Study of CMOS Sensing System for Radon and Alpha Radiation

Article
Study of CMOS Sensing System for Radon and Alpha Radiation
Roy Shor * and Yael Nemirovsky *

 Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 3200003, Israel
 * Correspondence: roy.shor@campus.technion.ac.il (R.S.); nemirov@ee.technion.ac.il (Y.N.)

 Simple Summary: Radon gas represents one of the main sources of exposure to radioactivity in
 humans. It’s the second most important cause of lung cancer and the main cause of lung cancer in
 non-smokers. Commercial radon detectors suffer from low resolution and lack of on-line monitoring
 capability, while continuous radon detectors are expensive. This study presents a new semiconductor
 sensing system for radon and alpha radiation in general, which is fabricated in a standard 180 nm
 CMOS process with a single polysilicon layer.

 Abstract: This study focuses on a CMOS sensing system for Radon and alpha radiation, which
 is based on a semiconductor device that is integrated monolithically on a single chip with the
 Readout Circuitry, thus allowing fabrication of a low-power and low-cost sensing system. The
 new sensor is based on a new mosaic design of an array of Floating Gate non-volatile memory-like
 transistors, which are implemented in a standard CMOS technology, with a single polysilicon layer.
 The transistors are electrically combined in parallel and are operated at subthreshold, thus achieving
 very high sensitivity and reduced noise. The sensing system’s architecture and design is presented,
 along with key operation concepts, characterization, and analysis results. Alpha and radon exposure
 


 results are compared to commercial radon detectors. The new sensor, dubbed TODOS-Radon sensor,
 
 measures continuously, is battery operated and insensitive to humidity.
Citation: Shor, R.; Nemirovsky, Y.
Study of CMOS Sensing System for Keywords: radon; alpha radiation; CMOS sensor; silicon radiation detector; ionizing radiation
Radon and Alpha Radiation. sensor; floating gate transistor
Radiation 2021, 1, 250–260. https://
doi.org/10.3390/radiation1030021

Academic Editor: Alexandros 1. Introduction
G. Georgakilas
 Radon gas (Rn-222) is a noble, odorless, invisible, and chemically inert radioactive
 gas that emerges naturally from soils and rocks. It represents one of the main sources of
Received: 5 August 2021
 exposure to radioactivity in humans, as it emits alpha particles that may damage lung
Accepted: 15 September 2021
Published: 17 September 2021
 tissues after inhalation [1]. It is by now well-established that radon gas is the second most
 important cause of lung cancer and the most important cause of lung cancer in non-smokers,
Publisher’s Note: MDPI stays neutral
 while every 1 in 15 homes in the US suffers from high level of radon concentration [2,3].
with regard to jurisdictional claims in
 The commercial solid state semiconductor radon sensors are reviewed in [4,5] and
published maps and institutional affil-
 are implemented in PIN diodes. Low dark current PIN diodes are expensive since large
iations. area photodiodes often have defects leading to large dark currents; thus, the sensors
 are expensive.
 The goals of this study were to specify the requirements for Radon gas sensors for
 smart homes and report how to meet these requirements. It is obvious that for consumer
 applications, the sensor must be low-cost, small, require low power, and exhibit high
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
 sensitivity and reliability. It is obvious to the authors that a standard CMOS technology is
This article is an open access article
 mandatory, and a transistor will exhibit higher sensitivity than a PIN diode.
distributed under the terms and
 The innovation of the new sensor, dubbed TODOS-Radon Sensor is outlined in the
conditions of the Creative Commons sections below: Section 2.3 presents the new sensor and system architecture and innova-
Attribution (CC BY) license (https:// tions. Device characterization is reported in Section 3.1. Section 3.2 presents experimental
creativecommons.org/licenses/by/ results of Radon gas exposure. Section 4 concludes this study.
4.0/).

Radiation 2021, 1, 250–260. https://doi.org/10.3390/radiation1030021 https://www.mdpi.com/journal/radiation

Radiation 2021, 1, FOR PEER REVIEW 2

 The innovation of the new sensor, dubbed TODOS-Radon Sensor is outlined in the
Radiation 2021, 1 sections below: Section 2.3 presents the new sensor and system architecture and innova- 251
 tions. Device characterization is reported in Section 3.1. Section 3.2 presents experimental
 results of Radon gas exposure. Section 4 concludes this study.
 2. Materials and Methods
 2. Materials and Methods
 2.1. Floating Gate Transisor
 2.1. Floating Gate Transisor
 The operation principle to sense radiation using Floating Gate (FG) transistor is to first
 The operation
 pre-charge principle
 the FG using to sense radiation
 Fowler–Nordheim (FN) using Floating
 Tunneling by Gate
 applying(FG) atransistor
 large enoughis to
 first pre-charge the FG using Fowler–Nordheim (FN) Tunneling by
 electric field on the FG itself [6]. Once the FG is charged, an alpha particle incident will applying a large
 enough
 generateelectric fieldthat
 e–h pairs on will
 the FG
 be itself [6]. Once
 separated by an theapplied
 FG is charged,
 electricalanfield
 alpha and particle incident
 will discharge
 will
 the FGgenerate e–h pairs that
 [7]. Considering the will
 fact be separated
 that by anvoltage
 the threshold appliedofelectrical
 the device fieldis and will dis-
 proportional
 charge the FGwithin
 to the charge [7]. Considering the fact that
 the FG as described bythe threshold
 Equation (1),voltage
 an alpha ofincident
 the device willis shift
 propor-
 the
 tional to the
 device’s chargevoltage
 threshold within the FG as described
 as illustrated by Equation
 in Figure 1. (1), an alpha incident will shift
 the device’s threshold voltage as illustrated in Figure 1.
 Qss,ox Q Q Q
 Vth = φms − ,− ss,sti , + FG + D.max . + 2φ f (1)
 = − Cins −Csti +Csti + Cins + 2 (1)
 
