Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 2496–2500
Part 1, No. 4B, April 1999c
°1999
Publication Board, Japanese Journal of Applied Physics

Measurement of Energetic and Lateral Distribution of Interface State Density in Fully-Depleted
Silicon on Insulator Metal-Oxide-Semiconductor Field-Effect Transistors
Tran Ngoc D UYET1, ∗ , Hiroki I SHIKURO1 , Yi S HI1 , Makoto TAKAMIYA1 , Takuya S ARAYA1 and Toshiro H IRAMOTO1,2
1 Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8558, Japan
2 VLSI Design and Education Center, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan(Received October 1, 1998; accepted for publication December 4, 1998)

A new modification of the charge pumping (CP) technique based on the effect of reverse pulse bias on suppression of thegeometric component is proposed in order to accurately determine the energetic and lateral distribution of interface state density(Dit ) in fully-depleted silicon on insulator metal-oxide-semiconductor field-effect transistors (FD SOI MOSFETs). A comparison of the conventional CP techniques with the proposed method is also presented. It is demonstrated that using the proposed
method, the precise estimation of the energetic and lateral distribution of Dit can be simply obtained without interference fromthe geometry-dependent effect which often leads to the great difficulties in data interpretation in the conventional CP methods.
KEYWORDS: FD SOI MOSFET, charge pumping, geometric component, rise and fall time method, DC reverse bias, pulse
reverse bias

1. Introduction
The interface between the silicon substrate and the gate oxide in the active region of a metal-oxide-semiconductor fieldeffect transistor (MOSFET) plays a crucial role in determining the device performances and affects the reliability andlifetime of the device. The measurement and characterization of the interface states are needed to understand the origin
and physical properties of the interface states in a MOS system. Interface states can capture and emit charge carriers andthe amount of charges in the interface states determines thedevice parameters.
The charge pumping (CP) technique1–3) has evolved intoa sensitive and reliable method to study the interface characteristics in a MOSFET. The major advantage of the CP
technique is that it is available for direct measurement of theinterface characteristics not only in a bulk MOSFET but also
for characterizing both the front and back interface properties
in a SOI MOSFET.4, 5)
The experimental setup for charge pumping measurements
in a SOI MOSFET is shown in Fig. 1. The gate is connected
to a pulse generator. The source and drain are connected together while the body is grounded through a DC ammeter.
The principle of conventional CP is described in detail inrefs. 2 and 3. When a gate pulse of sufficient amplitude to
invert the surface is on, electrons flow to the surface from thesource and drain. The surface states capture some of the inversion electrons and become negatively charged. When thepulse is off, the inversion layer electrons immediately flow
back to the source and drain, while the captured ones are stillretained by the interface states and recombine with the incoming holes from the body. Similarly, when the gate surface
is pulsed from accumulation to inversion, the trapped holes
recombine with electrons from the source and drain. These
recombinations give rise to a DC current Icp . By measuring
this current, the interface state information can be obtained.
A major problem of the CP measurement on MOSFETs is
the so-called “geometric component” of the CP current,6, 7)
which occurs if all mobile carriers of one type, electrons (inNMOSFET) during the fall and holes during the rise of thegate pulse, are not removed rapidly enough before the arrival∗ E-mail address: duyet@nano.iis.u-tokyo.ac.jp

of the other type of carrier. The carriers that are left behind
will recombine with carriers of the other type and therefore
an additional component of CP current that does not involvethe interface states arises. This parasitic component gives riseto an overestimation of interface state density (Dit ). The geometric component is not often observed in bulk or partiallydepleted (PD) SOI MOS transistors.
However, in FD SOI MOSFETs, particularly in very thinfilm SOI structures, where the resistivity of the body regionis generally high, the CP current tends to have a very large
geometry-dependent component6) resulting in an overestimation of Dit . A detailed discussion of the geometric effect in
FD SOI MOSFETs can be read in ref. 7. When the rise andfall times are relatively long, i.e. the measurement frequency
is low, this undesirable component is negligible but the sensitivity is severely reduced due to a low signal/noise ratio of Icp
and thus the use of the CP technique is limited to only largearea devices. Therefore, the new method to suppress thiscomponent and enhance the sensitivity is strongly requiredas the devices are scaled down. We have already developed a
new measurement technique using the reverse pulse (RP) bias
to suppress the geometric component in FD SOI MOSFET.7)
In the present work, we extend this method and apply it for
the accurate measurements of energetic and lateral distributions of Dit in FD SOI MOSFET by the RP charge pumping
method.

Back Gate
Pulse
Generator

Source
n+

e-

h+

Gate

e-

Drain
n+

Body

A

Buried Oxide

Fig. 1. Experimental setup for conventional charge pumping measurement
on SOI MOSFETs.
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T. N. D UYET et al.

Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 4B

2. Device
The devices used in this study are fully-depleted SOINMOSFETs as shown in Fig. 2. The thicknesses of the gate
oxide, Si, and buried oxide films are 5 nm, 100 nm, and 98 nm,
and the effective gate length and width are 2 µm and 20 µm,
respectively. For source/drain and the main part of gate poly
regions, phosphorus ions were implanted, while for the bodycontact, boron ions were used. It should be noted that boron
ions were also implanted in the gate poly region near the bodycontact. More information on the fabricated device parameters is given in ref. 7.

trons contribute to the CP recombination. Some of the trappedelectrons located near the conduction band will be emitted
during the fall time before the gate surface becomes accumuPulse voltage
applied to the gate

V
V gh

t
Vp

V gl

Accumulation

Pulse voltage
applied to the body(a)3. Geometric Component and Reverse Pulse Method
Figures 3(a) and 3(b) show the waveform used in the RP
method and the typical CP current characteristics obtained in
the variable amplitude mode with different rise and fall times(tr , tf ).7) The observed two stepwise increases of Icp are theresults of the two different n+ and p+ poly gate regions.7)
When tr and tf are 1 µs, no geometric component is observed
and the Icp at the first step corresponds accurately to Dit . As
tr decreases to 0.1 µs, the first step increases significantly due
to the additional geometric component resulting from the inefficiency of hole collection by the body contact. As shown in
Fig. 3 (b), by applying the RP bias (Vp ) equal to −1.6 V to thebody contact only during the rise time, the geometric component by holes is completely suppressed even if tr is 0.1 µs.
The result at a very low Vp (−2 V) is nearly the same asthat of Vp = −1.6 V, showing that the RP method also avoids
the reduction of the effective channel length which is undesirably observed in the DC body bias technique.6) The geometric component due to electrons, which is not observed in thissample, can also be suppressed by the RP bias to the sourceand drain only during the fall time.

2497

40

f = 100KHz, tw = 5µs

Vp = 0V, tr,f = 1µs

30

Not having GeometricComponent

V gl =- 2.5V, Vback =- 4V

tr = 0.1µs, tf = 1µs

Including GeometricComponent

V p = 0VV p = -1.6V (RP Method)V p = -2V (RP Method)

20

Without GeometricComponent

10

0
-2

-1

0

1

2

Vgh [V](b)
Fig. 3. (a) Illustration of pulse voltages applied to the gate and the body in
the RP technique. (b) Charge pumping current as a function of the top levelof the gate pulse under various rise and fall times in a FD SOI MOSFET.
Results at a long tr,f (1 µs) do not have any geometric component.

4. Energetic Distribution of Interface State Density

Vg

The energy range of interface states contributing to Icp isdetermined by emission processes occurring in depletion andweak inversion, assuming all traps are filled with majority
carriers during accumulation and with minority ones during
inversion.2) As shown in Fig. 4(b), when the gate surface ischanged from inversion to accumulation, not all trapped elec-t em,hInversion

tem,e

V th

t
tr

tf

V fb

Accumulation(a)Body(p+)p+-polyn+-poly

Lg

During tr

During tf
Emission
Eem,eEc

Interfacerecombination
Ev

Eem,h

Ec
Interfacerecombination
Emission

W

Source(n+)

Drain(n+)

GateFig. 2. Top view of the fabricated SOI devices.

(b)

Ev

(c)

Fig. 4. (a) Waveform of applied gate pulse used in CP measurements. tem,hand tem,e determined by tr and tf are the times for trapped holes and electrons to be emitted when the gate surface is pulsed from accumulation into
inversion and from inversion into accumulation, respectively. (b), (c) Illustrations of emission processes of the trapped electrons and holes during tf
and tr . Only traps having an energy between E em,h ∼ E em,e contribute to
the Icp .2, 8)

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T. N. D UYET et al.

Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 4B

lated. The fall time of the gate pulse determines the electron
emission level (E em,e ) and only the trapped electrons below it
contribute to Icp . Similarly, when the gate surface is pulsed
from accumulation to inversion, there is a hole emission level(E em,h ) and only trapped holes located above it contribute to
Icp as shown in Fig. 4(c). When tr and tf of the gate pulse andconsequently the emission times of the trapped holes (tem,h )and the trapped electrons (tem,h ) are long, the emission levels are shifted to the midst of the band gap. On the contrary,when tr and tf and correspondingly the times for these holes
and electrons to be emitted are short, these levels are locatednear the band edges. The principle of the conventional tr,fmethod8) is that by changing tr and tf of the gate pulse as illustrated in Fig. 4(a), the energy range that is attributed to theCP will change and from the derivative of the Icp with respect
to these times, the energy distribution of Dit can be calculated.
Figure 5 shows the tr and tf dependences of Icp in a FD
SOI MOSFET. When tf is 1 µs and tr is varied, Icp increases
rapidly at tr = 0.1 µs due to the geometric component, indicating that an accurate energetic distribution cannot be obtained. The geometric component due to the electrons does
not occur in this device because the channel length (2 µm inthis case) is short enough for the inversion electrons to flow
back to the source and drain during the fall time of the gate
pulse. Therefore, when tr is 1 µs and tf is varied, Icp just increases gradually because of an increase in an energy range
which can be scanned by the gate pulse with different tf . The
result using the RP method is also demonstrated in this figure.
The reverse pulse bias is applied to the body contact in order to improve the hole response and subsequently effectively
suppress the geometric effect. The parasitic geometric component is negligible and no rapid increase in Icp is observed
even at tr = 0.1 µs.
Dit obtained from the derivative of Icp with respect to the tr
and tf can be written as follows9)
d Icp
tr
·
Dit (E em,h ) = −(1)
q Ag kT f dtr
d Icp
tf
·
Dit (E em,e ) = −q Ag kT f dtf(2)

where q is the electron charge, Ag is the channel area of thetransistor and f is the frequency of the gate pulse.
The following expressions of E em,h and E em,e can be derived according to the theory of emission of carriers from thesurface states.2)¯¯ ¶µ¯ Vth − Vbf ¯¯ tr
E em,h = E i + kT ln vth σ n i ¯¯
(3)
1Vg ¯
¯
¯ ¶
µ
¯ Vth − Vbf ¯
¯
¯ tf
(4)
E em,e = E i − kT ln vth σ n i ¯
1Vg ¯
where E i and n i are the intrinsic Fermi energy and concentration, vth is the thermal velocity of the carriers, Vth and Vbf are
the threshold and flat band voltages of a device.
In the RP method, the reverse bias applied to the body contact leads to an increase in Vth and a decrease in Vbf . As aresult, the time for the trapped holes to be emitted becomes
longer compared with that of the conventional method. It
should be noted that a change in Vth and Vbf by the reverse
bias can be simply estimated using the conventional variablebase CP mode.2)
Using eqs. (1)–(4), the energetic distribution of Dit withand without the RP body bias can be evaluated as shown in
Fig. 6. When the RP method is not used, the extracted Dit
increases rapidly near the valence band edge. However, byapplying to the RP bias Vp = −1.6 V, an accurate Dit can beevaluated. As explained above, due to the reverse bias at therise time, the hole emission time becomes longer, therefore,
the measurable energy range is shifted to the midst of the energy gap in the RP method.
As shown in Figs. 5 and 6, when tr larger than 0.3 µs isused, the influence of the geometric effect is avoidable for thedevices used in this study. However, the long rise and falltimes of the gate pulse cannot be freely chosen in the highfrequency measurements that are essential for scaled devices.
Therefore, the proposed RP method will become more important as the device size shrinks.
5. Lateral Distribution of Interface State Density
The lateral profile of Dit can be obtained by applying theDC reverse bias9, 10) to the source and drain or the body contact. The principle of this DC technique is to use charge
pumping to detect the integrated amount of interface traps

20
f = 100KHz, tw = 5µs1014

V gl = - 2.5V, Vback = - 4V15

= -1.6V (RP Method)

Influence ofGeometric Effect

f = 100KHz, tw = 5µs
V gl = -2.5V, Vback= - 4V
tf : Varied
tr : Varied (Conv. Method)
tr : Varied, Vp = -1.6V

1013

10

(RP Method)

1012

5

t r = 1µs, tf : Varied
t r: Varied, tf = 1µs
t r: Varied, tf = 1µs, Vp

00.1

tr, tf [µs]Influence ofGeometric Effect1011

1

Fig. 5. Rise and fall time dependence of Icp in a FD SOI MOSFET. Accurate rise time dependence of Icp is obtained when applying a reverse pulse
bias of Vp = −1.6 V to the body.

-0.36 -0.30

-0.24

0.24

0.30

0.36

E-Ei [eV]Fig. 6. Energetic distribution of interface state density with and without
the body reverse pulse bias. An overestimation of Dit in the conventional
method is resolved by applying the reverse pulse bias to the body.

