Qamma and^eavy Ion Chapter- 5 Irradiation on...

Qamma and^eavy Ion Irradiation on MOSJTTs Chapter- 5

5.1 Introduction

Even though the MOS capacitor has the advantage of being the simplest test structure

available for characterizing oxide and oxide-semiconductor interface, its area is several

thousand times larger than the gate area of MOSFET used in actual integrated circuits.

Based on the understanding developed for MOS capacitor results, we decided to study

the effects of radiation on MOSFETs for gamma and heavy ions. Gamma rays interact

with matter mainly in three different ways: Photoelectric effect, Compton scattering and

Pair production. All these process are associated with the release of electrons. In the

photoelectric effect the incident gamma ray is completely absorbed by a tightly bound

atomic electron and this bound electron, called the photoelectron is ejected from the

atom. Compton scattering involves an inelastic collision v^th a loosely bound or a free

electron. However, as the energy of incident gamma ray increases, Compton scattering

can take place with bound electron also. In Compton scattering the gamma ray is

scattered at reduced energy and the reminder of the incident gamma ray's energy

becomes the part of kinetic energy of the scattered electron. In pair production, the

gamma ray passes near a nucleus and converts into an electron-positron pair. This

requires the gamma ray energy to be at least 1.02 MeV. In silicon the photoelectric effect

dominates at photon energies less than 50 keV and pair production dominates at energies

greater than 20 MeV with Compton scattering dominating in the intervening energy

range. Gamma rays are one of the basic radiation source used to test the device for space

applications. Charged particles such as electron, proton and heavy ions interact with

atoms primarily by Coulomb scattering and cause both ionization and atomic

displacement [1, 2]. As discussed before the presence of heavy ions in space can never

be neglected which can cause the device to malfunction. NMOS being more sensitive to

radiation than PMOS, have been studied by number of research groups for various

sources of radiations [3, 4], However, the study on effects of heavy ions on P-MOSFET

120

requires more attention because of its extensive use in circuits used in radiation rich

space environments. P-MOSFETs are considered to be the best choice for High-Side

Switching which can simulate a high current, high power CMOS when paired with an n-

channel MOSFET [5]. The ionization effects in these devices can be related to either the

total amount of radiation that is absorbed (total dose) or the rate at which radiation is

absorbed (dose rate) [6]. In the present experiments the devices are evaluated for total

dose effects.

Major concern in the total dose effects is the creation of hole-electron pairs in silicon-

dioxide. In silicon technology, the silicon-dioxide is in contact with low acceptor doped

Si, hence the concern for total dose effects is warranted. The dominant effects are due to

holes being trapped at the oxide causing free electrons to be attracted to the Si-Si02

interface and effectively resulting in an inversion of the doping near the interface [7].

Thus the electrons in between the two p-regions of a p-channel MOSFET cause leakage

currents and change the electrical parameters of the MOSFETs. The hole trapping

phenomenon in a p-channel MOSFET is schematically represented in Figure 5.1.

+v I

Potyiilkon

S O j

•.

p^drari

gate

n substrat<

f 1

p'^iource

Uhir radiated

+V I

Potyslicon gite

• + + + + • s

p^ drari

n substrat

R

• '

* source

f

47 Irapped final diarge

I ralysncon gate

+ • + +

+ -•

p'*'drain

.

+ - + - + - + - +

- + - + - + - + -•« + • + - + - + • +

+

- + - +

p^Murce

^ n substrate

1 Right after

radiation burst

i +V I

Po^ilicongate

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

+ + + + + + r -V

p'*'drain

n substrat

±

p* (ource

f

Holes left after electron transport

Figure 5.1: The hole trapping phenomenon in p-channel MOSFET 121

In addition to hole trapping, interface states are also generated at Si-Si02 interface. In a

negatively biased p-channel MOSFET, positive interface charges causes the threshold

voltage to shift towards less negative side while negative interface charges causes the

threshold voltage to shift towards the more negative side. HoJes transporting through p-

channels undergo Coulomb scattering from the charged interface states resulting in

reduction in carrier channel mobility and increase in channel ON resistance. Because of

this RC charging and discharging time-constants are increased and the circuit speed is

significantly reduced in CMOS integrated circuits.

This chapter is dedicated to study the effects of * Co gamma and heavy ions on P-

channel MOSFETs. The chapter is divided into seven sections. Section 5.2 gives the

details of the MOSFETs used in the present study. It also briefs the experimental details

relating to irradiation and characterization of devices. Section 5.3 discusses the effect of

^"Co gamma rays on ALD1102 MOSFETs. Section 5.4, 5.5, 5.6 discusses the effect of

50 MeV Li- ions, 80 MeV Oxygen ions and 120 MeV Si- ions on 3N163 P-Channel

MOSFETs respectively. Section 5.7 concludes the chapter.

5.2 Experimental Details

The irradiation experiments were conducted on two different P- channel MOSFETs.

ALDl 102 was exposed to ^ Co gamma rays and 3N163 were exposed to heavy ions. The

detailed operating specifications of the devices are given below.

