K. H. Lee

GaN-Based Schottky Barrier Photodetectors With a 12-Pair Mg

GaN-based ultraviolet (UV) photodetectors (PDs) separately prepared with a conventional single low-temperature (LT) GaN buffer layer and a 12-pair Mg_xN_y-GaN buffer layer were both fabricated. It was found that we could reduce threading dislocation (TD) density and thus improve crystal quality of the GaN-based UV PDs by using the 12-pair Mg_xN_y-GaN buffer layer. With a -2-V applied bias, it was found that the reverse leakage currents measured from PDs with a single LT GaN buffer layer and that with a 12-pair Mg_xN_y-GaN buffer layer were 4.57 times 10^-6 and 1.44 times 10^-12 A, respectively. It was also found that we could use the 12-pair Mg_xN_y-GaN buffer layer to suppress photoconductive gain, enhance UV-to-visible rejection ratio, reduce noise level, and enhance the detectivity.

_{\rm x}

InGaN-GaN multiquantum-well (MQW) metal-semiconductor-metal (MSM) photodetectors (PDs) with the unactivated Mg-doped GaN cap layer were successfully fabricated. It was found that we could achieve a dark current by as much as six orders of magnitude smaller by inserting the unactivated Mg-doped GaN cap layer. For MSM photodetectors with the unactivated Mg-doped GaN cap layer, the responsivity at 380 nm was found to be 0.372 A/W when the device was biased at 5 V. The UV-to-visible rejection ratio was also estimated to be around 1.96 times 103 for the photodetectors with the unactivated Mg-doped GaN cap layer. With a 5-V applied bias, we found that minimum noise equivalent power and normalized detectivity of our PDs were 4.09 times 10-14 W and 1.18 times 1013 cmmiddotHz0.5W-1, respectively. Briefly, incorporating the unactivated Mg-doped GaN layer into the PDs beneficially brings about the suppression of dark current and a corresponding improvement in the device characteristics.

N

We report an AlGaN/GaN high electron mobility transistors (HEMTs) based on InGaN/GaN multiquantum-well (MQW) structure. When InGaN/GaN MQW structure was inserted, InGaN layer has an opposite piezoelectric polarization field compared to AlGaN, which results in a very sharp rise of the conduction band. The raised potential barrier can help to improve carrier confinement and obtain a larger main peak transconductance of 111 mS/mm and satellite peak transconductance of 24 mS/mm, corresponding to AlGaN/GaN heterojunction and InGaN layer. MQW-based metal-oxide-semiconductor-HEMT was also fabricated and significantly reduced the leakage current and increased transconductance as a result of passivation by Ta2O5 gate oxide.

_{\rm y}

AlGaN/GaN heterostructure Schottky barrier photodetector (PD) with multi-MgxNy/GaN buffer was proposed and fabricated. Compared with AlGaN/GaN heterostructure PD prepared on conventional low-temperature GaN buffer, it was found that we can reduce dark leakage current by more than three orders of magnitude. It was also found that we can use the multi-MgxNy/GaN buffer to suppress photoconductance gain, enhance UV-to-visible rejection ratio, reduce noise level and enhance the detectivity.

–GaN Buffer Layer

The performances of InGaP/AlxGa1−xAs/GaAs heterojunction bipolar transistors (HBTs) with different doping concentrations of AlxGa1−xAs graded layers are theoretically studied. The use of the AlxGa1−xAs graded layer plays a key role in affecting the direct current and radio frequency performances of the studied HBTs. It is found that the studied devices with suitable doping concentrations of AlxGa1−xAs graded layers exhibit lower offset voltages, saturation voltages, and base and collector current ideality factors. Furthermore, due to the use of proper doping concentrations of AlxGa1−xAs graded layers, the studied devices show high values of the unity current gain cut-off frequency (fT) and maximum oscillation frequency (fmax). It is known that, from the theoretical analysis, the appropriate doping concentration of the AlxGa1−xAs graded layer is 1 × 1016 to 1 × 1018 cm−3. Consequently, this work is promising for device engineers to design high-performance HBT structures.