 where φ ms isisthe
 where thecombined
 combined work function
 work difference,
 function Qss,ox and Q
 difference, ss,sti 
 and are thearefixed
 thecharge
 fixed
 , ,
 densities
 charge in the read
 densities in theout oxide
 read out and
 oxide inand
 the in
 control oxide oxide
 the control respectively and Cand
 respectively ins is the serial
 is the
 oxide-STI capacitance, given by (see Figure 1a):
 serial oxide-STI capacitance, given by (see Figure 1a):
 11 11 −1
  
 Cins = = ++ (2)
 (2)
 
 Csti C ox

 ∆QΔ Ne–h ·⋅q 
 ∆VΔ = = FG =
 = (3)
 (3)
 th
 Csti ε 0 ε ox A/t
 / 
 sti
 is the
 Ne–h is the number
 number of of generated
 generated e–h pairs by
 e–h pairs by aa single
 single alpha
 alpha incident,
 incident, A is the
 A is the sensing
 sensing
 capacitor area and t sti isisthe
 capacitor area thesensing
 sensingcapacitor
 capacitorthickness.
 thickness.

 VG>>VBulk
 Control Gate

 Control Gate
 3500A Csti
 Source Drain 3500A
 Floating Gate
 Floating Gate
 110A Cox Source Drain
 VD
 n+ n+ 110A

 n+ n+

 p-substrate p-substrate
 (a) (b)
 ΔVT=Qinj/Csti
 ID
 ΔVT=Qion/Csti

 Uncharged Discharged Charged
 FG FG FG

 VG
 (c)
 Figure 1. FG transistor radiation response illustration: (a) Sensing capacitor charging. (b) e–h
 generation and separation. (c) Threshold voltage shift: Csti —sensing area capacitance; Qinj —injected
 charge by FN tunneling; Qion —ionized charge in an alpha incident.

Radiation
 Radiation 2021,
 2021, 1,
 1, FOR
 FOR PEER
 PEER REVIEW
 REVIEW 33

Radiation 2021, 1 Figure
 Figure 1.
 1. FG
 FG transistor
 transistor radiation
 radiation response
 response illustration:
 illustration: (a)
 (a) Sensing
 Sensing capacitor
 capacitor charging.
 charging. (b)
 (b) e–h
 e–h gen-
 gen-
 252
 eration
 eration and
 and separation.
 separation. (c)
 (c) Threshold
 Threshold voltage shift: 
 voltage shift: 
 
 —sensing
 —sensing area
 area capacitance;
 capacitance; 
 —injected
 —injected
 charge
 charge by
 by FN tunneling; 
 FN tunneling; —ionized
 —ionized charge
 charge inin an
 an alpha
 alpha incident.
 incident.
 
 The
 The number
 Thenumber
 numberof of e–h
 ofe–h pairs
 e–hpairs generated
 pairsgenerated
 generatedby by
 byanan alpha
 analpha incident- 
 incident- e–h–– , ,,can
 alphaincident-N can
 canbe be calculated
 becalculated
 calculatedbyby
 by
 dividing
 dividing the
 the absorbed
 absorbed energy
 energy of of
 a a single
 single alpha
 alpha particle
 particle
 dividing the absorbed energy of a single alpha particle with the with
 with the the
 e–h e–h
 pair pair generation
 generation en-
 energy.
 e–h pair generation en-
 .
 ergy.
 In the In
 ergy. In the
 the proposed
 proposed RadonRadon
 proposed Sensor,Sensor,
 Radon the STIthe
 Sensor, STI
 layer
 the layer
 layer thickness
 STIthicknessthickness isA.3500
 is 3500is The 
 3500 .. The
 The absorbed
 absorbed energyen-
 absorbed is
 en-
 ergy is
 calculatedcalculated using
 using popular
 ergy is calculated popular SRIM
 SRIM software
 using popular software [8].
 [8]. As[8].
 SRIM software As
 shown shown
 in Figure
 As shown in Figure
 2e–h pair
 in Figure 2e–h pair
 pair gener-
 2e–hgeneration
 gener-
 ation
 ation energy
 energy energy inis SiO
 in SiO2in 17 eVis
 SiO 17
 17 eV
 is[9]. eV [9].
 [9].

