Integrated Circuit Design for Radiation-Hardened Charge-Sensitive Ampliﬁer Survived up to 2 Mrad

sensors

Article Integrated Circuit Design for Radiation-Hardened Charge-Sensitive Ampliﬁer Survived up to 2 Mrad

Changyeop Lee 1,2, Gyuseong Cho 1, Troy Unruh 3, Seop Hur 2 and Inyong Kwon 2,*

1 Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea; [email protected] (C.L.); [email protected] (G.C.) 2 Korea Atomic Energy Research Institute, Daejeon 34057, Korea; [email protected] 3 Idaho National Laboratory, Idaho Falls, ID 83415-3531, USA; [email protected] * Correspondence: [email protected]

Received: 31 March 2020; Accepted: 8 May 2020; Published: 12 May 2020

Abstract: According to the continuous development of metal-oxide semiconductor (MOS) fabrication technology, transistors have naturally become more radiation-tolerant through steadily decreasing gate-oxide thickness, increasing the tunneling probability between gate-oxide and channel. Unfortunately, despite this radiation-hardened property of developed transistors, the field of nuclear power plants (NPPs) requires even higher radiation hardness levels. Particularly, total ionizing dose (TID) of approximately 1 Mrad could be required for readout circuitry under severe accident conditions with 100 Mrad around a reactor in-core required. In harsh radiating environments such as NPPs, sensors such as micro-pocket-fission detectors (MPFD) would be a promising technology to be operated for detecting neutrons in reactor cores. For those sensors, readout circuits should be fundamentally placed close to sensing devices for minimizing signal interferences and white noise. Therefore, radiation hardening ability is necessary for the circuits under high radiation environments. This paper presents various integrated circuit designs for a radiation hardened charge-sensitive amplifier (CSA) by using SiGe 130 nm and Si 180 nm fabrication processes with different channel widths and transistor types of complementary metal-oxide-semiconductor (CMOS) and bipolar CMOS (BiCMOS). These circuits were tested under γ–ray environment with Cobalt-60 of high level activity: 490 kCi. The experiment results indicate amplitude degradation of 2.85%–34.3%, fall time increase of 201–1730 ns, as well as a signal-to-noise ratio (SNR) of 0.07–11.6 dB decrease with irradiation dose increase. These results can provide design guidelines for radiation hardening operational amplifiers in terms of transistor sizes and structures.

Keywords: charge-sensitive amplifier; total ionizing dose; radiation effects; application-specific integrated circuit (ASIC); readout circuit; nuclear power plant; preamplifier; operational amplifier

1. Introduction Radiation detectors have been widely used in nuclear power plants (NPPs). For instance, measuring neutron flux in a reactor core of a NPP delivers critical information for safety operations. An up-to-date neutron detector known as a micro-pocket-fission detector (MPFD) has been introduced that can combine the operational concept of the coaxial fission chamber with radiation hardening geometry [1]. Generally, a containment building in NPPs presents a relatively high total dose and dose rate, 108–109 krad and 103–106 krad/h, unlike the outer space field with 103–104 krad total dose and 4 2 10− –10− krad/h dose rate, respectively [2]. For this harsh environment, readout circuits should be radiation hardened and installed as close as possible near the sensor to minimize voltage degradation and signal interferences caused by long coaxial cables.

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Previous work on circuits for total ionizing dose (TID) effects have been systemically conducted for the last forty years, although it has predominantly focused on the outer space field because of the many projects organized by NASA [3,4]. This research for space applications has been mainly investigated for degradation of single transistor performances such as threshold voltage shift, leakage current increase, trans-conductance reduction, and electrical noise increase caused by radiation [5–10]. Furthermore, radiation influences for unit circuits, for instance, bandgap reference circuit and bipolar CMOS (BiCMOS) amplifier, were analyzed in terms of output direct current (DC) voltage balance, spurious-free dynamic range (SFDR), etc. [11–15]. Similar to the developed circuits for space environments, suitable radiation hardened readout circuits for NPP conditions have become more highly required in these days. Particularly, a preamplifier, which directly amplifies current signals from detectors to voltage signals, is used for downstream signal processing to a shaping amplifier and an analog digital converter (ADC). Thus, the circuit is a key factor determining the performance of the radiation measurement system such as energy resolution and photon detection efficiency (PDE) [16,17]. One major architecture of preamplifiers is a charge-sensitive amplifier (CSA), which consists of an operational amplifier (OP-Amp), a feedback capacitor, and a feedback resistor [18,19]. It could be considered as a front-end readout circuit in a wide range of applications using detectors, since this circuit converts collected charges from a detector into voltage pulses with excellent converting linearity despite temperature, DC bias voltage, and gain variations [20]. However, these advantages could be debased from amplitude and fall time variations as well as electrical noise increase caused by induced radiation. For the reason, this paper provides the radiation-hardened designs of a CSA operating in harsh radiating environments such as NPPs by using channel width optimization of CMOS and BiCMOS transistors in comparison of 130 nm SiGe and 180 nm Si fabrication technologies. These schemes are associated with electrical noise of transistors, transistor type (bipolar junction transistor (BJT) and metal-oxide semiconductor field effect transistor (MOSFET)), and gate-oxide thickness depending on fabrication technologies. In order to clear the effect of numerous control variables on the circuit level, experimental results for amplitude change, fall time variation, and electrical noise are presented within a dose rate range corresponding to NPP environments.

