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From their discovery in 1888 to their first application within displays in the 1960s, liquid crystals have become a mainstream material choice with many impactful applications in the world of electronics.
As the key component behind liquid crystal displays (LCD), these materials change light polarization to create vibrant, high-resolution images on digital screens. The growth of LCD technology has helped propel the larger display panel market enormously, with industry valuation projected to reach $178.20 billion by 2026.
In this post we’ll explore the various types of LCD technology currently on the market and some of their distinct applications and advantages.
Types of LCD Technology and Terminology
A prolific variety of LCD types has been developed to best meet their exact use-cases and end-environments. Displays may be optimized for power consumption, contrast ratio, color reproduction, optimal viewing angle, temperature range, cost, and more.
When choosing which type of LCD is best suited for a project, consider the following:
Passive Matrix LCDs are addressed with common and segment electrodes. A pixel or an icon is formed at the intersection where a common and a segment electrode overlap. Common electrodes are addressed one-at-a-time in a sequence. Segment electrodes are addressed simultaneously with the information corresponding to all pixels or icons connected to the current common electrode. This method is referred to as multiplexing.
For multiplexing to work, the liquid crystal structure must have a threshold voltage (below which it does not respond to the applied voltage), and a significant ‘steepness’ in optical effect as a function of applied voltage once the threshold voltage is exceeded. This optical steepness is directly related to the number of common electrodes which can be addressed. The twisting of the liquid crystal helps create both the threshold voltage and a steep response.
Passive Matrix LCDs offer a cost advantage (both parts and tooling) and are highly customizable. The counterpart to Passive Matrix displays are Active Matrix displays.
Active-Matrix LCDs were developed to overcome some of the limitations of Passive Matrix LCDs – namely resolution, color, and size. Within an Active-Matrix LCD, an “active element” is added to each pixel location (the intersection between a horizontal row and vertical column electrode). These active elements, which can be diodes or transistors, create a threshold and allow control of the optical response of the liquid crystal structure to the applied voltage. Transistors are used as switches to charge a capacitor, which then provides the voltage to the pixel. Whenever a row is turned on, one at a time, all transistor switches in that row are closed and all pixel capacitors are charged with the appropriate voltage. The capacitor then keeps the voltage applied to the pixel after the row is switched off until the next refresh cycle.
Furthermore, the processes used for manufacturing Active-Matrix LCDs can create much finer details on the electrode structure. This allows splitting each pixel in three sub-pixels with different color. This together with the better voltage control allows full color displays.
The transistor switches used in Active Matrix Displays must not protrude significantly above the surface of the display substrates lest they might interfere with a uniform liquid crystal layer thickness. They must be implemented in thin films of suitable materials. Hence, the name Thin Film Transistors (TFT). While AM and TFT have a different meaning, they are often used interchangeably to indicate a higher performance display.
TFTs can be formed by amorphous silicon (denoted α-Si TFT), by poly-crystalline silicon (LTPS for Low Temperature Poly Silicon), or by semiconducting metal oxides (Ox-TFT or IGZO-TFT for Indium Gallium Zinc Oxide).
Currently the most common Electronics Display Technology on the market is LCD technology and among LCD technologies, TFT display technology is the most widely used across consumer applications (laptops, tablets, TVs, Mobile phones, etc.) as well as many industrial, automotive, and medical applications.
Nematic refers to one of the common phases of liquid crystals (LC). In this phase, rod-like molecules tend to self-align more or less parallel to each other.
As the first commercially successful LCD technology, Passive Matrix Twisted Nematic (TN) LCDs use a 90° twist of the nematic LC fluid between two polarizers to display information. The twist of the LC fluid either blocks light from passing through the LCD cell or allows light to pass, depending on the applied voltage. The applied voltage changes the twisted nematic orientation into an orientation that does not change the polarization of tight. This is called the TN effect.
Example of a Segmented TN DisplayTN displays can be normally white (NW) when they use two orthogonal liner polarizers or normally black (NB) when parallel linear polarizers are used. ‘Normally’ refers to what happens when no voltage is applied.
Initially, Passive TN LCDs were used in segmented, icon, or character displays where an image element was turned “on” and “off” depending on how the fluid was driven. Improvements were made along the way to address the limited viewing angle of TN technology, which can suffer from contrast loss or even inversion at shallow angles.
The TN effect is also used in some Active Matrix liquid crystal displays (AM-LCDs).
It can be advantageous to twist the director of the nematic phase a bit more than 90 degrees, but less than 180 degrees. Displays like this are a subset of TN displays and are sometimes caller Hyper Twisted Nematic Displays.
The numbers of rows or icons that can be addressed in a TN display without Active Matrix addressing is very limited. This is related to how strongly the liquid crystal responds to the applied voltage. Twisting the LC nematic fluid more than 180 degrees (typically between 210 and 270 degrees) causes the display to require a much smaller voltage difference between on and off pixels. This in turn allows addressing of many more rows without an active matrix. Displays with a twist between 210 and 270 degrees are called Super Twisted Nematic displays.
The higher display resolution of STN displays comes at a price. The optical effect is no longer neutrally black and white as in a TN display. Rather these displays are naturally yellow and black or blue and white. The color can be somewhat compensated with colored polarizers, but that comes at the expense of brightness and contrast.
The color in STN displays is caused by birefringence. Adding the same birefringence in the opposite direction can compensate for the effect. Initially this was done by stacking two STN displays on top of each other. This is referred to as Double STN or DSTN, but this is of course thicker and more expensive.