 SiO
 SiO22 -- 5.5MeV
 5.5MeV Alpha
 Alpha Absorbed
 Absorbed Energy
 Energy
 60
 60
 [KeV]
 energy[KeV]

 50
 50
 40
 absorbedenergy

 40
 30
 30
 Alphaabsorbed

 20
 20
 10
 10
 00
 Alpha

 00 0.05
 0.05 0.1
 0.1 0.15
 0.15 0.2
 0.2 0.25
 0.25 0.3
 0.3 0.35
 0.35
 SiO
 SiO22 Depth
 Depth [um]
 [um]

 Alpha
 Alpha Energy
 Energy

 Figure
 Figure2.2. Absorbed
 2.Absorbed energy
 Absorbedenergy of
 ofaaa5.5
 energyof 5.5 MeV
 5.5MeV alpha
 MeValpha particle
 alphaparticle crossing
 particlecrossing SiO layer
 SiO layer
 crossingSiO with
 layerwith range
 withrange
 range of 3500 
 .
 3500 .
 Figure 2 ofof
 3500 A. .
 The total
 Thetotal
 The
 absorbed
 totalabsorbed energy
 absorbedenergy within
 energywithin the
 withinthe STI
 theSTI layer
 layerisis
 STIlayer 53.7
 is53.7 KeV,
 53.7KeV, so
 KeV,so the
 sothe number
 numberofof
 thenumber e–h
 ofe–h pairs
 e–hpairs
 pairs
 produced by
 producedby
 produced byanan alpha
 analpha particle
 alphaparticle is:
 particleis:is:
 
 53.7 
 53.7 ≈ 3150
 
 – == absorbed energy =
 =
 53.7 KeV ≈ 3150 (4)
 (4)
 – 
 Ne–h = 
 = 17 
 17 ≈ 3150 (4)
 pair generation energy 17 eV

 2.2.
 2.2. C-Sensor
 2.2.C-Sensor
 C-Sensor
 Fabricating
 Fabricatinganan
 Fabricating anFGFG
 FG transistor
 transistor in
 in standard
 in standard
 transistor standardCMOSCMOS
 CMOS process
 process with with
 process aa single
 a single
 with polysilicon
 polysilicon
 single layer
 polysilicon
 layer
 layer was done in the mature, well-proven TowerJazz High Voltage standard 180 nm
 was was
 done done
 in the in the
 mature, mature, well-proven
 well-proven TowerJazzTowerJazz
 High High
 Voltage Voltage
 standard standard
 180 nm 180
 CMOS nm
 CMOS
 Process Process
 CMOS [10]. [10].
 TowerJazz
 Process TowerJazz
 [10]. had had
 had been
 been developing
 TowerJazz been developing
 Floating-Gate Floating-Gate
 developingtechnology technology
 for memory
 Floating-Gate for
 devices,
 technology for
 memory
 which haddevices,
 been which
 later had
 further been later
 developed further
 and developed
 extended for and extended
 direct sensing
 memory devices, which had been later further developed and extended for direct sensing for
 of direct sensing
 radiation and
 of
 of radiation
 dubbed and
 C-cell.
 radiation dubbed
 andThis C-cell.
 C-cell. This
 technology
 dubbed andtechnology
 This and
 its reliability
 technology its
 its reliability
 andhave have
 have been
 been documented
 reliability documented
 been in the open
 documented
 literature
 in
 in the [11–16].
 the open
 open The [11–16].
 literature
 literature essence of
 [11–16]. The
 The this technology
 essence
 essence of thisistechnology
 of this shown in Figure
 technology is
 is shown
 shown3. in
 in Figure
 Figure 3.
 3.

 Tunneling
 Tunneling capacitor
 capacitor Sensing
 Sensing capacitor
 capacitor Read-out
 Read-out NMOS
 NMOS
 Floating Gate
 Floating Gate
 TG
 TG CG
 CG Source Drain Bulk
 Source Drain Bulk
 Poly-Silicon
 Poly-Silicon
 p+ n+ p+ STI n+ n+ p+
 n-well p+ n+ n-well p+ STI (SiO2)
 (SiO2) n-well n+ n+ p+
 n-well Isolated p-well
 n-well Isolated p-well
 n-well
 Isolated p-well Isolated p-well

 Deep
 Deep n-well
 n-well
 p-substrate
 p-substrate
 Figure
 Figure3.3.
 Figure Implementation
 3.Implementation of
 Implementationof C-Sensor
 ofC-Sensor in
 inaaasingle-poly
 C-Sensorin single-poly CMOS
 single-polyCMOS process-schematic
 CMOSprocess-schematic cross
 process-schematiccross section.
 crosssection.
 section.
 2.3. System Implementation
 A way to effectively increase the sensing area of the device while keeping high yield,
 is by using a mosaic array of C-Sensors, where all cells are connected in parallel as shown

Radiation 2021, 1, FOR PEER REVIEW 4

 Radiation 2021, 1 253
 2.3. System Implementation
 A way to effectively increase the sensing area of the device while keeping high yield,
 is by using a mosaic array of C-Sensors, where all cells are connected in parallel as shown
 ininFigure
 Figure4.4.Each
 Eachcell
 cellininthe
 themosaic
 mosaicarray
 array is
 is considered
 considered as as sub-pixel.
 sub-pixel. Although
 Although aa defect
 defect in
 ina
 single sub-pixel will disable the sub-pixel itself, it will be negligible considering that
 a single sub-pixel will disable the sub-pixel itself, it will be negligible considering that there
 are thousands
 there of sub-pixels.
 are thousands of sub-pixels.

