Tangsheng Chen

Picosecond, narrow‐band, widely tunable optical parametric oscillator using a temperature‐tuned lithium borate crystal

A highly efficient, picosecond, narrow‐band, widely tunable optical parametric oscillator using a temperature‐tuned LBO crystal synchronously pumped by cw, Q‐switched, mode‐locked pulses is described. A signal conversion efficiency of 30% with an output linewidth of 0.14 nm is obtained. The latter can be further reduced to 0.01 nm.

Low Interface Trap Densities and Enhanced Performance of AlGaN/GaN MOS High- Electron Mobility Transistors Using Thermal Oxidized Y 2 O 3 Interlayer

AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors (MOS-HEMTs) with Y₂O₃ interlayer have been investigated to improve the interface quality. With the HfO₂/Y₂O₃ stack gate dielectrics, the devices show a saturated drain current density of up to ~943 mA/mm. Meanwhile, the pulsed Id-Vg measurement indicates interface traps are as low as 5.2 × 10¹¹ cm^-2, and the pulsed-IV measurement exhibits great resistance to current collapse. Furthermore, the devices also present good reliability under negative bias stress. Therefore, the interface engineering based on Y₂O₃ has a potential to open up a new avenue to high-performance AlGaN/GaN MOS-HEMTs.

Monolithic Integration of E/D-Mode AlGaN/GaN MIS-HEMTs

Monolithic integration of enhancement/depletion (E/D)-mode AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) are demonstrated for mixed signal applications. The gate length for the E-mode and D-mode devices are 0.2 μm and 0.35 μm, respectively, and drain current of 496 (Vgs=+5 V) and 720 mA/mm (Vgs=0 V) with a peak transconductance of 147 and 131 mS/mm are measured. Small-signal measurements result matched current-gain cutoff frequency (fT)/maximum oscillation frequency (fmax) of 29.9/55.2 GHz for the E-mode and 27.5/47.5 GHz for the D-mode devices. Direct-coupled FET logic E/D MIS-HEMT inverter and 51-stage ring oscillator are fabricated. The incorporation of gate dielectric enables the inverter a large logic voltage swing of 3.71 V at a supply voltage VDD of 5 V. The 51-stage ring oscillator implemented with 106 transistors shows an oscillation frequency of 427.6 MHz at VDD=5 V, corresponding to a stage delay of 23 ps.

Monolithic integrated enhancement/depletion-mode AlGaN/GaN high electron mobility transistors with cap layer engineering

Monolithic integrated enhancement/depletion (E/D)-mode AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated on an AlGaN/GaN heterostructure with an engineered triple-cap-layer. The energy band of the cap layer is greatly tailored by the polarizations within it, which improves the controllability of D-to-E mode conversion with gate recess. The uniformity of the threshold voltage (Vth) across a 3″ wafer is assessed and the standard deviations of Vth are 0.1 V and 0.14 V for E-mode and D-mode devices, respectively. Direct-coupled field-effect transistor logic E/D HEMT inverter and 17-stage ring oscillator are demonstrated, and the latter shows a oscillation frequency of 201 MHz at a supply voltage of 1 V, corresponding to a propagation delay of 146 ps/stage and a power-delay product of 1.96 pJ/stage.

Identifying Interface States in AlInN/GaN Heterostructure by Photocurrent Method

In order to assess the residual leakage current mechanisms in electronic devices based on AlInN/GaN heterostructures, defect states were investigated by photocurrent method based on a metal-semiconductor-metal structure device. As a result, a continuous distribution of defect levels from 56 to 110 meV below the conduction band of GaN was identified at the AlInN/GaN interface by analyzing photocurrent spectra under different bias voltages and photogenerated carrier transport based on the simulation of electric field distribution. These interface states filled with electrons at zero bias can release electrons at reverse bias and provide a path of gate leakage.

Improved linearity in AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors with nonlinear polarization dielectric

We demonstrate highly improved linearity in a nonlinear ferroelectric of Pb(Zr0.52Ti0.48)-gated AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor (MIS-HEMT). Distinct double-hump feature in the transconductance-gate voltage (gm-Vg) curve is observed, yielding remarkable enhancement in gate voltage swing as compared to MIS-HEMT with conventional linear gate dielectric. By incorporating the ferroelectric polarization into a self-consistent calculation, it is disclosed that in addition to the common hump corresponding to the onset of electron accumulation, the second hump at high current level is originated from the nonlinear polar nature of ferroelectric, which enhances the gate capacitance by increasing equivalent dielectric constant nonlinearly. This work paves a way for design of high linearity GaN MIS-HEMT by exploiting the nonlinear properties of dielectric.

Influence of fin architectures on linearity characteristics of AlGaN/GaNFinFETs

We investigate the influence of fin architecture on linearity characteristics of AlGaN/GaNFinFET. It is found that the FinFET with scaled fin dimensions exhibits much flatter G m characteristics than the one with long fins as well as planar HEMT. According to the comparative study, we provide direct proof that source resistance rather than tri-gate structure itself dominates the G m behavior. Furthermore, power measurements show that the optimized FinFET is capable of delivering a much higher output power density along with significant improvement in linearity characteristics than conventional planar HEMT. This study also highlights the importance of fin design in GaN-based FinFET for microwave power application, especially high-linearity applications.

3-inch GaN-on-Diamond HEMTs With Device-First Transfer Technology

Based on a device-first transfer process, a 3-inch polycrystalline diamond substrate is bonded within

High performance ultra-thin quaternary InAlGaN barrier HEMTs with f T > 260 GHz

1.5~\mu \text{m}

Quaternary InAlGaN barrier high-electron-mobility transistors with f max > 400 GHz

of the junction in GaN high electron mobility transistors (HEMTs) to enhance heat removal of the high-power RF devices. Highly preserved electrical performance is demonstrated by comparison exactly on the same HEMT device prior and after substrate transfer. The residual compressive strain relaxation of the whole GaN epilayer does not reduce the 2-D electron gas sheet density. The dc characteristics show weakened self-heating in the GaN-on-diamond HEMT with maximum current density increasing from 968 to 1005 mA/mm. The power density increases from 4.8 to 5.5 W/mm with the PAE slightly reducing from 50.9% to 50.5%. On-wafer infrared measurement is performed on a 1.25-mm GaN HEMT at power dissipation of 10 W/mm, and the peak juncture temperature of the device decreases from 241 °C to 191 °C after transferring to the diamond substrate.