Shahrir R. Kasjoo

Room-temperature operation of a unipolar nanodiode at terahertz frequencies

We report on the room-temperature electrical rectification at 1.5 THz of a unipolar nanodiode based on symmetry breaking in a nanochannel. The exploitation of its nonlinear diodelike characteristic and intrinsically low parasitic capacitance enables rectification at ultrahigh speed. The zero-voltage threshold and unique planar layout make the nanodiode suitable for building large arrays. This is the highest speed reported in nanorectifiers to date.

Electrical detection of dengue virus (DENV) DNA oligomer using silicon nanowire biosensor with novel molecular gate control

In this paper, a silicon nanowire biosensor with novel molecular gate control has been demonstrated for Deoxyribonucleic acid (DNA) detection related to dengue virus (DENV). The silicon nanowire was fabricated using the top-down nanolithography approach, through nanostructuring of silicon-on-insulator (SOI) layers achieved by combination of the electron-beam lithography (EBL), plasma dry etching and size reduction processes. The surface of the fabricated silicon nanowire was functionalized by means of a three-step procedure involving surface modification, DNA immobilization and hybridization. This procedure acts as a molecular gate control to establish the electrical detection for 27-mers base targets DENV DNA oligomer. The electrical detection is based on the changes in current, resistance and conductance of the sensor due to accumulation of negative charges added by the immobilized probe DNA and hybridized target DNA. The sensitivity of the silicon nanowire biosensors attained was 45.0µAM(-1), which shows a wide-range detection capability of the sensor with respect to DNA. The limit of detection (LOD) achieved was approximately 2.0fM. The demonstrated results show that the silicon nanowire has excellent properties for detection of DENV with outstanding repeatability and reproducibility performances.

Low-frequency noise of unipolar nanorectifiers

Unipolar nanodiodes, also known as self-switching devices, have recently been demonstrated as terahertz detectors at room temperature. Here, we study their low-frequency noise spectra and noise equivalent power and show that both performance parameters are comparable to those reported for state-of-the-art Schottky diodes. The truly planar nanodiode layout enables building structures with thousands of devices connected in parallel, which reduce low-frequency noise without affecting sensitivity. The observed 1/f noise can be described by Hooge’s mobility fluctuation theory.

Terahertz Detection Using Nanorectifiers

We report on the low-temperature detection of free-space radiation at 1.5 THz using a unipolar nanodiode, known as the self-switching diode (SSD), coupled with a spiral microantenna. The SSD, based on an asymmetric nanochannel, has a diode-like characteristic that can be utilized in rectifying high-frequency electrical signals. The truly planar structure of the SSD not only provides intrinsically low parasitic capacitance that enables rectification at ultrahigh speed, but also allows the fabrication of a large SSD array in parallel without the need for interconnection layers. The extrinsic voltage responsivity of the SSD-based detector achieved was ~15.6 V/W, but the estimated intrinsic voltage responsivity was ~45 kV/W.

Low-Frequency Noise of a Ballistic Rectifier

A ballistic-electron transport-based semiconductor nanorectifier, also known as a ballistic rectifier, has been demonstrated to have an intrinsic zero threshold voltage. In this paper, we characterize its low-frequency noise properties and show that the zero-threshold property enables elimination of the flicker noise. As a potential terahertz detector, the ballistic rectifier exhibits a noise equivalent power in the range of commercially available, uncooled thermal terahertz detectors. The observed noise in the device at finite biases is modeled quantitatively. The derived simple formula reveals that the narrowest part of the electron channels has a dominant role in the device noise properties.