 (a) (b)
 Figure
 Figure4.4.Mosaic
 Mosaicconnectivity:
 connectivity:(a)(a)schematics;
 schematics;(b)
 (b)Layout.
 Layout.

 Thesensors
 The sensorsarray
 arrayisisbuilt by 90
 builtby 90 ×× 64
 64 sub-pixels,
 sub-pixels, where
 whereeacheachsub-pixel
 sub-pixelsensing
 sensingarea
 areais
 17.5µm
 is17.5 ××12.5
 12.5µm
 ,, so
 sothat
 thatthe
 theentire
 entirearray
 arraysensing
 sensingarea 1.12mm
 areaisis1.12 ××1.12
 1.12mm ..
 InInorder
 ordertotodecrease
 decreaseDC DCandandtemperature
 temperaturedependencies,
 dependencies,aadifferential
 differentialmeasurement
 measurement
 isisperformed
 performedusingusingthe
 theconcept
 conceptofofactive
 activeand
 andblind
 blindsensor
 sensorarrays.
 arrays.Considering
 Consideringthat
 thatalpha
 alpha
 particle is a heavy particle, meaning its penetration depth is relatively small, few layers ofof5
 Radiation 2021, 1, FOR PEER REVIEW particle is a heavy particle, meaning its penetration depth is relatively small, few layers
 Kaptonfilm
 Kapton (~50 )
 film(~50 µm)werewereused
 usedtotocover
 coveroneoneofofthe
 thearrays
 arrayssosoititcould
 couldbebeconsidered
 consideredasasaa
 blinddetector.
 blind detector.TheThedevice
 devicelayout
 layoutisisshown
 shownininFigure
 Figure5.5.
 Another key concept in the proposed architecture is operating in the sub-threshold
 region, decreasing the power and noise, and also increasing the sensitivity due to the ex-
 ponential relation = ∝ exp at subthreshold.

 Figure 5. CMOS Sensor device layout.

 = ⋅ = ⋅ ⋅ (5)
 
 Operating in the subthreshold region introduces a linear relation between the output
 signal and the bias current— . This point is used as a feature to configure the sensitivity
 of the device, adapting it to long\short term measurements.
 Since the power consumption of the device is dominated by the bias current of the
 sub-pixels, working in the subthreshold will result in low power consumption which will

Radiation 2021, 1 254

 Another key concept in the proposed architecture is operating in the sub-threshold
 Figure 5. CMOS Sensor
 region, device layout.
 decreasing the power andnoise, and
  also increasing the sensitivity due to the
 −Vth
 exponential relation I = Ids ∝ exp VGSnKT at subthreshold.
 
 = ⋅ = ⋅ ⋅ (5)
 dI q
 isig = ·∆Vth = · I ·∆Vth (5)
 dVth nKT
 Operating in the subthreshold region introduces a linear relation between the output
 signal and the bias current— .
 Operating in theThis point is used
 subthreshold as aintroduces
 region feature to aconfigure the sensitivity
 linear relation between the output
 of the device, adapting it to long\short term measurements.
 signal and the bias current—I. This point is used as a feature to configure the sensitivity of
 Since the device,
 power consumption
 adapting it to of the deviceterm
 long/short is dominated
 measurements. by the bias current of the
 sub-pixels, working
 Sinceinthethepower
 subthreshold will result
 consumption of thein device
 low power consumption
 is dominated which
 by the biaswill
 current of the
 enable the device to beworking
 sub-pixels, operatedinby thestandard AAA will
 subthreshold batteries.
 result in low power consumption which will
 An incident
 enablealpha particle
 the device onoperated
 to be the activeby sensors
 standardarrayAAA willbatteries.
 increase current as shown
 in Equation (5). An
 Subtracting the blind array’s current from the
 incident alpha particle on the active sensors array will active array’s current
 increase pro-as shown in
 current
 vides a differential
 Equation measurement.
 (5). SubtractingHence,
 the blind thearray’s
 readout circuitfrom
 current wasthe designed as a precise
 active array’s current provides
 current subtractor followed
 a differential by a Trans Impedance
 measurement. Amplifier
 Hence, the readout (TIA),
 circuit wasto convert
 designed the
 asoutput
 a precise current
 signal into subtractor
 output voltagefollowed by a Trans
 (see Figure Impedance
 6). The differenceAmplifier
 between (TIA), to convert
 two adjacent the output
 samples of signal
 the readoutinto output
 voltage, voltagethe
 meaning (see Figurevoltage
 output 6). Thederivative,
 differencereflects
 between thetwo adjacent
 amount samples of the
 of alpha
 readout voltage, meaning the output voltage derivative, reflects the amount of alpha
 particles incidents.
 particles incidents.

