Want an interactive wearable device? The amplifier that makes dreams come true is here!

Text/Liu Yizhi, Li Junyi, Weng Jialing

Want an interactive wearable device? The amplifier that makes dreams come true is here!

Text/Liu Yizhi, Li Junyi, Weng Jialing | China? Taiwan Province province, China? Studying physics and electricity? Elective students

Remember when we imagined Google Glass? You can take pictures in the blink of an eye, and you can zoom the window and scroll the page at will with the shift of sight. However, have you ever paid attention to the technology behind these functions?

How to make the cool interactive wearable devices in science fiction movies become a reality has always been a dream of scientists. Pexel interactive wearable devices have always been one of the indispensable elements in science fiction movies, and the reason why these devices can only exist on the big screen is because there are many problems to be overcome in reality, besides cost considerations, there are also motion detection. Because most of the bioelectrical signals of human body are extremely weak, in order to detect these signals, the current effective technologies basically need to be equipped with an expensive and low endurance detection device to achieve the goal. The ideal detection equipment must be able to detect tiny movements. Therefore, the most important part of the detection equipment is the amplifier. However, if wearable interactive devices are to be popularized, this kind of amplifier should be low in power consumption, expandable, good in amplification effect, durable and low in production cost, so as to meet the needs of our long-term wear and use. ?

Amplifiers in electronic components-transistors At present, most amplifiers in electronic components use transistors to achieve the amplification effect, and these transistors are stacked by semiconductors.

Comparison of different conductive materials. Electrons in materials are originally bound in the valence band, but if they are given enough energy, they will have the opportunity to jump into the conduction band and become movable electrons, usually metal conductors, that is, the valence band is very close to the conduction band, and they can be turned into movable electrons with only a little energy, while insulators are just the opposite. Even given a lot of energy, few electrons can move.

As the name implies, semiconductors are between conductive and non-conductive. We can now decide whether a material can conduct electricity by adjusting the given energy. Using this characteristic, the semiconductor can become a simple automatic control device to control the on-off state of the circuit.

How do transistors made of semiconductors make amplification effects? From the root of the word transistor, we can find that it is composed of trans and resistance, that is, an extra contact is used to control the resistance in the transistor. Take the resistor as an example, it doesn't have any external control points, so it is assumed that a current of 1 amp flows in from one end, and the other end will output a current of 1 amp; The transistor has an extra contact. If a voltage is applied to this contact to "inform" the transistor to change the resistance at the output end, then we can control the output current, which is the principle of the transistor as an amplifier.

The schematic diagram of resistor (left) and transistor (right) shows that the transistor has an extra contact. Thin film transistor (TFT) mentioned below is a kind of transistor, which is often used in displays. According to whether the current passes or not, each pixel on the screen is indirectly controlled to generate different brightness, so that the liquid crystal display can display various pictures and colors. However, ordinary thin film transistors are difficult to form, so it is difficult to be applied to wearable devices.

Thin, short and cheap? "Organic" may be the key organic thin film transistor Note 1 (OTFT) is mainly made of polymer with * * * yoke bond Note 2. Common organic polymer materials, such as plastics and rubber, are insulators because their single-bond long-chain molecules composed of hydrocarbons do not move charges freely. The main chain of conductive polymer with * * yoke bond is alternately formed by single bond-double bond * * yoke bond. At this time, each carbon atom has an unpaired valence electron, which can move freely on the molecule without being bound by the bond end, but this valence electron is not easy to move along the whole long chain, so it needs to be doped-that is, to add a charged carrier (that is, to carry some kind of. ?

On the left is * * * yoke molecule 1, 3- butadiene, and on the right is non * * * yoke molecule 1, 4- pentadiene. ? Compared with ordinary thin film transistors, organic thin film transistors of Tui Tibara University have the following advantages: low temperature process, simple manufacturing steps, low cost and easy molding. However, because the molecules in organic materials are only bound together by weak gravity, unlike the molecules in inorganic semiconductors, they must be bound together by chemical bonds. This weak intermolecular interaction makes them easy to form defects, and carriers are easy to be captured by defects during transmission. At this time, it is necessary to apply a great voltage to provide energy to release them. This unresolved initial voltage problem is the reason why inorganic semiconductors are widely used in industry today. ?

Today, this set of Schottky barrier THIS thin film transistor amplification circuits (SB-OTFT amplification circuits) discovered by Chen Jiang team perfectly overcome the above obstacles. ?

