Silicon Depletion Layer MagnetometerJohn B. Flynn Citation: Journal of Applied Physics 41, 2750 (1970); doi: 10.1063/1.1659305 View online: http://dx.doi.org/10.1063/1.1659305 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/41/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Silicon depletion layer actuators Appl. Phys. Lett. 92, 184103 (2008); 10.1063/1.2920440 Photothermal microscopy of silicon epitaxial layer on silicon substrate with depletion region at the interface Rev. Sci. Instrum. 74, 553 (2003); 10.1063/1.1515893 Analysis of the Depletion Layer Transducer J. Appl. Phys. 35, 2106 (1964); 10.1063/1.1702799 Analysis of the Depletion Layer Transducer J. Acoust. Soc. Am. 34, 719 (1962); 10.1121/1.1937176 On the Injection of Carriers into a Depletion Layer J. Appl. Phys. 28, 513 (1957); 10.1063/1.1722786

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currents in the Nb junctions peak at about ~, this voltage iden tification is not sufficient to define the exact mechanism.' Further studies of the excess currents are underway.

The authors would like to acknowledge the helpful discussions with R. Rosenberg, J. Matisoo, A. F. Mayadas, A. A. Levi, and E. Alessandrini and the considerable assistance of J. Viggiano and J. Angelello.

1 J. E. Nordman, J. App!. Phys. 40, 2111 (1969); L. O. Mullen and D. R. Sullivan, J. App!. Phys. 40, 2115 (1969).

2 J. Matisoo (private communication). • I. Giaver and K. Meger!e, Phys. Rev. 122, 1101 (1961). • M. L. A. MacVicar and R. M. Rose, J. App!. Phys. 39,1721 (1968). • J. M. Rowell and W. L. Feldman, Phys. Rev. 172,393 (1968). • B. N. Taylor and E. Burstein, Phys. Rev. Lett. 10, 14 (1963). 7 C. J. Adkins, Phil. Mag. 8, 1051 (1963). S J. Matisoo, Phys. Lett. 29A, 473 (1969). • J. R. Schrieffer and J. W. Wilkins, Phys. Rev. Lett. 10, 17 (1963).

Silicon Depletion Layer Magnetometer

JOHN B. FLYNN

Honeywell Radiation Center, Lexington, Massachusetts 02173

(Received 8 September 1969; in final form 19 January 1970)

In this report we will describe a novel silicon magnetometer. This device utilizes the deflection of carriers in a junction deple­tion layer under the influence of a magnetic field. The depletion layer extends from the base in to the collector region of a transistor structure. A groove which divides the collector contact reaches the depleted region when the collector-base biasing voltage is at a sufficiently high leveL In this condition, both collector contact portions are electrically isolated.' The emitter, located above, and parallel to the groove injects carriers which are collected and traverse the depletion layer. Since recombination is negligible in the depletion layer, all of the injected carriers emerge at either contact. A previously established null condition will be unbalanced because of the Lorentz force deflection of the electrons in the depletion layer when a magnetic field is applied parallel to the groove. This is the basis of the device operation. Figure 1 shows a schematic drawing cross section.

The influence of magnetic fields on certain transistor parameters has previously been reported on conventional devices.2-' These reports did not demonstrate practical field measuring devices. More recently, Fry et aI.· have described an MOS transistor designed for sensing magnetic fields. Another magnetic field sensing device, using a transistor design with two collectors also exists6 but has not been described in the scientific or engineering literature other than editorially.

The device we describe in this report is different from the last two devices in design and operation. Detailed comparison suggests our device will show superior performance.

The essential difference between this device and a Hall mag­netometer is that the former has an output impedance orders of magnitude higher. Therefore, for the same power dissipated, it can be shown to give a correspondingly higher output signal for a given field.

The device we have studied was an n-p-n silcon structure. Therefore, the transport equation applicable to electrons in

BASE CONTACT WINDOW

r:====:!~~=~=:f= Si02

{+ M~

DEPLETED • } ·1O.ii_"F .... Oi1\\-_"F_iil---~t COLlECTOR

FIG. 1. Schematic drawing of a eros::; section of the device. Vo is measured between collector contacts.