Radiation 2021, 1, FOR PEER REVIEW 6

 (a)

 (b)
 FigureFigure
 6. (a) Readout schematic
 6. (a) Readout as a current
 schematic subtractor
 as a current followed
 subtractor by a TIA.
 followed by(b) current
 a TIA. (b) subtractor
 current subtractor
 design based on cascaded current mirrors.
 design based on cascaded current mirrors.

 Considering Equation (3), the threshold shift of a single sub-pixel caused by a single
 alpha incident is given in Equation (6).
 ⋅ 3150 ⋅ 1.6 ⋅ 10
 Δ , = = = 23 (6)
 / 8.84 ⋅ 3.9 ⋅ 10 ⋅ 12.5 ⋅ 17.5 ⋅ 10 /3500 ⋅ 10
 So that the average threshold voltage shift of the entire array is:
 Δ ,
 Δ = = 4 (7)
 64 × 90

(b)
Radiation 2021, 1 255
 Figure 6. (a) Readout schematic as a current subtractor followed by a TIA. (b) current subtractor
 design based on cascaded current mirrors.

 Considering
 Considering Equation
 Equation (3),
 (3), the
 the threshold
 threshold shift
 shift of
 of aa single
 single sub-pixel
 sub-pixelcaused
 causedby
 byaasingle
 single
 alpha
 alpha incident
 incident is
 is given
 given in
 in Equation
 Equation(6). (6).
 ⋅ ·q 3150 ⋅ 1.6 ⋅ 10
 ·1.6 ·10−19
 Δ ,∆V=th,α = Ne–h= 3150
 =⋅ 3.9 ⋅ 10 −⋅ 12.5 ==23
 23 
 mV (6)
 (6)
 ε / 8.84
 0 ε ox A/tsti 8.84·3.9·10 12 ·12.5·17.5⋅·10
 ⋅ 17.5 10−12/3500
 /3500·⋅10
 10−10

 So that the average threshold voltage shift of the entire array is:
 So that the average threshold voltage shift of the entire array is:
 Δ ,
 Δ =∆Vth,α = 4 (7)
 ∆Vth = 64 × 90 = 4 µV (7)
 64 × 90
 3. Results
 3. Results
 3.1. Device
 3.1. Device Characterization
 Characterization
 As
 As the first part of
 of the
 the device
 devicecharacterization,
 characterization,the
 thecharging
 chargingmechanism
 mechanismwas was tested
 tested to
 to achieve
 achieve controllability
 controllability over
 over thethe threshold
 threshold voltage
 voltage of of each
 each sensor
 sensor array.
 array. Figure
 Figure 7 shows
 7 shows an
 an initial
 initial calibration
 calibration process
 process to shift
 to shift the sub-pixels
 the sub-pixels threshold
 threshold voltagevoltage into and
 into higher higher and
 positive
 positive
 values. values.

 Initial Full Arrays IV curve Calibrated Full Arrays IV curve
 60 100
 50 80
 Current [mA]

 Current [mA]

 40
 60
 30
 40
 20
 10 20

 0 0
 -4 -3 -2 -1 0 1 2 3 4 5 6
 Vcg [V]
 Vcg [V]
 Blind Active Blind Active

 (a) (b)
 Figure7.7.Full
 Figure FullArrays
 ArraysIV
 IVCurves.
 Curves. (a)
 (a) initial
 initialIV
 IVcurve;
 curve;(b)
 (b)calibrated
 calibratedIV
 IVcurve.
 curve.

 The need to have such controllability over the threshold voltage comes from the need
 to: (a) minimize the offset between the arrays, keeping the readout circuit out of saturation,
 (b) configure high threshold voltage, to increase the e–h separation ratio, and (c) perform a
 sort of reset stage once the readout circuit is close to saturation.
 To characterize the device response to alpha particles radiation, the device was exposed
 to an alpha source, Po-210, with an alpha emission rate of 925 Bq for two periods of 6 min,
 with a reset stage before each exposure.
 The number of alpha particles reaching the detector is given by:

 AD
 (emission rate)· (8)
 2· R2 π
 where A D is the detector active sensing area and R is the distance of the detector from
 particles
 the alpha source. Incident alpha ratio is calculated to be 45 sec , so that the expected
 threshold voltage shift can be calculated by multiplying the threshold voltage shift caused
 by a single alpha incident, with the number of alpha particles reaching the detector:

 #α
 ∆Vth = 4 µV ·45 ·300 ≈ 52 mV (9)
 sec

where is the detector active sensing area and R is the distance of the detector from the
 alpha source. Incident alpha ratio is calculated to be 45 , so that the expected
 threshold voltage shift can be calculated by multiplying the threshold voltage shift caused
Radiation 2021, 1
 by a single alpha incident, with the number of alpha particles reaching the detector: 256
 # 
 Δ = 4 ⋅ 45 ⋅ 300 ≈ 52 (9)
 
