Introduction

I was about to write an article grouping several important technologies together, it kept being delayed as I needed more information on each one. I also could see that grouping the companies together would not give each company its due, as each is important to a different aspect of AR/MR. Therefore I decided to break up the one article into multiple shorter articles. This series started with Meta Materials’ Dimming Technology in Part 1 because I saw it as having an immediate impact and a technology I had wanted for years.

Porotech has the most impressive technology I saw at CES. What impressed me the most is that they have made major progress since I saw them just six months ago and are willing to prove that it works.

I should note that Jade Bird Displays (JBD) is already shipping monochrome green MicroLEDs in production to customers, including Vuzix (which I met with at CES and will discuss in a future article). But the real race is to do full color the best. JBD’s roadmap to full color on a single device is unclear. I also don’t like the JBL color cube approach shown at AWE in 2021 (News: WaveOptics & Jade Bird Display MicroLED Partnership) on technical grounds. I discussed my reservations about the color cube concept for AR/MR headsets in my AWE 2022 video with SadlyItsBradley. TCL demonstrated the JBD color cube at CES 2023 in a headset. Unfortunately, I did not see it personally, but I have gotten reports that the image quality was poor (as expected).

Porotech MicroLED

Porotech MicroLEDs was briefly discussed as a written add-on to the AWE 2022 MicroLED video article, as I had met Porotech a few weeks after the video was recorded. I have been looking at MicroLED technology for about six years, and bar none, Porotech has the most interesting MicroLED technology I have seen to date.

Even though the technology is still immature, Porotech keeps publicly and privately demonstrating that they have industry-leading MicroLED technology and that it works. Their CES 2023 demos showed they are progressing rapidly since I visited Porotech in Cambridge, UK, in June 2022.

The biggest breakthrough for Porotech since I visited them in June was that they have greatly improved Red’s efficiency, which is by far the hardest visible color to be made in GaN (Gallium Arsenide (GaAs) is used to make conventional red LEDs). According to Insight Media’s Chris Chinnock’s white paper on Porotech, Porotech’s “porous GaN” enabled them to produce such efficient reds (and greens) in GaN.

Porotech has been developing very high-performance and high efficient monochromatic MicroLEDs and their unique multiple colors from a single-diode/emitter technology.

Extremely High-Efficiency Monochrome (Single-Color) MicroLEDs

Porotech says they are achieving External Quantum Efficiency (EQE) efficiencies of 70% for blue, 50% for green, and 15% the red single-color MicroLEDs. These EQE numbers appear to be exceptionally good. For reference, see MicroLED-Info’s 2020 article “A discussion on MicroLED efficiency,” which summarizes a 2020 Paper on LED efficiency.

The EQE is the efficiency of the photons created exiting the material and not the overall “wall plug efficiency” (WPE, Watts-electrical-in to Watts-light-out). Porotech said their WPE is not much lower than their EQE percentages but could not give the exact values.

Below are hand-held (and thus not perfectly focused) Macro pictures I took at CES 2023 of their 0.26-inch 1280x720p with a 3.5um pixel which Porotech has measured as delivering 700K nits for Blue, 3 million nits for Green, and 700k nits for red. There is a significant variation (click on each image for a high-resolution picture) in the intensity, which I have seen with all MicroLED devices to date, even JBD. The variation is likely due to bonding issues and LED-to-LED differences. The top-to-bottom variation in the pictures (most noticeable in the top of the “AR” in with the blue device) is due to the camera’s shutter “beating” against the pulse width modulation of the LEDs and not an issue with the LEDs.

It should be noted that for a given white point, you need approximately (it depends dramatically on the wavelengths of each color and the desired white point) 68% Green, 28% Red, and only 4% blue nits. Nits (and lumens) are based on human subjective analysis and the human eye as opposed to Watts, which is a measure of light energy. The chart on the right shows the relationship between light power in Watts and lumens. Notice how few Lumens (and nits) there are per blue Watt relative to green. See my 2011 article on the relationship between electrical and optical power (Watts) and lumens and my 2017 article on the relationship between lumens and nits.

Single Emitter (Dynamic Pixel Tuning^®) Makes Porotech Unique

Porotech is the only company I have seen develop a MicroLED with a single emitter LED that can produce all visible (and some invisible) light colors, what they call DynamicPixelTuning^® (DPT^®). All other MicroLEDs require three emitters, either side by side or stacked, to produce red, green, and blue. With Porotech, each LED changes color based on the current level, and the driving duration controls the intensity.

