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 432 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. 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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. 434