Wen-Kai Wang

Performance enhancement by using the n/sup +/-GaN cap layer and gate recess technology on the AlGaN-GaN HEMT fabrication

Due to the low mobility and wide bandgap characteristics of the undoped AlGaN layer used in the conventional AlGaN-GaN HEMT as a cap layer, the RF performance of this device will be limited by its high contact resistance and high knee voltage. In this letter, we propose using the n/sup +/-GaN cap layer and the selective gate recess etching technology on the AlGaN-GaN HEMTs fabrication. With this n/sup +/-GaN instead of the undoped AlGaN as a cap layer, the device contact resistance is reduced from 1.0 to 0.4 /spl Omega//spl middot/mm. The 0.3 /spl mu/m gate-length device demonstrates an I/sub ds,max/ of 1.1 A/mm, a g/sub m,max/ of 220 mS/mm, an f/sub T/ of 43 GHz, an f/sub max/ of 68 GHz, and an output power density of 4 W/mm at 2.4 GHz.

Photonic crystal directional couplers formed by InAlGaAs nano-rods.

This study demonstrates the use of photonic crystal directional couplers to separate light of wavelengths 1.31 and 1.55microm. The photonic crystal structure consists of InAlGaAs nano-rods arranged in square lattice. The coupling length of the light in the directional coupler at a wavelength of 1.31microm was designed to be four times greater than that at 1.55microm. This behavior helps in designing devices to split the two wavelengths. The devices are fabricated by e-beam lithography and conventional photolithography. The measurement results confirm that 1.31microm/1.55microm directional couplers can be realized in PC structures formed by nano-rods.

Low damage, Cl/sub 2/-based gate recess etching for 0.3-/spl mu/m gate-length AlGaN/GaN HEMT fabrication

The traditional dry etching for GaN using the Ar/Cl/sub 2/ mixture gas in the reactive ion etching system has been developed. In order to reduce the surface damage, the additional CH/sub 4/ gas is introduced. However, this approach still has the problems of the residual surface damage and low etching selectivity between the AlGaN and GaN materials. Therefore, the following rapid thermal annealing (RTA) at 700/spl deg/C is necessary to recover the surface properties. In this study, we proposed the Ar/Cl/sub 2//CH/sub 4//O/sub 2/ for the GaN gate-recess etching in AlGaN/GaN HEMTs fabrication, which achieves a low surface damage and a high etching selectivity simultaneously. The 0.3 /spl mu/m gate-length AlGaN/GaN HEMTs present a transconductance of 230 mS/mm, an f/sub T/ of 48 GHz, and f/sub max/ of 60 GHz, respectively.

Transient pulsed analysis on GaN HEMTs at cryogenic temperatures

A pulsed measurement of AlGaN/GaN high electron mobility transistors (HEMTs) current-voltage (I-V) output characteristics from 100 to 300 K temperatures has been systematically investigated, and a significant kink is clearly observed, which is more severe at cryogenic temperatures. By comparing the pulsed and dc I-V curves, the kink effect is more significant in the pulsed mode evaluation, which indicates a time constant related mechanism involved in the carrier transport. Moreover, a weak impact ionization by monitoring the gate current in the on-state of device has also been observed, and it is more significant at cryogenic temperatures.

BCB-bridged distributed wideband SPST switch using 0.25-/spl mu/m In/sub 0.5/Al/sub 0.5/As--In/sub 0.5/Ga/sub 0.5/As metamorphic HEMTs

In/sub 0.5/Al/sub 0.5/As--In/sub 0.5/Ga/sub 0.5/As metamorphic high-electron mobility transistor (mHEMT) dc-30 GHz distributed single-pole-single through (SPST) switches were designed and fabricated using the low-/spl kappa/ benzocyclobutene (BCB) bridged technology. The current gain cutoff frequency, and the electron transit time of In/sub 0.5/Al/sub 0.5/As--In/sub 0.5/Ga/sub 0.5/As mHEMTs have been investigated. By analyzing the extrinsic total delay time, the effective velocity of electrons can be estimated, and the average velocity is 2.3/spl times/10/sup 7/cm/s. The dc-30 GHz distributed wideband SPST switch exhibits an insertion loss of less than 5.5 dB, and an isolation larger than 30 dB, which is the first demonstration of the high-isolation of InAlAs-InGaAs mHEMTs monolithic switch. As to the power performance, this switch can handle the power up to 12 dBm at 2.4 GHz. After over 250 h of 85-85 (temperature =85/spl deg/C, humidity =85%) environmental evaluation, this BCB passivated and bridged microwave and monolithic integrated circuit switch demonstrates reliable RF characteristics without any significant performance change, which proves that this process using the low-/spl kappa/ BCB layer is attractive for millimeter-wave circuit applications.

