Unipolar spin transistors and magnetic bipolar transistors

Abstract

Until recently the field of spintronics has focused on magnetic metals for conducting components [1]. Multilayer spintronic devices, such as giant magnetoresistive (GMR)[2] and magnetic tunnel junction (MTJ) [3] devices, have revolutionized magnetic sensor technology and hold promise for reprogrammable logic and nonvolatile memory applications. In these devices an electric current flows through two ferromagnetic regions, and the device function relies on a significant change in the resistance depending on whether the magnetizations of the two regions are oriented parallel or antiparallel to each other. The device performance improves as the spin polarization of the constituent material approaches 100%, and thus there are continuing efforts to find 100% spin-polarized conducting materials. Interest in magnetic semiconductors for semiconductor spintronics can be traced in part to their near-100% spin polarization; the high spin polarization comes from the small Fermi energies of these materials compared to magnetic metals.Many recent semiconductor spintronic device designs rely on spin-sensitive material properties other than the resistivity, including spin-sensitive gain and spin-selective optical properties, to provide new functional behavior. Two of these are the recently proposed unipolar spin transistors[4-6] and magnetic bipolar transistors[7]. In both cases the different electronic structure for spin-up and spin-down carriers in an inhomogeneous magnetic semiconductor is used to separately manipulate the spin-up and spin-down currents and densities. These semiconductor spintronic device designs offer the potential for novel reprogrammable logic circuits, sources of highly spin-polarized current, differential amplification of spin currents, and polarization-selective optical switches.The unipolar spin transistor was motivated by a fundamental analogy between unipolar ferromagnetic materials and nonmagnetic bipolar materials. The two types of carriers in the bipolar materials, electrons and holes, are analogous to the spin-up and spin-down carriers of one charge type (for specificity electrons) in the ferromagnetic material. In bipolar materials the doping level determines whether electrons or holes are the majority carriers, and the doping level is set when the material is grown. In a nearly 100% spin-polarized ferromagnet, however, the magnetization orientation determines whether spin-up or spin-down carriers are majority carriers. The magnetization orientation, in contrast to the doping level of a material, can be simply manipulated in a device, which provides new opportunities for reprogrammable logic.The analogy between nonmagnetic bipolar materials and unipolar ferromagnetic materials can be extended to devices by comparing a unipolar spin diode (a ferromagnetic material with a 1800 domain wall) and a traditional p-n diode. Shown in Fig. 1(a) are the band edges of the conduction and valence band for a traditional p-n diode in equilibrium. The quasifermi levels are shown as dashed lines. To assist in expl...