TECHNOLOGY LEDs
(see Figures 3 and 4). These characteristics show that it is feasible to integrate a GaN LED with a GaN MOS-channel HEMT process. Current- voltage characteristics for the integrated LED and HEMT are consistent with the expected behaviour for a transistor and diode connected in series: Current is limited by the LED before it turns on, and afterwards it is restricted by the transistor’s saturation current. The intensity of the LED light is fully modulated by the gate voltage of the MOS- HEMT with good linearity (see Figure 5). High temperature operation of the integrated GaN LED/transistor pair has been demonstrated up to 225 °C.
Figure 3. Optical images of the integrated GaN LED/HEMT pair in off-state (left), and with the LED lighted up (right). Inset in (a): the schematic view of the circuit configuration
Our monolithic integration demonstrates the process compatibility of GaN LEDs and GaN transistors, and in particular an approach to unifying a GaN transistor with a MOS gate. This is a noteworthy achievement, because the MOS gate process usually requires a higher thermal budget than the Schottky gate process used in the GaN HEMT.
Figure 4. Characteristics of the integrated GaN LED
multiple quantum well structure and finally p-type GaN layers.
Formation of our circuit began with a chlorine- based, inductively couple plasma reaction-ion etch of selected regions of the LED structure to expose the n-doped GaN. Further etching defined trenches between GaN LED and HEMT, isolating these devices. After that, we removed the remaining n-type GaN that was on top of the HEMT, located where the MOS-channel HEMT would be fabricated. Following this, electron beam lithography patterned submicron recess channels of the MOS-channel HEMTs. These are etched, with a subsequent wet chemical process removing any damaged that occurred.
After cleaning this structure, we deposited SiO2 as a gate dielectric, followed by polysilicon as
the gate electrodes. The ohmic contacts of the MOS-channel HEMTs and the cathode contact of the LEDs were formed at the same time. After this, the anode contacts of the GaN LED were added. Finally, the cathode of the LED was connected to the drain of the MOS-channel HEMT.
The blue-emitting LED that resulted has a dominant wavelength of 459 nm, a full-width half- maximum as narrow as 22 nm and produces a relatively linear increase in light output with current
42
www.compoundsemiconductor.net January / February 2014
The MOS gate is not just an essential element in a power transistor − it also holds the key to the future of the GaN CMOS IC platform. Integrating a GaN LED and transistor is the first step towards the creation of the light-emitting power ICs (LEPICs) platform, where a single chip contains LEDs, power transistors and logic ICs. Constructing LEPICs could revolutionise solid-
Driver circuit board of a commercial LED light bulb (Courtesy to Casey Goodwin, Smart Lighting ERC, RPI)
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