5.2.1 Device Specifications

ALD1102

The ALD 1102 is a monolithic dual P-channel matched transistor pair intended for a

broad range of analog applications. These enhancement mode transistors are

manufactured with Advanced Linear Devices' enhanced ACMOS silicon gate CMOS

process. The ALD 1102 is intended as a building block for differential amplifier input

stages, transmission gates and multiplexer applications. These devices also finds suitable

for use in precision applications which require very high current gain, beta, such as

current mirrors and current sources. Some of the features of this device is listed below

• Low threshold voltage

122

• Low input capacitance

• High input impedance

• Low input and output leakage currents

• Enhancement mode (normally off)

• High DC current gain of 10'*.

The operating electrical characteristics at room temperature are specified in the

manufacturer's manual [8]. The Pin configuration and Block diagram of the ALD 1102

is shown in Figure 5.2

SOURCE 1 1

GATEi 2

DRAIN 1 3

NC 4

w T ] SUBSTRATE

T ] SOURCE 2

T ] GATE 2

" 7 1 DRAIN 2

GATE 1 (2)

1. DRAIN 1 (3)

DRAIN 2 (5)

cU"

^

SOURCE 1 (1)

0 SUBSTRATE (8)

r—o SOURCE 2 (7)

TOP VIEW

(a)

GATE 2 (6)

(b)

Figure 5.2: Pin diagram (a) and Block diagram (b) of ALD 1102 P-channel

MOSFET

LS3N163

3N163 is a P-channel enhancement mode MOSFET manufactured by Linear Integrated

Systems. This device is contained in a cylindrical hermetic metal can, type TO-72,

composed of an external nickel layer 0.4mm thick. The transistor is placed on the surface

of a cubic die with an area of 0.3mmx0.3mm located at the centre of the cylinder. Its

structure and typical dimensions are shown in Figure 5.3 [9,10]. The device is generally

used for switching purposes. Some of the important features of 3N163 are

• High gate breakdown

• Ultra low leakage

• Fast switching

• Low capacitance

123

Nickel casing

pMOS transistor

Plastic support

Gold-plated circle

Sealing resin

Longitudinal axis

Longitudinal section Q

Bottom view

Case

4 . 3 — • ;

Figure 5.3: Longitudinal section and bottom view of tlie 3N163 transistor

The electrical operating characteristics of 3N163 at room temperature are specified by

the manufacturer's user manual [9].

5.2.2 Irradiation and Characterization Techniques

P-channel MOSFETs were exposed to Co gamma radiations using the Blood irradiator-

2000 at ISRO (Indian Space Research Organization) Satellite Centre, Bangalore.

ALD1102 has a DIP (Dual Inline package) and gamma rays being highly penetrating,

does not require to de-cap the devices. The details of the construction and working of the

irradiation setup is given in Chapter 2. The devices were irradiated for different gamma

doses varying from 1 krad to 1 Mrad. All the leads of the devices were shorted and

grounded during irradiation as P-channel MOSFETs are very sensitive to smallest

parasitic currents. P-channel MOSFETs 3N163 were irradiated with 50 MeV Li ions, 80

MeV Oxygen ions and 120 MeV Si ions for various fluences ranging from 5x10'° ions

cm' to IxlO'^ ions cm'^. Heavy ions with very little penetrating power, needs the device

to be decapped. Hence 3N163, a metal can packed device was chosen for heavy ion

irradiation so that the devices could be easily decapped. The devices were grounded and 124

maintained at ultra high vacuum during irradiation. Heavy ion irradiation was performed

at Inter University Accelerator Centre (lUAC), New Delhi.

The effects of ' X o gamma and heavy ions on P-channel MOSFETs were studied for

changes in the Threshold Voltage (Vj), Transconductance (g,n), Drain Current (ID) and

Subthreshold swing (S), Oxide charge density (Not) and interface charge density (Nu) due

to irradiation.

Threshold Voltage (VT)

The threshold voltage was determined from ID - VGS characteristics. Among several

methods available to measure the threshold voltage, one method is to choose a current

level and define the gate voltage (VGS) required to produce the drain-source current ID

[11]. For 3N163 devices, the VT was noted for b = -100 ^A and for ALD 1102

MOSFETs the Vj was noted for ID = -10 |iA

The threshold voltage of an irradiated MOSFET shifts towards the positive for interface

trapped charge and towards negative side of the voltage axis for oxide trapped charges.

The net threshold is shifted towards negative side because of the higher density of oxide

charges compared to interface charges. Therefore

AVT = AV„, + AVi,

where

AVT - total threshold voltage shift

AVoi - threshold voltage shift due to oxide trapped charges

AVii - threshold voltage shift due to interface trapped charges.

Drain Saturation Current (losat)

The drain current at any particular drain-source voltage (VQS) in the saturation regime of

ID-VDS curves gives the drain saturation current.

