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Magnetic control and switching of current filaments in a semiconductor
Solid State Communications, Vol. 58, No. 5, pp. 323-325, 1986. Printed in Great Britain.
0038-1098/86 $3.00 + .00 Pergamon Press Ltd.
MAGNETIC CONTROL AND SWITCHING OF CURRENT FILAMENTS IN A SEMICONDUCTOR J. Mannhart, R.P. Huebener, J. Parisi and J. Peinke Physikalisches Institut II, Universit/it TiJbingen, D-7400 Ttibingen, West Germany
(Received 20 December 1985 by ilL. Cardona) The filamentary current flow in the avalanche breakdown regime ofp-Ge at 4.2 K has been studied as a function of an applied perpendicular magnetic field. Magnetically induced switching of the current filaments between different ohmic contacts has been observed, and the principle of a new magnetic field effect transistor has been demonstrated. IMPACT IONIZATION AND avalanche breakdown in semiconductors have been investigated for many years [ 1 - 3 ] . At low temperatures in a homogeneously doped semiconductor most charge carriers are frozen out at the impurities. Since the ionization energy is only about 10 -2 eV, breakdown already occurs at fields of a few V cm -1 . In this case the current-voltage characteristic (IVC) is highly nonlinear. The details of the electric conduction process can become highly complex as indicated by the observation of temporal and spatial structures in the sample conductance. Spatially, the simplest structures consist of current filaments or high-field domains, resulting in a "S" shaped or "N" shaped 1VC, respectively, (current plotted vs voltage). Current filaments in a homogeneously doped semiconductor represent channels with high conductivity generated by impurity impact ionization. Recently, several experiments verified the f'flamentation of the current flow during avalanche breakdown [4, 5]. If an external magnetic field is applied perpendicular to the current direction, the current filaments are spatially shifted due to the Lorentz force. This results in a corresponding shift of the IVC (magneto-resistance), including an increase of the critical electric field at which breakdown sets in ("cooling effect" of the magnetic field on the hot carriers). At low temperatures where the carrier mobility, and hence the Lorentz force, is high, large shifts of the IVC are achieved with relatively small applied magnetic fields. In this paper we study the influence of small applied magnetic fields upon the IVC in the avalanche breakdown regime of p-Ge at 4.2 K. We have observed magnetic switching of the current filaments between different ohmic contacts on the specimen. Using a few windings of superconducting Nb wire attached to the surface of the Ge sample for generating the magnetic field, a threeterminal device has been fabricated acting as a magnetic field effect transistor. Power amplification has been observed up to frequencies of about 1 MHz. Additional
functions which can be performed by magnetic manipulation of the current filaments include logic operations, data storage, and A - D conversion [6]. Our experiments were carried out using singlecrystalline p-doped germanium with an accepter concentration of about 1014 cm -3 corresponding to a room temperature resistivity of 1012cm. The sample dimensions were typically 0.1 x 5 x 6 mm a. The arrangement of the ohmic A1 contacts (L, R, and R ' ) evaporated on one of the two polished crystal surfaces are indicated in the inset of Fig. 1. A more detailed description of the preparation method can be found elsewhere [7].
i,
.a
(mA) 10
"d
i
1./.,,
1.9
V LR{W)
Fig. 1. Current-voltage characteristics at different magnetic fields B = 0 G (a), B = 20 G (b), B = 40 G (c), and B = 60 G (d) measured between the contacts L and R of the sample sketched in the inset. T = 4.2 K.
323
During the experiments the sample investigated was in direct contact with the liquid-helium bath kept at 4.2 K. For protection against external irradiation (visible or FIR), the sample was completely surrounded by a copper shield. Small openings in the shield covered with baffles served for letting the liquid helium reach the interior of the shield. A magnetic field perpendicular to the broad sample surfaces could be applied using a superconducting coil surrounding the Cu shield. The following results were obtained by applying voltage
324
CURRENT FILAMENTS IN A SEMICONDUCTOR
bias to a series combination of the sample and a 1 ~2 resistor. The sample current was found from the voltage drop along the 1 ~ resistor. First we consider the main characteristics of the sample configuration. Figure 1 displays typical IVC's at different magnetic fields, clearly confirming the expected nonlinear behavior during electric avalanche breakdown. In indicates the current measured as a function of the voltage VR applied to the contacts L and R. Contact R ' is disconnected. The magnetoresistance leading to distinct shifts of the IVC to higher critical breakdown voltages is already evident at small applied magnetic fields. Figure 2 further illustrates the sensitive dependence of the sample current 1R upon the external magnetic field B at different values of the bias voltage Vn.
