A Novel Model for Implementation of GammaRadiation Effects in GaAs HBTs Jincan Zhang,Yuming Zhang,Senior Member,IEEE,Hongliang Lu,Member,IEEE,Yimen Zhang,Senior Member,IEEE,and Min LiuAbstract—For predicting the effects of gamma radiation on gal-lium–arsenide(GaAs)heterojunction bipolar transistors(HBTs), a novel model is presented in this paper,considering the radiation effects.Based on the analysis of radiation-induced degradation in forward base current and cutoff frequency,three semiempirical models to describe the variation of three sensitive model parame-ters are used for simulating the radiation effects within the frame-work of a simplified vertical bipolar inter-company model.Its va-lidity was demonstrated by analysis of the experimental results of GaAs HBTs before and after gamma radiation.Index Terms—Cutoff frequency,forward base current,gamma radiation effects,heterojunction bipolar transistor(HBT),semi-conductor device modeling,vertical bipolar inter-company (VBIC).I.I NTRODUCTIONG ALLIUM–ARSENIDE(GaAs)heterojunction bipolartransistors(HBTs),due to their superior performance, are widely used in space radiation environments,and the recent boost of wireless and other high-end communications continue to draw more and more attention to its reliable long-term per-formance under radiation.Earlier studies on radiation effects on GaAs HBT have shown that GaAs HBTs are very attractive candidates for applications in space-based communication sys-tems[1],[2].In this case,many integrated circuits(ICs)have been designed with a GaAs HBT process[3]–[5].However, to improve the radiation hardness of HBT ICs,designers need electrical models taking account for the degradation induced by radiation.However,most of the radiation studies on GaAs HBTs re-ported thus far have mainly focused on the radiation induced changes from the experimental results with the measured elec-trical characteristics of the devices(e.g.,excess base current, cutoff frequency,etc.)[1],[2],[6],[7].To our knowledge,there is not much published information on modeling the electrical characteristics of HBTs subjected to high-energy radiations[8]. The work of modeling the effects of gamma radiation on the dc characteristics of GaAs HBTs has been studied in our pre-vious work[9].However,the complexity of the radiation-in-Manuscript received May03,2012;revised September16,2012;accepted September20,2012.This work was supported by the National Basic Research Program of China under Grant2010CB327505,the Advance Research Project of China under Grant51308030306,and the Advance Research Foundation of China under Grant9140A08030511DZ111.The authors are with the Microelectronics Institute,Xidian University,Xi’an, Shaanxi710071,China(e-mail:zjc850126@).Digital Object Identifier10.1109/TMTT.2012.2221137duced degradation processes makes it difficult to develop a de-tailed physical model of the device after radiation.An alter-native semiempirical approach is to develop an improved ver-tical bipolar inter-company(VBIC)model to describe electrical characteristics of the device before and after irradiation.One can use the extracted model parameters to describe the degra-dation of sensitive model parameters as a function of radiation dose,and then assemble the device model.We believe that such an approach is very useful to predict the degradation effects of devices.There were reports for simulating radiation-induced degradation of dc characteristics in bipolar transistors of silicon based on the semiempirical approach[10],[11].However,very little progress has been made in modeling the degradation of ac characteristics as a function of radiation dose in bipolar transis-tors based on the semiempirical approach,which is now studied in this work.In this paper,a novel model for implementation of gamma ra-diation effects in GaAs HBTs is developed.This paper is orga-nized as follows.The novel model based on a simplified VBIC model is presented in Section II.To validate the validity of the model,the experimental results of GaAs HBTs under gamma radiation are shown in Section III.The modeled results are com-pared with the measured results in Section IV and conclusions are presented in Section V.II.M ODELThe VBIC model was defined by a group of representa-tives from IC and computer-aided design(CAD)industries to overcome the shortcomings of the Spice Gummel Poon(SGP) model.The equivalent network of VBIC is given in[12].There are several improvements comparing with the SGP model, such as temperature-dependence modeling,quasi-saturation modeling,and decoupling of base and collector currents. However,special characteristics of HBTs make it possible to consider a simplified VBIC model,as shown in Fig.1.In this simplified VBIC model,the following assumptions are consid-ered.1)There