Yeon-Sik Chae

94-GHz Log-Periodic Antenna on GaAs Substrate Using Air-Bridge Structure

We fabricated a compact, 94-GHz log-periodic planar antenna on GaAs substrate. We used millimeter-wave monolithic integrated circuit (MMIC) process technology to fabricate the on-chip antenna. An air-bridge crossover is used to replace the conventional bonded structure for crisscross connection, which is compatible with active devices and circuits. The fabricated antenna chip size is 1.2 times 2.6 mm2 . Measurement results show that this antenna can operate from 87 to 99.5 GHz with its return loss better than -10 dB. At 94 GHz, the log-periodic antenna has a return loss of -11 dB and 4.8 dBi of gain.

All-inorganic spin-cast quantum dot based bipolar nonvolatile resistive memory

We introduce an all-inorganic solution processed bipolar nonvolatile resistive memory device with quantum dot/metal-metal oxide/quantum dot structure. The two terminal device exhibits excellent switching characteristics with ON/OFF ratio >103. The device maintained its state even after removal of the bias voltage. The switching time is faster than 50 ns. Device did not show degradation after 1-h retention test at 150 °C. The memory functionality was consistent even after multiple cycles of operation and the device is reproducible. The switching mechanism is discussed on the basis of charge trapping in quantum dots with metal oxide serving as the barrier.

Design and Fabrication of Planar GaAs Gunn Diodes

We studied planar graded-gap injector GaAs Gunn diodes designed for operation at 94GHz. Two types of planar Gunn diodes were designed and fabricated. In the first diode, a cathode was situated inside a circular anode with a diameter of 190μm. The distance between the anode and cathode varied from 60μm to 68μm depending on the cathode size. Also, we designed a structure with a constant distance between the anode and cathode of 10μm. In the second diode, the anode was situated inside the cathode for the flip-chip mounting on the oscillator circuits. The fabrication of the Gunn diode was based on ohmic contact metallization, mesa etching, and air-bridge and overlay metallization. DC measurements were carried out, and the nature of the negative differential resistance, the operating voltage, and the peak current in the graded-gap injector GaAs Gunn diodes are discussed for different device structures. It is shown that the structure with the shorter distance between the cathode and anode has a higher peak current, higher breakdown voltage, and lower threshold voltage than those of the structure with the larger distance between the cathode and anode.

Development of GaAs Gunn Diodes and Their Applications to Frequency Modulated Continuous Wave Radar

In this work, we have designed and fabricated the GaAs Gunn diodes for a 94 GHz waveguide voltage controlled oscillator (VCO) which is one of the important parts in a frequency modulated continuous wave (FMCW) radar application. For fabrication of the high power GaAs Gunn diodes, we adopted a graded gap injector which enhances the output power and conversion efficiency by effectively removing the dead-zone. We have measured RF characteristics of the fabricated GaAs Gunn diodes. The operating current, oscillation frequency, and output power of the fabricated GaAs Gunn diodes are presented as a function of the anode diameters. The operating current increases with anode diameters, whereas the oscillation frequency decreases. The higher oscillation frequency was obtained from 60 µm anode diameters of the fabricated Gunn GaAs diodes and higher power was obtained from 68 µm. Also, for application of the 94 GHz FMCW radar system, we have fabricated the 94 GHz waveguide VCO. From the fabricated GaAs Gunn diodes of anode diameter of 60 µm, we have obtained the improved VCO performance.

High mobility polymer gated organic field effect transistor using zinc phthalocyanine

Organic thin film transistors were fabricated using evaporated zinc phthalocyanine as the active layer. Parylene film prepared by chemical vapour deposition was used as the organic gate insulator. The annealing of the samples was performed at 120 °C for 3 h. At room temperature, these transistors exhibit p-type conductivity with field-effect mobilities ranging from 0·025–0·037 cm2/Vs and a (Ion/Ioff) ratio of ∼103. The effect of annealing on transistor characteristics is discussed.