Novel Terahertz nanodevices and circuits

Terahertz (THz) technology has attracted rapidly increasing attention due to a very broad range of potential applications, e.g., medical imaging and homeland security. Perhaps more importantly, developing electronic devices capable of operating at THz frequencies will have great impact on future generation computation and communication. Despite enormous effort in recent years, THz field is still largely unexploited due to the bottleneck issue of the lack of compact, solid-state, room-temperature detectors and emitters. Here we overview our recent work on the THz operations of novel nano-diodes that can detect and emit THz waves at room temperature. Apart from the very high speed, these novel diodes also have characteristics such as zero threshold and quadratic rather than exponential current-voltage response, which are particularly important for applications including THz imaging and energy harvesting. These unique characteristics are possible because the planar nanodevices are based on completely new working principles from conventional diodes, i.e., the rectifying functionality does not rely on any pn junction or tunneling barrier. In our experiments, different antenna structures including spiral, dipole and bow-tie are fabricated to couple the nanodevices to free-space THz waves up to a few THz. Apart from THz imaging and communications, the possibility to extend the technology to mid-infrared frequencies for heat energy harvesting is very attractive because of the potentially high efficiency and low cost as compared with conventional thermoelectric devices.

Fabrication of nanodiodes using atomic-force microscope lithography

Unipolar nanodiodes, known as the self-switching diodes (SSDs), have recently been demonstrated as terahertz (THz) detectors at room temperature. The SSDs have also shown promising properties as THz emitters and nanomemory devices. Here, we report the fabrication of SSDs on a GaAs/AlGaAs substrate using an atomic-force microscope (AFM) lithography which utilizes AFM-tip ploughing technique and the use of a suitable polymethyl methacrylate layer with thermal-annealing treatment. This approach has successfully overcome some typical problems associated with the tip-ploughing method including the refilling of the SSDs trenches by debris generated during the ploughing process. In this report, all SSDs defined using the AFM lithoghraphy have shown standard diode-like I-V characteristics, indicating the reproducibility of the abovementioned approach. In addition, this method might be useful to realize electronic devices in nanoscale dimensions.

Characterization of self-switching diodes as microwave rectifiers using ATLAS simulator

A planar nanodevice, known as the self-switching diode (SSD), has a nonlinear current-voltage (I-V) characteristic which can be exploited as a high-speed rectifier in a wide range of applications. This diode-like I-V behaviour is achieved due to the structure of the device which consists of an asymmetric nanochannel. In this work, the rectification properties of silicon-based SSDs with different geometry of the nanochannel were reported. The characterization have been performed by means of ATLAS device simulator. The results obtained can be used to facilitate in improving the rectifying performance of the device.

RF characterization of zero-bias planar nanodiodes coupled with a narrowband dipole antenna

In this report, we developed a simple rectenna (rectifying antenna) which consists of a large array of unipolar nanorectifiers, namely the self-switching diodes (SSDs), coupled to a narrowband dipole antenna with resonant frequency of 890 MHz. Due to the planar nature of the SSDs structure, the large array was able to be fabricated in a single lithography step in which the layers for interconnection were not required. This allows for a simple and low-cost fabrication process. The RF characterization of the SSD-based rectenna was performed in the near- and far-field regions at unbiased condition which offers less consumption of the dc power.

An overview of self-switching diode rectifiers using green materials

A unipolar two-terminal nanodevice, known as the self-switching diode (SSD), has recently been demonstrated as a room-temperature rectifier at microwave and terahertz frequencies due to its nonlinear current-voltage characteristic. The planar architecture of SSD not only makes the fabrication process of the device faster, simpler and at a lower cost when compared with other rectifying diodes, but also allows the use of various materials to realize and fabricate SSDs. This includes the utilization of ‘green’ materials such as organic and graphene thin films for environmental sustainability. This paper reviews the properties of current ‘green’ SSD rectifiers with respect to their operating frequencies and rectifying performances, including responsivity and noise-equivalent power of the devices, along with the applications.A unipolar two-terminal nanodevice, known as the self-switching diode (SSD), has recently been demonstrated as a room-temperature rectifier at microwave and terahertz frequencies due to its nonlinear current-voltage characteristic. The planar architecture of SSD not only makes the fabrication process of the device faster, simpler and at a lower cost when compared with other rectifying diodes, but also allows the use of various materials to realize and fabricate SSDs. This includes the utilization of ‘green’ materials such as organic and graphene thin films for environmental sustainability. This paper reviews the properties of current ‘green’ SSD rectifiers with respect to their operating frequencies and rectifying performances, including responsivity and noise-equivalent power of the devices, along with the applications.