is no parasitic pnp transistor in npn HBTs[13];there-fore,the parameters to describe the parasitic transistor can be eliminated.2)The extrinsic base–emitter current and chargecan be neglected compared to the intrinsic base–emitter current and charge,respectively.3)Since de-embedding the parasitic parameters has beendone,base–collector small-signal capacitance and base–emitter small-signal capacitance can be ig-nored.0018-9480/$31.00©2012IEEEFig.1.Simplified VBIC model.4)In HBTs,both early voltages and knee currents for the for-ward and reverse operations can be considered to be infi-nite[14],therefore the normalized base charge tends to be1.The validity of the model to describe HBTs characteristics has been verified in our earlier work[15].Unfortunately,the simplified VBIC model also has not taken account of the par-ticular effects of radiation on the electrical behavior of devices. However,HBTs are significantly degraded when exposed to ra-diation.Forward base current and cutoff frequency are mainly affected.A.Forward Base CurrentIn the measurement of forward-mode Gummel plot,the base current is measured when isfixed at zero while the base–emitter junction is in forward bias.The presence of the relatively large valence band discontinuity at the base–emitter heterointerface leads to effective suppression of the hole current injected from the base region into the emitter.Thus,the base current is mostly determined by the recombination of:1)in the bulk and along the periphery of the base–emitter space-charge region(BE-SCR)and2)in the bulk and at the surface of the neutral base region(NBR).In the simplified VBIC model,the total forward base current for low-level injection where voltage drop across parasitic re-sistances can be neglected is followed by(1) where is the thermal voltage.As can be seen from (1),the base current includes a component,formed by the NBR recombination modeled with saturation current and ideality factor,and a component,caused by the BE-SCR recombination modeled with saturation currentand ideality factor.Radiation-induced degradation in the forward base current of HBTs is attributed to excess carrier recombination including ra-diation-induced traps in the BE-SCR[9],whereas excess base current defined as the difference between the post-radi-ation and pre-radiation base current can be experimentally ex-tracted by the variations of and with radiation dose. In this case,(1)will be improved as(2),in which radiation-in-duced excess saturation current and excess ideal factor are included.(2)B.Cutoff FrequencyPhysically,can be expressed as(3)(4)(5) where is the base–emitter junction capacitor charge time,is the base–collector junction capacitor charge time,is the base transit time,and is the base–collector space-charge region delay time.It has been shown that capacitance and resistance are slightly or even not degraded under radiation for HBTs in our previous works[16],Therefore,the degradation of is mainly caused by the increase of the transit time,which is just the sum of the variations of and in the compact model[17].In the simplified VBIC model,the transit time is modeled as(6) where is the forward transit time,is the variation of with basewidth modulation,is the coefficient of bias dependence,is the coefficient of dependence on, is the coefficient of dependence on,and is the forward collector current.Inserting a parameter related to the variation of due to radiation effect,the equation of the transit time can be im-proved as(7) where radiation-induced excess forward transit time is used to predict the increase of,in turn to describe the degra-dation of.III.E XPERIMENTSIn order to determine the regulations of,,and with radiation dose,and verify the validity of the presented model,the following radiation experiment has been performed.ZHANG et al.:NOVEL MODEL FOR IMPLEMENTATION OF GAMMA RADIATION EFFECTS IN GaAs HBTs3Fig.2.Radiation-induced degradation of base current for different total dose levels.The devices applied in the experiment are GaAs HBTs with a single fabrication batch from the WIN Semiconductors Corpo-ration,Tao Yuan Shien,Taiwan(type Q1H201B1).The width and length of each emitter mesa for Q1H201B1are1and20 m,respectively.Radiation of devices,without bias,was im-plemented in a“Gamma-Cell”with a Co source providing a dose rate of about50rd(Si)/s rd Si rd GaAs, and radiation time of5.5,16.5,38.5,and55h,equivalent to a gamma total dose of1,3,7,and10Mrd(Si),respectively.In order to get enough accurate test data,there were four test sam-ples under every radiation total dose mentioned above.Before the experiment,the samples were carefully selected to ensure the differences of performances among the16tested HBTs to be less than3%and the spread of the measured data for the four