Jinwook Burm

Microwave performance of a 0.25 μm gate AlGaN/GaN heterostructure field effect transistor

We fabricated a 0.25 μm gate length AlGaN/GaN heterostructure field effect transistor (HFET) with a maximum extrinsic transconductance of 27 mS/mm (at room temperature) limited by the source series resistance. The device exhibited an excellent pinch‐off and a low parasitic output conductance in the saturation regime. We measured the cutoff frequency fT and the maximum oscillation frequency fmax as 11 and 35 GHz, respectively. These values are superior to the highest reported values for field effect transistors based on other wide band‐gap semiconductors such as SiC. These results demonstrate an excellent potential of AlGaN/GaN HFETs for microwave and millimeter wave applications.

Ultra-low resistive ohmic contacts on n-GaN using Si implantation

Implanted ohmic contacts were made on molecular beam epitaxy grown GaN materials. Si was implanted at a doping density of about 4×1020  cm-3 to decrease the contact resistance of the contact, followed by an activation anneal at 1150 °C for 30 s. The overlay metal Ti/Au was evaporated. Four-probe measurements were performed on transmission line model patterns. The measured maximum contact resistance was 0.097 Ω mm and the apparent specific contact resistance was 3.6×10−8 Ω cm2.

CW operation of short-channel GaN/AlGaN doped channel heterostructure field effect transistors at 10 GHz and 15 GHz

We report on a 0.15-/spl mu/m gate length AlGaN/GaN doped channel heterostructure field effect transistor (DC-HFET) with maximum frequency of oscillation in excess of 97 GHz. HFETs based on our doped channel design exhibited CW microwave operation up to 15 GHz with a maximum output power of approximately 270 mW/mm at 10 GHz. These values are still limited by parasitics and can be significantly improved by optimizing the device design.

75 Å GaN channel modulation doped field effect transistors

A III–V nitride modulation doped field effect transistor (MODFET) layer structure grown by organometallic vapor phase epitaxy (OMVPE) on a sapphire substrate was employed for transistor fabrication. The MODFET layer structure contained a 75 A GaN channel, 50 A Al0.16Ga0.84N spacer, 20 A Si doped charge supply layer, 130 A Al0.16Ga0.84N barrier, and 60 A Al0.06Ga0.94N cap layer. The thin channel (75 A) was chosen to improve the carrier confinement in the channel. The fabricated MODFET’s had 0.25 μm long gates, and utilized a Au–Si alloy Ohmic metal and a Ti/Pd/Au gate metal. The measured transconductance was 40 mS/mm. From the microwave measurements on devices with 0.25 μm long gates, ft and fmax were determined to be 21.4 and 77.5 GHz, respectively.

GaN based heterostructure for high power devices

Abstract We discuss the potential of GaN-based field effect transistor for high-power, high-temperature operation. At room temperature, the GaN/AlGaN doped channel HFETs (DC-HFETs) demonstrated highest frequency operation among all wide band gap semiconductor devices because of excellent transport properties of two dimensional electron gas at the AlGaN/GaN heterointerface and a large sheet carrier concentration in the device channel. CW operation at 10 and 15 GHz was also recently reported. Monte Carlo simulations indicate that short-channel GaN devices should have transported superior even to GaAs. However, improved thermal and microwave designs are required in order to take advantage of these material properties for applications in high power devices.

Optimization of high-speed metal-semiconductor-metal photodetectors

Circular-aperture Metal-Semiconductor-Metal (MSM) photodetectors have been fabricated in order to decrease the device capacitance. The frequency-response of the MSM photodetectors has been calculated using a first-order approximation. The design of the circular-aperture detectors and conventional square-aperture detectors has been optimized using this approximation. The largest 3 dB bandwidths are obtained with 33 fingers for the circular detectors and 29 for the square detectors. These numbers do not change regardless of the size of the MSM photodetectors.<<ETX>>

Low-frequency gain in MSM photodiodes due to charge accumulation and image force lowering

To understand the nature of low-frequency gain in MSM photodiodes, Schottky barrier height was measured for an MSM photodiode fabricated on GaAs-based layers. The Schottky barrier height showed a dependence on the light irradiation and bias. This can be explained by a lowering of the Schottky barrier due to charge accumulation at surface states and image-force lowering at the edges of metal electrodes where electric field is extremely high. Thermionic hole emission is proposed as a source of low-frequency gain of MSM photodiodes.

Recessed gate GaN MODFETs

Abstract MODFETs, with and without gate recess, were fabricated on a GaN Al 0.27 Ga 0.73 N heterostructure. The gate recess etch was performed with an ECR etch. The gate recess etch improved the maximum transconductance from 23 to 45 mS/mm, ft from 11.4 to 14 GHz, and fmax from 21.2 to 42.5 GHz. The physical gate length of 0.25 μm before recess etch increased to 0.4 μm due to the etching of the resist. Through gate recess, the decrease of the distance between the gate and 2DEG increased the maximum transconductance and the decrease of the effective gate length increased ft and fmax.

High-frequency, high-efficiency MSM photodetectors

Metal-semiconductor-metal (MSM) photodiodes with submicron spaced interdigitated Schottky barrier fingers have been developed for applications in monolithic integrated optical receiver circuits capable of detecting a millimeter-wave modulation signal. Each photodetector layer, is designed for optimal absorption about a narrow linewidth centered on a specific wavelength between 700 and 800 nm. The MBE grown layers consist of an Al/sub x/Ga/sub 1-x/As cap layer, to prevent any surface recombination of carriers and to minimize top surface reflections; a thin GaAs absorption layer (375 nm), to achieve a high-frequency response (>39 GHz) by minimizing the collection times of optically generated carriers; and a buried Bragg reflector stack which reflects unabsorbed light back into the GaAs absorption layer. Using this layer design, we are able to fabricate detectors that have millimeter-wave bandwidths without sacrificing quantum efficiency. The measured internal quantum efficiency of an MSM photodiode, fabricated on such a layer structure, was 82% at 5 V and close to 94% at 10 V. >

Suppression of avalanche multiplication at the periphery of diffused junction by floating guard rings in a planar InGaAs-InP avalanche photodiode

We obtained a series of experimental results showing the effects of floating guard rings (FGRs) in InGaAs-InGaAsP-InP separate absorption, grading, charge, and multiplication avalanche photodiodes. It was confirmed from the scanned photocurrent curves that the essential role of FGRs is to disperse the curved equipotential lines at the lateral junction periphery and to give a low field route for a carrier beneath the FGRs. FGR effect mainly depends on guard ring spacing and it also depends on the magnitude of the applied bias. In our optimum guard ring condition, the current gain at the active planar region found to be 1.4 times larger than that at the curved edge.