2. Total Ionizing Dose Effects on MOSFET and BJT When radiation is injected into a metal-oxide semiconductor field effect transistor (MOSFET) and bipolar junction transistor (BJT), electron hole pairs (EHP) are formed along most of the radiation path, including the oxide layer. In the substrate, EHPs can be immediately recombined, while the holes of EHPs can be trapped in an oxide layer due to a hole-hopping-mechanism [21]. These holes result in total ionizing dose (TID) effects, which cause electrical malfunctions by triggering threshold voltage shifts, transconductance (gm) degradation, electrical noise increase, and leakage current increase [5–7].

2.1. Threshold Voltage Shift of MOSFET

The threshold voltage shift is the result of charges trapped inside the oxide (∆VOT) and the interface (∆VIT) between silicon dioxide (SiO2) and the channel of a MOSFET [8]. First, we have: q q ∆VOT = ∆NOT = tOX∆NOT , (1) −COX −εOX where q is elementary charge, and COX is the speciﬁc capacitance of the MOS capacitor that is expressed by C = ε A . ∆N is the density of oxide-trapped holes per area unit, given by OX OX tox OT R t N = OX n (x)dx V t2 ∆ OT 0 th . According to this equation, ∆ OT is proportional to OX [8,9] Second, interface-trapped holes change the MOSFET channel charges given by:

∆QIT ∆QIT ∆VIT = = tOX, (2) − COX εOX Sensors 2020, 20, 2765 3 of 11

where ∆QIT is the interface-trapped charge density per area unit, which breaks the channel charge balance, depending on the type of MOSFET. The overall threshold voltage shift is expressed by [8]:

∆VOV = ∆VOT + ∆VIT . (3)

The Equations (1)–(3) indicate that thin tOX (gate-oxide) with large COX leads to reduced threshold voltage shift. In terms of quantum mechanics, thin gate-oxide increases the probability of quantum tunneling of electrons. The increased probability enables most of the trapped holes caused by induced radiation to be recombined with electrons [10,11]. Therefore, minimizing tOX helps the MOSFET device become more radiation-tolerant due to the reduced threshold voltage shift. The previous irradiation test results of threshold voltage shift show a variation from 3 mV up to 20 mV depending on the gate-oxide thickness [7,22]. Therefore, deep submicron CMOS technologies with a thin gate oxide help MOSFET devices become more robust to radiation. In this paper, CSAs fabricated by SiGe 130 nm and 180 nm CMOS technologies with diﬀerent gate oxide thickness are compared, since threshold voltage shift can be expressed by amplitude variations of CSA’s output signals.

2.2. Noise Analysis Based on MOSFET Electronic noise is classified into two mechanisms: electron velocity fluctuations corresponding to thermal noise and electron number fluctuations representing 1/f noise. Both types of electric noise are relatively increased by induced radiation into MOSFET. The noise in MOSFET is expressed by:

2 de A f 4kBTΓ K f = Sw + = + , (4) d f f gm COXWL f where e2 is a variance of a voltage source represented by means of power spectral density. 4kBTΓ , gm the frequency independent term, is white noise (Sw) dominated by the channel thermal noise and consists of Boltzmann’s constant (kB), absolute temperature (T), channel thermal noise coeﬃcient W K f (Γ), and channel transconductance (gm ). The term, , which is known as the 1/f noise or ∝ L COXWL f the ﬂicker noise, is inversely proportional to the frequency and consists of 1/f noise parameter (K f ), capacitance (COX) of the MOSFET, and channel width and length (W, L). According to Equation (4), it is possible to reduce the electric noise by increasing the channel width [5]. Optimizing sizes of MOSFETs would lead to a more radiation tolerant circuit when designing a radiation hardening device, such as reducing electric noise and increasing the channel width. For the comparison, CSAs have been designed with two types of the channel widths of 1 µm and 2 µm.