Transconductance (gm)

The transconductance of a MOSFET is defined as the small change in drain current with

unit increase in gate voltage at constant source-drain voltage

125

g,n - dh/ d VGS @ Constant VQS

Subthreshold Swing (S)

When the gate voUage is below the threshold voltage and the semiconductor surface is in

weak inversion, the corresponding drain current is called the subthreshold current. The

subthreshold region is very important for devices used for switching applications as it

describes how the switch turns on and off [12]. Subthreshold measurement is a powerful

tool in characterizing radiation induced interface traps. The subthreshold swing is a

change in gate voltage necessary to reduce transistor current by one decade. The

subthreshold swing measures the change in the slope of subthreshold curves obtained by

plotting VGS along X axis and In ID along Y axis. The subthreshold swing is calculated

using the equation-

5 = lnlO ^-mV I decade dilnl^)

where

VG = gate voltage

ID = drain current.

The interface trap density can be estimated from subthreshold measurements using the

equation-

N IT

' S ^f q^

InlO kT

C. ox

A <i cm -' [13].

Where

S = subthreshold swing

kT/q = thermal voltage (0.0259 V)

Cox = oxide capacitance per unit area

q = electron charge (1.6 x 10"' C).

Charge Separation Method

The threshold voltage shift due to interface charges (AVjt) was separated from that due to

oxide trapped charges (AVot) by subthreshold measurements using the technique

126

proposed by Mc Whoter and Winokur [14]. Using this technique, it is possible to split

the total threshold voltage shift (AVT) into contribution due to interface trapped charge

and oxide trapped charge. There are different methods for separating these two

components, but they all use the assumption that interface traps are net neutral at midgap

so that voltage shift at midgap (AVmg) is a measure of oxide hole trapping i.e.,

AV,„g = AVot.

Then, the shift due to interface traps is given by

AVi, = AVT-AV,„g.

For a capacitor, one can use the stretch-out between midgap and inversion, or the stretch

out between threshold and midgap on the I-V characteristic of a transistor (which usually

requires extrapolating the subthreshold current to midgap) [15]. We note that the

assumption of midgap neutrality for interface traps was first used by Lenahan and

Dressendorfer [16], reexamined later by McWhorter [17], and still later by Lenahan [18]

again. It is then possible to determine the change in the interface charge density (ANu)

and oxide charge density (ANQT) using the equations given below

AN,T = ( AVi, CoxVq cm'

ANoT = (AVo, CoxVq cm-^

5.3 Results and Discussion of "Co Gamma Irradiation on P-IMOSFET

Current-Voltage (I-V) characteristics at room temperature were carried out on ALDl 102

MOSFETs using Keithley I-V Source Measure Unit. The devices were characterized for

ID - VDS and ID - VGS characteristics before and after irradiation with various doses of

gamma rays. The changes in the threshold, subthreshold and transfer characteristics are

analyzed and reported.

5.3.1 ID - VGS Characteristics

Threshold voltage is extracted from Ip - VGS Characteristics by keeping the drain-source

voltage (VDS) constant at -8 V. Figure 5.4 shows the ID - VGS curves of Virgin and

gamma irradiated (1 krad, 10 krad, 100 krad, 500 krad and 1 Mrad) devices. It can be

noticed that the curve shifts towards more negative voltage with increase in gamma dose.

127

-600.0)1

-200. Oti-

-100.0|J -

0.0'

s o

-500.0W -

-400.0(J -

-300.0|J^

- H ^ - Virgin - • - 1 Krad -^•^ 10 Krad

100 Krad — ^ 500 Krad —»— 1 Mrad

0.0 -3.0

Figure 5.4: ID - VGS characteristics of gamma irradiated P-channel MOSFETs

The threshold voltage of the devices were found to be -0.69 V for unirradiated (Virgin)

device and shifts to -2.41 V for the device irradiated to a total gamma dose of 1 Mrad.

The negative shift in the threshold voltage can be attributed to the buildup of positive

oxide charges. Even though the interface charges contribute to the shift in the threshold

voltage, the effect of oxide charges dominates. The individual contributions of oxide and

interface charges for the threshold voltage shift are reported in the coming sections.

The transconductance (gm) is directly related to the drain current and is one of the

important parameters of a MOSFET. A high transconductance is always preferred when

it comes to transistor performance. The transconductance of P-channel MOSFETs were

found to decrease from 30.90 X 10" mho (Virgin) to 4.04 X 10" mho for a total gamma

dose of 1 Mrads. The decrease in transconductance is the result of decreasing slope in the

saturation region of ID - VGS curves [19].

5.3.2 ID - VDS Characteristics

The drain saturation current (losat) is extracted from ID - VDS Characteristics by keeping

the gate-source voltage (VGS) constant at -6 V. Figure 5.5 shows the ID - VDS curves of

Virgin and gamma irradiated (1 krad, 10 krad, 100 krad, 500 krad and 1 Mrad) devices.

It can be noticed that the drain current saturates early with increase in gamma dose. The 128

drain saturation current can be measured at any point on the ID - VDS curve in the

saturation region.