Low-/spl kappa/ BCB passivation on AlGaN-GaN HEMT fabrication

Due to the stress-induced polarization effect on the GaN HEMTs, the surface passivation of the device is critical and is deserved to conduct a detailed study. It has been proven that the GaN HEMTs demonstrate nondispersive pulsed current-voltage (I-V) characteristics and better microwave power performances after passivating the Si/sub 3/N/sub 4/ film on the GaN surface. In this letter, we proposed to use the BCB material, a negative photoresist with a low-/spl kappa/ characteristic, as the surface passivation layer on GaN HEMTs fabrication. After comparing the dc I-V, pulsed I-V, RF small-signal, microwave power characteristics, and device reliability, this BCB-passivated GaN HEMT achieved better performance than the Si/sub 3/N/sub 4/ passivated device.

Single step electron-beam lithography for asymmetric recess and gamma gate in high electron mobility transistor fabrication

We have developed an asymmetric recess etch in conjunction with a gamma metal gate process for the fabrication of high electron mobility transistors (HEMTs). This process is based on direct electron-beam lithography with a single exposure of a three stack poly(dimethylglutarimide) and poly(methylmethacrylate) photoresists. It can modify the voltage drop over a wider region fabricated by the so called wide-recess technique as compared with the conventional approach, while maintaining a low source resistance, to reduce the electric field intensity at the gate edge, and therefore enhance the breakdown voltage. This gate engineering process for the 0.2 μm InAlAs/InGaAs metamorphic HEMTs achieved a 45% off-state breakdown voltage improvement as compared with the traditional bilayer T-gate device.

Dual-Gate E/E- and E/D-Mode AlGaAs/InGaAs pHEMTs for Microwave Circuit Applications

In this paper, we developed dual-gate enhancement/enhancement-mode (E/E-mode) and enhancement/depletion-mode (E/D-mode) AlGaAs/InGaAs pHEMTs for high-voltage and high-power device applications. These dual-gate devices had a higher breakdown voltage (Vbr) and maximum oscillation frequency (fmax). This could be obtained because there were two depletion regions, and the total electrical field was shared between the two regions, leading to lower output conductance (go) and lower gate-to-drain capacitance (Cgd). The dual-gate device can be operated at a higher drain-to-source voltage (Vds), resulting in better linear gain and output power performance, as compared to a conventional single-gate E-mode GaAs pHEMT device. The maximum oscillation frequency obtained using the dual-gate E/E-mode device increased from 78 to 123 GHz. When operated at 2.4 GHz, the maximum RF output power of the single-gate E-mode and dual-gate E/D-mode devices increased from 636 to 810 mW/mm, respectively. We also produced a 2.4-GHz high-gain and high-power density two-stage power amplifier using dual-gate E/E and E/D-mode transistors. A linear gain of 40 dB and a maximum output power of 24 dBm were obtained.

Leaky-wave photodiodes with a partially p-doped absorption Layer and a distributed Bragg reflector (DBR) for high-power and high-bandwidth-responsivity product performance

In this letter, we describe a novel edge-coupled photodiode (PD) structure, which can greatly relax the dependence of the responsivity on the cleaved length of an evanescently coupled optical waveguide. The integration of a leaky optical waveguide with a distributed Bragg reflector (DBR) and a partially p-doped photoabsorption layer allows the demonstrated device to exhibit a higher saturation current-bandwidth and responsivity than a control sample without the DBR structure. We achieved excellent speed (40-50 GHz), responsivity (0.8 A/W), and saturation current-bandwidth products (720 mAmiddotGHz at 40 GHz), comparable to the high-performance of an evanescently coupled PD, whose variation of responsivity is more sensitive to the cleaved waveguide length (30% versus 7%) than is our demonstrated device

Submicron RIE recessed InGaP/InGaAs doped-channel FETs

High performance submicron In/sub 0.49/Ga/sub 0.51/P/In/sub 0.15/Ga/sub 0.85/As doped-channel heterostructure field effect transistors (HFETs) have been developed and characterized. In order to achieve a high uniformity of device characteristics crossing the wafer, BCl/sub 3/+CHF/sub 3/ reactive ion etching technology in gate-recessed process is applied to fabricate the InGaP/InGaAs doped-channel FETs. The unity current gain cut-off frequency (f/sub T/), maximum frequency of oscillation (f/sub max/), and threshold voltage have been investigated versus the gate-length. The improved microwave performance in smaller gate-length devices is mainly associated with the reduction of the input capacitance. The 0.2 /spl times/ 200-/spl mu/m/sup 2/ gate-dimension DCFET exhibits a saturated P/sub out/ of 18.9 dBm, a power density of 388 mW/mm, a PAE of 35%, and an associated gain of 14 dB at 2.4 GHz.