INTRODUCING A NEW HALL EFFECT SENSOR

Novel Dynamic Offset Reduction Method

Vlassis N. Petoussis, Panos Dimitropoulos, George Stamoulis and Elias Houstis

University of Thessaly, Department of Computer & Communication Engineering

37th Glavani & 28th October Str., Volos, Greece

Keywords: Hall effect sensor, Offset reduction.

Abstract: Silicon Hall plates show an offset of a few millitesla. A large portion of this offset is caused by mechanical

stress in the device. The offset can be reduced when the spinning - current principle is applied. In this paper

we present a model and numerical analysis of a new Hall effect sensor which using a novel offset reduction

method. We call it “Wheel Hall Senor” and senses all 3 filed dimensions. The flux-density of B

Z

is

proportional to the DC component of the output signal, whereas the B

X

and B

Y

components are proportional

to the first harmonic of the output signal. Furthermore we calculate the function which governs the changes

in the electric field inside the new Hall effect sensor in presence of magnetic field. This function help us to

control in MatLab environment the equipotential lines and to monitor the changes in biasing conditions.

About the new sensor, the combination of his pioneering form and the elaborate sequence of using the

dynamic spinning current technique, could be lead to satisfactory results of produced Hall voltage with

small noise in a presence of external magnetic field.

1 INTRODUCTION

Hall plates are used in a wide variety of applications

to measure static and dynamic magnetic fields. A

major drawback of these sensors is their high offset

voltage, i.e., an output signal in the absence of a

magnetic induction. According to (A. Bellekom,

1994) the effects which contribute to the offset

voltage are piezoresistive effects, geometrical errors,

temperature gradients, nonhomogeneities, etc.

However, the offset voltage with a spatially periodic

nature differs from the Hall voltage, which is

constant. One way to reduce the offset caused by

orthogonal asymmetries is to use coupled symmetric

Hall plates (Maupin, 1980). However, the offset can

never be completely cancelled because there will

always be small differences between the Hall

devices. To overcome the mismatch, the bias current

of a single four contact Hall device is spun by

contact commutating and the resulting output

voltages are averaged over time. The orientation

dependence of the offset sources in silicon suggests

that orthogonal switching is not sufficient. To cancel

components of higher asymmetries a multicontact

Hall plate with more than four terminals is

necessary. The method presented in this paper is

based on Hall device with sixteen outside and

sixteen inside contacts. The spinning current vector

is generated by two harmonic biasing currents. As a

result, the output signal becomes spatially

continuously accessible, hence, asymmetries of

higher order can be cancelled out. Aim of present

work is the presentation of new Hall sensor with

pioneering form that reminds cogwheel with two

lines teeth, one internally and one externally (for this

reason we name it “Wheel Hall Sensor”) but also

with different way in the cancellation of offset

voltage and finally 1/f noise rejection. Initially the

designing constituted fundamental objective that as

aim had the respect and compatibility in the existing

techniques of reduction 1/f noise and also offset

voltage. For this aim was selected the circular

structure.

2 OFFSET VOLTAGE

In a standard IC process Hall plates have the

advantage of cheap and small. Therefore they are

quite often used in applications where the permanent

magnet is combined with the sensor. Because Hall

plates show a large offset the magnet used in these

464

Petoussis V., Dimitropoulos P., Stamoulis G. and Houstis E..

INTRODUCING A NEW HALL EFFECT SENSOR - Novel Dynamic Offset Reduction Method.

DOI: 10.5220/0003397304640469

In Proceedings of the 1st International Conference on Pervasive and Embedded Computing and Communication Systems (PECCS-2011), pages

464-469

ISBN: 978-989-8425-48-5

Copyright

c

2011 SCITEPRESS (Science and Technology Publications, Lda.)

applications have to be a strong magnet. The offset

of the Hall plate is the voltage that is measured when

no magnetic field is applied. Eq.1 represents the

theoretical situation, when an offset is added this

equation becomes:

offsetH

VISBV

(1)

Where I is the biasing current and S is the current –

related sensitivity. To reduce the offset, which is

time variant, spinning current Hall plates was

developed (Popovic, 1991). These are symmetrical

Hall plates with more than 4 contacts. In this multi

contact Hall plates the bias current is switched in for

example, eight directions through the Hall plate and

the corresponding voltages are measured, see Fig

1.(S.Bellekom, 1997).

Figure 1: An eight contact spinning-current Hall plate with

the possible bias current directions.

The measured voltages represent one period of a

(radial spatial) periodic signal. The spinning current

method uses this periodicity, it separates

transduction effects according to their spatial radial

periodicity. The Hall effect itself does not depend on

bias current direction and is therefore present in the

DC component. So, a spinning-current Hall plate

solves one of the most important drawbacks in

integrated silicon Hall plates, their offset.