-20.0m -

-16.0m -

-12.0m -

-8.0m -

-4 .0m-

0 0 c

•»

• - Virgin

1 Krad iOKrad 100 Krad 500 Krad IMrad

/ .4

Jr •*

/ * *' ^ ^ » •

»'

' 1 •2

• « '

* •

_ ~V '

_ . ^ . ^ - - — ^ - * - ^ " ^ * *

' 1 ' 1 • 1 - 4 - 6 - 8

v„(V)

Figure 5.5: ID - VDS characteristics of gamma irradiated P-channel MOSFETs

The losat was found to be -19.06 mA for Virgin device and reduces to -8.13 mA for a

device irradiated to a total gamma dose of 1 Mrads. The reduction of drain current due to

gamma exposure can, in principle, be explained by a shift of threshold voltage (VT)

and/or a decrease of mobility (^) [20]. The reduction in the drain current can also be

attributed to the increased channel resistance caused due to carrier removal effect in

irradiated devices. The pronounced coulomb scattering in the channel due to radiation

induced interface traps also causes the drain current to reduce.

5.3.3 Subthreshold I-V Characteristics

Figure 5.6 shows the subthreshold characteristics of Virgin and gamma irradiated ALD

1102 P-channel MOSFETs. The decrease in slope of In ID VS VQS curves with increase in

total dose can be clearly observed. The slope of the pre-irradiated curve was measured to

be 26.66 while the one irradiated to a total gamma dose of 1 Mrads was found to be

11.04. The decreasing slope was analogous to the distortion of the C-V characteristics

and is due to an increase in the density of interface traps [19]. A decreased slope means

129

that a larger swing in gate voltage is required to bring the transistor into strong inversion.

Therefore interface traps reduce the switching speed of the MOSFETs.

3> c

• « - _ -

-10

- 18 -

O

8 -20 H

CO

-25

-3.0 — I — -2.e

Virgin IKrad lOKrad 100 Krad SOOKrad IMrad

-2.0 —1 ' 1— -1.6 -1.0 -0.5 0.0

Figure 5.6: Subthreshold characteristics of gamma irradiated P-channel MOSFET

The subthreshold swing is found to increase from 9.10 mV/decade (Virgin) to 16.10

mV/decade for a total gamma dose of 1 Mrad. The experimentally obtained values of Vj,

gm, S and ID for unirradiated and gamma irradiated devices are summarized in Table 5.1.

Table 5.1: Experimental results of Gamma (y) irradiated P-channel MOSFETs

y- Dose (rads)

Virgin

I K

10 K

100 K

500 K

I M

V T ( V )

-0.69

-0.72

-0.84

-1.39

-1.95

-2.41

gn, ( X lO'* mho)

30.90

30.60

29.30

21.32

11.07

4.04

S (mV/decade)

9.0

9.62

10.12

10.58

13.34

16.10

ID (mA)

19.06

18.81

18.17

14.63

11.12

8.13

130

5.3.4 Oxide and Interface Trapped Charge Density

The effect of oxide and interface charges on the threshold and subthreshold

characteristics of a MOSFET has been briefed in the earlier sections. As discussed before

both the charges (oxide and interface) contribute to the total threshold voltage shift

(AVT) and the individual contribution to the AVT can be identified by using the charge

separation technique. Figure 5.7 shows the total voltage shift and the voltage shifts due

to oxide (AVot) and interface trapped charges (AVit) for various doses of gamma

radiation.

T ' 1 ' I ' I Virgin 1 Krad 10 Krad 100 Krad 500 Krad 1 Mrad

Gamma Dose

Figure 5.7: Contribution of oxide and interface charges to AVT of Gamma

irradiated P-channel MOSFETs

It can be observed fi^om the figure that the interface charges shift the threshold voltage

towards the positive voltage while the oxide charges causes the VT to shift towards more

negative voltage. Since the oxide charge density is large compared to interface charge

density, the voltage shift due to oxide trapped charges becomes dominating resulting in

the total negative shift in the threshold voltage. The AVj for a MOSFET irradiated with 1

Mrads of gamma rays was found to be -1.72 V for which AVot contributes -1.84 V and

AVit contributes 0.12 V. Similar results were observed for other gamma doses which are

summarized in Table 5.2. The changes in oxide charge density (ANQT) and interface

131

charge density (ANu) are calculated from AVot and AVit. Figure 5.8 shows the variation

in ANoT and ANu for various gamma doses.

1.4x10 -

T ' 1 • 1 ' 1 ' 1 • r Virgin 1 Krad 10 Krad 100 Krad 500 Krad 1 iVIrad

Gamma Dose

Figure 5.8: ANQT and ANix of P-channel MOSFETs for various Gamma doses

The ANoT and ANu of 1 Mrad gamma irradiated MOSFETs were found to be 1.33 x lO'

cm' and 8.72 x 10'° cm" . The calculated values of ANQT and ANu for various doses of

gamma rays are summarized in Table 5.2.

