World Academy of Science, Engineering and Technology 39 2008
Introducing a High Sensitive Hall Effect Sensor
Reza Hosseini, and Alireza Kashaninia
Abstract—The sensitivity of conventional Hall Effect sensors is
strongly limited by the well-known short-circuit effect. Many
researches were directed to reduce offset and noise, but few works
were carried out to improve the sensitivity of these sensors. Here, a
new shape of integrated horizontal Hall Effect device is presented.
This particular shape has been developed to minimize the shortcircuit effects via reduction of “length to width ratio” of the sensor.
This results in reduction of sensor’s average resistance. A resolution
(minimum detectable magnetic field) of 590 p T has been reached
with a 100nm× 35nm ×10nm sensor biased with a current of 500 µA.
RH
I .B z |
t
r
RH = n
qnc
(1)
| VH |=|
(2)
where RH denotes the Hall coefficient, t is the thickness of the
plate, q is the magnitude of the electron charge, nc shows the
carrier density, and rn stands for the scattering factor of
material.
Keywords—Hall Effect, High sensitivity, Multiple Strips, Shortcircuit Effect.
T
I. INTRODUCTION
HE sensitivity of conventional hall effect devices is
strongly limited by the well known short-circuit effects
[1]. As well, in order to improve the signal to noise ratio of
magnetic field measurement systems, many researches were
performed to reduce the offset and the low frequency noise
level.
In this paper, the effective methods are investigated to
increase the sensitivity of devices and meanwhile, present
some new shapes of horizontal Hall Effect devices. In order to
show the merit of the presented devices, the sensitivity of
conventional rectangular device will be compared with 3, 4, 5
strips hall plates.
These new shapes allows to minimize the short-circuit
effects due to the biasing contacts and consequently allows to
increase the sensitivity of the devices.
Fig. 1 Conventional Hall Effect Device
B. Rectangular Hall Device
In practical rectangular Hall effect devices (Fig. 1), the
biasing (B1 and B2) and the sensing contacts are responsible
for the short-circuit effects which reduce the Hall voltage by a
geometry dependent correction factor. Under low magnetic
induction, the geometrical correction factor of a long
rectangular Hall plate (L/W >1.5 ) with small sensing contacts
(S/W <0.18 ) can be approximated by the following formula
[3]:
II. HALL EFFECT DEVICES
A. Hall Effect
Fig. 1 presents a conventional rectangular Hall Effect
device. In a thin and infinitely long device with lateral sensing
contacts, the Hall voltage VH appearing between these sensing
contacts (S1 and S2 on Fig. 1) is proportional to the biasing
current (I) and the magnetic field component perpendicular to
the plate (BZ):
G rect ≅ [1 − exp(−
π L
2W
)].[1 −
2 S
]
πW
(3)
where L and W denote the length and the width of the device,
and S is the sensing contacts size (Fig. 1). The first term of
Eq.3 represents the Hall voltage reduction due to the large
current supplying contacts B1 and B2 while the last one
considers the current lines deflection near the sensing contacts
S1 and S2.
To keep Grect close to unity, L has to be greater than W.
Consequently, the resistance Rrect between the biasing contacts
of conventional rectangular Hall devices is generally large [3]:
This work was supported in part by the Department of Electrical
Engineering, University of Tehran.
Reza Hosseini is with the Islamic Azad University, Science & Research
Branch,Tehran, Iran (e-mail:hosseini.elec@ gmail.com).
Alireza Kashaninia is with the Islamic Azad University, central Tehran
Branch, Tehran, Iran (e-mail: kashaninia@ iauctb.ac.ir).
Rrect = R⊥
431
L
W
(4)
World Academy of Science, Engineering and Technology 39 2008
R⊥ =
1
qμnc t
III. Bias Current
Increasing bias current can improve sensitivity of device. It
can be shown from Eq. 6 that if Hall current increase,
minimum detectable field will be decreased.
(5)
R⊥ is the square resistance of the device material and µ is the
mobility.
