Post on 21-Jun-2018
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.
5.4.1 ID - VGS Characteristics
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" .
5.4.2 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 5.10 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.
5.4.3 Subthreshold I-V Characteristics
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
5.4.4 Oxide and Interface Trapped Charge Density
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
irradiated P-channel MOSFETs
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
irradiated P-channel MOSFETs
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
5.5.1 ID - VGS Characteristics
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
5.5.2 ID - VDS Characteristics
The drain saturation current (losat) is extracted from ID - VDS Characteristics by keeping
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
5.5.4 Oxide and Interface Trapped Charge Density
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
irradiated P-channel MOSFETs
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
irradiated P-channel MOSFETs
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.
5.6.1 ID - VGS Characteristics
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'^.
5.6.2 ID - VDS Characteristics
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.
5.6.3 Subthreshold I-V Characteristics
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
5.6.4 Oxide and Interface Trapped Charge Density
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
irradiated P-channel MOSFETs
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
irradiated P-channel MOSFETs
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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149