 Figure
 Figure88describes
 describesthe experiment
 the experiment flow andand
 flow the the
 threshold voltage
 threshold shift.shift.
 voltage Table Table
 1 sum-1
 marizes the results over three different devices, where each device was exposed
 summarizes the results over three different devices, where each device was exposed twice twice to
 the alpha source, while a reset stage was performed before each exposure (see
 to the alpha source, while a reset stage was performed before each exposure (see Figure 9).Figure 9).
 Table
 Table 11 indicates the bias
 indicates the bias current
 currentofofthe
 theactive
 activedevice
 deviceinineach
 eachexposure,
 exposure, the
 the threshold
 threshold volt-
 voltage
 age
 shiftshift
 andandthe the readout
 readout voltage
 voltage change
 change through
 through the exposure.
 the exposure.

 Initial IV curve IV Curve post first exposure
 8 10
 8

 Current [mA]
 Current [mA]

 6
 6
 4
 4
 2 2

 0 0
 3.5 4 4.5 5 3.5 4 4.5 5
 Vcg [V] Vcg [V]

 Iactive Iblind Iactive Iblind
Radiation 2021, 1, FOR PEER REVIEW Iactive sat line Iblind sat line 8
 Iactive sat line Iblind sat line

 (a) (b)

 IV Curve After Reset IV Curve post second exposure
 8 8
 Current [mA]
 Current [mA]

 6 6

 4 4

 2 2

 0 0
 3.5 4 4.5 5 3.5 4 4.5 5
 Vcg [V] Vcg [V]

 Iactive Iblind Iactive Iblind
 Iactive sat line Iblind sat line Iactive sat line Iblind sat line

 (c) (d)
 Figure
 Figure 8.
 8. Blind
 Blind and
 and Active arrays IV
 Active arrays IV curve.
 curve. (a)
 (a)Initial
 InitialIV
 IVcurve.
 curve.(b)
 (b)post
 post6 6min
 minexposure
 exposureIVIV curve.
 curve. (c)(c)
 IVIV curve
 curve after
 after reset.
 reset. (d)
 (d) post second 6 min exposure.
 post second 6 min exposure.

 Table 1. Summary of CMOS Sensor exposure to Alpha radiation.
 First Alpha Radiation Exposure - Vout Second Alpha Radiation Exposure - Vout
 4 #Device #Measurement 4 Bias Current ∆Vth ∆Vout
 1 1 103 µA 57 mV 1.75 V
 1 2 3.5 102 µA 53 mV 1.81 V
 3.5
 2 1 90.5 µA 60 mV 1.8 V
 Vout [V]
 Vout [V]

 2 2 85 µA 52 mV 1.76 V
 3 3 1 3 98 µA 44 mV 1.92 V
 3 2 110 µA 44 mV 1.45 V
 2.5 2.5

 2 2
 0 2 4 6 8 10 12 0 2 4 6 8 10 12
 Time [minutes] Time [minutes]

Iactive Iblind Iactive Iblind
 Iactive sat line Iblind sat line Iactive sat line Iblind sat line

Radiation 2021, 1
 (c) (d) 257
 Figure 8. Blind and Active arrays IV curve. (a) Initial IV curve. (b) post 6 min exposure IV curve. (c) IV curve after reset.
 (d) post second 6 min exposure.

 First Alpha Radiation Exposure - Vout Second Alpha Radiation Exposure - Vout
 4 4

 3.5 3.5

 Vout [V]
 Vout [V]

 3 3

 2.5 2.5

 2 2
 0 2 4 6 8 10 12 0 2 4 6 8 10 12
 Time [minutes] Time [minutes]

 Vout Vout

 Figure
 Figure 9. profile
 9. V profileduring
 duringalpha
 alpharadiation exposure. V isismeasured
 radiationexposure. measuredatatthe
 theoutput
 outputofofthe
 theTIA
 TIA(see
 (seeFigure
 Figure6a).
 6a).
 out out

 Table 1. Summary of CMOS Sensor exposure to Alpha radiation.
Radiation 2021, 1, FOR PEER REVIEW3.2. Radon Gas Exposure 9
 #Device #Measurement
 Four instances of the proposed radonBias sensors to radon gas in
 Current were exposed concentra-
 