3 THE

SPLIT – CURRENT MODEL

In this section, we will represent the magnetic field

resolution achievable with Hall split-current, sensor

and the offset voltage for a Hall plate. The presented

results have been checked modeling the Hall devices

with an equivalent electrical circuit, consisting of a

network of resistances and voltage controlled current

sources, as proposed in (Maier, 1999, Popovic,

1985). Fig. 2 shows the structure of a split-current

device, placed in a static magnetic field B=

z

B

ˆ

, and

biased with a current I. The current difference

between the two split contacts is (Popovic, 1991):

IBFI

(2)

where l and w are the length and width of the device,

d is the distance between the split contacts, and F is

a magneto-geometrical factor Thus:

BI

RkT

B

/

/4

min

(3)

F is the maximum for l/w 1, d/w 1, and μB<<1.

In these conditions, F

1. Optimum magnetic field

resolution for a given area is obtained for l w and

μB<<1, where:

w

kTR

twen

kT

B

sat

sat

opt

0

min,

44

(4)

Figure 2: The split-current Hall Effect Device.

4 THE CHANGES IN THE

ELECTRIC FIELD INSIDE THE

SENSOR

In this section we calculate a set of functions which

governs the changes in the electric field inside the

new Hall effect sensor in presence of magnetic field

B. This function help us to control in MatLab

environment the equipotential lines and to monitor

the changes on them when biasing conditions are

change. In our split-current model we use Dirichlet

and Neumann boundary conditions as we can see in

Fig.3. Use the following empirical expressions

(Vinal, 1982):

2

16 3 0.72

1265

(65.0 )

1[ /(8.510 )]

n

T

cm

Nxcm Vs

(5)

INTRODUCING A NEW HALL EFFECT SENSOR - Novel Dynamic Offset Reduction Method

465

2

16 3 0.72

447.3

(46.7 )

1[ /(6.310 )]

p

T

cm

Nxcm Vs

(6)

where N

T

denotes the total ionized impurity

concentration. For the Hall mobility we use the

approximation (Smith, 1978), μ* = rμ, with

r = <τ

2

>/<τ>

2

(Vinal, 1982). We consider

semiconductor slabs of rectangular geometry with a

magnetic field perpendicular to the device surface: B

= (0, 0, B,). The distributions of carriers and the

potential in the device depend only on x and y and

we solve a two-dimensional problem.

2

()

1( )

p

p

B

cB

x

B

2

()

1( )

n

n

B

cB

x

B

(7)

where:

y

J

Bc

pB

ˆ

)(

And we get two equations (Eq.7) for a p and n type

respectively. These equations give us the

dependence of the electric field inside the sensor in

presence of magnetic field.

(a) (b)

Figure 3: The boundary conditions for a split-current

element Hall effect device. a) The area of the received

Hall voltage tacked in four places of the sensor. We can

see that the margins of the field in our model is well

separated each other (black trapezoid areas). b) Graphic

representation of a simple model in even phase, with D

and N presented the boundary conditions. Dirichlet

boundary conditions in contacts with positive or zero

potential and Neumann boundary conditions elsewhere.

5 THE WHEEL HALL SENSOR

A novel Hall sensor device introduced which uses

elaborate spinning current technique. The novel Hall

device that we call “Wheel Hall Senor” is presented

in Figures 4a and 4b. The current enters the device,

as presented in the aforementioned Figures, in two

phases namely the even phase (PHASE-P) and the

odd phase (PHASE-N). The device exploits the

signals attributed to Hall voltage, Hall current and

geometric MR effect. As a result the device is

equivalent to an “ideal” voltage or current Hall

sensor with geometrical factor of one (G

H

= 1 or

F = 1). Moreover it provides for high-speed

spinning, given that the voltage distribution changes

moderately between different phases. This is

equivalent to minimum charge injection that 0 in

turn – allows spinning frequency increase.

(a)

(b)

Figure 4: The novel Hall device that we call “Wheel Hall

Senor” or WHS. (a) The even phase (PHASE-P); (b) the

odd phase (PHASE-N).

Rotating in the clock wise direction we changing

periodically between the two phases and we are able

to have in each phase simultaneously 0

0

and 90

0

degree representation (as in spinning current

technique) of the Hall voltage. Due to the periodicity

PECCS 2011 - International Conference on Pervasive and Embedded Computing and Communication Systems

466

of the piezoresistive constants in the chip plane the

sensor’s offset is inverted if the current is switched .

When averaged, the offset-related part of the signal

is suppressed, whereas the component related to the

magnetic field remains unchanged. Single-sensor

applications have been implemented also in the past

with 2, 8, and 16 current directions (Munter. 1990,

Gottfried. 1991, Biolotti, 1997).

