Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Magnetic field
Reexamination Certificate
1999-08-02
2001-04-17
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Magnetic field
C257S423000, C257S565000
Reexamination Certificate
active
06218718
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a spin transistor which is a transistor which relies upon two separate populations of charge carriers distinguished by the direction of the spin or magnetic moments of the carriers in the same way that in a conventional semiconductor transistor the carrier families are distinguished by their having different effective electrical charges.
The original spin transistor, or magnetic transistor as it is also known, is described in Science, 260, 320 Apr. 16, 1993, Bipolar Spin Switch by M Johnson. The spin transistor described in this journal report was derived from a metallic, giant magnetoresistance (GMR) magnetic trilayer with contacts to each of three layers. The transistor is biased so as to pump a spin polarized current from one of the magnetic layers into a base layer; the latter suffers a consequent divergence in the chemical potentials of the up and down spin channels and this in turn induces magnetically dependent current flow in a collector (the second magnetic layer). This spin transistor design has a significant problem with its practical implementation in that the transistor offers no power gain. Also, the voltages involved are of the order of nanovolts.
More recent developments in the spin transistor are described in Phys. Rev. Lett., 74, 26, 5260, (1995), D J Monsma et al. The spin transistor described in this journal paper is a metal-semiconductor hybrid in which the rectifying properties of semiconductor junctions are exploited. The transistor consists of two layers of silicon which sandwich a metal GMR multilayer. The transistor is biased to pass current from one silicon layer to the metal multilayer stack and the latter's magnetic configuration then governs what proportion of the current eventually penetrates to the second silicon layer. In this respect, the multilayer stack operates as a normal magnetic multilayer which has a large resistance in a low magnetic field and a small resistance in a high magnetic field. The current gain of the transistor is magnetically controllable by a factor of around 2, with the GMR of the multilayer on its own being only about 3%. Even with this design, however, while the collector/base current gain &bgr; varies by a large factor, its actual value is very small and, in fact, is less than unity, whereas for a commercial silicon transistor &bgr; is generally 200 or more.
SUMMARY OF THE INVENTION
The present invention is a further development of the spin transistors described above which seeks to increase the gain &bgr; of the transistor to a value greater than unity. To achieve this, the present invention relies on the generation of a spin polarized diffusion current which is controlled by means of a magnetically controlled barrier such as a switchable density of states for example.
The present invention provides a spin transistor comprising a hybrid semiconductor emitter, base and collector, wherein the emitter includes spin polarizing means for spin polarizing the charge carriers at the emitter, and a magnetically controllable barrier provided between the base and collector to control the arrival of charge carriers at the collector.
Thus, the charge carrier populations in the spin transistor are distinguished by magnetic moment instead of electrical charge.
Preferably, a semiconductor bridge containing a charge carrier diffusion layer, which essentially forms the base, is provided, the charge carrier diffusion layer being formed between the emitter and the collector. Preferably, a p-n junction is formed between the emitter and the base and the other side of the base abuts the magnetically controlled barrier. The semiconductor bridge is ideally in the form of a membrane having a thickness of less than 1 &mgr;m. The magnetically controllable barrier has a thickness less than the spin diffusion length of the carriers and presents different bandstructures to the carriers in dependence on an externally applied magnetic field.
Additionally, the spin polarizing means preferably includes a layer substantially consisting of cobalt or Chromium dioxide and exchange pinning means which may be comprised of a layer of an alloy of manganese and iron in contact with the cobalt layer.
P-n and n-p junctions are provided between the emitter and base and between the barrier and collector connection. Ideally, the magnetically controllable barrier is in contact with the p-n junction and this junction between the barrier and the p-n junction functions as a Schottky or as an Ohmic barrier. A metallic layer may also be provided between the magnetically controllable barrier and the p-n junction preferably of silver, which may improve the characteristics of the barrier.
Furthermore, the magnetically controllable barrier may extend beyond the area of the spin polarizing means and may consist of a material including chromium dioxide. Ideally, the magnetically controllable barrier consists of a layer of chromium dioxide and the diffusion layer extends beyond the area of the spin polarizing means and the magnetically controllable barrier.
In a preferred embodiment, the emitter and the spin polarizing means are provided on a first side of a semiconductor wafer and the base, the collector and the magnetically controllable barrier are provided on a second side of the semiconductor wafer.
A further aspect the present invention provides a method of fabricating a spin transistor comprising: providing a semiconductor wafer; etching the wafer to form a pit on a first surface of the wafer; forming a p-n junction in a semiconductor bridge between the base of the pit and an opposing second surface of the wafer with at least one of the p and n doped regions forming the junction extending beyond the area of the pit; forming a layer of a spin polarizing material in the base of the pit, and establishing an electrical connection to the spin polarizing layer, forming a magnetically controllable barrier layer on the second surface of the wafer over the p-n junction in the semiconductor bridge; establishing an electrical connection to the magnetically controllable barrier layer; and establishing an electrical connection to that portion of the p or n doped regions extending beyond the area of the pit.
The method may further comprise forming an exchange pinning layer on the spin polarizing material; forming a p-n junction on a separate semiconductor wafer and coating with silver; coating the magnetic barrier layer with silver; cold welding the two silver layers to form the transistor structure; and making a contact to the p-n junction structure on the second wafer on the side opposite to the silver layer.
REFERENCES:
patent: 3818328 (1974-06-01), Zinn
patent: 5432373 (1995-07-01), Johnson
patent: 5973334 (1999-10-01), Mizushima et al.
D.J. Monsma et al., “Perpendicular Hot Electron Spin-Valve Effect in a New Magnetic Field Sensor: The Spin-Valve Transistor,” Physical Review Letters, vol. 74, No. 26, Jun. 26, 1995, pp. 5260-5263.
Mark Johnson, “Spin Injector in Metal Films: The Bipolar Spin Transistor,” Materials Science and Engineering B31 (1995), pp. 199-205.
Mark Johnson, “Bipolar Spin Switch,” Science, vol. 260, Apr. 16, 1993, pp. 320-323.
Gregg John Francis
Sparks Patricia Dresel
Crane Sara
Isis Innovation Limited
Wenderoth Lind & Ponack, LLP.
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