosng: P j =, j =, 4, () P ( V ) GS =, () P =. () The dran current ntensty becomes: I ( VGS ) VS VS, (4) where Z = nc L (5) and [ ( )] = F + sqn a F. C (6) where the Ferm potental was measured wth respect to the mddle of the un-allowed energy band. The parameters P (=, 4) were obtaned from the comparson of the epermental and theoretcal I-U curves, by mnmzng the test functon Here, V and S N = [ I ( V ) I ] F. (7) S I are the epermental values, N the number of the epermental data, and the functon I (V S ) was defned frst, by the theoretcal general model Eq. () and second, by the smple model Eq. (4). We used a Levenberg-Marquart type method for optmzaton the test functon (7) and we compute the moblty ( n) and the Ferm energy ( F) values, before and after the rradaton process, for dfferent V GS voltage values. Usng ths values, we can fnd the conductance for V S and the threshold voltage (the mnmum voltage appled to the gate electrode up to whch the voltage on the semconductor surface vansh) from the equatons, respectvely and Moreover, usng a dependence of the type [] n db w G = VGS e s ( F ) (8) Lw w d ( ) th V GS = s F. (9) d e
Fast electron rradaton effects on MOS transstor mcroscopc parameters epermental data 6 we can compute the uraton voltages ( V currents ( I [] L I V () Z C n ), for the epermentally measured uraton dran ). Consequently, we can fnd the decreasng of the dran channel length from the equaton a ( V V ) / L s S. () qn Fgs. 4, 5 llustrate the applcaton of the prevous models for I-U characterstcs of MOS transstor ROS 5, at V GS =- V. Smlar results were obtaned for ROS 4 and ROS, at dfferent V GS values [4]. 5 (ma) 5 5 4 6 8 4 6 V S(V) Fg. 4. Epermental and theoretcal characterstcs (general model) for ROS 5 transstor, at V GS =- V. -before rradaton, after rradaton. 5 (ma) 5 5 4 6 8 4 V S(V) Fg. 5. Epermental and theoretcal characterstcs (smplfed model) for ROS 5 transstor, at V GS =- V. -before rradaton, after rradaton.
64 G. Stoenescu, G. Iacobescu 4. Results and dscusson After rradaton wth fast electrons, the transfer characterstcs for all studed transstors proved to have a strong dsplacement n the sense of ncreasng the threshold voltage from 4-5 V up to appro. 9- V. The slopes of the curves were practcally unmodfed and ths seems to be a result of the accumulaton of the postve charge n the ode layer placed between the gate and the substrate of the transstors. The fast electrons crossng ths layer durng the rradaton process produce the onzaton of the atoms [5]. Consequently, a secondary electron emsson from the ode layer took place, whch fnally leads to accumulaton of the postve charges n the ode substrate. Ths postve charge creates an electrc feld whch has a major contrbuton n the transfer characterstcs dsplacement towards hgh voltage values. The theoretcal curves derved from the general and smple models are n good agreement wth the epermental I-U characterstcs for all studed transstors. We have computed the Ferm potental and the parameters, for ROS 5 transstor and we found that the values of the Ferm potental (. ev) and of the parameter (.48-6 ), whch depends on the acceptors concentraton, reman unchanged after the fast electron rradaton. By contrast, the electron moblty was proved to be modfed as a result of the rradaton process, ts value decreasng from.74 m / Vs down to. m / Vs, for V GS =- V. The change of the dran channel length was L=.4 Å, showng a good agreement wth the epermental data and the phenomenologcal analyss. 5. Conclusons In our work we propose two theoretcal approaches for study the changes of the ntenstyvoltage characterstcs of MOS transstors subjected to fast electron rradaton. We have computed the mcroscopc and macroscopc parameters of these devces, under the same condtons, also. We have found that the electron moblty decreased after rradaton process, whle the Ferm energy was not changed, both parameters dependng on the gate voltage values. These theoretcal results are n good agreement wth the epermental measurements. References [] H. L. Hughes, R. A. Grou, Warnng Space Radaton affects FET S, Electroncs 7,, 58-6 (964). [] H. K. Ihantola, I. L. Moll, Sold State Electroncs, 7, June, 6 (964). [] G. Stoenescu, V. Calan, I. Spanulescu, N. Baltateanu, Proc. of SIOEL 95. [4] G. Stoenescu, Thess, Unv. of Buc, Fac. of Phys., Bucharest, 996. [5] K. H. Zannger, Appl. Phys. Letts. 8, 6 (986).