We investigate thermo-electronic behaviors of flexible AlGaN/GaN heterostructure field-effect transistors (HFETs) for high-power operation of the devices using Raman thermometry, infrared imaging, and current-voltage characteristics. A large negative differential conductance observed in HFETs on polymeric flexible substrates is confirmed to originate from the decreasing mobility of the two-dimensional electron gas channel caused by the self-heating effect. We develop high-power transistors by suppressing the negative differential conductance in the flexible HFETs using chemical lift-off and modified Ti/Au/In metal bonding processes with copper (Cu) tapes for high thermal conductivity and low thermal interfacial resistance in the flexible hybrid structures. Among different flexible HFETs, the ID of the HFETs on Cu with Ni/Au/In structures decreases only by 11.3% with increasing drain bias from the peak current to the current at VDS = 20 V, which is close to that of the HFETs on Si (9.6%), solving the problem of previous flexible AlGaN/GaN transistors.
A pixel array of micro‐light emitting diodes (µ‐LEDs) based on a flip‐chip structure and driven by a passive matrix is fabricated. The array is fabricated through multi‐level metallization using a photosensitive polyimide (PSPI) inter‐metal dielectric (IMD) layer. The device consisted of 256 pixels in a 2 × 2 mm2 array (the individual pixel area is 115 × 115 µm2). This device facilitates the control of individual µ‐LEDs in the array. To investigate the effect of reflectivity and coverage to p‐GaN on mesa area of p‐type electrode, controlled and uncontrolled µ‐LED arrays is fabricated with different reflectivity and coverage of the p‐type electrode. The results show that the devices has similar electrical properties. The light output power and maximum electroluminescence intensity of the controlled µ‐LED array is improved by 3.7 and 5.1 times at injection currents of 100 and 60 mA compared to the uncontrolled µ‐LED array, respectively. The variation of dark spaces in several emission images (total 9 pixels) is investigated as a function of the pixel pitch for the controlled µ‐LED array at an injection current of 20 mA. The results shows that the dark space between pixels almost disappears at a pixel pitch of 125 µm.
This paper investigates the thermal distribution of an LED headlight for vehicles based on the thermal conductivity of thermally conductive plastics (TCP). In general, heat dissipation structures used for LED headlights are made from metallic materials. However, headlight structures made from TCP have not been investigated. The headlights made from TCP having a various thermal conductivity were fabricated by injection molding with and without a metal plate insert. The temperature characteristics were compared and analyzed using thermal simulations and measurement. The inserted metal in TCP greatly reduced the temperature at solder point, indicating that the fast heat dissipation from the high power LED package to TCP though the inserted metal is essential. The measured temperature at solder points decreased as the thermal conductivity of TCP increased, which is well matched to the simulation results. The measured temperature at the solder point was lower than 150 °C when the thermal conductivity of the TCP was 10 W/mK.
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