Spin Polarized Feild Effect Transistor (Spin-FET)

Spin polarized field effect transistor (Spin FET) was proposed by Datta Das in 1990 [1]. This has not been realized yet, but is regarded as one of the most advanced applications of spintoronics in the future.

In these devices a non magnetic layer which is used for transmitting and controlling the spin polarized electrons from source to drain plays a crucial role. For functioning of this device first the spins have to be injected from source into this non-magnetic layer and then transmitted to the collector. These non-magnetic layers are also called as semimetals, because they have very large spin diffusion lengths. The injected spins which are transmitted through this layer start precessing as illustrated in Figure 1 before they reach the collector due to the spin-orbit coupling effect.

Fig. 1 Spin polarized field effect transistor. Vg is the gate voltage. When Vg is zero the injected spins which are transmitted through the 2DEG layer starts precessing before they reach the collector, thereby reducing the net spin polarization. Vg is the gate voltage. When Vg >> 0 the precession of the electrons is controlled with electric filed thereby allowing the spins to reach at the collector with the same polarization.

Hence the net spin polarization is reduced. In order to solve this problem an electric field is applied perpendicularly to the plane of the film by depositing a gate electrode on the top to reduce the spin-orbit coupling effect as illustrated in Figure 4. By controlling the gate voltage and polarity can the current in the collector can be modulated there by mimicking the MOSFET of the conventional electronics. Here again the problem of conductivity mismatch between the source and the transmitting layer is an important issue. The interesting thing would be if a Heusler alloy is used as the spin source and a semimetallic Heusler alloy as the transmitting layer, the problem of conductivity mismatch may be solved. For example from the Slater-Pauling curve Mt = Zt - 24, Heusler alloys with Mt >>0 can act as spin sources and alloys with Mt ~ 0 can act as semimetals. Since both the constituents are of same structure the possibility of conductivity mismatch may be less.

1. S. Datta, B. Das, et al., Appl. Phys. Lett. 56 665 (1990).