Kumjae Shin

Carbon-Nanotube-Modified Electrodes for Highly Efficient Acute Neural Recording

Microelectrodes are widely used for monitoring neural activities in various neurobiological studies. The size of the neural electrode is an important factor in determining the signal-to-noise ratio (SNR) of recorded neural signals and, thereby, the recording sensitivity. Here, it is demonstrated that commercial tungsten microelectrodes can be modified with carbon nanotubes (CNTs), resulting in a highly sensitive recording ability. The impedance with the respect to surface area of the CNT-modified electrodes (CNEs) is much less than that of tungsten microelectrodes because of their large electrochemical surface area (ESA). In addition, the noise level of neural signals recorded by CNEs is significantly less. Thus, the SNR is greater than that obtained using tungsten microelectrodes. Importantly, when applied in a mouse brain in vivo, the CNEs can detect action potentials five times more efficiently than tungsten microelectrodes. This technique provides a significant advance in the recording of neural signals, especially in brain regions with sparse neuronal densities.

A scanning microscopy technique based on capacitive coupling with a field-effect transistor integrated with the tip

We propose a method for measuring the capacitance of a thin layer using a Tip-on-Gate of Field-Effect Transistor (ToGoFET) probe. A ToGoFET probe with a metal-oxide-semiconductor field-effect transistor (MOSFET) with an ion-implant channel was embedded at the end of a cantilever and a Pt tip was fabricated using micro-machining. The ToGoFET probe was used to detect an alternating electric field at the dielectric surface. A dielectric buried metal sample was prepared; a sinusoidal input signal was applied to the buried metal lines; and the ToGoFET probe detected the electric field at the tip via the dielectric. The AC signal detected by the ToGoFET probe was demodulated by a simple AC-to-DC converter. Experimentally, it was shown that an electric field could be measured at the surface of the dielectric layer above a buried metal line. This promising result shows that it is possible to measure the surface local capacitance.

A micro-machined hydrophone using piezoelectricity on gate of a field-effect transistor

This paper presents the design and fabrication of a miniaturized hydrophone made by applying the electric field of a micro-sized piezoelectric body directly on the gate of a field-effect transistor (FET). Changes in the bound surface charge density of the piezoelectric body and the corresponding changes in the electric field in response to the applied pressure affect the channel current of a FET regardless of its size. The proposed transduction overcomes the sensitivity limitations of a micro-sized piezoelectric body for various underwater applications, especially at low frequencies. A complementary metal oxide semiconductor (CMOS)-integrated underwater sensor system can also be realized by using the proposed CMOS-compatible fabrication.

A Micro-Machined Microphone Based on a Combination of Electret and Field-Effect Transistor

Capacitive-type transduction is now widely used in MEMS microphones. However, its sensitivity decreases with reducing size, due to decreasing air gap capacitance. In the present study, we proposed and developed the Electret Gate of Field Effect Transistor (ElGoFET) transduction based on an electret and FET (field-effect-transistor) as a novel mechanism of MEMS microphone transduction. The ElGoFET transduction has the advantage that the sensitivity is dependent on the ratio of capacitance components in the transduction structure. Hence, ElGoFET transduction has high sensitivity even with a smaller air gap capacitance, due to a miniaturization of the transducer. A FET with a floating-gate electrode embedded on a membrane was designed and fabricated and an electret was fabricated by ion implantation with Ga+ ions. During the assembly process between the FET and the electret, the operating point of the FET was characterized using the static response of the FET induced by the electric field due to the trapped positive charge at the electret. Additionally, we evaluated the microphone performance of the ElGoFET by measuring the acoustic response in air using a semi-anechoic room. The results confirmed that the proposed transduction mechanism has potential for microphone applications.

A micromachined low frequency microphone based on a field effect transistor and an electret

Recently, several Internet of Things (IoT) devices using low-frequency acoustic sound have emerged as promising sensor applications. Unfortunately, the detection of low-frequency sound using miniaturized microphones is restricted due to the low cut-off frequency of capacitive type transduction. To overcome this limitation, a micromachined microphone based on a field-effect transistor (FET) and an electret was reported and its feasibility as a low-frequency microphone was demonstrated in 2015. However, the proposed microphone was realized by bonding two chips mechanically and the FET was embedded in a membrane, which was disadvantageous for sensitivity enhancement. To realize stable highly sensitive modulation, we devised and fabricated a structure in which the electric field due to an electret embedded in the membrane modulates the channel of the FET. The acoustic signal causes the electret mounted on the membrane to vibrate, which changes the distance between the channel of the FET and the electret. The ...