Example of a FSTN displayThe birefringence of an STN display can be approximated with a stretched transparent plastic film. Adding such a film to an STN display instead of the 2nd STN display is a lot more attractive and has almost the same performance. This is referred to as a Film Compensated STN display (FSTN, or sometimes if two films are used as FFSTN).
FSTN displays are used commonly in consumer, medical and industrial display applications that require low cost and do not need high resolution images or full color.
Another development to the TN display was to use the same concept as in FSTN displays on TN displays. However, the film cannot just be a stretched polymer. Instead, a twisted liquid crystal structure is made and polymerized into a film that is used as a compensation film for TN displays. As this method mostly improves the display characteristics at shallow viewing angles while preserving the excellent performance in straight on viewing, this technology is called Wide View Twisted Nematic (WVTN).
The above display technologies have liquid crystal molecules that are aligned nearly parallel to the display surface with more or less twisting when going from one substrate to the other. In VA (also called VAN) displays, the liquid crystal molecules are aligned vertically with respect to the display surface. Applying a voltage causes the molecules to lay flat, with or without twist.
The advantage of this arrangement is a very dark black state with very little light leakage. This allows making displays with a black mask and colored icons or symbols. These displays look like color displays with brilliant colors, however each image element or icon can only have its assigned color or black.
Sometimes these displays are referred to as PMVA displays to distinguish them from VA displays using an active matrix.
Due to the ability in AM displays to address one row while the other rows are isolated, the demands on the electro-optical performance of the Liquid Crystal Configuration are less stringent. In principle, all the above mentioned configurations can be used in AM displays. In practice, TN, and WVTN are frequently used as well as some versions of VA technology described below. (MVA, AIFF-MVA, PVA, ASV).
In-Plane Switching (IPS) and Fringe Field Switching (FFS) are technologies that apply the electric stimulus between electrodes on only one substrate unlike all other technologies described here where the electric stimulus is applied between electrodes on both substrates.
The advantage of these technologies is a much wider and more symmetrical viewing angle along with the elimination of the contrast inversion (or color shift) seen in TN TFT LCDs when viewed from various angles. IPS and FFS displays also are less sensitive to pressure, which is a big advantage in touchscreen displays.
Throughout the development of these technologies, there were the initial type, super, advanced, pro, etc. versions, which led to a lot of acronyms like (S-IPS, AS-IPS, H-IPS, FFS-Pro)
PVA and MVA are active-matrix equivalents to the VA or VAN technology described above.
Here, each color sub-pixel is further divided into zones (called domains) having a different direction of the molecular movement when voltage is applied. Again, the purpose is a much wider and more symmetric viewing performance and the elimination of color shifts and contrast inversion at shallow angles. MVA technology achieves that with carefully designed protrusions on the inside surfaces of the display, while PVA uses fine patterning of the electrodes on both substrates.
Note: PMVA is reserved for passive matrix vertically aligned and does not mean patterned multidomain vertically aligned.
ASV is a version of MVA where instead of two or four domains per pixel the liquid crystal switches in radial directions all around the center of the pixel. This technology was developed and used exclusively by Sharp and is no longer in production.
Even more exotic, AIFF-MVA is a technology attempting to combine the benefits of VA type displays with fringe field switching.
The integrated circuit is a patterned piece of silicon or other type of semiconducting material. A modern IC contains millions or even billions of tiny transistors. Their tiny size allows for the fabrication of smaller, faster, more efficient, and less expensive electronic circuits. The driver chips addressing electronics displays are ICs.
Legacy LCDs normally have the driver ICs (integrated circuit) mounted on a printed circuit board (PCBA) which consists of a flat sheet of insulating material used to mount and connect the driver IC and electronic periphery to the LCD. PCBs can be a single-sided, double-sided or multi-layer.
Often PCBAs are connected to the display with flexible printed circuits. It’s also possible to mount all necessary components on FPCs without the need of a PCBA in the display module.
Display driver IC’s are often mounted directly onto the display glass. This is done with an anisotropic conductive film (ACF) adhesive.
Low Voltage Differential Signaling (LVDS) is an interface to the display, not a display technology itself. This technology is not specific to displays, as it’s used in many other applications as well. It’s a high-speed signal that provides some noise immunity for the display. It also allows for longer distances compared to parallel interfaces.
As display resolutions increase, data transmission rates must increase as well. At high frequencies, single-ended signaling circuits can begin to act as antennas to radiate and receive radiated noise. Low Voltage Differential Signaling (LVDS) addresses many of these shortcomings by using differential signaling at low voltages to transmit display data at high speeds.
Mobile Industry Processor Interface (MIPI®) is a high-speed Display Serial Interface (DSI) between the host processor and the display module. It has a low pin count, high bandwidth, and low Electro Magnetic Interference (EMI), and is commonly used in cameras, cell phones and tablets.
New Vision Display (NVD) has decades of experience designing and manufacturing custom display and touch panel assemblies for some of the world’s largest original equipment manufacturers (OEMs) in the automotive, medical, industrial, and consumer markets.
NVD’s state-of-the-art factories are equipped to build solutions using a wide range of display and touch technologies. To view our extensive portfolio, visit our Products Page.
With industry-leading certifications and expert engineers on staff, NVD can handle your design needs, even for the most rigorous and complex end environments. To learn more about what makes us the display manufacturer for your needs, contact us today.
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