6 THE OFFSET REDUCTION

METHOD

To reduce the offset, the new Hall effect sensor uses

spinning-current technique in the symmetrical Hall

plate (Fig.4). In the offset reduction sequence the

direction of the bias current is splited right and left

and the corresponding output voltage is measured on

the contacts in

45

0

to the current direction in each

phase. The total number of measurements at each

phase is in four places like a cross and all phases

give as the total Hall voltage. When sixteen outside

and sixteen inside contacts are used, the bias current

is switched

45

0

for each measurement, the voltage

contacts are switched outside to inside respectively

(Fig. 5). For each phase of the bias current, four

output (V

H+

and V

H-

) and input (V

H-

and V

H+

)

voltages are measured in a rotating clock wise.

(a)

(b)

Figure 5: The even phases (a) and the odd phases (b). With

I we denote the current bias in 45

0

direction in each

measurement and V

H+

and V

H-

denoted the two Hall

Voltage references in each phase (for n-type material).

Finally for each turn in PHASE-P and PHASE-

N, totally sixteen Hall voltages are measured. Each

harmonic biasing current in each phase produces an

offset voltage witch totally in turns gives us the

offset cancellation. So offset caused from current I

AD

in phase P cancel the offset caused from current I

CD

in phase N and I

AD

in phase N cancel the offset

caused from current I

AF

in phase P. Finally the one

offset in one phase cancel the other in next phase

(Petoussis, 2009).

7 THE REAL STRUCTURE

OF THE SENSOR

Figure 6: The real structure of the “Wheel Hall Sensor”.

A triple group of p-n junction diffusion with proposed

dimensions. We see only a part of the 144 p-n junction

diffusions divided in triple groups, each group has a

contact for measuring and biasing.

The main shape of the sensor is a circle (Fig.4). This

shape to be in real material contains problems

especially on the offset caused by stress.

Furthermore the designed shape must satisfy a way

to be easy functionally for a spinning-current

INTRODUCING A NEW HALL EFFECT SENSOR - Novel Dynamic Offset Reduction Method

467

technique. The final decision was to develop the

sensor in a row with a two lines of small Hall Effect

devices with up and down parts in a reversed

trapezoid shape resemble the real cycled structure of

the sensor. We need totally 144 p-n junction

diffusions pieces (Fig.6) divided in 32 triple

trapezoid groups, each group has a contact for

measuring or biasing and finally we use 32 contacts.

8 FIELD SIMULATIONS

In this part we present simulations of the device

structure with a MatLab simulator. The main goal

was to see the behavior of the electric field inside

the device when this exposed in external magnetic

field Fig (7a, 7b). In a way to control the

equipotential lines in it, we solve the Laplace

equation using the boundary conditions that we saw

in the split current model previously. The results

show us that the shape and the place of the contacts

is in the right position and are well combined in

even and odd phase during the spinning current

technique.

(a)

(b)

Figure 7: The novel Hall device that we call “Wheel Hall

Senor” or WHS. (a) The electric field and the equpotential

lines in the even phase (PHASE-P) for B

0; (b) The

electric field and the equpotential lines in the odd phase

(PHASE-N) for B

0.

9 CONCLUSIONS

In this paper we analyze the theoretical and the

technical way of the structure of a new Hall Effect

Sensor which uses an elaborate spinning current

technique. Using as shape with a row of a two lines

of small Hall Effect devices with up and down parts

we are able to reduce the stress offset and we able to

support reduced the 1/f noise measurements using

fast spinning current technique. Moreover it

provides for high-speed spinning, given that the

voltage distribution changes moderately between

different phases. This is equivalent to minimum

charge injection that 0 in turn – allows spinning

frequency increase. The device senses all 3 filed

dimensions, namely the flux-density of B

Z

is

proportional to the DC component of the output

signal, whereas the B

X

and B

Y

components are

proportional to the first harmonic of the output

signal. Finally the device can be made in a way to

reuse the current, if integrated in a BiCMOS

technology providing for matched JFETs.

ACKNOWLEDGEMENTS

I would like to thank Pr. Dr. George Stamoulis, Dr.

Panos Dimitropoulos for the useful advices and Pr.

Dr. Manolis Vavalis for the helpful discussions.

furthermore I would like to thank the electrical

engineering & computer engineering department of

the university of thessaly for the technical support

and fulfillment of this work.

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Vlassis N. Petoussis. P. Dimitropoulos, George Stamoulis,

A Novel Hall Effect Sensor Using Elaborate Offset

Cancellation Method, Sensors & Transducers Journal,

Vol. 100, January 2009, pp. 85-91.

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