2.3. Gain Degradation of BJT A p-type MOSFET device has radiation-hardening characteristics, because major carriers are not electrons but holes. Additionally, an npn BJT exhibiting lower 1/f noise than n-type MOSFET has been widely used for applications requiring low noise and high SFDR [15,23]. Therefore, an OP-Amp combining npn BJTs with p-type MOSFETs should be investigated regarding the radiation tolerance by replacing the n-type MOSFETs. The base currents for the npn BJTs are composed of recombination current (IB1) and injection current (IB2 ) caused by the carrier transfer between base and emitter terminals [24]. First, the recombination current is expressed by: QB IB1 = , (5) τb Sensors 2020, 20, 2765 4 of 11

where QB is total quantity of electrical charges of minority carriers inside the base region. The small minority carrier life-time (τb) means that the most of the carriers have been recombined faster than large τb. Second, the injection current provoked by a hole injection from base into emitter region is given by:

2 qADp qADp ni VBE IB2 = PE(0) = exp , (6) Lp Lp ND VT where Lp is the diffusion length of minority carriers inside the emitter region, Dp is the diffusion coefficient of holes, q is the magnitude of charges, A is the sectional area, and PE(0) is the concentration of the minority carriers injected into the emitter from the junction of the base region. The total base current can be obtained by summations (IB = IB1 + IB2). Then, the current gain ( βF) is expressed by [20]: Sensors 2020, 20, x FOR PEER REVIEW 4 of 11 I 1 β = C = , (7) F 2 D concentration of the minority carriers injectedIB intoWB the+ emitterp WB NfromA the junction of the base region. 2τbDn Dn Lp ND The total base current can be obtained by summations ( = +). Then, the current gain ( ) is where WexpressedB is the widthby [20]: of the base region, Dn is the diffusion coefficient of electron, and the NA/ND is the relative doping ratio of the base region and the emitter region. Previous studies have indicated = = , (7) that a dominant factor of the gain degradation in case of npn BJTs from Equation (7) is reduction of carrier life-time (τ ) due to electrons inside the base region recombined with trapped holes provoked where isb the width of the base region, is the diffusion coefficient of electron, and the / by radiationis the relative inside doping the isolation ratio of oxidethe base between region an thed the emitter emitter andregion. the Previous base terminals studies have [6]. indicated that a dominant factor of the gain degradation in case of npn BJTs from Equation (7) is reduction of

2.4. Noisecarrier Analysis life-time Based () ondue BJT to electrons inside the base region recombined with trapped holes provoked by radiation inside the isolation oxide between the emitter and the base terminals [6]. The noise source of BJTs is composed of the thermal noise of minority carriers spreading resistance from the2.4. base Noise to Analysis the emitter Based region,on BJT shot noise associated with the junction potential barrier of the base and correctorThe current, noise source and 1 /off noise BJTs ofis thecomposed base current of the thermal [25]. Similar noise toof theminority gain degradationcarriers spreading mechanism of BJTs,resistance a predominant from the noise base to source the emitter by induced region, shot radiation noise associated is 1/f noise with rather the junction than thermal potential and barrier shot noise due to generationof the base and of corrector the additional current, and traps 1/f noise [26]. of In th oure base experiments, current [25]. Similar average to the mean-square gain degradation variations of outputmechanism voltages of were BJTs, measureda predominant from noise Gaussian source by distributions induced radiation of amplitudesis 1/f noise rather at the than baseline thermal of CSA output signals.and shot noise due to generation of the additional traps [26]. In our experiments, average mean- square variations of output voltages were measured from Gaussian distributions of amplitudes at the 3. Designedbaseline Charge-Sensitive of CSA output signals. Amplifier As3. illustrated Designed Charge-Sensitive in Figure1, CSAs Amplifier were designed with a two-stage operational amplifier, an external feedback resistorAs illustrated (R f ) and in anFigure external 1, CSAs feedback were designed capacitor with (C f ).a Intwo-stage the figure, operationalM1 and amplifier,M2 are considered an as differentialexternal inputs feedback of resistor the first ( stage,) and an and externalM3-M feedback5 are placed capacitor as current (). In mirrors.the figure,The andM6 and areM 7 act as considered as differential inputs of the first stage, and - are placed as current mirrors. The the active load and M8 acts as a common source of the second stage. and act as the active load and acts as a common source of the second stage.

Figure 1.FigureSchematic 1. Schematic of theof the designed designed CSAs CSAs including including a feed aback feedback resistor resistor and a capacitor and a with capacitor the input with the input conﬁguration.configuration.

For various samples, CSAs were designed with different variables, as shown in Table 1. In order

to analyze the amplitude and the fall time variation from induced radiation, the output voltage ()

= , () (8) () of the CSA should be considered by: where and are impedance of detector capacitance and a feedback capacitance depending on the frequency. The open-loop gain () of two stage OP-Amps at low frequency is expressed by:

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For various samples, CSAs were designed with diﬀerent variables, as shown in Table1. In order to analyze the amplitude and the fall time variation from induced radiation, the output voltage (Vout) of theSensors CSA 2020 should, 20, x FOR be PEER considered REVIEW by: 5 of 11

Q V = s , (8) out =2(jw//)(//) , (9) jwC f + C f + CD AOL(jw) where and are respectively the trans-conductance and the output resistance of MOSFET from Figure 1. Considering these equations and methods, the CSAs were designed with the similar where 1 and 1 are impedance of detector capacitance and a feedback capacitance depending on specificationsjwCD jwC of fapproximately 40 dB gain and 60˚ phase margin, as shown in Table 1 and Figure 2. the frequency.These CSAs The fabricated open-loop in 130 gain nm (SiGeAOL and) of two180 nm stage Si were OP-Amps packaged, at low as shown frequency in Figure is expressed 3. by:

