High-k dielectrics as bioelectronic interface for field effect transistors

Ion-sensitive field-effect transistors (ISFETs) are employed as bioelectronic sensors for the cell-transistor coupling and for the detection of D NA sequences. For these applications, thermally grown SiO2 films are used as standard gate dielectric. In the first part of this dissertation, the suitab lity of high-k dielectrics was studied to increase the gate capacitance and hence the sign al-tooise ratio of bioelectronic ISFETs: Upon culturing primary rat neurons on the c orresponding high-k dielectrics, Al 2O3, yttria stabilised zirkonia (YSZ), DyScO 3, CeO2, LaAlO3, GdScO3 and LaScO3 proved to be biocompatible substrates. Comprehensiv electrical and electrochemical current-voltage measurements and capacitance-voltag e measurements were performed for the determination of the dielectric properties of t he high-k dielectrics. High dielectric constants of the produced Al 2O3, YSZ und DyScO3 films facilitated the processing of ISFETs with considerably higher input capacitances than the standard SiO 2 ISFETs. The achieved breakdown fields of the Al 2O3, YSZ und DyScO3 films ensured their reliable employment as ISFET gate dielectric. Good stability against electrostatic charging and small interface state densities were concluded from small hysteresis in the capacitancevoltage measurements. In the second part of the dissertation, standard S iO2 ISFETs with lower input capacitance and high-k dielectric Al 2O3, YSZ und DyScO3 ISFETs were comprehensively characterised and compared with each other regardin g their signal-to-noise ratio, their ion sensitivity and their drift behaviour. One of the r esults was that the transconductance and the signal-to-noise ratio of the high-k ISFETs did not improve according to their higher input capacitance. It is assumed that high source a nd drain feed line resistances and the invalidity of the long-channel field-effect transis tor equation for the employed ISFETs are the underlying reasons. The ion sensitivity measure ments showed that the YSZ ISFETs were considerably more sensitive to K + and Na ions than the SiO 2, Al2O3 und DyScO3 ISFETs. Hence YSZ ISFETs are suited to study the im pact of the ion sensitivity of the gate dielectric in the cell-transistor coupling mechanis m. In long-term drift measurements, the YSZ ISFETs drifted much longer and further than the other ISFETs. Higher porosity of the YSZ films is assumed as underlying reason. In the final third part of the dissertation, bioele ctronic experiments were performed with the high-k ISFETs. Alternating deposition of o ppositely charged polyelectrolyte layers, a model experiment for DNA detection, was s uccessfully carried out with Al 2O3 and DyScO3 ISFETs. In first DNA detection experiments with Al 2O3 ISFETs, small hybridisation signals were measured. Coupling betwe n YSZ and DyScO3 ISFETs and the HL-1 heart cell line was performed successfully in proof of principle experiments. The shape of the signals, which were measured from HL-1 cells with YSZ ISFETs, differed considerably from the corresponding measurements wi th SiO2 and DyScO3 ISFETs: After the onset of the K + current, the action potentials measured with YSZ I FETs showed a strong drift in the direction opposite to the K + current signal. In the YSZ ion sensitivity measurements, it was shown that this drift is an in trinsic property of YSZ. Therefore, in a general conclusion, it is assumed that the signal s hapes measured in cell-transistor coupling experiments are influenced by the chemical properti es of the gate oxide surface. First coupling experiments between HEK 293 cells, which w ere transfected with a K + ion channel, and YSZ ISFETs affirmed the assumption fro m the HL-1 experiments that the characteristic drift behaviour of YSZ ISFETs influe nc s the measured signal shapes. In summary, especially DyScO 3 has proven to constitute an excellent bioelectroni c interface for field-effect transistors with high in put capacitance: The DyScO 3 films exhibited a high dielectric constant and good chemi cal stability in the ion sensitivity measurements and the long-term drift measurements. Furthermore, the biocompatibility of the DyScO3 films was demonstrated in successful coupling expe riments with electrically