mosfets dosimetry

MOSFETS dosimetryMiguel Ángel Carvajal Rodríguez

[email protected]

ECsens, CITIC-UGR, Department of ElectronicsUniversity of Granada, E-18071 Granada, Spain

mailto:[email protected]

OUTLINE

Introduction Radiation effects Constant current measurements Linear range improvement: Two currents method Thermal dependence:

• Thermal model for lateral transistors• Thermal compensation: Three currents method

Sensor module: Biased and unbiased modes Readout process: deferred mode Results and discussion

Online ELICSIR Summer School (Sept. 2020)

Introduction

Why MOSFET dosimeters for in-vivo dosimetry?• Small size and low cost• Immediately readout• Linearity• Easy calibration• Reproducibility

SiO2

n

p+ p+

Puerta

GSD

B


Radiation effects

par e--huecohuecoelectrónhuecoatrapadotrampa

SiO2

Si

VGB>0

Puerta

(a)

SiO2

Si

VGB>0

Puerta

(b)

SiO2

Si

VGB>0

Puerta

(c)

SiO2

Si

VGB>0

Puerta

(d)

SiO2

Si

VGB>0

Puerta

(e)

SiO2

Si

VGB>0

Puerta

(f)

SiO2

Si

VGB>0

Puerta

(g)

E

GATE

GATE GATEGATE

GATE GATE GATE

Electron-hole pairHoleElectronTrapped holeTrap


Unbiased mode:• No external voltage.• Low sensitivity

Biased mode:• External VGB during irradiation• Higher sensitivity and linearity.


M1VGB

M2

Radiation effects

Ionizing radiation createscharge in the oxide and traps inthe Si-SiO2 interface.

Charge in oxide and positivetraps produce an increment inthe threshold voltage, |Vt|.

Interface traps degrade thecarriers mobility and then, the βparameter is reduced.

( )22 tGSD VVI −−=β

Radiation effects

SiO2

n

p+

Gate

GSD

B

+ + + + ++ + + + + + +++ + +p+

SiO2

Gate

+ + + + ++ + + + + + +++ + +

+ ++- -

+ ++- -


Simplified measurement of Vt : VS at constantdrain current (VGD = 0).

00

<∆>∆

β|V| t

ID

VS

VDD

S

G B

D

⇒≈ constant β St VV ∆≈∆

( )22 tGSD VVI −−=β

Constant current measurements

βIVV St

2−=

Ionizing radiation creates charge in the oxide: Thresholdvoltage, Vt ,is the dosimetric parameter


Radiation effects in electrical parameters

|Vt| increases and β reduces with absorbed dose

y = 0.0297x + 0.0037R2 = 0.9999

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

0 10 20 30 40 50 60Dose (Gy)

∆|V T

| (V)

5.65E-04

5.70E-04

5.75E-04

5.80E-04

5.85E-04

5.90E-04

5.95E-04

6.00E-04

6.05E-04

∆β (A

/V2 )∆|Vt|

∆β

• 60Co Source

• Semicondcutor analyser HP-4145B

• Unbiased mode

3N163, Vishay-Siliconix (USA)


Linear range improvement:Two currents method (2CM)

Accounting for ∆β effect in ∆Vt

Pre- and post-irradiation parameters at constant current I:

⇒−=βIVV St

2

prepre

Spre

tIVV

β2

−=

postpost

Spost

tIVV

β2

−={

−−∆=∆ prepostSt IVV

ββ112}

( )22 tS VVI −=β


Using two drain currents during read-out phase:

Threshold voltage shift without neglecting β degradation

Problem: Thermal compensated source voltage at twodifferent currents are needed, thus the thermal compensationmust be made using the thermal coefficient of Vt.


ββ112 11


ββ112 22

1

2

121

1II

VVVV SSSt

−

∆−∆+∆=∆

Linear range improvement:Two currents method (2CM)


Both |Vt| and β decrease withtemperature.

Therefore, at constant current:• If T ↑ |Vt| ↓ VS ↓• If T ↑ β ↓ VS ↑

( )

tD

S

tGSD

VI

V

VVI

+=

⇓

−−=

β

β

2

22

Thermal dependence


Di

SGv

ZTC

321 TTT <<3T2T

1T

ID

VS

VDD

S

G B

D

Thermal dependence

Both effects are compensated at IZTC current (ZeroTemperature Coefficient)

The thermal dependence of VS is minimized at IZTC current


Thermal model for lateral transistors

Thermal compensation: Model• Starting hypothesis: linear thermal trend of voltages