3-D 94 GHz single balanced mixer for low conversion loss with ring coupler by DAML

A 94 GHz single balanced mixer in 3-dimension was successfully fabricated. The GaAs-based low loss transmission lines, dielectric-supported air-gapped microstrip lines (DAMLs), have been developed using surface micromachining technology, and the DAML-based hybrid ring coupler is successively fabricated. We have developed the high-performance 94 GHz single balanced active mixer using hybrid ring coupler with 70 nm gate length MHEMT. The mixer showed the conversion loss of 2.5 dB ~ 2.8 dB and isolation characteristics less than −30 dB, in the range of 93.65 GHz ~ 94.25 GHz. These results are the best performances demonstrated from a 94 GHz single balanced mixer utilizing GaAs-based HEMTs in terms of conversion loss as well as isolation characteristics.

W-band Waveguide-to-Coplanar Waveguide Transition for 94 GHz MIMIC applications

In this paper, we report our recent research works on the W-band waveguide-to-CPW transitions for various MIMIC applications. For this, the transitions operating in a frequency range of 85 ~ 100 GHz are designed, fabricated and characterized. The designed waveguide-to-CPW transition is optimized to achieve low loss by using an EM field solver of HFSS. From the measurement, an insertion of -2.2 dB and a return loss of -23.86 dB, respectively, were obtained at 94 GHz. The average insertion loss of waveguide-to-CPW back-to-back transition is -2.5 dB in a frequency range of 85 ~ 100 GHz.

Design and fabrication of MMW module for 94 GHz radar sensor applications

We present design and fabrication of a 94 GHz radar sensor module. The 94 GHz sensor module has four components including 94 GHz single balanced diode mixer part, waveguide VCO (voltage controlled oscillator) part, magic tee part and bias PCB part. The 94 GHz single balanced diode mixer is developed on Duroid RT 5880 substrate with DC 1346 Schottky diode. The mixer has advantage of good conversion loss at high LO power and isolation characteristic. 94 GHz single balanced diode mixer has advantage of comparatively easy fabrication. The waveguide VCO consist of GaAs Gunn diode, varactor diode, two-bias post and cavity. The waveguide VCO is operated at 94 GHz. The magic tee has four waveguide arm regions with standard WR-10 and divides output power from VCO. Transmission frequency of 94 GHz MMW sensor module is 93.607 ∼ 94.727 GHz. Bandwidth is 1.12 GHz. 2% linearity range is 680 MHz. Power is 11.03 ∼ 11.47 dBm. Conversion loss about −7 dB at IF 500 MHz

Two-Stage 94 GHz drive Amplifiers Using 0.1-μm Metamorphic HEMT Technology

In this paper, millimeter-wave 94 GHz drive amplifiers based on metamorphic high electron mobility transistors (MHEMTs) were designed and fabricated. The fabricated 100 nm gate length MHEMT devices exhibit DC characteristics with a drain current density of 690 mA/mm and an extrinsic transconductance of 770mS/mm. The current gain cutoff frequency (fT) and the maximum oscillation frequency (fmax) are 185 GHz and 230 GHz, respectively. The matching circuit of amplifier was designed using CPW (coplanar wave-guide) transmission line. The fabricated amplifier shows a good S21 gain of 7.79 dB, an input return loss (S11) of -16.5 dB and an output return loss (S22) of-15.9 dB.

Fabrication of GaAs Gunn diodes using trench method

We developed the improved fabrication technology using the trench method for a graded-gap injector GaAs Gunn diode to avoid the formation of cracks which are caused by the difference in the thermal expansion coefficients between the GaAs epi-layer and the gold integral heat sink. We observed the negative-differential-resistance in the fabricated Gunn diode. The process we developed facilitates further process and handling because of the integral heat sink with thickness of 60 mum. Also, the formation of cracks in GaAs epi-layer was reduced during ohmic alloy process because of the reduced stress between the epi-wafer and the gold integral heat sink.