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Taking an active interest in display technology
Choosing the right display technology for your next application requires an eye for detail
A color display is hard to beat in any application and continued price erosion means it is becoming commercially viable to use graphical color displays in a wider range of applications.
If the product only needs to convey simple information, then a LED is still a good option. However, in the industrial, medical and automotive markets, the trend is towards graphical user interfaces that provide more than the basics.
So what kind of display technology is the ‘right’ one for a given application? There is a wide range of display solutions available, creating a design dilemma for engineers.
The latest chapter in the display technology saga is, arguably, the active matrix organic LED, or AMOLED. Although the technology is relatively mature, it has yet to become as prolific as LCDs, but that may be changing.
While most graphic color displays employ thin-film transistor (TFT) technology to control the color and light intensity of every pixel, only AMOLEDs are truly emissive; they generate their own light instead of modulating and filtering the light provided by a single, common source.
AMOLEDs use TFT technology to implement an active pixel addressing scheme, which sets them apart from their passive matrix cousins (PMOLED). Both types generate light through an emissive electroluminescent layer, formed from a thin layer of an organic compound. However, PMOLEDs use a simpler ‘row/column’ pixel addressing scheme inherited from earlier LCD designs, which imposes the need for refresh rates high enough to maintain an acceptable image.
AMOLEDs, on the other hand, allow each pixel to maintain its state between refresh cycles, through the integration of an active transistor and capacitor circuit on each pixel. This removes the need for high refresh rates purely for image persistence and normally results in lower power consumption.
While AMOLEDs have many advantages over PMOLEDs, there are still many applications where a PMOLED would be more suitable. Similarly, other forms of displays using backlit LCD technology may represent a better solution in specific applications.
The biggest application area for AMOLEDs is currently high-end smartphones, but with new manufacturing facilities and advanced processes coming online, costs and minimum order quantities (MoQs)are coming down. It is, therefore, easier for the embedded sector to access AMOLED technology, bringing the benefits of OLED technology to a wider range of products.
Availability and performance are always the key factors to consider when selecting a display, with viewing angle, brightness, response time, power and size varying between different technologies.
AMOLED displays offer a wider viewing angle than TFT LCDs and are significantly thinner. Their construction also allows more light to reach the top layer (normally glass), so they are generally perceived to be brighter. Data shows that, for a given luminance of, say, 150cd/m2, AMOLEDs can appear almost 50% brighter than TFT-LCDs.
In practice, this means a TFT-LCD would need to use more power to appear as bright as an AMOLED, particularly in sunlight. Conversely, the AMOLED could operate at lower power and still be readable in direct sunlight.
By way of illustration, Truly Semiconductors is a Chinese manufacturer of small and medium-sized display modules, producing both AMOLED and TFT-LCD displays. Its data shows that AMOLED offers superior performance in terms of contrast ratio and response times.
This alone would make it more applicable in many applications, but the benefits extend to include lower dynamic power, broadening the range of potential applications to include portable equipment.
AMOLED technology offers much higher response rates than TFT-LCD. High response times directly influence the update rate; a frame rate of 60Hz requires a minimum response time of 16ms. It follows that a higher response rate results in a clearer image. This can be particularly important in industrial or medical applications, where the clarity of the image could be crucial.
Although AMOLED technology normally wins in terms of performance and size, it is certainly not right for every application.
The most immediate factor is cost; AMOLED technology carries a 100% premium compared to standard twisted nematic TFT displays and is 50% more expensive than TFT in plane switching technology, which offers similar viewing angles and brightness to AMOLED, at the expense of size and power consumption.
MoQs for AMOLED technology is also higher. Although AMOLEDs are available for industrial applications, MOQs are something like 50,000 per year – much less than the millions required even a short time ago, but still too high for many industrial applications. Lead times also are longer on AMOLED – roughly double that of TFT at present.
Finally, the lifetime can rule out AMOLED in professional electronics applications. You should consult the datasheet for the operational hours on your preferred AMOLED or TFT but expect in the region of 20,000 to 30,000hr for AMOLED and 50,000hr for TFT.
Truly Semiconductors recently invested in a 4.5G production line that will use low-temperature polysilicon to make AMOLEDs with a density of up to 500pixel/in. It is also running an amorphous silicon process to manufacture 300pixel/in TFT-LCDs.
There are strong technical and commercial reasons for choosing AMOLED technology when developing next-generation industrial, medical and automotive applications. Its superior performance, flexibility – often literally – and availability compare favorably with existing display technologies and it is likely to become a popular choice with OEMs and their customers.