High-Detectivity Nitride-Based MSM Photodetectors on InGaN–GaN Multiquantum Well With the Unactivated Mg-Doped GaN Layer

The temperature-dependent dc characteristics of an interesting heterojunction bipolar transistor with an InGaAsP spacer and an InGaAs∕InGaAsP composite-collector structure are studied and demonstrated. By employing the intermediate band-gap In0.72Ga0.28As0.61P0.39 material at the emitter-base and base-collector heterojunction, the electron blocking effect is effectively eliminated. The studied device gives the promising dc performances including the small offset and saturation voltages without degrading the breakdown behaviors. The typical incremental current gain of 114 and the maximum dc current gain of 118 are obtained. It is worthwhile to note that the desired current amplification over 11 decades of the magnitude of collector current IC is obtained in the studied device. Moreover, the switching or hysteresis phenomenon usually observed in InP-based devices is not seen in the studied device.

AlGaN/GaN high electron mobility transistors based on InGaN/GaN multiquantum-well structures

Abstract The characteristics of polysilicon resistors for CMOS ULSI applications have been investigated. Based on the presented sub-quarter micron CMOS borderless contact, both n+ and p+ polysilicon resistors with Ti- and Co-silicide self-aligned process are used at the ends of each resistor. A simple and useful model is proposed to analyse and calculate some important parameters of polysilicon resistors including electrical delta W(ΔW), interface resistance Rinterface, and pure sheet resistance Rpure. Furthermore, the characteristics of voltage-coefficient resistor, temperature-coefficient resistor, and resistor mismatching are also studied. An interesting sine-wave voltage-dependent characteristic due to the strong relation to the Rinterface has been modelled in this paper. This approach can substantially help engineers in designing and fabricating the precise polysilicon resistors in sub-quarter micron CMOS ULSI technology.

AlGaN/GaN Schottky Barrier Photodetector With Multi-

A normal amorphous silicon-based separate absorption and multiplication avalanche photodiode (SAMAPD) with very high optical gain of the avalanche photodiode has been developed successfully by plasma-enhanced chemical vapor deposition (PECVD). Based on experimental results, using undoped /spl alpha/-Si:H as avalanche layer material and /spl alpha/-Si/sub 1-x/Ge/sub x/:H as absorption layer material, the hole-injection (HI) type SAMAPD yields a very high optical gain of 686 at a reverse bias of 16 V under an incident light power of P/sub in/=1 /spl mu/W and has a rise time of 145 /spl mu/s at a load resistance R=10 k/spl Omega/. Thus the amorphous silicon-based SAMAPD is a good candidate for the long-range optical communication applications. >

{\rm Mg}_{\rm x} {\rm N} _{\rm y}

InGaN-GaN multiple-quantum-well metal-semiconductor-metal photodiodes (PDs) with in situ grown 40-nm-thick unactivated semi-insulating Mg-doped GaN cap layer were successfully fabricated. The dark leakage current of this PD was comparably much smaller than that of conventional PD without the semi-insulating layer, because of a thicker and higher potential barrier of semi-insulating cap layer, and also a smaller number of surface states involved. For the PDs with the semi-insulating Mg-doped GaN cap layers, the responsivity at 380nm was 0.372A/W when biasing at 5 V. In short, incorporating a semi-insulating Mg-doped GaN cap layer into the PDs beneficially leads to the suppression of dark current and a corresponding improvement in the ultraviolet-to-visible rejection ratio

/GaN Buffer

InGaN epitaxial films grown by metalorganic chemical vapor deposition using trimethylgallium and triethylgallium as precursors exhibited different optical and electrical properties. The films were characterized by x-ray diffraction, photoluminescence, secondary ion mass spectroscopy, and atomic force microscopy. Impacts of unactivated Mg-doped GaN in situ grown cap layers on InGaN and GaN films were further investigated. Current-voltage and spectral response measurements combined with Hall-effect measurement and analytical modeling have been used to assess possible current transport mechanisms of reverse dark and photo current flow in metal-semiconductor-metal photodetectors fabricated from InGaN and GaN. Unlike the dominant thermionic emission, which can be blocked by higher and thicker potential barrier in GaN, the trap-assisted tunneling is more pronounced in InGaN. The passivation effect on high density surface states in InGaN is proposed to explain the improved device performances after the incorpora...