devices at each radiation to be within0.1%.All of samples were measured at room temperature K before and after radiation.On-chip forward dc Gummel characteristic measure-ments were made with an HP4142Semiconductor Analyzer. Scattering parameters(-parameters)were measured using an HP8510C vector network analyzer from100MHz to40GHz, and in a wide bias current range based on circuit applications.A.Forward Base CurrentFig.2shows the plot of measured versus with the base–collector junction shorted for different total dose radiation, while the collector current remains approximately unchanged. As can be seen from thisfigure,at low current levels,the curve shows significant change after radiation.In the high current regime,almost has no change.The increase rate of base current,defined as the ratio of excess base current to pre-radiation base current,is plotted with incremental dose values,as shown in Fig.3.The increase rate increases with the total dose,and reaches620%at V after a gamma total dose of10Mrd(Si).However,as can be seen from Fig.3,the increase rate of base current versus decreases.In the regime of V,the increase rate becomes close to0.Fig.4shows the effect of the total dose on the excess base current for different total dose levels.The results presented are limited in the low injection current region where the excess base current remains significant compared with thevalue of the total base current.The excess basecurrent is approximately linear throughout the biasrange with a slope of the idea factor Fig.3.Increase rate of forward base current.Fig.4.Radiation-induced excess base current.Fig.5.versus collector current for different total dose levels..These results indicate that radiation-induced recombi-nation mechanism in the BE-SCR is more dominant in the ex-cess base current.Thus,it is reasonable that only the and parameters associated with the BE-SCR are improved in the novel model,as shown in(2).There are two possible recombination mechanisms in the BE-SCR to be consistent with the measured base current ide-ality factor[18].One is trap-assisted tunneling due to gamma radiation induced traps.The second possible mecha-nism is the recombination from a nonuniform distribution of Shockley–Read–Hall centers within the BE-SCR.B.Cutoff FrequencyThe cutoff frequency was extracted using-parameters measurements in the common-emitter configuration by extrap-olating.Fig.5shows measured versus collector current for different total dose levels.In general,relatively obvious degradation is observed for the GaAs HBT after10-Mrd(Si)ra-diation.4IEEE TRANSACTIONS ON MICROWA VE THEORY ANDTECHNIQUESing fly-backing fly-back measurement.The emitter series resistance was measured with the fly-back technique in which the emitter is grounded and current is forced into the base.The open circuit collector voltage was measured.The emitter resistance is taken as the slope of the linear segment of the curve.Fig.6shows the comparison of from fly-back measurement curves before and after 10-Mrd(Si)radiation.As can be seen from this figure,there is almost no change in .In the simpli fied VBIC model,includes the extrinsic col-lector resistance and intrinsic collector resistance .Fig.7shows from fly-back measurement curves to be similar to measurement.The value of is equal to the slope of the linear segment of the curve,and has no change after radiation.can be determined by optimizing the fitting to quasi-saturation region data of common-emitter char-acteristics.It can then be obtained that there is almost no change in .The possible reason for almost unchanged and after radiation is that the doped concentration in HBT devices is high,which makes radiation induced a little reduction of carrier con-centration causing no obvious increment in and .Fig.8shows the comparison of the capacitances for the GaAs HBT.The curves nearly coincide,suggesting that even after 10-Mrd(Si)total dose gamma radiation,the capacitances almost do not change.According to the measured results,it can be concluded that the degradation of is only due to the change of ,which validates the correctness of the discussion in Section II-B.IV .A NALYSIS AND D ISCUSSIONTo predict the electrical behavior of ICs for a given radiation total dose,designers usually need an ef ficient evaluation oftheFig.8.Capacitances (and )for GaAs HBT.TABLE I V ALUES OF,,,AND P ARAMETERS FOR P RE -R ADIATIONAND P OST -R ADIATION OF D IFFERENT R ADIATION L EVELSradiation parameters embedded in sensitive device model pa-rameters to determine the degradation of these parameters with dose.Such an approach permits an easy implementation for ra-diation-induced degradation in the electrical simulator,such as the Advanced Design System (ADS),by means of symbolically de fined device (SDD),which is an