The Advantages of a single emitter MicroLED include:

• Much better light coupling efficiency (lower etendue) since the area of light emission is from a single emitter
• Smaller pixels
• Only two contacts versus six for full-color pixels
• 1/3rd the diodes to produce

Porotech’s single emitter is that it is much more complicated to drive. With three-emitter full-color devices, either current or pulse-width controls the brightness. With Porotech’s LED, the current and the pulse width must be controlled simultaneously. This means the driving silicon backplane will be much more complex and difficult to develop. But what I like so much is that Porotech is addressing the fundamental issue and shifts the complexity to a “domain” that will be solved eventually.

It is also unclear whether DPT will be as efficient or bright as individual single-color LEDs. Porotech says that while behind their single-color LEDs, they are processing rapidly on the efficiency and brightness of the DPT LEDs and are already seeing “double-digit” efficiency results.

Because no silicon backplane can yet drive their devices, they showed different capabilities in separate demos. In one demo, they produced white (and other colors) by rapidly time-sequencing red, green, and blue colors. Impressively, they adapted an LCOS-type 1280×720 pixel backplane to show a moving bitmapped display where they could make all “on” pixels any one color and change that color at will. Unfortunately, I did not have the setup or time to get a good video of the demo at CES, as this was their most impressive new demo.

Pixel Shifting MicroLEDs (and an Alternative to DPT)

Not infrequently, when discussing the uniformity and color issues with MicroLEDs, the concept of pixel-shifting (or even sub-pixel shifting) comes up. MicroLEDs, with their typically extremely fast switching times (typical in the nanoseconds), would be more adept at pixel shifting than other technologies. Pixel shifting is already being used with digital cameras and front projectors to enhance resolution and could play and have major uses with MicroLEDs, including:

• Shifting several LEDs to the same location to average out their brightness and to correct for any dead pixels to greatly improve yields. Porotech’s current samples and every MicroLED I have seen show a high degree of LED-to-LED variability. More than other display technologies, MicroLEDs might need pixel shifting to get acceptable yields.
• Shifting spatial color subpixels (red, green, and blue) to the same location for a full-color pixel (as an alternative to the Porotech’s single emitter for all colors). This would be a way to reduce the effective size of a pixel and “cheat” the etendue issue caused by a larger spatial color pixel. I would expect that Porotech’s technology would allow them to have advantages in supporting spatial color if that proved more effective.
• Similar to cameras, it would be a way to improve resolution. The MicroLED emission area is typically much smaller than the pitch between pixels, so there might be no overlap when switching and thus give the full resolution advantage. This technique could provide even fewer pixels with fewer connections, but there will be a tradeoff in maximum brightness that can be achieved.

Any or all of the above might be used in some combination to get multiple benefits.

There are many possible pros and cons of a shifted spatial color versus Porotech’s DPT that can be imagined and debated. The yield, efficiency, and brightness might be better with spatial pixels (or not). It would be simpler to design a backplane to control time sequential spatial pixels than to control the current to change the color of DPT pixels.

When I first saw Porotech’s DPT technology, I thought, “this has to win,” but then you have to realize that rather than give up, companies will seek out alternatives such as pixel shifting, which may or may not prove to be better. Still, I don’t think X-Cubes with multiple devices or spatial color without sub-pixel shifting will be effective in the long run for AR.

Other Uses

Porotech is setting up to be an LED manufacturer and not a display builder. This will enable Porotech to sell MicroLEDs for everything from microdisplays to watches, cell phones, and large displays. Ultimately, it will be up to someone else to develop the backplanes and a production LED-to-backplane transfer and connection process. I could easily see the highly efficient monochrome MicroLEDs being singulated and used to make products like watches that could lead to cell phone displays if a company develops a transfer process.

Porotech says that visible light MicroLEDs are only a small percentage of the expected application of their Porous Gallium Nitride technology. They claim that their technology could revolutionize the whole field of Gallium Nitride devices. It is nowhere in my technical wheelhouse to understand these claims as an old digital CPU designer, but I can say that their MicroLED developments are the most impressive I have seen to date.

Conclusion

In a nutshell, Porotech has the most efficient monochrome, red, green, and blue GaN technology to date and seems to have the best potential to deliver a full-color approach to MicroLEDs (either with their DPT technology or if they chose to do a spatial color pixel).

While Pototech seems to be making rapid progress in developing its MicroLEDs, they are far from having a production-ready device. They will still need to improve their devices’ yields and uniformity, and someone must develop a high-volume transfer process of the LEDs to the driving substrates. This might include developments with pixel-shifting technology.

As a former CPU architect (back in my days as a Texas Instrument Fellow) and LCOS backplane designer (as CTO of two companies), I see both the difficulty while believing that the complex backplane drive that Porotech requires will eventually be developed. Porotech DPT technology has moved the problem to one where highly skilled “turning the crank” can solve it.

After six years of watching the MicroLED developments, I was getting discouraged at the slow progress made in MicroLEDs, particularly concerning full color. Porotech’s technology has renewed my enthusiasm.