MEMS microphone based on the membrane with bias voltage and FET (field effect transistor) mechano-electrical transduction

Most commercially available microphones use a capacitive method, and their structure and performance are saturated to some extent. However, due to the limitation of the capacitive transduction method, the roll-off at the low frequency is inevitable, and there is a limit in reducing the mechanical thermal noise caused by the squeeze film damping that occurs between the membrane and backplate structure. Proposed FET (field effect transistor) based MEMS microphone detects a change in the source-drain current according to the gate voltage change of the FET induced by vibrating the membrane with the electric field. In the case of a MEMS microphone using the proposed FET, low frequency roll-off according to the energy conversion does not occur, and the size of the backplate can be drastically reduced as compared with the conventional MEMS microphone, thereby further reducing the mechanical thermal noise, leading to the possibility of achieving the higher SNR (signal to noise ratio). In this study, design and fabrication, performance test of the proposed FET based MEMS microphone are conducted. [Work supported by CMTC, UM15304RD3.]

A scanning probe mounted on a field-effect transistor: Characterization of ion damage in Si

We have examined the capabilities of a Tip-On-Gate of Field-Effect Transistor (ToGoFET) probe for characterization of FIB-induced damage in Si surface. A ToGoFET probe is the SPM probe which the Field Effect Transistor(FET) is embedded at the end of a cantilever and a Pt tip was mounted at the gate of FET. The ToGoFET probe can detect the surface electrical properties by measuring source-drain current directly modulated by the charge on the tip. In this study, a Si specimen whose surface was processed with Ga+ ion beam was prepared. Irradiation and implantation with Ga+ ions induce highly localized modifications to the contact potential. The FET embedded on ToGoFET probe detected the surface electric field profile generated by schottky contact between the Pt tip and the sample surface. Experimentally, it was shown that significant differences of electric field due to the contact potential barrier in differently processed specimens were observed using ToGOFET probe. This result shows the potential that the local contact potential difference can be measured by simple working principle with high sensitivity.

A micromachined microphone based on the membrane with bias voltage and field-effect-transistor mechano-electrical transduction

Recently, capacitive-type microphone dominates the MEMS (Micro-Electro-Mechanical-System) microphone market. Since sizes of the electrodes on the membrane and backplate determine the sensitivity of the microphone, there is a limit in reducing the size of the MEMS microphone. Here, we propose a micro-machined microphone consist of the membrane with bias voltage and field-effect-transistor (FET). The difference between the conventional capacitive MEMS microphone and proposed microphone is the Mechano-Electrical transduction mechanism. By biasing the voltage on membrane, the gate voltage is induced depending on the vibration of the membrane, eventually changes the source drain current. In case of a MEMS microphone using the proposed FET and biased membrane microphone, low frequency roll-off according to the energy conversion does not occur, and the size of the membrane and backplate can be reduced as compared with the conventional MEMS microphone, without the degradation of sensitivity. In addition, dependin...

A micromachined microphone based on the electret membrane and field-effect-transistor mechano-electrical transduction

Most commercially available MEMS (Micro-Electro-Mechanical-System) microphones use a capacitive method, and their structure and performance are saturated to some extent. However, due to the limitation of the capacitive transduction method, the roll-off at the low frequency is inevitable, and there is a limit in reducing the mechanical thermal noise caused by the squeeze film damping that occurs between membrane and backplate structure. Proposed electret membrane and FET (Field Effect Transistor) based MEMS microphone detects a change in the source-drain current according to the gate voltage change of the FET induced by vibrating the membrane with the fixed charges. In the case of a MEMS microphone using the proposed FET, low frequency roll-off according to the energy conversion does not occur, and the size of the backplate can be drastically reduced as compared with the conventional MEMS microphone, thereby further reducing the mechanical thermal noise, leading to the possibility of achieving the higher S...

Field-effect transistor-based transduction and acoustic receiving transducers

Microphones and hydrophones are representative acoustic receiving transducers. To properly receive sound waves, a receiver must be smaller than the wavelength of the target sound. The target wave characteristics do not impose any lower limits on the size of microphones. When the performance of a smaller microphone or hydrophone will be satisfactory, users generally choose a smaller device since smaller receivers are easier to install and use. However, miniaturized microphones are less sensitive at low frequencies and conventional infrasound detectors are considerably larger than those for higher frequency sounds. These trends in receiver size can be explained by considering the transduction characteristics of microphones and hydrophones. We describe two transduction mechanisms based on field-effect transistors (FET) and use them to develop new microphones and hydrophones. We used theoretical analysis and experiments to show that the sensitivity and frequency response functions of FET-based microphones and...