Spin Valve Transistor
Published on Dec 12, 2015
In a world of ubiquitous presence of electrons can you imagine any other field displacing it? It may seem peculiar, even absurd, but with the advent of spintronics it is turning into reality.
In our conventional electronic devices we use semi conducting materials for logical operation and magnetic materials for storage, but spintronics uses magnetic materials for both purposes. These spintronic devices are more versatile and faster than the present one. One such device is spin valve transistor.
Spin valve transistor is different from conventional transistor. In this for conduction we use spin polarization of electrons. Only electrons with correct spin polarization can travel successfully through the device. These transistors are used in data storage, signal processing, automation and robotics with less power consumption and results in less heat. This also finds its application in Quantum computing, in which we use Qubits instead of bits.
Two experiments in 1920's suggested spin as an additional property of the electron. One was the closely spaced splitting of Hydrogen spectralines, called fine structure. The other was Stern -Gerlach experiment, which in 1922 that a beam of silver atoms directed through an inhomogeneous magnetic field would be forced in to two beams. These pointed towards magnetism associated with the electrons.
Spin is the root cause of magnetism that makes an electron tiny magnet. Magnetism is already been exploited in recording devices. Where data is recorded and stored as tiny areas of magnetized iron or chromium oxide. To access that information the head detects the minute changes in magnetic field. This induces corresponding changes in the head's electrical resistance - a phenomenon called Magneto Resistance.
EVOLUTION OF SPINTRONICS:
Spintronics came into light by the advent of Giant Magneto Resistance (GMR) in 1988. GMR is 200 times stronger than ordinary Magneto Resistance. It results from subtle electron - spin effects in ultra multilayers of magnetic materials that cause a huge change in electrical resistance.
The discovery of Spin Valve Transistor (GMR in magnetic multilayers) has let to a large number of studies on GMR systems. Usually resistance of multilayer is measured with the Current in Plane (CIP). For instance, Read back magnetic heads uses this property. But this suffers from several drawbacks such as; shunting and channeling, particularly for uncoupled multilayers and for thick spaced layers diminish the CIP magneto resistance. Diffusive surface scattering reduces the magneto resistance for sandwiches and thin multilayers.
In spin valve transistor (SVT) electrons are injected in to metallic base across a Schottky barrier (Emitter side) pass through the spin valve and reach the opposite side (Collector side) of transistor. When these injected electrons traverse the metallic base electrons are above Fermi level, hence hot electron magneto transport should be considered in Spin Valve Transistor (SVT).
The transport properties of hot electrons are different from Fermi electrons .For example spin polarisation of Fermi electrons mainly depends on Density Of States (DOS) at Fermi level, while the spin polarisation of hot electron is related to the density of unoccupied states above the fermi level.
For the preparations of transistor we apply direct bonding, both to obtain device quality semiconductor material for the emitter and to allow room temperature processes.The starting material for both emitter and collector is a 380um, 5-10Ocm, n-si (100) wafer. After back side n++ implantation ,wafer is dry oxidised to anneal the implant and to form a SIO2 layer .
After depositing a Pt ohmic contact on to the back side, wafer is sawn in to 10X10mm collector and 1.6X1.6mm emitters. Collector is subsequently dipped in HNO3, 2% HF to remove the native oxide on silicon fragments,5% Tetra methyl Ammonium Hydroxide at 90°, and buffered HF to remove thermal oxide .following each step the collector is rinsed in demineralised water.
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