Oxland, Richard K.
An HBT magnetic sensor with integrated 3-dimensional magnetic structures.
PhD thesis, University of Glasgow.
Full text available as:
The applicability and functionality of high frequency digital and millimetre wave circuits can be enhanced by the integration of sensor elements into the circuits. It is furthermore advantageous to utilise or modify the pre–existing fabrication process flow in creating this added functionality. This thesis describes a work on magnetic field sensors based on an InP/InGaAs heterojunction bipolar transistor (HBT) which has been fabricated to be compatible with high frequency epilayer structure and processes.
In this work, the complete fabrication process for the HBT magnetic sensors has been developed, using standard, transferrable process modules. Ohmic contact metallisations were optimised and D.C. electrical characterisations are also reported upon. The effects of several surface treatments on device performance have been studied and characterised. Surface passivation using two distinct sulphur containing compounds of different phases was shown to enhance performance and an ion bombardment process was developed that degraded surface quality and increased surface leakage currents for enhanced sensor performance.
In order to improve the sensitivity of an HBT to magnetic field 3–dimensional magnetic structures were designed to be incorporated onto the surface of the extrinsic base.
This design process was informed by simulation of magnetic field profiles of the magnetic elements and fabrication processes were created that would allow for arbitrary 3–dimensional structures.
The response to magnetic field applied both parallel and perpendicular to the normal of the wafer of an as–fabricated HBT was investigated. Two different emitter structures were compared, a simple square emitter and a multiple finger emitter, and the ability of the devices to resolve applied field angle was uncovered. The effects of device bias on the field response was also looked at and the optimal bias conditions determined. An analysis of the temperature variation of the magnetic field response was conducted with lower temperatures resulting in higher sensitivity to applied field.
Finally, the response of an HBT with integrated 3–dimensional magnetic structures was investigated. A passivated device was found to be less sensitive to applied magnetic field and a device treated with ion bombardment to be more sensitive to magnetic field applied parallel to the normal. The signal to noise ratio for an HBT with integrated magnetic structures was 36.4 dB with an equivalent noise of 0.002 T. The maximum magnetic field strength sensitivity was 0.339 T^(−1) and the maximum magnetic field applied angle sensitivity was 0.119 rad^(−1). The maximum change in normalised D.C. current gain was 0.019. A mathematical description of the change in current gain caused by a given magnetic field applied at a given angle was also determined.
Actions (login required)