150 150

0

100 6t 100 '7'-~s 8

x ~o E

~61 / c:

~ "'-50 50

/ 10 20 30 110

VCSIVOlTI

F'G. 2. Depletion layer width versus collector-base voltage V CB; b., left ordinate. Interelement resistance Ri versus V CB; O. right ordinate.

silicon will be used. For small magnetic fields the current density vector J is related to the magnetic induction B and electric field E by 7

J=O'E+R0'2ExB, (1)

where 0' is the conductivity and R is the Hall constant. The coordinate system is shown in Fig. 1.

Equation (1) neglects transport by diffusion which is reason­able because of the high fields of the depletion layer. Since B. = By=O we may write the current density components as

J.=O'E.+Ra"EyB •.

J y = 0' E y - Ra" E.B.,

Since no accumulation of mobile charge is possible in the depletion layer which would cause the Hall field E y to develop, this quantity remains zero.S Hence, noting E.=O,

J.=O'E.,

J y= _R0'2E.B.= -P.eHJ.B.,

where P.'H is the Hall mobility. The output voltage Vo from Fig. 1 is given by

where

Iy=Z jW Jydx. o

(2)

Here Z is the length of the emitter and W the thickness of the depletion layer.

As we will show, it is not necessary for the undepleted portion to reach the groove for this treatment to be correct, merely that the sum of Rl and R2 from Fig. 1 be lower than the resistance of the undepleted portion above the groove. Of course, for the highest output impedance, it would be necessary that this un­depleted "neck" be eliminated. Assuming J. is uniform in the y direction, we obtain

TABLE I. Data used for theory check.

RL IcCae) B. f Ic(dc) W, Collector-base area Device thickness Groove depth

930 10 p.A (rms) 1.25 kG

400Hz 0.35mA 0.0188 cm 0.125cm2

0.0137 ±O. 0003 em 0.oo66±0.OOO8 em

(3)

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FIG. 3. Schematic used to test Eq. (3).

Here W. is the width of the emitter, Ie the total collector current, and we neglect the sign.

The width of the depletion layer was calculated from the collector capacitance measurements as a function of bias. Also by connecting a bridge through a large capacitor where Vo is measured and making the resistors 1 Mn (see Fig. 1) the resistance was measured as a function of bias. Figure 2 shows a plot of interelement resistance and space charge width as a function of collector-to-base voltage. The groove depth, sample thickness, and collector-base area were also measured for the foregoing calculations. The arrow in Fig. 2 indicates the measured distance from the top of the groove to the collector-base junction.

In Fig. 3 we show the schematic used to test Eq. (3). The device is represented as a transistor with two collectors. The collector was reverse bi~sed and a small de current was injected at the emitter, then a small ac signal was impressed on the emitter. The unbalanced ac signal appearing was nulled by adjusting the center tap on RL and the variable capacitor. The magnetic field B. was then applied and the size of the unbalance signal measured.

In this way, the output signal was measured as a function of the depletion layer width at a constant magnetic field (1.25 kG). Since all of the quantities in Eq. (3) are known or measurable, it may be unambiguously verified. Values of I'CH calculated from these measurements using (3) with the data in Table I lay between 1600-1700 cm'/V sec. The value reported by Messier and Flores9

is 1880 cm'/V sec. While the uncertainty in both this value and our measurements are sufficient to account for the difference, slight "current crowding" under the emitter may be operative.

Finally, using a dc magnetic field of 1.3 kG, the signal from another, similar device was measured as a function of injected current Ie. This device had a groove on the bottom isolating a small border under reverse bias which served to shunt surface leakage noise to ground. The data are shown in Fig. 4. From these we calculate a value of 10 V /(G·A). This is superior to a value of 6 V/G·A from (5) and 0.4 V/G·A from (6). A silicon Hall element gave 0.085 V/G·A.lO

In this report we have described a new device for detection of magnetic fields. The theory of operation has been derived and has been found to agree very welI with experimental results. The device has a very high output impedance, and has been shown to

FIG. 4. Output voltage versus collector current for 1.3 kG.