 1 Bq 1 Bq 103 57 1.75 
 tions of 200 m3 up to 800 m3 for two periods of 9 days and 7 days respectively. The sensor
 1 2 102 53 1.81 
 Figure
 results were10 presents with
 compared 27 mV threshold voltage
 commercial shift
 detectors: afterEye
 Radon 9 days
 plusofand
 radon exposure
 Airthings for a
 wave.
 2 1 90.5 60 1.8 
 singleFigure
 sensor,
 10 while Figure
 presents 27 mV 11threshold
 presents voltage
 the readout circuit
 shift after derivative
 9 days of radon ofexposure
 all instances,
 for a
 2 2 85 52 1.76 
 single sensor,
 which while
 represents theFigure 11 difference
 voltage presents the readouttwo
 between circuit derivative
 measures of allbyinstances,
 caused which
 alpha particles
 3 1 98 44 1.92 
 represents
 incident the voltage difference
 3 on the active detector.
 2 Allbetween two measures
 instances
 110 present
 caused
 the same by alpha particles
 44trend,
 while the 1.45
 incident
 distribution
 
 onthe
 of the results
 active detector. All instances
 reflects each present the which
 sensor’s sensitivity, same trend, while the
 is configured bydistribution
 the sensor’sofbias
 the
 results reflects each sensor’s sensitivity, which is configured by the sensor’s bias current.
 current.
 3.2. Radon Gas Exposure
 Four instances of the proposed radon sensors were exposed to radon gas in concen-
 CMOS Sensor
 trationsIV 200
 ofCurves up to 800
 before and after 9 days
 for two of Radon
 periods exposure
 of 9 days and 7 days respectively. The
 9 sensor results were compared with commercial detectors: Radon Eye plus and Airthings
 wave.
 8
 7
 6
 current [mA]

 5
 4
 3
 2
 1
 0
 3.8 4 4.2 4.4 4.6 4.8 5
 Vcg [V]

 Iactive_initial Iblind_initial
 Iactive_post_9_days_radon_exposure Iblind_post_9_days_radon_exposure

 Figure
 Figure 10.
 10. IV
 IV curve
 curve of
 of blind
 blind and
 and active
 active arrays,
 arrays, before
 before and
 and after
 after 99 days of Radon
 days of Radon exposure.
 exposure.

 9 days Radon Exposure - Delta Vout
 2
 1.8
 1.6

3.8 4 4.2 4.4 4.6 4.8 5
 Vcg [V]

 Iactive_initial Iblind_initial
Radiation 2021, 1 Iactive_post_9_days_radon_exposure Iblind_post_9_days_radon_exposure 258

 Figure 10. IV curve of blind and active arrays, before and after 9 days of Radon exposure.

 9 days Radon Exposure - Delta Vout
 2
 1.8
 1.6
 Delta Vout [mV]

 1.4
 1.2
 1
 0.8
 0.6
 0.4
 0.2
Radiation 2021, 1, FOR PEER
 0 REVIEW 10
 20 40 60 80 100 120 140 160 180 200
 Time [hours]
 Figure 12 shows the comparison between commercial radon sensors to the proposed
 Device 1 In order
 CMOS sensor. Device 2
 to convert theDevice 3 voltage
 readout Device
 into4 radon concentration, a calibra-
 tion factor is suggested:
 Figure
 Figure 11.
 11. Readout
 Readout voltage
 voltage derivative.
 derivative. 
 = (10)
 ⋅ 
 Figure 12 shows the comparison between commercial radon sensors to the proposed
 CMOS Calculating
 sensor. In the calibration
 order to convertfactor for thevoltage
 the readout compared
 intodevice in Figure 12, isa given
 radon concentration, by:
 calibration
 factor is suggested: 
 501 REFavg 
 = Factor =
 Calibration = 9.09 (11)
 (10)
 CRSavg · Ibias ⋅ ⋅ 
 0.58 ⋅ 96 

 Comparison to Commercial Radon Monitors
 900 0.8
 Radon Eye + 0.75
 800

 700 0.7

 0.65
 Delta Vout [uV/sec]

 600
 0.6
 500
 Bq/m3

 0.55
 400
 0.5
 300
 Airthings Wave CMOS Radon Sensor 0.45
 200 0.4
 100 0.35

 0 0.3
 2021/5/3 14:24 2021/5/5 14:24 2021/5/7 14:24 2021/5/9 14:24
 Time

 Radon Eye Plus 24h average Airthings Wave RADON CMOS Sensor 24h Avg

 Figure
 Figure 12.
 12. CMOS
 CMOS Sensor
 Sensor comparison
 comparison to
 to commercial detectors [17,18].
 commercial detectors [17,18].

 Figure 13 shows radon measurement of a different CMOS sensor device, with a
 higher sample rate, at a different period of time. Calibration factor is extracted again:
 
 180.5 
 = = 9.6 (12)
 0.22 ⋅ 85 ⋅ ⋅ 

Radiation 2021, 1 259

 Calculating the calibration factor for the compared device in Figure 12, is given by:
 Bq
 501 m3 Bq
 Calibration Factor = = 9.09 (11)
 0.58 mV ·96 µA m3 ·mV ·µA

 Figure 13 shows radon measurement of a different CMOS sensor device, with a higher
 sample rate, at a different period of time. Calibration factor is extracted again:
 Bq
Radiation 2021, 1, FOR PEER REVIEW
 180.5 m3 Bq 11
 Calibration Factor = = 9.6 (12)
 0.22 mV ·85 µA m3 ·mV ·µA

 Radon Eye Plus Vs Radon CMOS Sensor
 350 0.43

 300 0.38

 Delta Vout [uV/sec]
 250
 0.33
 200
 Bq/m3

 0.28
 150
 0.23
 100

 50 0.18
 Radon Eye CMOS Radon
 0 0.13
 2021/5/14 7:12 2021/5/16 7:12 2021/5/18 7:12
 Time

 Radon CMOS Sensor 12 hours Average Radon Eye Pluse 12h averge

 Figure 13.
 Figure 13. CMOS Sensor comparison
 CMOS Sensor comparison to
 to commercial
 commercial detectors.
 detectors.