Table 5.2: Threshold voltage shifts and trapped charge densities of Gamma

irradiated F-channel MOSFETs

y- Dose (rads)

I K

lOK

lOOK

500K

I M

A V T ( V )

-0.03

-0.15

-0.7

-1.26

-1.72

AV„,(V)

-0.041

-0.169

-0.727

-1.333

-1.840

AVi,(V)

0.011

0.019

0.027

0.073

0.120

ANoT (cm"^)

3.00x10'°

1.23x10"

5.27x10"

9.67x10"

1.33x10"

ANiT (cm"^)

8.33 xlO^

1.44x10'"

2.00x10'°

5.36x10'°

8.72x10'°

132

5.4 Results and Discussion of 50 MeV Li- Ion Irradiation on P -MOSFET

3N163 P-channel MOSFETs were irradiated with 50 MeV Li ^ ions for three different

fluences ranging from IxlO" to IxlO'^ ions cm' . The devices were decapped and the

contact leads were grounded during irradiation. The I-V characteristics of Li ion

irradiated devices were performed to study the changes in the electrical parameters.


The ID - VGS Characteristics of 3N163 P-channel MOSFETs were carried out by keeping

the drain-source voltage (VDS) constant at -8 V. The b - VQS curves of Virgin and Li ion

irradiated (IxlO'^ 5xlO" and IxlO'^ ions cm' ) devices are as shown in Figure 5.9.

-S.Om

-Z5tn -

- 2 . 0 m -

-1.Sm -

-1.0m -

-500.0|J-

0.0 *«<Mt<H>«i«i<>«i»m»iHM •»•«<«><*<«•»

Virgin 1x10 ions cm 5x1 o" ionscm^ 1x10" ions cm''

•4 -12 I

-16 •20

Figure 5.9: ID - VGS Characteristics of Li- ion irradiated P-channel MOSFETs

It can be observed that there is a negative shift in the curves after irradiation. The drain

current for device irradiated with 1x10 Li ions cm" fails to rise above -500 ^A even

after the gate bias exceeds -20 V. This shows that the device is damaged to a greater

extent due to heavy ion irradiation. The threshold voltage of the device was found to

shift from -4.42 V (for Virgin device) to -15.02 V for device irradiated with IxlO'^ Li

ions cm' . In this case the threshold voltage was considered to be the gate voltage

133

required to raise the drain current to -100 \iA. The transconductance (gm) which varies

according to the slope of the b - VDS curves of P-channel MOSFETs were found to

decrease from 12.5 x 10" mho (Virgin) to 1.4 x 10^ mho for Li ion fluence of IxlO'^

ions cm" .



the gate-source voltage (VGS) constant at -6 V. Figure 5.10 shows the ID - VDS curves of

Virgin and Lithium irradiated (IxlO", 5xl0" and IxlO'^ ions cm' ) devices. The early

saturation of the drain current with increase in ion fluence can be clearly observed.

-40.0in

-35.0in -

-30.0m -

-25.0m -

Virgin 1x10" ions cm' 5x10" ions cm' 1x10" ions cm""

Figure 5.10: ID - VDS Characteristics of Li- ion irradiated P-channel MOSFETs

The drain saturation current was found to be -38.09 mA for Virgin device and reduces to

-1 mA for device irradiated with Lithium fluence of 1x10* ions cm' . The reduction of

drain current is in accordance with the increased threshold voltage in Li ion irradiated

devices.


Figure 5.11 shows the subthreshold characteristics of Virgin and Li- ion irradiated

3N163 P-channel MOSFETs. 134

-«-

c -10 ^

0)

t 3 O 5 -18

(A

£ 5 -"

-25

^' '*«^,

-18 — I — -16

Virgin

1x10" ions cm

5x10" ions cm

1x10" ions cm^

V„(V)

Figure 5.11: Subthreshold characteristics of Li- ion irradiated P-channel MOSFET

The decreasing slope of In ID VS VGS curves causes the increase in subthreshold swing (S)

with increase in ion fluence. The subthreshold swing is found to increase from 20.7

mV/decade (Virgin) to 80 mV/decade for a Lithium fluence of IxlO'^ ions cm' . The

experimentally obtained values of VT, gm, S and ID for unirradiated and Li-ion irradiated

devices are summarized in Table 5.3.

Table 5.3: Experimental results of Li ion irradiated P-channel MOSFETs

Li Fluence (ions cm'^)

Virgin

IxlO"

5x10"

1x10'^

V T ( V )

-4.42

-5.89

-7.56

-15.02

gm (X 10"* mho)

12.5

11.6

4.3

1.4

S (mV/decade)

20.07

28.9

35.4

80.0

ID (mA)

38.09

31.25

20.5

1.0

135


The effect of oxide and interface charges on the total threshold voltage shift of Lithium

ion irradiated MOSFET has been explored by subthreshold measurements. Figure 5.12

shows the total voltage shift (Vj) and the voltage shifts due to oxide (AVot) and interface

trapped charges (AVit) for various fluences of Li ions.

I

2.0x10 4.0X10 e.oxio' 8.0x10 1.0x10"

Lithium (ions cm")

Figure 5.12: Contribution of oxide and interface charges to AVT of Li- ion


It can be observed from Figure 5.12 that the voltage shift due to oxide trapped charges is

much greater than the shift due to interface charges and the net threshold shift is

negative. The AVj for a MOSFET irradiated with IxlO'' Li ions cm" was found to be -

10.6 V for which AVot contributes -11.596 V and AVu contributes 0.994 V. The ANQT

and ANiT for various fluences of Li ions are shown in Figure 5.13.