LO

,8

~ .6 o 2: « z '" .4

, 2

I °

/ L-__ ~L-____ L-__ ~'~

20 40 60 80

COLLECTOR CURRENT IMICROAMPERESI

be capable of far greater signals for the same magnetic field and current than a conventional Hall device.

It is a pleasure to acknowledge the assistance of H. RandalI and L. MacDonald in fabricating the devices used and valuable discussions with J. Hancock and W. Webb with regard to circuitry.

1 J. B. Flynn. J. M. Epstein. D. R. Palmer. and J. V. Egan. IEEE Trans. Electron Devices 16, 877 (1969).

2 c. B. Brown. Phys. Rev. 76, 1736 (1950). 'c. B. Brown. Electronics 23, 81 (1950). 4 P. C. Trivedi and G. P. Srivastava. Electronic and Radio Engineer.

36, 368 (1959). 'P. W. Fry and S. J. Hoey. IEEE Trans. Electron Devices 16, 35 (1969). 'Editorial. Electronic Design News. Feb. 15. 1969. p. 73. , R. A. Smith. Semiconductors (Cambridge University Press. New York.

1961). p. 122. S We are indebted to the observation of one reviewer who pointed out

that because of surface accumulation this may be true only for carriers injected at a point away from the surface.

9 J. Messier and J. M. Flores. J. Phys. Chern. SoIids,24., 1539 (1963). 10 O. J. Mengali and T. S. Shilliday. Solid-State Electron. 7, 379 (1964).

Evidence for Hole Traps in CdS Crystals*

Ho B. 1M. t HERMAN E. MATTHEWS. t AND RICHARD H. BUBE

Department of Materials Science. Stanford University. Stanford. California 94305

(Received 5 January 1970)

Indirect evidence from luminescence studies indicates the existence of hole traps in CdS crystals lying about 0.14-0.17 eV above the valence band. I ., Deep hole traps lying about 1.2 eV about the valence band are also known from their effects as sensitizing centers for photoconductivity." Recent studies of photochemical changes in CdS crystals' have led us to investigate the possibility of detecting hole traps by photo-Hall or photo­thermoelectric measurements. Measurements were made on CdS crystals previously irradiated by electrons (165-400 keV) and then annealed at 200°C under photoexcitation,' and on crystals of CdS(75)CdSe(25) grown by Clevite Corporation.

The temperature dependence of the Hall mobility between 200° and 3200 K is describable approximately by I'HcxT-3/2, and can be explained in terms of a combination of optical phonon scattering and piezoelectric scattering.6•7 Figure 1 shows the Hall mobility measured for carriers released from traps in the course of a measurement of thermally stimulated conductivity. The varia­tion of Hall mobility with temperature is similar to that of the conductivity itself, showing maxima and minima at about the same temperatures. The maxima of the Hall mobility for the thermally stimulated conductivity (TSC) measurement are close to the values of Hall mobility under full intensity photo­excitation in most cases. Thus the minima of HalI mobility or any deviation from I'HcxT-3/2 obtained for full intensity photo­excitation can be due either to the possibility that the Hall voltage equilibrium is not reached or to some participation by hole traps. Hall measurements during TSC are difficult to measure accurately, and so recourse was taken to measurements of thermo­electric power for the detection of possible hole traps.

The measured thermoelectric power for photoexcited carriers agreed well with calculated values of the electronic contribution above 1500 K; below that temperature phonon drag contributions became appreciable. General results were similar to those re­ported by Morikawa.s Attempts to use the thermoelectric power generated by carriers freed in a thermally stimulated conductivity measurement have been made by Lawrance and Bube9 in CdS, and by Grigas and MikalkevichyslO in Sb"Se,. An unambiguous determination of carrier type in this method, however, is not possible unless a complete change in the sign of the thermo-

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