T. N. D UYET et al.

Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 4B

Gate

Gate
Pulse
Generator
Source

XS

XD

2499

Pulse
Generator

Drain

BOX

Source

A

Drain

p

BOX

A

Depletion

V back

4x1012

Depletion

Vbody

(a)

V backV gl = -2.5V, Vgh = 0.8V, Vback =- 4V

Body

DC Method, tr,f = 1µs

3x1012DC Method, tr,f = 0.1µs

2x1012

Influence ofgeometric component(Short tr and tf)

1x1012(Long tr and tf)

(a)
V ,V
g body
V
gh

0

0
0.0
Source

0.1

0.2

1.8

X [µm]
(b)

1.9

Vp

while varying the size of the region from which the interface
traps can contribute, by modulating the source and drain depletion widths.
In our experiment, the reverse bias Vb is applied to the bodycontact to control the depletion widths as illustrated in Fig.
7(a). In general, the CP current is given by11)
Z XD
Icp = f · q · W
q19s dx
(5)
XS

s

¤
2²si ²0 £× (Vb + 28F )1/2 − (28F )1/2q NAs¤2²si ²0 £× (Vb + 28F )1/2 − (28F )1/2
X D = L eff −
q NA
XS =

t
Vbase_p2.0
Drain

Fig. 7. (a) Experimental setup for determining lateral distribution of Dit in
the DC reverse bias method. The DC reverse bias is applied to the bodycontact to vary the depletion layer widths near the source and drain. (b)Interface state density as a function of distance between the source anddrain. The DC reverse technique is inapplicable when tr and tf are short.

where

Pulse
Generator

(6)

V
gl

(b)
Fig. 8. (a) Experimental setup for determining lateral distribution of Dit in
the proposed RP method. Instead of the DC bias, the reverse pulse bias is
applied to the body contact. (b) Waveform of the pulse biases applied to
the gate and the body in the RP technique. Vp is fixed to suppress the geometric component while Vbase_p is varied in order to control the depletion
layer width.

4x1012DC Method tr,f = 0.1µs

3x1012RP Method tr,f = 0.1µs

2x1012

Influence ofgeometric component(Conventional)(Short tr and tf)

19s is the change of surface potential, f is the frequency of
the gate pulses, 8F is the built-in potential, L eff and W are theeffective length and width of a device.
With a floating source contact and a different body bias, thesource end of the depletion region X S in the channel is fixed,
but the drain depletion edge increases. After measuring the
Icp given by eq. (5), the lateral distribution Dit near the drain
end can be obtained by straightforward calculation as follows
µ
¶ µ
¶
d Icp
d XD
1
×
·
(8)
Dit (X D ) =f qW 19s (X D )
d Vb
d Vb
The distribution of Dit near the source end can be similarly
obtained by floating the drain contact and varying the bodybias.
The experimental results are shown in Fig. 7(b). When tr,fSuppression ofGeometric Component(RP Method)

1x10120(7)

V gl = -2.5V, Vgh = 0.8V, Vback =- 4V

0.0
Source

0.1

0.2

1.8

X [µm]

1.9

2.0
Drain

Fig. 9. Comparison between the measured lateral distributions of Dit by
the DC and the proposed RP methods for a very short tr,f (0.1 µs). An
accurate result of Dit is obtained by the proposed RP technique while theoverestimation of Dit is evaluated in the conventional DC method.

(= 1 µs) is long enough for holes to flow back to the bodyduring the rise of the gate pulse, a precise value for the lateral
distribution can be estimated. However, in the short tr andsmall DC body bias, the geometric component leads to an
overestimation of Dit .
To overcome this difficulty, instead of using the DC bias, areverse pulse bias is applied to the body contact as illustrated
in Fig. 8(a). Figure 8(b) shows the new RP waveform used in
the proposed method, where the top level (Vp ) of the RP bodybias is kept constant during the rise time to suppress the ge-

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Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 4B

ometric effect and its base level (Vbase_p ) is changed in orderto vary the depletion widths. This waveform expands the depletion regions while suppressing the geometric component.
Therefore, the lateral Dit distribution can be accurately determined.
Figure 9 shows the lateral profiles of Dit obtained by the
DC reverse bias and the present RP bias methods for tr andtf being as short as 0.1 µs. In the DC method the extracted
Dit is dramatically overestimated due to the geometric component. However, in the proposed RP method, the geometriccomponent is not involved even when tr and tf are 0.1 µs. Thismethod is sensitive with a high frequency and can be applied
to the scaled FD SOI MOSFETs.
6. Conclusions
A new method for determining energetic and lateral distributions of the interface state densities is proposed. The main
advantage of this method is that it allows us to accurately
measure the interface state properties in FD SOI MOSFETs
where the conventional CP method is inapplicable due to theparasitic geometric component. The proposed method can be

T. N. D UYET et al.

expected to be more powerful in scaled FD SOI devices, inwhich a high frequency CP pulse, and consequently a small
rise and fall time must be used to improve the signal/noise
ratio.

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