Cheng-Kuo Lin

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.

A

This study presents a W-band injection-locked frequency divider (ILFD) with a wide locking range characteristic by using 0.15 mum GaAs pHEMT techniques. Based on the cascode circuit topology, the oscillation and the injection parts can be designed individually without the trade-off between the input matching and the oscillation condition. Including with a characteristic of the active capacitance in this ILFD, a free-running oscillation frequency about 50 GHz was obtained with a frequency tuning function, in which the tuning range was about 1.2 GHz (50.5-49.3 GHz). By injecting a signal of around 100 GHz into this ILFD, the maximum locking range was measured up to 400 MHz, while the injected power was set to -5 dBm under a 3 V supply with a power consumption of 21 mW in the ILFD core.

W

A V-band cross-coupled sub-harmonic injection-locked oscillator has been designed and fabricated using 0.15-mum GaAs pHMET technology. Based on the known harmonic injecting circuit topology, this oscillator was designed by a differential output approach, a low-Q microstrip-line resonator, and a current mirror, which has a free-running oscillation frequency around 60GHz with a tuning range of 2.5GHz (from 57.8GHz to 60.3GHz). The maximum single-end output power is 3.8dBm with a dc dissipation of 225mW under a -3V supply voltage. Within the input matching network for second (30GHz) and fourth (15GHz) sub-harmonic signals injection, it demonstrates the maximum locking ranges close to 120MHz and 30MHz, respectively

-band Injection-Locked Frequency Divider Using GaAs pHEMTs and Cascode Circuit Topology

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.

V-Band GaAs pHEMT Cross-Coupled Sub-Harmonic Oscillator

A K-band (20 GHz) monolithic amplifier was developed and fabricated by adopting a low-/spl kappa/ benzocyclobutene (BCB) coplanar waveguide (CPW) line and InGaP-InGaAs doped-channel HFETs (DCFETs). This monolithic microwave integrated circuit (MMIC) utilizes a high impedance BCB CPW microstrip line (Z/sub 0/=70 /spl Omega/) for the biasing circuits, and a Z/sub 0/=50 /spl Omega/ line for the RF signal transmission. The low dielectric constant characteristic of the BCB interlayer is beneficial for a common-ground bridge process, which reduces the parasitics. The calculated loss tan/spl delta/ is 0.036 for the BCB at 20 GHz. The one-stage MMIC amplifier achieves an S/sub 21/ of 5 dB at 20 GHz, which is the first demonstration of the K-band InGaP-InGaAs DCFET monolithic circuit.