Sun-Chin Wei

ESD engineering of nitride-based LEDs

GaN-based light emitting diodes (LEDs) with p-cap layers grown at various temperatures were fabricated. It was found that the LED with 900/spl deg/C-grown p-cap layer could only endure negative 1100 V electrostatic discharge (ESD) pulses while the LED with 1040/spl deg/C-grown p-cap layer could endure ESD pulses as high as negative 3500 V. It was also found that the ESD performances of the LEDs with 900 and 1040/spl deg/C-grown p-cap layers were limited by the V-shape defects and the bonding pad design, respectively.

Nitride-based LEDs with n/sup -/-GaN current spreading layers

Nitride-based light-emitting diodes (LEDs) with n/sup -/-GaN current spreading layers were proposed and fabricated. With a 0.1-/spl mu/m-thick n/sup -/-GaN current spreading layer, it was found that the output power could be enhanced by 35% without increasing the operation voltage of the LEDs at 20 mA. In addition, implementing the n/sup -/-GaN current spreading layer also significantly improved the electrostatic discharge characteristics of nitride-based LEDs.

Nitride-based MQW LEDs with multiple GaN-SiN nucleation layers

Nitride-based light emitting diodes (LEDs) separately prepared with a conventional single low-temperature (LT) GaN nucleation layer and multiple GaN-SiN nucleation layers were both prepared. It was found that we could reduce defect density and thus improve crystal quality of the GaN-based LEDs by using multiple GaN-SiN nucleation layers. With a 20-V applied reverse bias, it was found that the reverse leakage currents measured from the LED with a single LT GaN nucleation layer and the one with 10-pair GaN-SiN nucleation layers were 1.5/spl times/10/sup -4/ and 2.5/spl times/10/sup -6/ A, respectively. It was also determined that we could use the multiple GaN-SiN nucleation layers to enhance the output intensity of near ultraviolet (UV) LEDs and to improve the reliability of nitride-based LEDs.

Flicker noise of GaN-based heterostructure field-effect transistors with Si-doped AlGaN carrier injection layer

Aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure field effect transistors (HFETs) with and without Si-doped AlGaN layer were fabricated and investigated. HFETs with the Si-doped AlGaN carrier-injection layer show better DC performance, and the transconductance is 150 mS/mm. However, the HFETs with Si-doped AlGaN layer present the deviation from the 1/f noise at low frequency. The Lorentz shape was observed in the noise spectrum. It suggests that traps might be more pronounced in this kind of structure. Therefore, the DC characteristics of HFETs can be improved by the insertion of Si-doped AlGaN layer, but it can result in more low-frequency noise with the carrier-injection layer.

Nitride-based LEDs with MQW active regions grown by different temperature profiles

Nitride-based light-emitting diodes (LEDs) with multiple quantum-well active regions were separately prepared by metal-organic vapor phase epitaxy in different temperature profiles. Compared with conventional samples, the reduced reverse leakage current and improved electrostatic discharge characteristics of the LEDs can both be achieved using temperature ramping and temperature cycling methods. However, using the temperature ramping may degrade the optical properties of devices due to desorption of In atoms and/or impurity incorporation. With an emission wavelength of 465 nm, the 20-mA output powers measured were 5.5, 6.0, and 7.9 mW for temperature ramping LED, conventional LED, and temperature cycling LED, respectively.

Nitride-Based Dual-Stage MQW LEDs

Nitride-based light emitting diode (LED) with dual-stage multiquantum well (MQW) structure is proposed and fabricated. It was found that we could improve crystal quality, reduce reverse leakage current, and reduce forward voltage of the LED by inserting the electron emitter MQW structure. With 20 mA current injection, it was found that measured output powers were 3.2 and 4.7 mW for the conventional single-stage MQW LED and the dual-stage MQW LED, respectively. Furthermore, it was found that electrostatic discharge characteristics of the dual-stage MQW LED are better.

Stability of measurement of heterojunction bipolar transistors current-voltage characteristics with thermal effect

To obtain a reliable design, a careful determination of model parameters is needed. In this study, the thermally stable time of collector current (IC) of heterojunction bipolar transistors (HBTs) operated at different biasing points is observed. It is observed that the stable time of IC is different with different biasing points. The maximum thermally stable time is about 72 ms in this study. For the conventional measurement method, the default biasing time of current–voltage (IC–VCE) curves at each bias point is about 20 ms. The calculated thermal resistance and junction temperature obtained by the conventional measurement are quite different from those obtained by thermally stable measurement. In this work, we found that the thermal resistance obtained by thermally stable measurement (i.e. sufficiently long biasing time) is smaller than that obtained by conventional measurement. Moreover, the junction temperature estimated from the former is smaller than that from the latter. It is obvious that the biasing time of every measurement point must be ascertained by using the curve of IC versus biasing time with the operation VCE.

Nitride-based devices fabricated by wet etching

Photo-enhanced chemical (PEC) wet etching technology was used to etch GaN and AlGaN epitaxial layers. Figure 1 shows PEC etch rate for the GaN and Al/sub x/Ga/sub 1-x/N epitaxial layers in aqueous KOH and H/sub 3/PO/sub 4/ solutions. It was found that the maximum etch rates were 510 nm/min, 1960 nm/min, 300 nm/min and 0 nm/min for GaN, Al/sub 0.175/Ga/sub 0.825/N, Al/sub 0.23/Ga/sub 0.77/N and Al/sub 0.4/Ga/sub 0.6/N, respectively. Nitride-based Schottky diodes and heterostructure field effect transistors (HFETs) were also fabricated by PEC wet etching. As shown in figures 2, 3 and 4, it was found that we could achieve a saturated I/sub D/ larger than 850 mA/mm and a maximum g/sub m/ about 163 mS/mm from PEC wet etched HFET with a 0.5/spl mu/m gate length. Compared with dry etched devices, the leakage currents observed from the PEC wet etched devices were also found to be smaller.

Nitride-based HFETs with carrier confinement layers

Nitride-based Al/sub 0.24/Ga/sub 0.76/N/GaN heterostructure field effect transistors (HFETs) with carrier confinement layers were fabricated. The results found that the enhanced 2 dimensional electron gas (2DEG) carrier mobility from 1070 to 1180 cm/sup 2/V/sup -1/sec/sup -1/ by the insertion of a Mg-doped semi-insulating carrier confinement layer with a Cp/sub 2/Mg flow rate of 2.36/spl times/10/sup -8/ mole/min and smoother sample surface. The DC and RF characteristics of these HFETs were also good.

Thermal resistance variation of HBT with high junction temperature and bias condition

Abstract Thermal resistance of heterojunction bipolar transistor is usually considered to be a linear function of ambient temperature. However, in power applications, the thermal effects become obvious. To obtain well design characteristics, the thermal resistance cannot only be considered to vary with ambient temperature in linear relation, but also the function of junction temperature. In this paper, the variations of thermal resistance are discussed, and the thermal resistance is almost a linear function of junction temperature.