biology

Low-temperature polycrystalline silicon

Low-temperature polycrystalline silicon is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Low-temperature polycrystalline silicon rather than just read about it. In short: Low-temperature polycrystalline silicon (LTPS) is polycrystalline silicon that has been synthesized at relatively low temperatures (~650 °C and lower). LTPS is important for display industries, since the use of large glass panels prohibits exposure to deformative high temperatures.

Key takeaways

Low-temperature polycrystalline silicon belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Low-temperature polycrystalline silicon to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Low-temperature polycrystalline silicon from memory before moving on to harder problems.

Reference excerpt

Low-temperature polycrystalline silicon (LTPS) is polycrystalline silicon that has been synthesized at relatively low temperatures (~650 °C and lower). LTPS is important for display industries, since the use of large glass panels prohibits exposure to deformative high temperatures. More specifically, the use of polycrystalline silicon in thin-film transistors (LTPS-TFT) has high potential for large-scale production of electronic devices like flat panel LCD displays or image sensors. LTPS is polycrystalline silicon that has been synthesized at ~650 °C and lower, compared to traditional methods (above 900 °C).

Development of polycrystalline silicon Polycrystalline silicon (p-Si) is a pure and conductive form of the element composed of many crystallites, or grains of highly ordered crystal lattice. In 1984, studies showed that amorphous silicon (a-Si) is an excellent precursor for forming p-Si films with stable structures and low surface roughness. Silicon film is synthesized by low-pressure chemical vapor deposition (LPCVD) to minimize surface roughness. First, amorphous silicon is deposited at 560–640 °C. Then it is thermally annealed (recrystallized) at 950–1000 °C. Starting with the amorphous film, rather than directly depositing crystals, produces a product with a superior structure and a desired smoothness. In 1988, researchers discovered that further lowering the temperature during annealing, together with advanced plasma-enhanced chemical vapor deposition (PECVD), could facilitate even higher degrees of conductivity. These techniques have profoundly impacted the microelectronics, photovoltaic, and display enhancement industries.

Use in liquid-crystal displays

Amorphous silicon TFTs have been widely used in liquid-crystal display (LCD) flat panels because they can be assembled into complex high-current driver circuits. Amorphous Si-TFT electrodes drive the alignment of crystals in LCDs. The evolution of LTPS-TFTs can have many benefits such as higher device resolution, lower synthesis temperature, and reduced price of essential substrates. However, LTPS-TFTs also have several drawbacks. For example, the area of TFTs in traditional a-Si devices is large, resulting in a small aperture ratio (the amount of area which is not blocked by the opaque TFT and thus admits light). The incompatibility of different aperture ratios prevents LTPS-based complex circuits and drivers from being integrated into a-Si material. Additionally, the quality of LTPS decreases over time due to an increase in temperature upon turning on the transistor, which degrades the film by breaking the Si-H bonds in the material. This would cause the device to suffer from drain breakdown and current leakage, most notably in small and thin transistors, which dissipate heat poorly.

Processing by laser annealing

XeCl Excimer-Laser Annealing (ELA) is the first key method to produce p-Si by melting a-Si material through laser irradiation. The counterpart of a-Si, polycrystalline silicon, which can be synthesized from amorphous silicon by certain procedures, has several advantages over widely used a-Si TFT:

High electron mobility rate; High resolution and aperture ratio; Available for high integration of circuits. XeCl-ELA succeeds in crystallizing a-Si (thickness ranges from 500-10000Å) into p-Si without heating the substrates. The polycrystalline form has larger grains that yield better mobility for TFTs due to reduced scattering from grain boundaries. This technique leads to the successful integration of complicated circuits in LCD displays.

Development of LTPS-TFT devices

Apart from the improvement of the TFTs themselves, the successful application of LTPS to graphical displays also depends on innovative circuits. One recent technique involves a pixel circuit in which the outgoing current from the transistor is independent of the threshold voltage, thus producing uniform brightness. LTPS-TFT is commonly used to drive OLED displays because it has high resolution and accommodation for large panels. However, variations in LTPS structure would result in non-uniform threshold voltage for signals and non-uniform brightness using traditional circuits. The new pixel circuit includes four n-type TFTs, one p-type TFT, a capacitor, and a control element to control the image resolution. Enhancing the performance and microlithography for TFTs is important for advancing LTPS active-matrix OLEDs. These many important techniques have allowed the mobility of crystalline film to reach up to 13 cm2/Vs, and they have helped to mass-produce LEDs and LCDs over 500 ppi in resolution.

LTPO Low-temperature polycrystalline oxide (LTPO) is a type of OLED display backplane technology developed by Apple that combines LTPS TFTs and oxide TFTs (indium gallium zinc oxide, or IGZO). In LTPO, the switching circuits use LTPS while the driving TFTs use IGZO materials. LTPO allows for more efficient use of power due to the lower leakage of the oxide gate insulation. LTPO displays are known for their improved battery life and can be found in some smartphones, smartwatches, and other mobile devices. Although the core technology in LTPO is developed by Apple, Samsung also has its proprietary technology for LTPO AMOLED panels using a combination of LTPS TFTs and hybrid-oxide and polycrystalline silicon (HOP).

See also Indium gallium zinc oxide Light-emitting diode Monocrystalline silicon Photovoltaics Wafer (electronics)

References

Illustrations

Worked examples

Example 1 — a first encounter with Low-temperature polycrystalline silicon

Start with the simplest possible case. Write down what Low-temperature polycrystalline silicon claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Low-temperature polycrystalline silicon before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Low-temperature polycrystalline silicon ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Low-temperature polycrystalline silicon

In research: Low-temperature polycrystalline silicon appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Low-temperature polycrystalline silicon in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Low-temperature polycrystalline silicon is common in secondary-school and first-year university syllabi. It links to neighbouring topics Allotropes of silicon, Crystals, Electronics manufacturing, so understanding it makes those chapters shorter.
In everyday life: Look for Low-temperature polycrystalline silicon outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Low-temperature polycrystalline silicon in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Low-temperature polycrystalline silicon means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Low-temperature polycrystalline silicon out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Low-temperature polycrystalline silicon in simple terms?

Why does Low-temperature polycrystalline silicon matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Low-temperature polycrystalline silicon?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Low-temperature polycrystalline silicon.