I
Vol. 58, No. 5
|
i
i
i
IR
I
(rnA)
i I
(rn,~
i
1
t(min)
Fig. 3. Temporal structures of the applied magnetic field (upper trace) and the measured sample currents In and I n ' (lower traces) at the bias voltages VR = 1.5 V and Vn' = 1.6 V. T = 4.2 K.
h
-200
-100
0
100
200
B((3)
Fig. 2. Sample current Ii~ vs applied magnetic field at different bias voltages Vn . T = 4.2 K. In order to experimentally verify the existence of current filaments, we have applied a time-dependent magnetic field of the profile shown in the upper trace of Fig. 3. The bias voltages Vn (between the contacts L and R) and Vn' (between the contacts L and R ' ) were kept at 1.5 V and 1.6 V, respectively. The measured temporal structures of the corresponding sample currents I n and I n ' are shown in the lower traces of Fig. 3. Starting at t = 0 and B = - - 10G, we have IR' > 0 and I n = 0. As the magnetic field increases, the current I n sets in abruptly, while I n ' is reduced simultaneously. Before B = 10G is reached, l n ' is switched off. The traces in Fig. 3 clearly demonstrate the typical switching behavior to be expected if filamentation of the current flow occurs. Apparently, the switching process is controlled by the external magnetic field and occurs between the two stable channels of high conductance established between the contacts L and R (current I n ) and the contacts L and R ' (current In'), while the sample always remains insulating between the r.h.s, contacts R and R'. For plotting the measurements on a X - Y recorder, all curves of Fig. 3 were taken at a relatively slow time scale.
For studying the switching behavior at higher frequencies, external r.f. magnetic fields were generated using a superconducting Nb coil of 20 windings and about 1.5 mm diameter attached to the surface of the Ge crystal. A thin capton foil of about 50/am thickness ensures electric insulation and magnetic coupling between the crystal surface and the overlay coil. This configuration represents a three-terminal device operating as a magnetic field effect transitors. We have measured its power amplification as a function of the frequency of the applied magnetic field. The results are shown in Fig. 4. We see that the transistor configuration used in our experiments operates at frequencies up to about 1 MHz. Of course, by proper geometric modifications, this range can be extended to higher frequencies. (The structure in Fig. 4 appearing near 10MHz may be due
I.,t 0 -20 -t+O i
101
10~
i
103
10 ~
10s
106
107
f(Hz)
Fig. 4. Amplitude of the r.f. sample current IR vs the frequency of the applied r.f. magnetic field. The average of the current amplitude at frequencies up to 1 MHz was arbitrarily used to define the 0 dB level. T = 4.2 K.
Vol. 58, No. 5
CURRENT FILAMENTS IN A SEMICONDUCTOR
to resonances with the transit time and does not concern us in the context of this paper). In summary, we have experimentally demonstrated the operating principle of a new three-terminal transistor based on the magnetic manipulation of current filaments in the avalanche breakdown regime of p-Ge. The magnetic control of such current fdaments also opens up other electronic applications, including current measurement, r.f. generation, logic operations, data storage, and A - D conversion [6].
REFERENCES 1. 2. 3. 4. 5. 6.
Acknowledgements - The authors gratefully acknowledge discussions with E. SchOll.
325
7.
K. Seeger, Semiconductor Physics, SpringerVerlag, Berlin, (1982). G. Lautz, Halbleiterprobleme, Vol. 6, p. 21, (Edited by F. Sauter), Vieweg, Braunschweig (1961). W. Mtinch, Phys. Status. Solidi 36, 9 (1969). B.S. Kerner & V.F. Sinkevich, Zh. Eksp. Teor. F/z. 36, 359 (1982)[JETPLett. 36, 437 (1982)1. H. Baumann, D. J~ger, R. Marcus, T. Pioch & W. Rath, to be published (1986). German Patent Application P 35 41 290.9 from 22. Nov. 1985. J. Peinke, A. Muhlbach, R.P. Huebener & J. Parisi, Phys. Lett. 108A, 407 (1985).
