Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2003-03-27
2004-06-22
Abraham, Fetsum (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S183000, C257S192000
Reexamination Certificate
active
06753562
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to spintronic (spin electronic) device, comprising semiconductor structures in combination with ferromagnetic components. More specifically, it relates to information storage of integrated circuit memory devices using spintronic memory cells.
Semiconductor integrated circuit (IC) memory devices have replaced magnetic-core memory devices due to their lower fabrication cost and higher performance. An IC memory circuit includes a repeated array of memory cells which store one state of a two state information (0 or 1), or multi-state information (for example, 00, 01, 10, or 11 of 4 states), together with support circuitry such as a row decoder, a column decoder, a write circuit to write to the memory cell array, a control circuitry to select the correct memory cell, and a sense amplifier to amplify the signal.
One early memory circuit is a flip-flop that has an output that is stable for only one of two possible voltage levels. SRAM (static random access memory) circuit stores information in flip-flops where the information can be read from any memory cell at random (random access memory), and where the stored information can be kept indefinitely as long as the circuit receives power.
The next generation memory cell is a DRAM (dynamic random access memory) cell. A DRAM cell typically consists of a transistor and a capacitor. The capacitor stores information in the form of electrical charge and the transistor provides access to the capacitor. Because of the inherent leakage of the capacitor charge, DRAM cells must be rewritten or refreshed at frequent intervals.
SRAM and DRAM memories cannot retain the stored information without a power source, therefore they belong to a class of memory called volatile memory. Another class of memory is called non-volatile memory which will still retain the stored information even after the power is turned off.
A typical non-volatile memory is ferroelectric random access memory (FRAM). Similar to a DRAM cell, a FRAM cell consists of an access transistor and a storage capacitor. The difference is that FRAM cell uses ferroelectric material for its capacitor dielectric wherein the stored information is the polarization of the ferroelectric material. Ferroelectric material can be polarized by an electric field with a polarization lifetime of about 10 years.
Recent developments of materials that have changeable electrical resistance have introduced a new kind of non-volatile memory, called RRAM (resistive random access memory). The basic component of a RRAM cell is a variable resistor. The variable resistor can be programmed to have high resistance or low resistance (in two-state memory circuits), or any intermediate resistance value (in multi-state memory circuits). The different resistance values of the RRAM cell represent the information stored in the RRAM circuit.
The advantages of RRAM are the simplicity of the circuit which leads to smaller devices, the non-volatile characteristic of the resistor memory cell, and the stability of the memory state.
Since resistor is a passive component and cannot actively influence nearby electrical components, a basic RRAM cell can be just a variable resistor, arranged in a cross point resistor network to form a cross point memory array. To prevent cross talk or parasitic current path, a RRAM cell can further include a diode, and this combination is sometimes called a 1R1D (or 1D1R) cross point memory cell. To provide better access, a RRAM can include an access transistor, as in DRAM or FRAM cell, and this combination is sometimes called a 1R1T (or 1T1R) cross point memory cell.
The resistance states of the RRAM can be represented by different techniques such as structural, polarization, or magnetization state. Chalcogenide alloy is an example of structural state RRAM device. Chalcogenide alloys can exhibit two different stable reversible structural phases, namely an amorphous phase with high electrical resistance and a polycrystalline phase with lower electrical resistance. Resistive heating by an electrical current pulse can change the phases of the chalcogenide materials. One example of polarization state is a polymer memory element. The resistance state of a polymer memory element is dependent upon the orientation of polarization of the polymer molecules. The polarization of a polymer memory element can be written by applying an electric field.
MRAM (magnetic random access memory) is another class of RRAM circuits using magnetic properties for storing information based on magnetoresistance effect wherein the resistance of a magnetic material can be programmed. The magnetoresistance effect in MRAM devices are caused by the spins of electrons (corresponding to the rotation of the electron around its own axis).
In ferromagnetic materials, the electron spins can be aligned in one direction under the influence of an external field, and can keep their alignment even after the external field is removed (the hysteresis effect). At high temperatures (above Curie temperature), the ferromagnetic materials become paramagnetic (non-magnetic) because of the loss of the spin alignment due to high thermal energy.
In an MRAM cell employing magnetoresistance effect, conduction carriers (electrons or holes) with certain spins alignment are generated from a spin-polarized ferromagnetic source, and are injected into a non-ferromagnetic channel, and then detected at a spin-analyzer ferromagnetic drain. The conduction carriers are allowed to move more freely from the ferromagnetic source to the ferromagnetic drain if the magnetization in those source and drain are parallel than if they are antiparallel or partially antiparallel. The variation of the resistance is the magnetoresistance effect and the device is often called a spin valve device in view of the fact that the magnetization states of the ferromagnetic source and drain act like a valve for spin-polarized carriers.
In the typical spin valve devices, the non-ferromagnetic channel is a non-ferromagnetic metal. To improve the magnetoresistance effect, the channel can be a thin insulator, and the spin valve device is called a magnetic tunnel junction (MTJ). In a MTJ, the magnetoresistance results from the spin-polarized tunneling of conduction electrons between the two ferromagnetic layers. The tunneling current depends on the relative orientation of the magnetic moments of the two ferromagnetic layers.
The operation of the spin valve devices depends on the difference in the magnetization states of the two ferromagnetic layers. In practical devices, one of the ferromagnetic layers is pinned while the other ferromagnetic layer is free to change polarization. This free layer stores information based on the direction of the magnetic polarization with respect to the pinned layer. A variation of the spin valve structure is a pseudo spin valve wherein the pinned ferromagnetic layer is replaced by a thicker ferromagnetic layer with higher coercive strength. The higher coercive strength prevents the thicker ferromagnetic layer to change magnetic polarization during the change of magnetic polarization of the other ferromagnetic layer.
Further developments of the spin valve devices have led to a hybrid field of semiconductor and magnetic materials called spintronic (spin electronic). Semiconductors and magnetic materials have been studied extensively, but only recently devices having a combination of properties and functions of both materials were studied.
In a spintronic device, the non-ferromagnetic channel is a semiconductor material. However, spin injection from a ferromagnetic material to a semiconductor material is difficult due to the heterojunction between the ferromagnetic material and the semiconductor material. The heterojunction represents an important quantity which may place fundamental constraints on the expected efficiency of the injection process. Interfacial problems and the differences in carrier number and energy levels across the interface are possible difficulties in the heterojunction between the fe
Hsu Sheng Teng
Sakiyama Keizo
Tang Jinke
Abraham Fetsum
Rabdau Matthew D.
Ripma David C.
Sharp Laboratories of America Inc.
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