III. NUMERICAL SIMULATIONS
C. Increasing Sensitivity
The minimum detectable field of a Hall sensor can be
defined as [6]:
Bmin =
Vnoise
RH I H
A. Conventional Rectangular Hall Device
To verify the effect of charge carrier mobility in device
sensitivity, it has been compared the simulated behavior of a
rectangular plate with materials of different mobilities. These
simulations were carried out with ISE-TCAD. First, the plate
of 100nm× 35nm ×10nm with Nd=6×1016 and bias current 500
μA was simulated with Si as its constituent material. The
resultant sensitivity was 39µT (Fig. 3), For GaAs from the
other hand, it was achieved as 11μT (Fig. 4) and for InAs as
650nT (Fig. 5) (The estimated sensitivities are corresponded
to the lowest break points in the relative characteristic curves).
Electron mobility in InAs is greater than GaAs and in GaAs is
greater than Si. So, it can be seen that high mobility materials
increase Hall plate sensitivity.
(6)
The main noise component is due to Johnson noise
Vnoise = (4k B TR s Δf )1 / 2 , where kB is the Boltzmann constant, Rs
is the series resistance, Δf is the measurement bandwidth, and
T is the absolute temperature.
The sensitivity of Hall Devices can be increased via
following ways:
I. Multiple Strips Hall Device
To overcome this sensitivity limitation, one can reduce the
input resistance while keeping the geometrical correction
factor close to one. To implement this idea, a new shape of
horizontal Hall Effect device has been developed. This
particular shape is based on a principle first proposed by
Popovic for magnetic field effect transistors (MagFETs). It
consists of supplying the current into a short Hall Effect active
zone with multiple “distributed current sources” (Fig. 2).
Since the output impedances of these sources are very high,
the device behaves as if it is infinitely long while it is kept
actually short.
It was proved that in n strips Hall Effect device the
resistance of device is n times smaller than conventional
rectangular device ( R = Rrect ) [3].
rnew
n
Fig. 3 Relative sensitivity of Si plate
Fig. 2 Biasing principle using distributed current source
II. Mobility
Using the materials with high charge carrier mobility in
rectangular hall plate will increase the sensitivity. If the
mobility is increased, resistance will be decreased (Eq. 5),
which results in decreasing in Vnoise (Eq. 6) and hence,
sensitivity of the device will be improved. Initial Hall plate
used Si with the mobility of about 1000cm2/v.s.The materials
are recently used, like InAs and InSb, have mobility of 22000
and 40000cm2/v.s respectively.
Fig. 4 Relative sensitivity of GaAs plate
B. Multiple Strips
In order to verify the effect of multi strips and distributed
current source in increasing the device sensitivity, the four and
five strips 100nm× 35nm ×10nm hall devices were simulated. In
4 strips hall plate, it was used four InAs strips with 3 GaAs
strips between them. These GaAs strips reduce short circuit
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World Academy of Science, Engineering and Technology 39 2008
effects. In this device, minimum detectable magnetic field
reaches down to 1.3nT which is smaller than that of
conventional hall plates (Fig. 6).
For 5 strips Hall sensor (Fig. 7) sensitivity is strongly
increased. This hall sensor can detect the magnetic field of
590 pT (Fig. 8).this model used 5 InAs strips with Nd=6×1016
and 4 GaAs strips with Nd=8×1017 .
Fig. 8 Relative sensitivity of 5 strips hall effect sensor
IV. CONCLUSION
A new shape of integrated horizontal Hall Effect devices
has been proposed. The resulting multiple strips device
exhibits a lower input resistance than conventional horizontal
Hall Effect devices, while keeping a geometrical correction
factor near to unity. Since the input resistance directly limits
the intensity of the biasing current, this low resistive device
presents a higher maximum sensitivity than conventional
rectangular devices. Thus, by increasing the number of strips
as much as needed, it is possible to enhance the sensitivity as
much as desired. This helps to improve the resolution of the
sensory system. With a biasing current of 500μA, the sensor
based on a 100nm× 35nm ×10nm five strips Hall Effect device
exhibits a sensitivity of 590 p T.
Fig. 5 Relative sensitivity of InAs plate
ACKNOWLEDGMENT
The authors would like to thank Dr. Morteza Fathipoor and
his students in CAD design center at the department of
Electrical Engineering, University of Tehran, for their
constructive collaborations.
