A magnetic sensor based on efficient spin injection of spin-polarized electrons from ferromagnets into semiconductors and rotation of electron spin under a magnetic field. Previous spin injection structures suffered from very low efficiency (less than 5%). A spin injection device with a semiconductor layer sandwiched between &dgr;-doped layers and ferromagnets allows for very high efficient (close to 100%) spin polarization to be achieved at room temperature, and allows for high magneto-sensitivity and very high operating speed, which in turn allows devising ultra fast and sensitive magnetic sensors.