Table 1. Designed variable OP-AmpsAOL = 2 withgm2 gdifferentm8(ro2// sizeros7 versus)(ro5// propertiesro8) , of gain, gain-bandwidth (9) product and phase margin. where g and r are respectively the trans-conductance and the output resistance of MOSFET from m O 130 nm 130 nm 180 nm 180 nm 180 nm 180 nm Si Figure1. Considering theseSiGe equations SiGe and methods, the CSAs were designedSi with the similar Si CMOS Si CMOS BiCOMS speciﬁcations of approximatelyCMOS 40 dB gainCMOS and 60◦ phase margin, as shownBiCMOS in Table 1 and Figure2. Variable CSA These CSAs fabricated in 130 nm SiGe and 180 nm Si were packaged, as shownTail in FigureTail3. Double Double Basic Basic Current Current6 Width Width Table 1. Designed variable OP-Amps with diﬀerent sizes versus properties of50 gain, ㎂ gain-bandwidth7 ㎂ product and phase margin. and 1/0.13 2/0.13 1/0.18 2/0.18 2/2 2/2 W/L (µm) 130 nm 130 nm 180 nm Si 180 nm Si 180 nm Si 180 nm Si W/L (µm) SiGe 4/0.13 CMOS SiGe8/0.13 CMOS 4/0.18CMOS 8/0.18CMOS BiCMOS 2/2 BiCOMS 2/2 Variable CSA and Double Double Tail Current Tail Current 3/0.13Basic 6/0.13 3/0.18Basic 6/0.18 10/0.18 10/0.18 W/L (µm) Width Width 50 µA 67 µA M1 andGateM2 oxideW/L(µ m) 1/0.13 2/0.13 1/0.18 2/0.18 2/2 2/2 M4 W/L(µm) 42/0.13 8 2/0.13 3.6 4/0.18 3.6 8/0.18 3.7 2 /23.7 2 /2 Mthickness6 and M7 W /(nm)L(µm) 3/0.13 6/0.13 3/0.18 6/0.18 10/0.18 10/0.18 GateShallow oxide thickness Trench (nm) 2 2 3.6 3.6 3.7 3.7 Shallow Trench 400 400 350 350 320 320 Isolation (nm) 400 400 350 350 320 320 Isolation (nm) GainGain (dB)(dB) 41.9 41.9 42.3 42.3 39.37 39.37 38.83 38.83 62.43 62.43 62.43 62.43 Gain Bandwidth Product Gain Bandwidth 316.2 377.3 202.25 289.5 115.24 115.24 (MHz) 316.2 377.3 202.25 289.5 115.24 115.24 Product (MHz) Phase Margin (◦) 56.46 57.6 58.72 57.5 44.2 44.2 Phase Margin (˚) 56.46 57.6 58.72 57.5 44.2 44.2

Figure 2. Figure 2.The The layout layout ofof CSAs fabricat fabricateded for for irradiation irradiation tests. tests.

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(a) (b)

Figure 3. The microphotographs of packaged CSAs (a) of 130 nm SiGe fabrication and (b) of 180 nm (a) (b) Si fabrication. FigureFigure 3. The 3. The microphotographs microphotographs(a) of of packaged packaged CSAs ( (aa) )of of 130 130 nm nm SiGe SiGe fabrication fabrication(b) and and(b) of (b 180) of nm 180 nm 4. ExperimentalSi fabrication.Si fabrication.Figure 3. Setup The microphotographs of packaged CSAs (a) of 130 nm SiGe fabrication and (b) of 180 nm Si fabrication. 4. Experimental4.The Experimental γ–ray irradiation Setup Setup tests were performed with Cobalt-60 of 490 kCi at Korea Atomic Energy Research4. Experimental Institute (KAERI Setup ). According to the standard dose rate of European Space Components The Theγ–ray γ–ray irradiation irradiation tests tests were were performed performed with Cobalt-60 Cobalt-60 of of490 490 kCi kCi at Korea at Korea Atomic Atomic Energy Energy Coordination (ESCC) 22900 [27], we exposed the CSAs to up to 2 Mrad with the dose rate of 104.43 ResearchResearch InstituteThe Institute γ–ray (KAERI). irradiation (KAERI) According .tests According were performed toto thethe standard with Cobalt-60 dose dose rate of rate 490 of ofkCiEuropean European at Korea Space Atomic Space Components Energy Components krad/h. The test boards and equipment for the measurements were placed away from the irradiation CoordinationCoordinationResearch (ESCC) Institute (ESCC) 22900 (KAERI 22900 [27 ],)[27],. weAccording exposedwe exposed to thethe the CSAsstandard CSAs to uptodose up to 2rateto Mrad 2 ofMrad European with with the the Space dose dose rateComponents rate of of 104.43 104.43 krad /h. room with 10 m BNC coaxial cables, as shown in Figure 4. The testkrad/h.Coordination boards The test and (ESCC)boards equipment 22900and equipment [27], for we the exposed measurements for the the meas CSAsurements to were up to were placed 2 Mrad placedaway with away the from dose from the rate the irradiation of irradiation 104.43 room krad/h. The test boards and equipment for the measurements were placed away from the irradiation withroom 10 m with BNC 10 coaxial m BNC cables, coaxial as ca shownbles, as shown in Figure in Figure4. 4. room with 10 m BNC coaxial cables, as shown in Figure 4.

