Abstract

A solid-state magnetic sensor made with semiconductor materials generates an electric signal under the presence of a magnetic induction. This electric signal is the result of the Lorentz force on the mobile carriers and of a series of physical phenomena known as galvanomagnetic effects in semiconductors. A solid-state magnetic sensor is designed in such a way that the interaction between the magnetic induction and the mobile carriers inside the semiconductor material gives a maximum response. The physical properties of the semiconductor material used to build up the sensor play an important role.

The carrier mobility is the most important parameter for building magnetic sensors with semiconductor materials. Because the carrier deflection is proportional to the mobility of the carriers, the higher the mobility, the higher the deflection, and smaller magnetic inductions can be detected. However, building a magnetic sensor with semiconductor materials that offers higher carrier mobilities is expensive and it is only worthwhile if there is no other given choice. Besides, integrating the electronics for the processing of the electric signal in the same chip of the magnetic sensor can be desirable.

Among the various well established semiconductor processes, the complementary metal-oxide-semiconductor (CMOS) technology is the most popular. Mainly based on silicon, one can produce millions of identical devices at very low cost. Building a magnetic sensor with silicon and taking advantage of the CMOS process steps results in low-cost integrated sensors, because the sensor electronics can be easily integrated. But this goal can be only reached if building the sensor structure does not modify the standard steps of the CMOS process. Post-processing steps cannot be made if the building cost of the integrated sensor has to be kept low.

The split-drain magnetic metal-oxide-semiconductor field-effect transistor, MAGFET for short, is the ideal choice for integrated magnetic sensors in CMOS technology. Its fabrication does not need any post-processing step and actually it is full compatible with the CMOS process. The electronics for the processing of the electric signal can be built in the same substrate as for the sensor. Actually, the CMOS technology is optimized for building MOSFETs and the MAGFET functionality is based on the inversion layer of a common MOSFET. However, the carrier mobility in the inversion layer of a MOSFET is not as high as in the bulk of silicon or even, of many other semiconductor materials.

The split-drain MAGFET has its drain split into two or more drains. Under zero magnetic induction operation, the drains equally share the total drain current. If a magnetic induction is applied perpendicular to the inversion layer of a MAGFET, the drain currents experience an imbalance. As a result, a differential current can be measured at the drains. This differential current is proportional to the mobility of the carriers. At room temperature, only large magnetic inductions can be detected. If the device is cooled to liquid Nitrogen temperature, the carrier mobility is increased and the differential current too. As a result, smaller magnetic inductions can be detected. Besides, cryogenic operation of silicon devices offers some advantages, as for example, better signal-to-noise ratios.

For a better understanding of the electro-magnetic interaction inside the semiconductor materials, three-dimensional simulations are required. The vectorial nature of the Lorentz force requires such analysis in the space that three-dimensional simulations can properly make. Also, variations on geometric parameters can be better analyzed, because analytical approximations does not take properly into account such variations. Details on the potential and carrier concentration under the presence of a magnetic induction can be better understood if simulations are carried out at both, room temperature and liquid Nitrogen temperature.

This thesis presents simulation results of split-drain MAGFETs. Measurements of two-drain MAGFETs are accurately reproduced by simulations at both, room temperature and liquid Nitrogen temperature. An analysis of the relative sensitivity, the main figure of merit of magnetic sensors, is made at various bias and geometric conditions for two-drain and three-drain MAGFET structures. By means of a discretization scheme that takes into account an arbitrary magnetic field, three-dimensional simulations have been performed with MINIMOS-NT.