Fig. 6 Relative sensitivity of 4 strips hall effect sensor
REFERENCES
[1]
[2]
[3]
[4]
Fig. 7 Five strips hall plate (black strips are InAs, bright ones are
GaAs and reds are bias contacts)
[5]
[6]
[7]
[8]
433
H. Baltes and R. Popovic “Integrated semiconductor magnetic field
sensors”Proc. IEEE, vol. 74, pp. 1107–1132, Aug. 1986.
P. C. de Jong, F. R. Riedijk, J. van der Meer “Smart Silicon Sensors –
Examples of Hall-effect Sensors”, 2002.
J. Kammerer, L. Hébrard,, V. Frick, P. Poure, and F. Braun, “Horizontal
Hall Effect Sensor With High Maximum Absolute Sensitivity” IEEE
Sensors Journal, Vol. 3, No. 6, December 2003.
A. Sandhu, A. Okamoto, I. Shibasaki , A. Oral , “Nano and micro Halleffect sensors for room-temperature scanning hall probe microscopy”
Microelectronic Engineering 73–74, 2004.
S. Takahashi, S.Maekawa, “Nonlocal spinHall effect and spin-orbit
interaction in nonmagneticmetals” Journal of Magnetism and Magnetic
Materials,2006.
A.Sandhu and H. Handa “Practical Hall Sensors for Biomedical
Instrumentation” IEEE Transaction On Magnetics, Vol.41, No.10,
October 2005.
A. Lapicki, H. Sanbonsugi, T. Yamamura, N. Matsushita, M. Abe, H.
Narimatsu, H. Handa, and A. Sandhu, “Functionalization of Micro-Hall
Effect Sensors for Biomedical Applications Utilizing Super
paramagnetic Beads” IEEE Transaction On Magnetics, Vol. 41, No. 10,
October 2005.
K. Togawa, H. Sanbonsugi, A. Lapicki, M. Abe, H. Handa and A.
Sandhu, “High-Sensitivity InSb Thin-Film Micro-Hall Sensor Arrays
for Simultaneous Multiple Detection of Magnetic Beads for Biomedical
World Academy of Science, Engineering and Technology 39 2008
Applications” IEEE Transaction On Magnetics, Vol. 41, No. 10,
October 2005.
[9] P. Kanungo, A. Imre, W. Bin, A. Orlov, G. Snider,W. Porod, N. P.
Carter “Gated hybrid Hall effect device on silicon” Microelectronics
Journal 36 (2005) 294–297.
[10] T. Aytur, P. R. Beatty, B. Boser, “An Immunoassay Platform Based on
CMOS Hall Sensors” Solid-State Sensor, Actuator and Microsystems
Workshop Hilton Head Island, South Carolina, June 2-6, 2002.
[11] D. L. Partin, J. P. Heremans, T. Schroeder, C. M. Thrush and L. A.
Flores-Mena “Temperature Stable Hall Effect Sensors” IEEE Sensors
Journal, Vol. 6, No. 1, February 2006.
[12] J. Lenz and A. S. Edelstein “Magnetic Sensors and Their Applications”
IEEE Sensors Journal,Vol. 6, No. 3, June 2006.
Reza Hosseini was born in 1982. He received the M.S.
degree in semiconductor devices from the Islamic Azad
University, Central Tehran branch, Iran, in 2007. He is
currently Ph.D. student in Electrical Engineering at the
Islamic Azad University, Science & Research branch,
Tehran, Iran
Alireza Kashaninia was born in 1972. He received the BEng in Electronics
Communication from University of Tehran, Iran, MEng in semiconductor
devices from the IAUSTB, Iran and PhD in Electronics Biomedical
Engineering from Islamic Azad University, Science & Research Branch,
Tehran, Iran in 1996, 2000 and 2005 respectively. He is now professor
assistant in Electrical Engineering in IAUCTB, Tehran, Iran. His current field
of interests contains semiconductor and nano_scale electronic devices, RF
characterization and advanced biomedical devices.
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