There are thousands of transistors, why can it win? This group of amplifiers uses C8-BTBT (a polymer material with * * * yoke bond) as the main component of organic semiconductor, so it can be produced by inkjet printing technology (for example, literally "printing"), so the manufacturing cost is much lower than that of the common thin film transistors in the past, and it has the potential for mass production. Because the crystal grain of C8-BTFT (>: 50μm) is relatively large, it can effectively cover the whole channel. Under the condition of constant volume, the number of large grains is small, which can also reduce the contact area between grains, effectively cover the whole channel, and reduce the formation of crystal defects such as grain boundary injection 4 and stacking fault, thus reducing the initial voltage of this organic semiconductor and overcoming the problems of fast power consumption and poor endurance when most organic semiconductors are used with batteries.

Schematic diagram of Schottky organic thin film transistor amplifier developed by Chen Jiang team. Chen Jiang et al, 20 19 In addition, due to its material characteristics, the amplifier also has a high transconductance Note 5 (38.2 S/A, close to the theoretical limit? About 38.7 S/A, while the average inorganic transistor is 20~30 S/A), and the power consumption is extremely low (

After three months of environmental exposure test, the drift of threshold voltage (i.e. working voltage) is less than 1mV, and the fluctuation of conduction efficiency is less than 1%, which is far lower than the performance of other organic thin film transistor elements under the same conditions (>: 100 mV, & gt20%), which means that it has excellent stability even after long-term operation.

The characteristics of this group of antimony OTFT are compared with those of other transistors. Chen Jiang et al., 20 19 For example, this amplifier can greatly improve the current technology of detecting EOG signals, that is, tracking eye movements by detecting corneal and retinal potentials. The above features improve the problems of large volume, high cost and high power requirement of current detectors. In addition, high magnification and high power make it possible to detect tiny fluctuation signals, so that we can understand the situation of the eyes facing the virtual environment (such as depth of field effect). This is very important information in constructing virtual reality technology.

Schematic diagram of this group of SB-OTFT detection eye signals. This eye signal is compared before and after amplification. Chen Jiang et al., 20 19 organic thin film transistor breaks through the imagination of science and technology. Compared with traditional inorganic thin film transistors, organic thin film transistors have the advantages of simple and diverse manufacturing processes and low cost. Furthermore, it is made of organic materials, which makes it more flexible, so the size of the object can be made smaller, lighter and more portable.

In the past research of organic thin film transistors, the carrier mobility note 4 and the current switch ratio note 5 have been pursued as improvements of digital switches. This study broke through the old stereotype and opened up a new research direction. This amplifier can meet the requirements of low power consumption, high amplification, high power and high stability at the same time. Not only that, it also has the advantages of excellent environmental stability and mass production, which is beneficial to the application in life, which other transistors can't do. ?

As more and more technologies focus on interactive wearable devices, perhaps the development of organic thin film transistors will continue this research and find more possibilities, so that interactive wearable devices can be widely used in life and break through the imagination of today's technology.

Thank you for this report from the Department of Physics, Taiwan Province Provincial University, China, and thank Professor Zhu Shiwei and Assistant Professor Cheng for their advice and help. ?

Attention:? 1. Thin Film Transistors (TFT): A type of field effect transistor, which is made by depositing various thin films on a substrate, such as a semiconductor active layer, a dielectric layer and a metal electrode layer as a channel region. ?

2.*** Conjugate bonding: refers to a bonding mode in which single bonds and double bonds alternate, in which a P orbital domain overlaps the middle single bond. It allows π electrons to drift through all adjacent aligned P orbital domains. This π electron is not a single bond or atom, but belongs to a group of atoms. The largest yoke system exists in graphene, graphite, conductive polymers and carbon nanotubes. ?

3. Transconductance efficiency: a parameter in a transistor that describes the proportional relationship between transconductance and the corresponding working current. The higher the value, the smaller the working current required for setting in the same working environment, thus reducing the overall power consumption. The commonly used definition is gm/IDS, where gm is transconductance and IDS is drain current. ?

4. Carrier mobility: refers to how fast carriers can move under the action of an external electric field (commonly used cm2? V- 1？ S- 1)?

5. On/off current ratio: When the given voltage is greater than the initial voltage, the transistor is in a conducting state, and vice versa. The current ratio in the on-off state is called the current-to-switch ratio, and the larger current switch is faster than the representative switch, which has obvious switching effect. ?