1

2

121

1II

VVVV SSSt

−

∆−∆+∆=∆

TVVTVV

SS

SS

∆+∆=∆

∆+∆=∆

2022

1011

α

α

TVV Vttt ∆+∆=∆ α0

} ⇒

−=

ZTCi

Vti II1αα


Thermal compensation: Three currents method (3CM)

ΔVS Thermal compensation: Adding a new current

1

11

01

1ααC

SSCSS

VVVV−

∆−∆+∆=∆TVTV

TVTV

CSCSC

SS

∆+∆=∆

∆+∆=∆

α

α0

1011

)()(

001 SCS VV ∆≈∆:radiationnoIf

C

SCS VVTαα −∆−∆

≈∆1

1}

( )C

ZTCSSCSS II

IIVVVV

−

−∆−∆+∆=∆

1

111

01

−=

ZTCVt I

I1αα


PROCEDURES OF MEASUREMENTThree currents method (3CM)

Thermal compensation• ∆|Vt| thermal compensated

• Simplification I1 = IZTC

• Additional current: IC

1

2

01

020

1

1II

VVVV SSSt

−

∆−∆+∆=∆

( )C

ZTCSSCSS II

IIVVVV

−−

∆−∆+∆=∆2

222

02

ZTC

ZTCSSZTCSt

IIVV

VV2

,02

,

1−

∆−∆+∆=∆


Thermal model: Experimental validation (I)

−=

ZTCi

Vti II1αα

3N163#1

0.0E+00

2.0E-04

4.0E-04

6.0E-04

8.0E-04

1.0E-03

2 2.5 3 3.5 4 4.5|VDS| (V)

I (A)

-40.4 ºC

-40.4 ºC

85.0 ºC

85.0 ºC

y = -0.0021x + 2.7181R2 = 0.9983

y = 0.0027x + 4.0762R2 = 0.9958

2.45

2.55

2.65

2.75

2.85

-50 -30 -10 10 30 50 70 90T (ºC)

Vs[I D

=20 µ

A] (V

)

3.85

3.95

4.05

4.15

4.25

4.35

2.00E-051.00E-03

Vs [I

D=1

mA]

(V)



3N163#1

-4.0

-2.0

0.0

2.0

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035ID

1/2 (A1/2)

α (m

V/ºC

)

20µA

−=

ZTCi

Vti II1αα

ID

VS

VDD

SG

B

D

Thermal model: Experimental verification (II)



Thermal model: Experimental verification (III)


CD4007#1

-4.0

-2.0

0.0

2.0

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035ID

1/2 (A1/2)

α (m

V/ºC

)

10µA

ZTCI

TVα−


Thermal model: Experimental verification (IV)


RADFET 400nm#1

-6.0

-3.0

0.0

3.0

6.0

9.0

12.0

0 0.01 0.02 0.03 0.04

ID1/2 (A1/2)

a (m

V/ºC

)

20 μA

700 μA

TVα−

ZTCI


Thermal model: IZTC summary



One current:• To thermal compensation ID=IZTC. (usual mode)

Two currents:• To increase the linear range use

• To thermal compensation (without using IZTC)

Three currents:• Increasing the linear range and thermal compensation

( )C

ZTCSSCSS II

IIVVVV

−−

∆−∆+∆=∆2

222

02

ZTC

ZTCSSZTCSt

IIVV

VV2

,02

,

1−

∆−∆+∆=∆

( )C

ZTCSSCSS II

IIVVVV

−

−∆−∆+∆=∆

1

111

01

1

2

121

1II

VVVV SSSt

−

∆−∆+∆=∆



Thermal compensation with two identical devices

M1 M2

During irradiation

Read-out configuration

∆VD

+ -

M1 M2

Sensing: Two identical pMOS with different sensibilities. Readout: Differential measurement.


Thermal compesation using two identical devices

During irradiation: Two different bias voltage, thereforedifferent sensitivity to radiation

The readout is carried out with the same conditionsminimizing the interference of the temperature.

( )DSSVDSVDSV

DS

S21

22

11 −=∆⇒

=∆=∆

M1 M2

During irradiation

Read-out configuration

∆VD

+ -

M1 M2


Thermal compensation techniques: Summary

ONE TRANSISTOR (Lateral MOSFET): One current: IZTC. Two currents: Out of IZTC zone. Three currents: IZTC and two currents for thermal

compensation and linear range improvement.

TWO TRANSISTORS: Different bias voltages during irradiation and differential

measurements.