(a) (b) (c) (d) (a) (b) (c) (d) (a) (b) (c) (d) Figure 4. The photographs of (a) the experiment test setup with 10 m BNC coaxial cables, (b ) the γ– Figure 4. The photographs of (a) the experiment test setup with 10 m BNC coaxial cables, (b) the γ– FigurerayFigure 4. irradiationThe 4. photographs The source photographs of ofCobalt-60, ( aof) the(a) the experiment(c )experiment the test board, test test setupsetup and ( dwith with) the 10 10test m m BNCconfiguration. BNC coaxial coaxial cables, cables, (b) the (b )γ the– γ–ray ray irradiation source of Cobalt-60, (c) the test board, and (d) the test configuration. irradiationray irradiation source of source Cobalt-60, of Cobalt-60, (c) the (c test) the board, test board, and and (d )(d the) the test test conﬁguration. configuration. 5. Experimental Results 5. Experimental5. Experimental Results Results 5. ExperimentalThe input Results current pulse from a function generator was set to 200 nA and 3–4 µs to supply all of the TheCSAs.The input inputInThe addition, input currentcurrent current positive pulse pulse supplyfrom from from a a voltagefunction function of generator generator1.8 generator V, gate-source was was set was toset 200voltage set to nA200 to and 200 ofnA cu 3–4 nAandrrent µs and 3–4 tomirror supply 3–4µs toofµ all ssupply0.6 toof V, the supply and all of allthe CSAs. In addition, positive supply voltage of 1.8 V, gate-source voltage of current mirror of 0.6 V, and ofCSAs. thepositive CSAs. In addition, input In addition, voltage positive of positive0.9 supply V were supplyvoltage applied. voltageof For 1.8 theV, of entiregate-source 1.8 V,experiments, gate-source voltage output of voltage cu signalsrrent of mirror of current CSAs of were mirror0.6 V, and of measuredpositive inputin real voltage time of using 0.9 V werea high-end applied. oscilloscopeFor the entire atexperiments, 20 GHz samplingoutput signals rate of and CSAs 200 were MHz 0.6positive V, and input positive voltage input of voltage0.9 V were of 0.9 applied. V were For applied. the entire For theexperiments, entire experiments, output signals output of CSAs signals were of bandwidth.measured Underin real these time testusing environment, a high-end transienoscilloscopet signals at 20 of GHzBiCMOS sampling CSA wererate andsaved, 200 as MHz shown CSAsmeasured were measuredin real time in real using time a using high-end a high-end oscilloscope oscilloscope at 20 at 20GHz GHz sampling sampling rate rate and and 200200 MHzMHz in bandwidth.Figure 5. Under these test environment, transient signals of BiCMOS CSA were saved, as shown bandwidth.bandwidth.in Figure Under Under 5. these these test test environment, environment, transient transien signalst signals of of BiCMOS BiCMOS CSA CSA were were saved, saved, as shownas shown in Figurein Figure5. 5.

Figure 5. During the irradiation test with Cobalt-60 γ-ray exposure up to 2 Mrad (SiO), transient signals were measured.

Figure 5. During the irradiation test with Cobalt-60 γ-ray exposure up to 2 Mrad (SiO2), transient signals were measured. Sensors 2020, 20, x FOR PEER REVIEW 7 of 11 Sensors 2020, 20, 2765 7 of 11

