On field-effect transistor concepts beyond traditional application
The metal-oxide-semiconductor field-effect transistor (MOSFET) has been the basis of state-of-the-art electronics for the last decades. Their introduction into integrated circuits (ICs) has opened the door to the immense increase in computational power we have experienced. However, the versatility and potential of MOSFETs far exceeds the mere use as an electric switch in integrated circuits. This thesis presents two novel devices, which are both based on the MOSFET concept, to create approaches to areas beyond conventional computing. Adding a floating gate and applying the specialised MemFlash wiring scheme transforms simple MOSFETs into memristive devices, promising candidates for the application as artificial synapses. Such novel devices are needed to emulate the functionalities of fundamental building block in the brain, in order to implement novel computing systems based on neuronal functionalities. Moreover, MOSFETs have been the basis of numerous sensors, such as Bio- and ChemFETs. In this work, a MOSFET based stress sensing device is presented, called the piezoelectric field-effect transistor. Utilising MOSFET based memory cells as memristive devices allows the integration of novel approaches to computing into existing fabrication technologies, since it combines neuromorphic engineering with modern silicon technology. However, present memory devices like EEPROM cells experience high power consumption. The same is seen for memristive devices based on the MemFlash concept. In this work, a way to not only reduce power consumption but at the same time adjust the learning behaviour of the memristive MemFlash devices by tuning of the tunnelling oxide thickness is shown. Furthermore, a novel MOSFET based stress sensor is presented in this work. A piezoelectric layer in the gate stack of the device allowed the transistor to react to externally applied stress. The combination of the highly integrable and CMOS compatible silicon technology of MOSFETs and the piezoelectric effect of low temperature AlN has great potential for application in many fields, such as stress and tactile sensing, accelerometers and even magnetic field sensing. For this novel sensor device fabrication and characterisation procedures were developed, allowing the application of a determined stress across the piezoelectric field-effect transistor. The usage of different piezoelectric materials allowed CMOS compatible devices with different sensitivities to be created. A touch sensor based on the piezoelectric field-effect transistor concept demonstrates a possible application of this novel device.
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