Browsing by Author "Khalid, Ata"
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Item Open Access Investigation of contact edge effects in the channel of planar Gunn diodes(IEEE, 2019-11-26) Mindil, Ahmed; Dunn, G. M.; Khalid, Ata; Oxley, C. H.The effect of the edge of the channel on the operation of Planar Gunn diodes has been examined using Monte Carlo simulations. High fields at the corner of the anode contact are known to cause impact ionization and consequent electroluminescence, but our simulations show that the Gunn domains are attracted to these corners, perturbing the formation of the domains which can lead to chaotic dynamics within the rest of the channel leading to uneven heating and reduced RF output power. We show how novel shaping of the electrical contacts at the ends of the channel reduces the attraction and restores the domain wave-fronts for good device operation.Item Open Access Investigation of high-frequency fine structure in the current output of shaped contact planar gunn diodes(IEEE, 2020-04-08) Mindil, Ahmed; Dunn, Geoffrey; Khalid, Ata; Oxley, Chris H.A novel gallium arsenide (GaAs) planar Gunn diode design with shaped anode and cathode contacts using Monte Carlo simulations has been shown to produce significantly higher frequency fine structure components in the output waveform than the natural transit time frequency of the diode. We have investigated devices without a feed- back potential and devices with a feedback potential (in the delayed mode) and have shown 350-GHz fine structure frequency components in a device with a nominal transit time frequency of 70 GHz is possible. This is the first observation of such stable repeating high-frequency components in a Gunn diode, giving potential for very high-frequency power generation and other wave-shaping applications.Item Open Access A low-cost miniature immunosensor for haemoglobin as a device for the future detection of gastrointestinal bleeding(Royal Society of Chemistry, 2024-01-11) Demirhan, Alper; Chianella, Iva; Patil, Samadhan B.; Khalid, AtaGastrointestinal bleeding (GIB) is a serious medical condition, which requires immediate attention to establish the cause of the bleeding. Here, we present the development of a miniaturised electrochemical impedance spectroscopy (EIS) device for the detection of GIB. The device performs EIS measurements up to 100 kHz. Following the development of an immunosensor for haemoglobin (Hb) on screen printed electrodes, the EIS device was used for detecting Hb as an early indication of bleeding. The sensor was able to detect Hb in a redox solution in a linear range between 5 μg mL−1 and 60 μg mL−1, with a limit of detection of 13.3 μg mL−1. It was also possible to detect Hb in simulated intestinal fluid, without the need for a redox solution, within a range of 10 μg mL−1 to 10 mg mL−1 with a limit of detection of 2.31 mg mL−1. The miniature EIS device developed in this work is inexpensive, with an estimated cost per unit of £30, and has shown a comparable performance to existing commercial tools, demonstrating its potential to be used in the future as an ingestible sensor to detect GIB. All these measurements were carried out in a purpose built flow cell with supporting hardware electronics outside the cell. Integration of the hardware and the sensing electrodes was demonstrated in pill form. This pill after integration sampling fluidics has potential to be used in detecting gastrointestinal bleeding.Item Open Access Resonant tunneling diode terahertz sources with up to 1 mW output power in the J-band(IEEE, 2019-12-12) Al-Khalidi, Abdullah; Alharbi, Khalid Hamed; Wang, Jue; Morariu, Razvan; Wang, Liquan; Khalid, Ata; Figueiredo, José M. L.; Wasige, EdwardTerahertz (THz) oscillators based on resonant tunneling diodes (RTDs) have relatively low output power, tens to hundreds of microwatts. The conventional designs employ submicron-sized RTDs to reduce the device self-capacitance and, as a result, realize higher oscillation frequencies. However, reducing the RTD device size leads to lower output power. In this article, we present RTD oscillators that can employ one or two RTD devices of relatively large size, 9-25 μm2, for high power and, at the same time, can oscillate at THz frequencies. This is achieved through low resonating inductances realized by microstrip or coplanar waveguide transmission line short stubs with low characteristic impedances (Z0), which have lower inductance values per unit length and so compensate the increase of the self-capacitance of large area RTD devices. Thus, fabrication using only photolithography is possible. It is also shown that device sizing, which is limited only by bias stability considerations, does not limit device bandwidth. Further, we report a new way to estimate the RTD oscillator output power with frequency. A series of oscillators with oscillation frequencies in the 245-309 GHz range and output powers from 0.1 to 1 mW have been demonstrated showing the feasibility of the proposed approach.Item Open Access Single-chip, mid-infrared array for room temperature video rate imaging(The Optical Society, 2017-12-20) Xie, Chengzhi; Aziz, Mohsin; Pusino, Vincenzo; Khalid, Ata; Steer, Matthew; Thayne, Iain G.; Sorel, Marc; Cumming, David R. S.The need for energy efficiency and lower emissions from industrial plants and infrastructures is driving research into novel sensor technologies, especially those that allow observing and measuring greenhouse gases, such as CO2CO2. CO2CO2 emissions can be captured using mid-infrared imagers, but at present, these are based on hybrid technologies that need expensive manufacturing and require cooling. The high price tag prevents a wider diffusion of mid-infrared imagers and hence their use for many low-cost and large-volume applications. Here we report a monolithic III-V technology that integrates GaAs transistors with an InSb photodiode array. The monolithic material system reduces costs and provides an excellent platform for the sensor system-on-chip. We present a focal plane array imaging technology operating at room temperature in the 3–6 μm wavelength range that will address the need for identification and measurement of a range of industrially important gases.