Figure 5. During the irradiation test with Cobalt-60 γ-ray exposure up to 2 Mrad (SiO), transient 5.1. Normalizedsignals Amplitudewere measured. After5.1. Normalized the irradiation Amplitude tests, amplitudes of CSAs were investigated, as shown in Figure6. In the case of the 130After nm SiGe the irradiation CSA in Figure tests, amplitudes6a, normalized of CSAs maximum were investigated, amplitude as shown variations in Figure of the6. In basic the case size of and doublethe channel 130 nm widthsSiGe CSA are in respectively Figure 6a, normalized shown as maxi 3.6%mum and amplitude 2.23% ((Amp variationsmax-Amp ofmin the) /basicAmp presize radand 100) − × duringdouble the totalchannel dose widths 2 Mrad are respectively (SiGeO). shown In addition, as 3.6% and 180 2.23% nm Si(( of Figure-6b shows)/ 2.85% and× 100 2.32%) for theduring basic the size total and dose the 2 Mrad double (SiGeO channel). In addition, widths, 180 respectively. nm Si of Figure The 6b maximum shows 2.85% amplitude and 2.32% for variations the couldbasic be generated size and the by double induced channeγ-rayl widths, radiation, respectively. as considered The maximu with previousm amplitude results variations [7] and could Equations be (8) andgenerated (9), thus by this induced phenomenon γ-ray radiation, is related as considered to the steadywith previo declineus results of trans-conductance, [7] and Equations (8) includingand (9), a thus this phenomenon is related to the steady decline of trans-conductance, including a threshold voltage threshold voltage term, and the steady increase of the output resistance [28,29]. Moreover, Figure6a,b term, and the steady increase of the output resistance [28,29]. Moreover, Figure 6a,b shows that increasing showsthe that channel increasing width without the channel modifying width fabrication without proce modifyingsses could fabrication deliver more processes radiation tolerance could deliver against more radiationamplitude tolerance variations. against These amplitude results are variations. associated Thesewith threshold results arevoltage associated shifts depending with threshold on channel voltage shiftswidths. depending The threshold on channel voltage widths. shift Thevaries threshold in a larger voltage range for shift the variesbasic size in adesign larger compared range for to the the basic size designdoubled compared channel width to the design doubled [30,31]. channel width design [30,31]. As shownAs shown in Figure in Figure6c, CSAs6c, CSAs based based on on BiCMOS BiCMOS with with didifferentfferent current current flows flows (50 (50 ㎂,µ 67A, ㎂ 67) µareA) are plottedplotted for amplitude for amplitude variations. variations. In In the the case case of of 50 50µ ㎂A,, 34.3% degradation degradation of ofamplitudes amplitudes appears appears in in comparison with 16.17% degradation of 67 µA. These sharp decreases in amplitude, unlike CMOS comparison with 16.17% degradation of 67 ㎂. These sharp decreases in amplitude, unlike CMOS designs, can be explained with Equations (5) and (6), since generated traps as a result of radiation designs, can be explained with Equations (5) and (6), since generated traps as a result of radiation impact events inside an oxide layer can lead to base current (IB) increase by reducing minority carrier impact events inside an oxide layer can lead to base current () increase by reducing minority carrier lifetime.lifetime. Furthermore, Furthermore, a low a low current current flow flow can can cause cause more amplitude amplitude degradation degradation than than a high a high current current flow, becauseflow, because the highthe high current current has has higher higher number number of of minorityminority carriers. carriers.

FigureFigure 6. During 6. During the irradiationthe irradiation test test with with Cobalt-60 Cobalt-60 γγ-ray exposure exposure up up to to 2 Mrad, 2 Mrad, normalized normalized amplitude amplitude are plottedare plotted for( afor) 130(a) 130 nm nm SiGe SiGe (b )(b 180) 180 nm nm Si, Si, and and ( c()c) BiCMOS. BiCMOS. Measured Measured maximum maximum amplitudes amplitudes have have variationsvariations from from 2.23%–34.3% 2.23%–34.3% depending depending on on the thedesigns designs of the CSAs CSAs with with different different fabrications, fabrications, channel channel widthswidths of metal-oxide of metal-oxide semiconductor semiconductor fieldfield ef efffectect (MOSFET), (MOSFET), and and current current flows flows in BiCMOS. in BiCMOS.

5.2. Fall5.2. Time Fall Time FigureFigure7 shows 7 shows that that all ofall theof the CSAs CSAs have have increased increased fallfall time from from 201–1730 201–1730 ns nscalculated calculated by by ( − ). As shown in Figure 7a, the 130 nm CSAs have fall time variations of 3.6 ns (FTpre rad FT 2 Mrad). As shown in Figure7a, the 130 nm CSAs have fall time variations of 3.6 ns and and− 2.23− ns depending on basic and double channel widths, respectively. The 180 nm CSAs, as shown 2.23 ns depending on basic and double channel widths, respectively. The 180 nm CSAs, as shown in in Figure 7b, have similar fall time variations of 2.85 ns of basic and 2.32 ns of double channel widths Figure7b, have similar fall time variations of 2.85 ns of basic and 2.32 ns of double channel widths and of devices fabricated by the 130 nm process. These fall time increases can explained by the carrier and ofrecombination devices fabricated of trapped by the holes 130 inside nm process. the oxide These layer fallcaused time by increases ionizing canradiation, explained sinceby the the slew carrier recombination of trapped holes inside the oxide layer caused by ionizing radiation, since the slew rate rate (SR= ) is determined by the current of M in Figure 1. Cc (SR = ) is determined by the current of M4 in Figure1. I4 In comparison between two CSAs of 50 ㎂ and 67 ㎂ current flows, as shown in Figure 7c, the In comparison between two CSAs of 50 µA and 67 µA current ﬂows, as shown in Figure7c, the fall fall times increase with 608 ns and 1730 ns, respectively. As with MOSFETs, the fall times of BiCMOS times increase with 608 ns and 1730 ns, respectively. As with MOSFETs, the fall times of BiCMOS are are clearly increased with the slew rate decrease due to the mobile carrier recombination. clearly increased with the slew rate decrease due to the mobile carrier recombination.