equation-based module to en-able designer to quickly and easily de fine custom and nonlinear components.Furthermore,this approach,which allows reason-able computation time,is generally preferred for the applica-tions of complex physics with large numbers of parameters.A.Forward Base CurrentTo extract the forward Gummel base current parameters,theand parameters can be determined from the inter-cept and the slope of the plot in the region of low .The values of and are then easily obtained by fitting the curve in the high injection region.The obtained ,,,and parameter values are listed in Table I for pre-radiation and post-radiation of different radiation levels.The extracted curves for and versus total dose are plotted in Fig.9(a)and (b),respectively.A saturation effect is exhibited for high total doses.The objective functions for fitting and are shown in (8)and (9),respec-tively,where Dose represents the gamma radiation total dose [in Mrd(Si)]and ,,,,,and are fitting parametersDose (8)Dose(9)As can be seen from Fig.9(a)and (b),the fitting curves of the first four dose levels (first four modeled)nearly coincide with that of all the five dose levels (modeled).The values of and at Dose Mrd(Si)obtained from the first four modeled,modeled,and measured are shown in Table II.ThereZHANG et al.:NOVEL MODEL FOR IMPLEMENTATION OF GAMMA RADIATION EFFECTS IN GaAs HBTs5(a)(b)Fig.9.(a)Comparison of the measured and modeled.(b)Comparisonof the measured and modeled.TABLE IIVALUES OFANDP ARAMETERS AT DoseMrd(Si)parison of the measured and first four modeled excess base cur-rent for 10-Mrd(Si)gamma radiation.are little deviations between the measured values and the firstfour modeled values.The error between the measured excess base current and the excess base current based on first four modeled is less than 5%for the fifth total dose 10Mrd(Si),as depicted in Fig.10.Thus,it can be concluded that the novel model should be able to pre-dict accurately the radiation-induced degradation in excess base current even after more than 10-Mrd(Si)gammaradiation.The measured and modeled forward base current for dif-ferent total doses is drawn in Fig.11.The modeled results parison of the measured and modeled forward base current for different total dose levels.TABLE IIIV ALUES OF,,,,AND P ARAMETER FOR P RE -R ADIATION AND P OST -R ADIATION OF D IFFERENT R ADIATION L EVELSmatch the measured data reasonably well (the error within 2%)up to V.The difference between the measured and modeled increases up to 3%for the high injection current region because our model does not account for the degradation of the NBR.However,this mismatch is still not bad.B.Cutoff FrequencyIn order to extract the transit time parameters,the forward transit time is obtained from the intercept of against the curve.The ,,,and param-eters of the transit time are further estimated by optimization.Table III presents the extracted ,,,,and parameter values for pre-radiation and post-radiation of dif-ferent radiation levels.It seems that parameters do not change much between pre-radiation and post-radiation.Since the most susceptible transistor materials to be sensitive to the total dose effect are insulators,the SiN insulator instead of oxides in the GaAs HBTs does not show serious degradation to the total dose effect.To describe the decrease of ,the following objective func-tion is used to describe excess forward transit time :Dose(10)where ,,and are fitting parameters.The measured and modeled versus total dose is pre-sented in Fig.12.The fitting curve based on the first four dose levels almost entirely coincides with that based on all five dose levels,suggesting that the novel model should be able to predict the degradation of cutoff frequency under more than 10-Mrd(Si)gamma radiation.The cutoff frequency versus is illustrated in Fig.13for different total dose levels with a maximum error less than 1%in the all-bias range.6IEEE TRANSACTIONS ON MICROWA VE THEORY ANDTECHNIQUESparison of the measured and modeled.parison of the measured and modeled cutoff frequency for dif-ferent total dose levels.V .C ONCLUSIONA novel model to include total dose effects for HBTs has been presented in this paper.To predict the behavior of ICs in space-like environments,semiempirical models for radiation parameters as a function of radiation total dose have been pro-posed.By incorporating the radiation parameters into sensitive model parameters,a novel model based on a simpli fied VBIC model has been implemented to simulate accurately the radia-tion-induced degradation in forward base current and cutoff fre-quency at least 10-Mrd(Si)gamma total dose.Our analysis also shows that the model can possibly predict the electrical charac-teristics of HBTs 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