Bringing The Inverter Onto The Chip

Monolithic integration of LEDs and transistors is helping to usher in a new era for solid-state luminaires, where the emitter and its control electronics are united on single chip.
By Zhongda Li and co-workers from The Smart Lighting Engineering Research Centre at Rensselaer Polytechnic Institute.

The driving conditionsfor the LED could hardly be more different from the output from the mains. While the LED needs just a few volts of direct current, mains delivers an alternating current at voltages ranging from 110 V to 250 V, depending on the country.

To address this difference, solid-state light bulbs contain an AC-to-DC power converter to modify the supply from the mains into a form that can drive the LEDs. The electronic driver actually carries out several roles, including maintaining a stable light output when the temperature of the LED chips change, and providing functions such as dimming and colour tuning.�

Today, the electronic driver circuits for LED bulbs are assembled from various discrete components, such as power transistors, capacitors and logic controller ICs. These are connected together on a printed-circuit board (PCB), which is usually bulky – it can take up as much space as the LEDs.

The perfect solution for the future of solid-state lighting is to monolithically integrate the LEDs, power transistors and controller ICs on a single chip. Since the LEDs in the bulb are based on GaN, all the devices must be built with this wide bandgap material.

Taking this approach will eliminate the parasitic inductance, capacitance and resistance between the driver board and the LED chips, and thereby allow the lighting system to operate at far higher switching speeds. In turn, this could boost efficiency and enable a trimming of the size of the passive components, as smaller inductors and capacitors can be used at higher frequencies. The upshot of all of this would be more frugal driver circuits with a smaller form factor than the current PCB solution.

Thanks to recent breakthroughs in GaN power device technologies, the building blocks are in place to form the electronic driver circuits with this material system. Power switching transistors, which are the key components in LED driver circuits, have already been made in GaN, and they show outstanding performance: Compared to conventional silicon transistors, they have a higher breakdown voltage, lower specific on-resistance and a higher operating frequency. In addition to these material property advantages over silicon, GaN can be made into HEMTs that exploit two-dimensional quantum confinement at the heterojunction to boost electron conduction. And on top of that, by combining a GaN HEMT structure with the metal-oxide-semiconductor (MOS) gate, it is possible to create a hybrid device structure called a MOS-channel HEMT. This has a positive threshold voltage, an extremely low gate leakage current, and is easy to drive.

At the Smart Lighting Engineering Research Centre at Rensselaer Polytechnic Institute we have drawn on all these device technologies to demonstrate the world’s first monolithic integration of GaN LEDs and MOS-channel HEMTs (see Figure 1). This accomplishment – which involved using a unified process to fabricate both classes of device on single GaN-on-sapphire wafer (see Figure 2) – is an important stepping-stone to making an LED bulb from an emitter and electronic control circuit on a single chip.

Figure 1. Monolithic integration of LEDs, power transistors and logic ICs in GaN

Figure 2. Process flow of the GaN LED and MOS-channel HEMT integration using selective epi removal

We commenced fabrication with MOCVD growth of a GaN LED structure on a commercial HEMT epiwafer, which was formed on a sapphire substrate. LED growth involved deposition of a�0.5 �m-thick n-type GaN film, followed by a 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 (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

Figure 4. Characteristics of the integrated GaN LED

Figure 5. LED current and light output intensity as a function of the supply voltage and the gate voltage of the GaN MOS-channel HEMT at room temperature (left) and at225 �C (right)

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.

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-state lighting, because such a platform could play an important role in LED control technologies and add functionalities required for emerging lighting applications. One example of this is visible light communication (VLC), also known as Li-Fi, where the visible spectrum is used for free-space optical communication.

This technology is ideal for localized high-volume data transmission in the office and the home. The frequency of visible light is in the range of hundreds of terahertz, so VLC has the potential for a much higher data capacity than that possible with a conventional wireless RF network. Flickering of the light source will not be an issue, because the on and off modulations that provide data transmission will occur at far higher speeds than the human eye can perceive. Meanwhile, the small form factors of the LEPICs also make them suitable for distributed lighting, with each light source serving as an independent data channel. Living in such a world, LED bulbs will not only be at the forefront of lighting – they’ll also be a big player in wireless technology.

Driver circuit board of a commercial LED light bulb� (Courtesy to Casey Goodwin, Smart Lighting ERC, RPI)

Further reading:

R. Karlicek, “Smart lighting – beyond simple illumination," in IEEE Photonics Soc. Summer Topical Meeting Series, Seattle, WA, 2012, pp. 147-148.

T. P. Chow and Z. Li, “Recent advances in high-voltage GaN MOS-gated transistors for power electronics applications," in GaN and ZnO-based Materials and Devices. Berlin, Germany: Springer, 2012, pp. 239-250.

Z. Li et. al.Appl. Phys. Lett. 102192107 (2013)

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