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FigureFigure 7. During 7. During the the irradiation irradiation test test with with Cobalt-60Cobalt-60 γγ-ray-ray exposure exposure up upto 2 to Mrad, 2 Mrad, fall times fall timeswere were measuredmeasured for (a for) 130 (a) nm130 SiGenm SiGe (b) 180(b) 180 nm nm Si, Si, and and (c )(c BiCMOS) BiCMOS at at 10% 10% andand 90% of of peak peak voltages. voltages. The The fall timesfall show times variations show variations from 201–1730from 201–1730 ns dependingns depending on on the the CSAs.CSAs. All All of ofthe the CSAs CSAs appear appear with the with the increased fall time. Especially, the BiCMOS shows relatively greater slope. increased fall time. Especially, the BiCMOS shows relatively greater slope.

5.3. SNR5.3. SNR The changes of signal-to-noise ratios (SNR) were measured from 0.07–11.6 dB according to the total The changes of signal-to-noise ratios (SNR) were measured from 0.07–11.6 dB according to the total dose increase, as shown in Figure 8. The 130 nm CSAs show small changes of −0.314 and −0.202 dB dose increase, as shown in Figure8. The 130 nm CSAs show small changes of 0.314 and 0.202 dB depending on the basic and the double channel widths exposed to up to 2 Mrad (SiGeO−) in total doses− in dependingFigureFigure on 8a. theOn 7. During basicthe other andthe hand, irradiation the doublethe SNRs test channel forwith 180 Cobalt-60 nm widths designs γ-ray exposed show exposure relatively toup up toto high 22 MradMrad,er degradations fall (SiGeO times) were inof total2.396 doses in Figureand8 measured0.07a. On dB thefor for the other (a )basic 130 hand, nmand SiGe the the double(b SNRs) 180 nmchannel for Si, 180 and widths, nm (c) BiCMOS designs respectively, at show 10% inand relatively Figure 90% of 8b. peak higherEspecially, voltages. degradations the The basic of fall times show variations from 201–1730 ns depending on the CSAs. All of the CSAs appear with the 2.396 andchannel 0.07 width dB fordesign the is basic sharply and decreased the double after 500 channel krad (SiO widths,) in total respectively, dose. These results in Figure are 8relatedb. Especially, to noiseincreased increase fall corresponding time. Especially, to 1/fthe and BiCMOS thermal shows noise. relatively Depending greater on slope. Equation (4), the electrical noise the basic channel width design is sharply decreased after 500 krad (SiO2) in total dose. These results are relatedof MOSFET to noise is relatively increase reduced corresponding by the increased to 1/ fchannel and thermal width. Furthermore, noise. Depending these noises on of Equation CSAs (4), 5.3.fabricated SNR in 130 nm SiGe are scarcely seen, because of the increased probability of the quantum tunneling the electrical noise of MOSFET is relatively reduced by the increased channel width. Furthermore, of electronsThe changes in the of thinner signal-to-noise gate oxide ratios rather (SNR) than were the 180 measured nm CSA, from which 0.07–11.6 can leave dB accordingfewer trapped to the holes total these noises of CSAs fabricated in 130 nm SiGe are scarcely seen, because of the increased probability doseinside increase, the oxide as layer; shown this in was Figure explained 8. The by 130 the nm previous CSAs work show for small a single changes transistor of − 0.314[32]. and −0.202 dB of the quantum tunneling of electrons in the thinner gate oxide rather than the 180 nm CSA, which can depending on the basic and the double channel widths exposed to up to 2 Mrad (SiGeO) in total doses in leaveFigure fewer 8a. trapped On the holesother hand, inside the the SNRs oxide for layer;180 nm this designs was show explained relatively by thehigh previouser degradations work of for 2.396 a single transistorand 0.07 [32 dB]. for the basic and the double channel widths, respectively, in Figure 8b. Especially, the basic

Inchannel the case width of design BiCMOS is shar designs,ply decreased SNR reductionsafter 500 krad of (SiO 11.6) dBin total and dose. 3.12 These dB are results shown are byrelated the currentto flowsnoise of 50 increaseµA and corresponding 67 µA, as shown to 1/f and in thermal Figure8 noise.c. As Depending mentioned on in Equation Section (4), 2.4 the, 1 electrical/f noise, noise which is associatedof MOSFET with theis relatively carrier numberreduced fluctuation,by the increased becomes channel much width. worse Furthermore, by accumulated these noises radiation of CSAs eff ects due tofabricated the recombination in 130 nm SiGe process are scarce betweenly seen, mobilebecause carriersof the increased and trapped probability holes. of the For quantum this reason, tunneling incident radiationof electrons can bring in the SNR thinner degradation. gate oxide Moreover,rather than the a high 180 nm current CSA, flow which with can largerleave fewer mobile trapped carriers holes can be inside the oxide layer; this was explained by the previous work for a single transistor [32]. more radiation tolerant than a low current flow with fewer mobile carriers for CSAs.