PROCEDURES OF MEASUREMENTPulsed biasing (PB)

SNR improvement• Read-time instabilities caused by low frequency noise (LFN)

(due to near-interface and interface traps)• How can LFN be reduced? Chopping the drain current as in

other experimental techniques (i.e. spectroscopy) and averaging

pMOS Switch


Sensor module

Unbiased mode• JFET as a switch (shortcircuited during irradiation and storage and

open during read-out)

pMOS

JFETRG

Irradiation and storage During readout


Sensor module

Biased mode: Two JFET as switches CD4007

RADFET

Irradiation Readout

Storage

VGB

Readout process: deferred mode

Dose measurement process• Zeroing:

• Measurement and storage of pre-irradiation VS at the two (three) draincurrents

• Irradiation• Timeout for short-term fading (2-3

minutes)• Dose measurement

• Recovery of the pre-irradiation values from the memory.• Measurement and storage of the post-irradiation VS at the two

(three) currents• Dose calculation (calibration is required)


Results for 3N163 (unbiased and deferred mode): (PB+2CM)

Linearity improvement• Irradiation source: Siemens KDS-6MV.

y = -3.35E-03x + 1.01E+00R2 = 9.17E-01

y = -1.06E-03x + 1.01E+00R2 = 5.20E-01

0.92

0.94

0.96

0.98

1.00

1.02

0 5 10 15 20 25Dose (Gy)

Nor

mliz

ed S

ensi

tivity

Sen VS (IZTC)Sen Vt , 2 Ids

MOSFET P2

Results for 3N163 (unbiased and deferred mode): (PB+2CM)

Linearity improvement• Sensitivity decay coefficient, mean sensitivity and the

linear limit (up to 5%).

DC modes PB

IZTC 2CM IZTC 2CM

m (mV/Gy2) -0.153 -0.095 -0.113 -0.057

Mean Sensitivity

(mV/Gy)20.6 19.7 20.0 19.2

Linear Range (Gy) 6.8 10.3 8.8 16.8

V

VPBVs

DCVs

µσ

µσ

4.36

8.45

=

=

20 % reduction of SD

Experimental results for 3N163 (3CM)

Thermal compensation:• Reduction in a factor of 50 in the thermal drift

• Compensated

AIAI

AI

ZTC µµµ

24712030

3

2

===

-20

-15

-10

-5

0

5

10

15

20

17 19 21 23 25 27 29 31T (ºC)

∆|V

t| (m

V)

∆|Vt| not compensated

∆|Vt| comp. Method (i)

∆|Vt| comp. Method (ii)CV

TVt /º70µ<∆∆


Unbiased and deferred mode (3CM+PB): 3N163

Radiation response of 3N163 Dpre=30 Gy (60Co)

y = 0.0233xR2 = 1.0000

y = 0.0240xR2 = 1.0000

y = 0.0240xR2 = 1.0000

y = 0.0243xR2 = 1.0000

y = 0.0232xR2 = 1.0000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 10 20 30 40 50 60D-Dpre (Gy)

∆V t

(V)

12345Li l (5)


Temperature range 10 – 40 ºC Resolution 1 cGy Accuracy ± 3 %

Linear range 15 Gy > 80 Gy*

Thermal drift < 3mGy/ºC Delay after irradiation 2 – 3 minutes * with recalibrations every 15 Gy

Note: For 3N163 transistor

Unbiased and deferred mode (3CM+PB): 3N163


Dosimetry characterization, resultsRADFET 400nm

No improvement with 3CM respect one current method.

y = -0.2862x2 + 55.918x

y = -0.2876x2 + 56.072x

0

500

1000

1500

2000

2500

0 10 20 30 40 50 60

D (Gy)

?VT (

mV)

3I_400nm

Iztc_400nm

∆VT

(mV)

LINAC 6MV


Sensitivity vs. dose

y = -3.1198x + 79.988R2 = 0.1846

y = -9.0124x + 68.031R2 = 0.6893

0

20

40

60

80

100

0 0.5 1 1.5 2 2.5 3 3.5D (Gy)

Sen

(mV/

Gy)

3N163 Biased 12.5VRADFET 400nm 0V

Deferred mode (3CM+PB): Summary


Thermal model has been applied to RADFETs andlateral commercial transistors

For RADFETs 3CM and IZTC method are equivalent 3N163 biased at 10 V presents a sensitivity similar to

RADFETs of 400 nm in unbiased model, but showingmuch more fading.

CD4007 5.24 ± 0.17 021.5 ± 1.1 048 ± 3 555 ± 3 10

RADFET 55.4 ± 1.2 0

3N163

Bias (V)Sensitivity (mV/Gy)

Deferred mode (3CM+PB): Summary


Future talks

Internal topology of the reader unit will be described Programmable current source Hardware for zeroing and resolution improvement

Real-time measurements DMOS transistor:

Thermal compensation Additional hardware required Radiation response

Etc.


AKNOWLEDGEMENTS

MMND 2014, Port Douglas (Australia)

mosfets dosimetry

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