The ANoT and ANn of devices irradiated with IxlO'^ Li ions cm"' were found to be 2.26

X 10* cm' and 1.94 x lO''* cm" . The voltage shifts and calculated values of ANQT and

ANiT for various fluences of Li ions are summarized in Table 5.4.

136

2.5x10"

2.0x10 -

E 1.5x10"-I

t ^ 1.0x10''H

8.0x10*' H

0.0-

0.0 2.0x10" 4.0x10" 1 ' 1 ' 1

8.0x10" 8.0x10" I.OxlO"

Lithium (ions cm')

Figure 5.13: ANQT and ANIT of P-channel MOSFETs for various Li- ion fluences

Table 5.4: Tlireshold voltage shifts and trapped charge densities of Li- ion


Li Fluence (ions cm" )

1x10"

5x10"

1x10'^

A V T ( V )

-1.47

-3.14

-10.6

AV„,(V)

-1.608

-3.387

-11.596

AV«(V)

0.137

0.246

0.994

ANoT (cm'^)

3.14x10'^

6.61 xlO''*

2.26x10'^

ANiT (cm-^)

2.67x10'^

4.80x10"

1.94x10"*

5.5 Results and Discussion of 80 MeV Oxygen Ion Irradiation on P -MOSFET

3N163 P-channel MOSFETs were irradiated with 80 MeV O ^ ions for three different

fluences ranging from 5x10*" to IxlO'^ ions cm' . The I-V characteristics of the devices

were performed to understand the effects of oxygen ions on the electrical parameters of

P-channel MOSFETs.

137


The ID - Vos Characteristics of 3N163 P-channel MOSFETs were carried out by keeping

the drain-source voltage (VDS) constant at -8 V. The ID - VQS curves of Virgin and

Oxygen ion irradiated (5xlO'°, IxlO'^ and IxlO'^ ions cm"' ) devices are as shown in

Figure 5.14.

^5*

< ^ M '

_ Q

-8.0m -

"

-6.0in -

-4.0m -

-2.0m -

0 .0 -

——Virgin • 5x10'° Ions cm""

1x10" Ions cm^ 1x10"lonscm^

'" "''"' 1 • 1

-4 -a

• A

7 1

J i I *•

1 i f i

i

f- * f *

i 4

. /

' ' " - -»^-»4*^<<» 1 H » - « - » * * ^ ^

1 • 1 •

-12 -16 -2(

V„.(V)

Figure 5.14: I© - VGS Characteristics of Oxygen ion irradiated P-channel MOSFETs

It can be noted that very less shift in the threshold voltage is observed in the device

irradiated with 5xlO'° Oxygen ions cm" as compared to unirradiated device. Above this

fluence the threshold voltage raises sharply shifting the ID - VGS curve to the more

negative side. The threshold voltage of the devices was found to shift from -4.31 V

(Virgin) to -13.71V for device irradiated with IxlO'^ O ions cm' . The threshold voltage

was considered to be the gate voltage required to raise the drain current to -100 ^A. The

transconductance (gm) of Oxygen ion irradiated P-channel MOSFETs were found to

decrease from 13.15 x 10" mho (Virgin) to 8 x 10" mho for Oxygen fluence of 1x10'

ions cm" .

138



the gate-source voltage (Vos) constant at -6 V. Figure 5.15 shows the ID - VDS curves of

Virgin and Oxygen ion irradiated (5x10* , IxlO" and IxlO'^ ions cm' ) devices. Similar

to the ID - VQS curves, only a small variation is observed between the unirradiated device

and the device irradiated with Oxygen fluence 5xlO'° ions cm"' . This fluence can be

considered as the threshold fluence after which the device degrades heavily.

-40.0m

-35.0m -

-30.0m -

-26.0m -

— Virgin -•—5x10" ions cm

1x10" ions cm^ 1x10"ionscm

4 * - . - * . + - * • < * *

12

Figure 5.15: ID - VDS characteristics of Oxygen ion irradiated P-channel MOSFETs

The drain saturation current was found to be -36.1 mA for Virgin device and reduces to -

6.33 mA for device irradiated with Oxygen fluence of IxlO'^ ions cm" .

5.5.3 Subthreshold I-V characteristics

Figure 5.16 shows the subthreshold characteristics of Virgin and Oxygen ion irradiated

3N163 P-channel MOSFETs.

139

2

w & x: CO

-e-

-12-

-16

- 2 0 -

- 2 4 -

• — A » » „ . . . " " " " " ' • —

V • * ^ ^ \

% * 1\ ^ 4 *•

4| A M • f ^

M JL n

^ M % ^ ff

- ^ V I i B i n \ ^ 1 — ^ 5x1 Onions cm* \ * | .

Ixio" ions cm'' '"•V.', 1 ^

• 1x10" ions cm" ' < . ^ ^ ' ! > ? / < l . X ' ^ l :

4

1 • 1 • 1 • 1 •

-16 - 1 2 - 8 - 4 0 V„(V)

Figure 5.16: Subthreshold characteristics of Oxygen ion irradiated P-channel

MOSFET

The Subthreshold swing is found to increase from 22.5 mV/decade (Virgin) to 110.86

mV/decade for a Oxygen fluence of IxlO'^ ions cm' The experimentally obtained values

of Threshold voltage (VT), Transconductance (gm), Subthreshold swing (S) and Drain

current (ID) for unirradiated and Oxygen ion irradiated devices are summarized in Table

5.5.