0038-1098/86 $3.00 + .00 Pergamon Press Ltd.
MAGNETIC CONTROL AND SWITCHING OF CURRENT FILAMENTS IN A SEMICONDUCTOR J. Mannhart, R.P. Huebener, J. Parisi and J. Peinke Physikalisches Institut II, Universit/it TiJbingen, D-7400 Ttibingen, West Germany
(Received 20 December 1985 by ilL. Cardona) The filamentary current flow in the avalanche breakdown regime ofp-Ge at 4.2 K has been studied as a function of an applied perpendicular magnetic field. Magnetically induced switching of the current filaments between different ohmic contacts has been observed, and the principle of a new magnetic field effect transistor has been demonstrated. IMPACT IONIZATION AND avalanche breakdown in semiconductors have been investigated for many years [ 1 - 3 ] . At low temperatures in a homogeneously doped semiconductor most charge carriers are frozen out at the impurities. Since the ionization energy is only about 10 -2 eV, breakdown already occurs at fields of a few V cm -1 . In this case the current-voltage characteristic (IVC) is highly nonlinear. The details of the electric conduction process can become highly complex as indicated by the observation of temporal and spatial structures in the sample conductance. Spatially, the simplest structures consist of current filaments or high-field domains, resulting in a "S" shaped or "N" shaped 1VC, respectively, (current plotted vs voltage). Current filaments in a homogeneously doped semiconductor represent channels with high conductivity generated by impurity impact ionization. Recently, several experiments verified the f'flamentation of the current flow during avalanche breakdown [4, 5]. If an external magnetic field is applied perpendicular to the current direction, the current filaments are spatially shifted due to the Lorentz force. This results in a corresponding shift of the IVC (magneto-resistance), including an increase of the critical electric field at which breakdown sets in ("cooling effect" of the magnetic field on the hot carriers). At low temperatures where the carrier mobility, and hence the Lorentz force, is high, large shifts of the IVC are achieved with relatively small applied magnetic fields. In this paper we study the influence of small applied magnetic fields upon the IVC in the avalanche breakdown regime of p-Ge at 4.2 K. We have observed magnetic switching of the current filaments between different ohmic contacts on the specimen. Using a few windings of superconducting Nb wire attached to the surface of the Ge sample for generating the magnetic field, a threeterminal device has been fabricated acting as a magnetic field effect transistor. Power amplification has been observed up to frequencies of about 1 MHz. Additional
functions which can be performed by magnetic manipulation of the current filaments include logic operations, data storage, and A - D conversion [6]. Our experiments were carried out using singlecrystalline p-doped germanium with an accepter concentration of about 1014 cm -3 corresponding to a room temperature resistivity of 1012cm. The sample dimensions were typically 0.1 x 5 x 6 mm a. The arrangement of the ohmic A1 contacts (L, R, and R ' ) evaporated on one of the two polished crystal surfaces are indicated in the inset of Fig. 1. A more detailed description of the preparation method can be found elsewhere [7].
i,
.a
(mA) 10
"d
i
1./.,,
1.9
V LR{W)
Fig. 1. Current-voltage characteristics at different magnetic fields B = 0 G (a), B = 20 G (b), B = 40 G (c), and B = 60 G (d) measured between the contacts L and R of the sample sketched in the inset. T = 4.2 K.
323
During the experiments the sample investigated was in direct contact with the liquid-helium bath kept at 4.2 K. For protection against external irradiation (visible or FIR), the sample was completely surrounded by a copper shield. Small openings in the shield covered with baffles served for letting the liquid helium reach the interior of the shield. A magnetic field perpendicular to the broad sample surfaces could be applied using a superconducting coil surrounding the Cu shield. The following results were obtained by applying voltage
324
CURRENT FILAMENTS IN A SEMICONDUCTOR
bias to a series combination of the sample and a 1 ~2 resistor. The sample current was found from the voltage drop along the 1 ~ resistor. First we consider the main characteristics of the sample configuration. Figure 1 displays typical IVC's at different magnetic fields, clearly confirming the expected nonlinear behavior during electric avalanche breakdown. In indicates the current measured as a function of the voltage VR applied to the contacts L and R. Contact R ' is disconnected. The magnetoresistance leading to distinct shifts of the IVC to higher critical breakdown voltages is already evident at small applied magnetic fields. Figure 2 further illustrates the sensitive dependence of the sample current 1R upon the external magnetic field B at different values of the bias voltage Vn.