Figure 8. During the irradiation test with Cobalt-60 γ-ray exposure up to 2 Mrad, standard deviation of the Gaussian distribution for signal-to-noise ratios (SNRs) were measured for (a) 130 nm SiGe, (b) 180 nm Si, and (c) BiCMOS. In the case of the 180 nm basic design, the SNR sharply drops as 2.59 dB after

500 krad (SiO) in total dose.

In the case of BiCMOS designs, SNR reductions of 11.6 dB and 3.12 dB are shown by the current flows of 50 ㎂ and 67 ㎂, as shown in Figure 8c. As mentioned in Section 2.4, 1/f noise, which is associated with the carrier number fluctuation, becomes much worse by accumulated radiation effects due to the recombination process between mobile carriers and trapped holes. For this reason, Figureincident 8. Duringradiation the can irradiation bring SNR test degradation. with Cobalt-60 Moreover,γ-ray exposure a high current up to 2 Mrad,flow with standard larger deviation mobile Figure 8. During the irradiation test with Cobalt-60 γ-ray exposure up to 2 Mrad, standard deviation of ofcarriers the Gaussian can be more distribution radiation for tolerant signal-to-noise than a low ratios current (SNRs) flow with were fewer measured mobile for carriers (a) 130 for nm CSAs. SiGe, the Gaussian distribution for signal-to-noise ratios (SNRs) were measured for (a) 130 nm SiGe, (b) 180 (b) 180 nm Si, and (c) BiCMOS. In the case of the 180 nm basic design, the SNR sharply drops as 2.59 dB nm Si, and (c) BiCMOS. In the case of the 180 nm basic design, the SNR sharply drops as 2.59 dB after after6. Conclusion 500 krad (SiO ) in total dose. 500 krad (SiO) 2in total dose. This paper provides a comparison of different CSA designs in order to be operated in high 6. Conclusions radiationIn the environmentscase of BiCMOS designed designs, withSNR reductionsdifferent fabrication of 11.6 dB andtechnologies, 3.12 dB are channel shown bywidths, the current and Thisflows paper of 50 provides㎂ and 67 a comparison㎂, as shown of in di Figurefferent 8c. CSA As designsmentioned in orderin Section to be 2.4, operated 1/f noise, in high which radiation is environmentsassociated designedwith the carrier with di numberfferent fabricationfluctuation, technologies,becomes much channel worse by widths, accumulated and transistor radiation types. Aftereffects the irradiation due to the recombination tests, although process fabricated between CSAs mobile survived carriers upand to trapped 2 Mrad holes.γ-ray For with this reason, generating validincident output signals,radiation the can influences bring SNR of degradation. radiation showed Moreover, amplitude a high current degradation flow with from larger 2.85% mobile to 34.3%, carriers can be more radiation tolerant than a low current flow with fewer mobile carriers for CSAs.

6. Conclusion This paper provides a comparison of different CSA designs in order to be operated in high radiation environments designed with different fabrication technologies, channel widths, and

Sensors 2020, 20, 2765 9 of 11 fall time increase of 201–1730 ns, as well as SNR decrease of 0.07–11.6 dB for all CSAs, as summarized in Table2. Note that the BiCMOS CSA with 50 uA shows the worst performance, with amplitude decrease of 34.3%, fall time increase of 1730 ns, and SNR reduction of 11.6 dB. However, the remarkable thing we came up with is that increasing current ﬂow for the BiCMOS CSAs can notably improve the performance of the CSA. Furthermore, in comparison with CMOS and BiCMOS technologies, the CSA designs that consisted of CMOS transistors showed more radiation tolerance in terms of amplitude, fall time, and SNR. These results are primariy associated with gain degradation of BiCMOS caused by operation mechanisms and structures, unlike CMOS.

Table 2. Summary for irradiation test results with amplitude, fall time, and SNR.

130 nm 130 nm 180 nm Si 180 nm Si 180 nm Si 180 nm Si SiGe CMOS SiGe CMOS CMOS CMOS BiCMOS BiCMOS Variable CSA Double Double Tail Current Tail Current Basic Basic Width Width 50 µA 67 µA Amplitude variation (%) 3.6 2.23 2.85 2.32 34.3 16.17 (Ampmax Amp ) − min Fall time variation (ns) 398 504 260 210 1730 608 (FTpre rad FT 2000 krad) − − − − − − − − SNR (dB) 0.314 0.202 2.396 0.07 11.6 3.12 (SNRpre rad SNR2000 krad) − − − −

The experimental results in this paper indicate that increasing channel width and current ﬂow and using deep submicron MOSFETs help CSAs become more radiation tolerant with excellent converting − linearity operating in harsh radiation environments such as NPPs. The data may be useful for circuit designers for applications requiring radiation hardened ability.

Author Contributions: C.L. analyzed the experimental results, and wrote the manuscript. G.C. and S.H. developed the experimental setup and conducted the experiment. T.U. experiment support and participated the experiment. I.K. proposed the method and funding acquisition. I.K. also revised the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported in part by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2017M2A8A4017932, 2020M2A8A1000830). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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