GIPO newsletter

Suppression of Current Collapse in Enhancement Mode AlGaN/GaN High Electron Mobility Transistors

Professor Jian-Jang Huang's laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

AlGaN/GaN high electron mobility transistors (HEMTs) for RF and power applications have been researched and developed for over 20 years now and have met/exceeded many of the goals initially pursued by its researchers. In recent years there is strong demand for GaN power devices on Si substrates. In this work, the phenomenon of current collapse of E-mode HEMTs with the p-GaN cap layer was studied with (device B) and without (device A) SiO₂ passivation. By extracting normalized R_on of our devices at different gate stress, it is demonstrated that device B has less severe current collapse within the gate stress of -15V. QSCV indicates that donor-like traps in SiO₂/GaN interface and within the SiO₂layer suppresses electrons on the epi-structure surface and within the p-GaN cap layer, thus maintaining the channel carriers and mitigating current collapse.

Fig. 1 (a) Illustration of the setup for current collapse measurement. (b) The waveform of the node V_Gand V_D in the dynamic measurement. V_DD is 40V in the test.

Fig. 2 Transient response of the dynamic measurement of (a) device A (without SiO₂ passivation) and (b) device B (with SiO₂ passivation) with the measurement duration 5µs. A longer measurement of 500µs was also performed and shown in (c) for device A and (d) for device B. The gate bias stress is 0, -5, -10 and -15V, and the post stress measurement is called “0V final”.

Fig. 3 Illustration of the carrier flows in the devices. (a) shows carrier trapping in device A while (b) is the case for device B. Carrier distributions at the off-state but under the gate stress bias are drawn in the left figures while the switched-on conditions are shown in the right.

Tunable Three-Leaf Trefoil-Type Corner Cube Retro-Reflector

Professor Jui-che Tsai

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We have developed a three-leaf trefoil-type corner cube retro-reflector (CCR) fabricated with the MUMPs polysilicon surface micromachining process. Unlike other MEMS CCRs, this CCR’s opening faces straight upward as its (1, 1, 1) direction, i.e., the direction along which the retro-reflection efficiency is maximum, is perpendicular to the substrate’s surface (Fig. 1). Simulation results show that the orthogonality of CCR plays an important role in the optical performance of the CCR. Two CCRs leaning against each other to form a CCR pair are also demonstrated (Fig. 2), showing the feasibility of achieving a linear CCR array. With the ability to independently set the on-off state of each CCR by employing an electrostatically actuated gap-closing mirror as one of the three reflecting “leaves,” a CCR array can be used to realize optical identification.

Figure 1

Figure 2

© 2015 IEEE
Y. F. Chen, B. J. Yang, and J. C. Tsai, “Surface-micromachined MEMS tunable three-leaf trefoil-type corner cube retro-reflector for free-space optical applications,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 21, No. 4, 2700907, Jul./Aug. 2015

InAsSb/InAsPSb multiple quantum well disk cavities with pedestal structures on a GaSb substrate for mid-infrared whispering gallery mode emission beyond 4 μm

Professor Ming-Hua Mao's laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The mid-infrared whispering gallery modes disk cavities with InAs_0.85Sb_0.15/InAs_0.53P_0.23Sb_0.24 multiple quantum wells active medium on a GaSb substrate were fabricated. For this material system in the mid-infrared range, fabrication techniques were developed to form the disk cavity structure. The smooth sidewalls of the disk cavities were achieved by appropriate gas mixture flow ratio of BCl₃/Ar in the inductively coupled plasma reactive ion etching. In addition, selective wet etching technique was used to from the pedestal of the disk cavity using dilute hydrofluoric acid with good selectivity. For efficient confinement of the whispering gallery modes along the radial direction, the extent of the lateral etching was carefully controlled. The processed 30-μm-diameter disk cavities were optically pumped, and the whispering gallery modes with wavelengths around 4.1 μm can be observed up to 90 K. This work has been published in Optics Letters, Vol. 40, No. 9, pp. 1904, (2015).

Fig. 1. Scanning electron microscope image of a fully processed disk cavity with a diameter of 30 μm.

Fig. 2. Emission spectrum of a 30-μm-diameter disk cavity at 90 K.

Modeling monochromatic light propagation through scattering medium via optical phase conjugation

Professor Snow H. Tseng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We model the propagation of monochromatic light through a macroscopic scattering medium by employing the pseudospectral time-domain (PSTD) simulation technique. Simulation results show that, with the appropriate amplitude and phase, monochromatic light can be directed through a scattering medium and focus into a narrow peak. Specifically, we compare: i) focusing monochromatic light through a macroscopic scattering medium, and, ii) focusing monochromatic light through vacuum. Based upon numerical solutions of Maxwell’s equations, we show that the sub-diffraction focusing can be achieved with monochromatic light of specific amplitude and phase; furthermore, simulation results demonstrate that this phenomenon is insensitive to the scattering medium.


Fig. 1. (a) Light propagation through vacuum. (b) Light propagation through a scattering medium exhibits a sinuous, complex pattern resembling a network of branching water channels.


Fig. 2. Simulation of light (λ = 4 μm) back-propagated through a scattering medium consisting of 1600 6-μm-diameter dielectric cylinders, each with a refractive index (a) n = 1.05, (b) n = 1.2, (c) n = 1.3, respectively. Inset-figures: Regardless of n, the back-propagated light converges coherently and reconstructs the original light profile.

Ionic polymer metal composite for an optical zoom in a compact camera

Professor Guo-Dung J. Su's laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The reflective method is utilized in the optical zoom function of a thin camera for the advantage of folding the optical path. An ionic polymer metal composite deformable mirror used in a reflective zoom system achieves large deformations to change optical power with a low bias voltage. Polydimethylsiloxane is used as a buffer layer to improve surface roughness. The surface roughness of this layer is about 17 nm. The optical focusing power of the deformable mirror reaches 73.8 m−1 diopters with 3 volts. A complete reflective camera module is fabricated using two ionic polymer metal composite deformable mirrors in the zoom function. The zoom ratio is about 1.6 ×. This work has been published in Optics Express, Vol. 23, Issue 10, pp. 13265-13277 (2015).


Fig. 1. Optical zoom module (a) Simulation image (b) Entity image


Fig. 2. Experimental results (a) Image at tele end (b) Image at wide end