Table 5.5: Experimental results of Oxygen ion irradiated P-channel MOSFETs

O- Fluence (ions cm" )

Virgin

5x10'°

1x10"

1x10'^

V T ( V )

-4.31

-4.45

-9.64

-13.71

gm (X 10"* mho)

13.15

12.5

9.52

8.0

S (mV/decade)

22.5

23

40.02

110.86

ID (mA)

-36.1

-34.8

-19.7

-6.33

140


Figure 5.17 shows the total voltage shift (VT) and the voltage shifts due to oxide (AVot)

and interface trapped charges (AVit) for various fluences of Oxygen ions.

zoxio 4.0x10 6.0x10 8.0x10 1.0x10

Oxygen (ions cm")

Figure 5.17: Contribution of oxide and interface charges to AVj of Oxygen ion


It can be observed from Figure 5.17 that the net threshold shift is negative due to the

higher density of positive oxide trapped charges. The A VT for a MOSFET irradiated with

IxlO'^ O- ions cm' was found to be -9.4 V for which AVot contributes -10.884 V and

AVit contributes 1.483 V. The changes in oxide charge density (ANQT) and interface

charge density (ANIT) for various fluences of Oxygen ions are as shown in Figure 5.18.

The ANoT and ANn of devices irradiated with IxlO'^ Oxygen ions cm" were found to be

2.12 X 10'^ cm' and 2.89 x 10 '* cm^ The total threshold voltage shift, voltage shift due

to oxide and interface charges and calculated values of ANQT and ANIT for various

fluences of Oxygen ions are summarized in Table 5.6.

141

2.5x10

T • r OxIO" 1.0x10"

Oxygen (ions cm')

Figure 5.18: ANQT and ANIT of P-channel MOSFETs for various Oxygen ion

fluences

Table 5.6: Threshold voltage shifts and trapped charge densities of Oxygen ion


O- Fluence (ions cm" )

5x10'°

1x10''

IxlO'^

A V T ( V )

-0.14

-5.33

-9.4

AV„.(V)

-0.149

-5.624

-10.884

AVi,(V)

0.008

0.294

1.483

ANoT (cm"^)

2.91 xlO'^

1.09x10'^

2.12x10'^

ANiT (cm" )

1.62x10'^

5.74x10'^

2.89x10"'

5.6 Results and Discussion of 120 MeV Si - Ion Irradiation on P-MOSFET

3N163 P-channel MOSFETs were irradiated with 120 MeV Si " ions for three different

Si- ion fluences viz. 5xlO'° ions cm' , IxlO" ions cm" and 5xlO" ions cm' . The

changes in the electrical parameters like threshold voltage drain saturation current and

142

subthreshold swing for various ion fluences were studied by performing I-V

measurements.


The ID - VGS Characteristics of 3N163 P-channel MOSFETs were carried out by keeping

the drain-source voltage (VDS) constant at -8 V. The ID - VQS curves of Virgin and Si ion

irradiated (5x10"^, IxlO" and 5xlO" ions cm") devices are as shown in Figure 5.19.

-10.0m

•8.0m -

-6.0m -

— -4.0m -

-2.0m-

0.0-

Virgln 5x10'° ions cm^

1x10" Ions cm^

5x10" ions cm^

-r~ -ie -20

Figure 5.19: ID - VGS Characteristics of Si- ion irradiated P-channel MOSFETs

The threshold voltage of the devices was found to shift from -4.42 V (for Virgin device)

to -13.63 V for device irradiated with 5xlO" Si ions cm'^. It can be observed from the

Figure 5.19 that the threshold voltage shifts more at lower fluences than at higher

fluences. The gm of P-channel MOSFETs was found to decrease from 12.04 x 10" mho

(Virgin) to 9 x 10" mho for Si fluence of 5xl0" ions cm'^.


The losat is extracted from ID - VDS Characteristics by keeping the gate-source voltage

(VGS) constant at -6 V. Figure 5.10 shows the ID - VDS curves of Virgin and Si- irradiated

(5x10"^, IxlO^' and 5x10*' ions cm" ) devices.

143

-40.0m

-35.0m -

-30.0m -

-25.0m -

-•— Virgin • 5x10" Ions cm''

Ix lO" ions cm 5x10" ions cm^

Vo.(V)

Figure 5.20: ID - VDS Characteristics of Si- ion irradiated P-channel MOSFETs

The drain saturation current was found to be -35.9 mA for Virgin device and reduces to -

8.99 mA for device irradiated with Silicon fluence of 5xlO" ions cm' . The drain current

is found to reduce sharply for lower fluences as compared to higher fluences.


Figure 5.21 shows the subthreshold characteristics of Virgin and Si- ion irradiated 3N163

P-channel MOSFETs.