I
Vol. 58, No. 5
|
i
i
i
IR
I
(rnA)
i I
(rn,~
i
1
t(min)
Fig. 3. Temporal structures of the applied magnetic field (upper trace) and the measured sample currents In and I n ' (lower traces) at the bias voltages VR = 1.5 V and Vn' = 1.6 V. T = 4.2 K.
h
-200
-100
0
100
200
B((3)
Fig. 2. Sample current Ii~ vs applied magnetic field at different bias voltages Vn . T = 4.2 K. In order to experimentally verify the existence of current filaments, we have applied a time-dependent magnetic field of the profile shown in the upper trace of Fig. 3. The bias voltages Vn (between the contacts L and R) and Vn' (between the contacts L and R ' ) were kept at 1.5 V and 1.6 V, respectively. The measured temporal structures of the corresponding sample currents I n and I n ' are shown in the lower traces of Fig. 3. Starting at t = 0 and B = - - 10G, we have IR' > 0 and I n = 0. As the magnetic field increases, the current I n sets in abruptly, while I n ' is reduced simultaneously. Before B = 10G is reached, l n ' is switched off. The traces in Fig. 3 clearly demonstrate the typical switching behavior to be expected if filamentation of the current flow occurs. Apparently, the switching process is controlled by the external magnetic field and occurs between the two stable channels of high conductance established between the contacts L and R (current I n ) and the contacts L and R ' (current In'), while the sample always remains insulating between the r.h.s, contacts R and R'. For plotting the measurements on a X - Y recorder, all curves of Fig. 3 were taken at a relatively slow time scale.
For studying the switching behavior at higher frequencies, external r.f. magnetic fields were generated using a superconducting Nb coil of 20 windings and about 1.5 mm diameter attached to the surface of the Ge crystal. A thin capton foil of about 50/am thickness ensures electric insulation and magnetic coupling between the crystal surface and the overlay coil. This configuration represents a three-terminal device operating as a magnetic field effect transitors. We have measured its power amplification as a function of the frequency of the applied magnetic field. The results are shown in Fig. 4. We see that the transistor configuration used in our experiments operates at frequencies up to about 1 MHz. Of course, by proper geometric modifications, this range can be extended to higher frequencies. (The structure in Fig. 4 appearing near 10MHz may be due
I.,t 0 -20 -t+O i
101
10~
i
103
10 ~
10s
106
107
f(Hz)
Fig. 4. Amplitude of the r.f. sample current IR vs the frequency of the applied r.f. magnetic field. The average of the current amplitude at frequencies up to 1 MHz was arbitrarily used to define the 0 dB level. T = 4.2 K.
Vol. 58, No. 5
CURRENT FILAMENTS IN A SEMICONDUCTOR
to resonances with the transit time and does not concern us in the context of this paper). In summary, we have experimentally demonstrated the operating principle of a new three-terminal transistor based on the magnetic manipulation of current filaments in the avalanche breakdown regime of p-Ge. The magnetic control of such current fdaments also opens up other electronic applications, including current measurement, r.f. generation, logic operations, data storage, and A - D conversion [6].
REFERENCES 1. 2. 3. 4. 5. 6.
Acknowledgements - The authors gratefully acknowledge discussions with E. SchOll.
325
7.
K. Seeger, Semiconductor Physics, SpringerVerlag, Berlin, (1982). G. Lautz, Halbleiterprobleme, Vol. 6, p. 21, (Edited by F. Sauter), Vieweg, Braunschweig (1961). W. Mtinch, Phys. Status. Solidi 36, 9 (1969). B.S. Kerner & V.F. Sinkevich, Zh. Eksp. Teor. F/z. 36, 359 (1982)[JETPLett. 36, 437 (1982)1. H. Baumann, D. J~ger, R. Marcus, T. Pioch & W. Rath, to be published (1986). German Patent Application P 35 41 290.9 from 22. Nov. 1985. J. Peinke, A. Muhlbach, R.P. Huebener & J. Parisi, Phys. Lett. 108A, 407 (1985).