 4. Conclusions
 4. Conclusions
 A new sensing system for detecting high energy alpha particle and Radon gas is re-
 A new sensing system for detecting high energy alpha particle and Radon gas is
 ported.
 reported. The system
 The system is isbased
 basedonona aCMOS
 CMOSIC ICSensor
 Sensorwhich
 which was
 was fabricated
 fabricated inin aa standard
 standard
 0.18 μm CMOS process with a single polysilicon layer. The sensor’s unique
 0.18 µm CMOS process with a single polysilicon layer. The sensor’s unique architecture is architecture
 is based
 based onon active
 active andand blind
 blind mosaic
 mosaic arrays
 arrays of of
 FGFG cells,
 cells, operating
 operating in in
 subsub threshold
 threshold region.
 region.
 The sensing system was tested by exposure to an alpha radiation source
 The sensing system was tested by exposure to an alpha radiation source in a controlled in a con-
 trolled environment as well as to Radon gas and showed high sensitivity
 environment as well as to Radon gas and showed high sensitivity and good correlation and good corre-
 to
 lation to other commercial
 other commercial radon detectors.radon detectors.
 In
 In summary,
 summary, thethe main
 main advantages
 advantages of of the
 the new
 new Sensing
 Sensing System
 System are:
 are: low
 low cost
 cost compared
 compared
 to commercialradon
 to commercial radonsensors,
 sensors,enabling
 enablingit it
 toto
 bebe applied
 applied in every
 in every smart
 smart home;home;
 low low
 powerpower
 and
 and
 may may be battery
 be battery operated
 operated usingusing standard
 standard AAA batteries;
 AAA batteries; and theand the integrated
 integrated digital
 digital sensor
 sensor
 allows allows for continuous
 for continuous sampling sampling
 of radon ofgas
 radon gas in air
 in indoor indoor air providing
 providing hourly reso-
 hourly resolution of
 lution of Radon levels. The CMOS die is passivated, and therefore
 Radon levels. The CMOS die is passivated, and therefore insensitive to humidity. insensitive to humidity.

 5. Patents
 Technion-TODOS Ltd.
 Technion-TODOS Ltd. Patent Application; # provisional July 2019, Current
 Current July 2021.
 2021.

 Conceptualization,Y.N.;
 Contributions: Conceptualization,
 Author Contributions: Y.N.;methodology,
 methodology,Y.N.Y.N.and
 and R.S.;
 R.S.; software,
 software, R.S.;
 R.S.; val-
 valida-
 idation,
 tion, R.S.;R.S.; formal
 formal analysis,
 analysis, Y.N.Y.N.
 and and
 R.S.;R.S.; investigation,
 investigation, Y.N. Y.N. and writing—original
 and R.S.; R.S.; writing—original draft
 draft prep-
 aration, R.S.; R.S.;
 preparation, writing—review and editing,
 writing—review Y.N.Y.N.
 and editing, and and
 R.S.;R.S.;
 supervision, Y.N.;Y.N.;
 supervision, project administration,
 project administra-
 Y.N.; funding acquisition, Y.N. Both All authors have read and agreed to the published version of
 the manuscript.
 Funding: This research was funded by TODOS TECHNOLOGIES Ltd. (https://www.todos-technol-
 ogies.com, (accessed on 1 August 2021)) TODOS TECHNOLOGIES holds exclusively the IP related

Radiation 2021, 1 260

 tion, Y.N.; funding acquisition, Y.N. Both authors have read and agreed to the published version of
 the manuscript.
 Funding: This research was funded by TODOS TECHNOLOGIES Ltd. (https://www.todos-
 technologies.com, (accessed on 1 August 2021)) TODOS TECHNOLOGIES holds exclusively the IP
 related to this work, Technion grant number “2024790”. The die fabrication was funded by TowerJazz.
 The patents related to the technology of fabrication are held exclusively by TowerJazz.
 Institutional Review Board Statement: Not applicable.
 Informed Consent Statement: Not applicable.
 Data Availability Statement: Not applicable.
 Acknowledgments: The authors would like to thank all the people who contributed to this work:
 Igor Brouk, Tanya Blank, Sara Stolyarova, Sharon Bar-Lev, and Gavriel Amar. Last but not least, the
 enormous contribution of Lena Zugman, the library of the ECE Dept. at Technion-Israel Institute of
 Technology is appreciated.
 Conflicts of Interest: The authors declare no conflict of interest. Roy Shor pursued this research as
 part of his M.Sc. thesis at Technion- Israel Institute of Technology. Yael Nemirovsky supervised the
 research and thesis.

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