3 c £ -10-

u

1 i CO

- 1 6 -

-20

-25

-*— Viryin • 5x10" ions cm" ^ 1x10^' ions cm'

-20 -T— -18

— I — -12

. S . - i - ^ . - ^ - r l -I*: W ' ^v-;

V„(V)

Figure 5.21: Subthreshold Characteristics of Si- ion irradiated P-channel MOSFET 144

The Subthreshold swing (S) is found to increase from 20.7 mV/decade (Virgin) to 62.56

mV/decade for a Silicon fluence of 5xl0" ions cm"' The experimentally obtained values

of VT, gm, S and ID for unirradiated and Silicon ion irradiated devices are summarized in

Table 5.7.

Table 5.7: Experimental results of Si- ion irradiated P-channel MOSFETs

Si Fluence (ions cm'^)

Virgin

5x10'°

1x10"

5x10'"

V T ( V )

-4.42

-8.72

-12.90

-13.63

g„ (X 10- mho)

12.04

11.76

9.25

9.00

S (mV/decade)

20.7

43.24

57.96

62.56

ID (mA)

-35.9

-25.77

-11.05

-8.99


Figure 5.22 shows the total voltage shift (VT) and the voltage shifts due to oxide (AVot)

and interface trapped charges (AV,t) for various fluences of Silicon ions.

(0

}

-10-

1x10' 2x10 3x10" 4x10' 5x10

Silicon (ions cm')

Figure 5.22: Contribution of oxide and interface charges to AVT of Si- ion


145

It can be noticed from Figure 5.22 that the voltage shift due to oxide trapped charges is

much greater than the shift due to interface charges and the net threshold shift is

negative. The AVT for a MOSFET irradiated with 5xlO" Si ions cm' was found to be -

9.21 V for which AVot contributes -9.913 V and AV,t contributes 0.702V. The changes in

oxide charge density (ANQT) and interface charge density (ANn) for various fluences of

Si ions are shown in Figure 5.23.

2.0x10 -

1x10 2x10' 3x10 4x10 SxlO'

Silicon (ions cm')

Figure 5.23: ANQT and ANIT of P-channel MOSFETs for various Si- ion fluences

Table 5.8: Threshold voltage shifts and trapped charge densities of Si- ion


Si Fluence (ions cm'^)

5x10'°

1x10"

5x10"

A V T ( V )

-4.30

-8.48

-9.21

AV„t(V)

-4.678

-9.105

-9.913

AVi,(V)

0.377

0.624

0.702

ANoT (cm'^)

9.13x10'"

1.77x10'^

1.93x10'^

ANirCcm-^)

7.38x10'^

1.22x10"^

1.37x10'"

The ANoT and ANn of devices irradiated with 5xlO" Si ions cm'' were found to be 1.93

X lO' cm' and 1.37 x lO'" cm' . It can be observed from Figure 5.23 that build up of

146

oxide and interface charges tends to saturate at Silicon fluence of IxlO" ions cm" . The

calculated values of ANQT and ANu for various fluences of Silicon ions are summarized

in Table 5.8.

5.7 Conclusion

P-channel enhancement mode MOSFETs were irradiated with ' Co gamma rays and

various species of heavy ions. Since Compton scattering dominates in case of gamma

rays and Coulomb scattering dominates with heavy ions, the impact of both the

radiations on the electrical characteristics of the MOSFET is different. The performance

degradation seems to be more in heavy ion irradiated devices compared to gamma

irradiated devices. The devices were experimented for several species of heavy ions viz.

50 MeV Li ions, 80 MeV Oxygen ions and 120 MeV Si ions for various fluencies. The

changes observed in the figures of merit of MOSFETs for various heavy ions at a

particular fluence of 1x10" ions cm' have been summarized in Table 5.9.

Table 5.9: The effects of Li, Oxygen and Si ions on MOSFET performance at ion

fluence of 1x10" cm'

Parameters / ions

V T ( V )

gm (X 10"* mho)

S (mV/decade)

ID (mA)

AVT (V)

AVo, (V)

AVi,(V)

ANoT (cm"')

ANiT (cm" )

50 MeV Li

-5.89

11.6

28.9

31.25

-1.47

-1.608

0.137

3.14x10'^

2.67x10'-^

80 MeV Oxygen

-9.64

9.52

40.02

-19.7

-5.33

-5.624

0.294

1.09x10'^

5.74x10'^

120 MeV Si

-12.90

9.25

57.96

-11.05

-8.48

-9.105

0.624

1.77x10"

1.22x10"

From the above table, it can be noticed that the threshold voltage shift, subthreshold

swing, oxide trap charge density and interface trap charge density are highest in 120

147

MeV Si ion irradiated devices and least in 50 MeV Li ion irradiated devices. The oxide

trapped charges are found to contribute much to the net threshold voltage shift and the

contribution of interface trapped charges was very minute. Similarly transconductance

and drain saturation current is found to be the least for 120 MeV Si ion irradiated devices

and highest for 50 MeV Li ion irradiated devices. The figures of merit for 80 MeV

Oxygen ion irradiated devices stands in between. This clearly indicates that the

irradiation induced device degradation is more for 120 MeV Si ions than 50 MeV Li

ions.

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148

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