Electrokinetics of all-around-gated ambipolar ionic field effect trnasistor

Abstract

In this dissertation, the advanced device was developed for overcoming the limitation of IFET which can be manipulate charged species pass through nanochannel. And the slow response time of nanofuidic device was studied for improving the device. we developed a versatile ionic field effect transistor (IFET) which has an ambipolar function for manipulating molecules regardless of their polarity and can be operated at wide range of electrolytic concentrations (10-5M~1M). The IFET has circular nanochannels completely covered by gate electrodes, called all-around-gate, with a aluminum oxide (Al2O3) of a near-zero surface charged oxide layer. Experimental and numerical validations were conducted for characterizing the IFET. We found that the versatility is originated from the zero-charge density of oxide layer and all-around-gate structure which increase the efficiency of gate effect 5 times higher than a previously developed planar-gate by capacitance calculations. Our numerical model adapts Poisson-Nernst-Plank-Stokes (PNPS) formulations with additional nonlinear constraints of fringing field effect and counter-ion condensation and the experiment and numerical results are well matched. The device can control the transportation of ions at severe concentration up to 1M electrolyte which resembles a backflow of a shale gas extraction process. Furthermore, while traditional IFETs can manipulate either positively or negatively charged species depending on the inherently large surface charge of oxide layer, the presenting device and mechanism provide effective means to control the motion of both negatively and positively charged molecules which is important in DNA sequencing through nanopore, medical diagnosis system and point-of-care system, etc. However the ambipolar IFET showed slow response time during ionic current measurement. Smeets analyzed the slow response time using a simple RC electric circuit, but it does not fit well to experiment data below t = 50 us. We hypothesized that the long response time comes from the electric double layer capacitor. Thus we developed a new model and equation that includes EDL capacitor. Silicon nitride nanopore device was made by traditional fabrication method. The device measurement was done by patch clamp. By matching the new model to experimental data, we showed the possibility of a better theory than Smeetsequation.