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System on Chips, Silently Powering Our Lives

In the modern world, System on Chips (SoCs) are used in diverse fields, ranging from streaming video to driving assistance, and deeply influences everyday life. The technological gap between SoCs and traditional computer processors is also narrowing, as newer SoCs perform better in certain workloads. Despite their wide adoption, however, not many people know what SoCs are. Therefore, the Sungkyun Times (SKT) will look through their features, embedded technology, shortcomings, and the future of SoCs.

What Exactly Are SoCs?

SoCs: Their Definition and Characteristics

SoCs are types of semiconductors that incorporate various computer parts, such as the Central Processing Units (CPUs), Graphics Processing Units (GPUs), and Random Access Memory (RAMs) into a single chip. Whereas a traditional computer system is comprised of different interchangeable parts and a central circuit called a motherboard, SoCs can run a full computer system without these complementary parts. SoCs were originally developed to be used in small electronic devices such as digital watches. Ever since, SoCs have primarily been used in analog phones and smart devices – and with the popularization of smartphones, SoC technology has developed at a rapid pace. Nowadays, SoCs are utilized in a lot of different products beyond just smartphones, such as Bluetooth earphones, cars, and even refrigerators. As some SoCs have even proved to outperform competing CPUs, they are now being adopted into desktop and laptop uses, threatening the position of traditional CPUs.

M1, One of the Fastest SoCs on the Market (apple.com)

Technological Differences of SoCs

A processor can be classified into two general groups by where it is used: computers, and portable devices. The main difference between the two is the type of instruction set used within the processor. An instruction set is a type of manual or guidebook for the processor on how to execute certain commands. Computers, such as desktops and laptops, use traditional CPUs instead of SoCs, which is a complex instruction set computer (CISC). As the word complex shows, CISC makes commands longer and more complex. This in turn raises the difficulty of designing the processor, and it also necessitates a larger code decoder, which takes up limited on-chip space. On the other hand, SoCs, which are mainly used in portable devices, are a reduced instruction set computer (RISC). An RISC is easier to decode and has higher efficiency, thanks to its simplicity. RISC-based SoCs have an edge over traditional CISC CPUs as they consume less energy, have a higher processing efficiency, and perform better with simpler commands. SoCs have taken full advantage of RISCs, and some have even begun to overtake traditional CPUs. For example, Apple’s M1 SoC, which was released in 2020, has grown anticipation for the future of SoCs as it proved to outperform a midrange CISC CPU.

SoCs and Their Future Challenges

Limits to Semiconductor Miniaturization

As processor technology has developed, the need to place more transistors in a single chip has naturally led to the miniaturization of semiconductor production. In particular, SoCs have relied heavily on miniaturization for performance and efficiency improvements because it is not possible to increase SoCs’ chip sizes with the limited internal space of portable devices. However, miniaturization is reaching a limit. According to Professor Hiroshi Iwai of the Tokyo Institute of Technology, there are three limitations to miniaturization: the ultimate, fundamental, and practical. The ultimate limit is caused by physical limitations, as production at scales smaller than the size of an atom is impossible. In addition, when circuit integration reaches a certain point, electrons stop acting like particles and start following the rules of quantum mechanics. This is when the fundamental limit occurs. At miniaturization levels beyond 3nm, electrons leak through designated paths and become extremely difficult to control. The practical limit is reached when the additional cost of miniaturization is greater than its advantages. According to semiconductor manufacturer AMD, a 7nm production process is four times more costly than a 45nm process. Facing physical and practical challenges, further miniaturization of semiconductor production seems difficult, and this will in turn hinder the development of SoCs.

Delicate Production of Semiconductors (upenn.edu)

Overheating and Instability Problem

As SoCs are mainly used in portable devices that contact directly with the user’s skin, overheating may cause discomfort for the user. Whereas traditional computers have enough space to employ active coolers that use air or water for cooling, portable devices with SoCs have limited space – so active cooling is not possible. If cooling performance is not sufficient to handle the heat of the SoC, portable devices can easily overheat and cause performance degradation and sudden breakdowns. Most SoCs feature an automatic temperature control function called thermal throttling to protect the delicate inner circuits. Thermal throttling controls the device’s temperature by limiting the performance of an SoC when the device starts to overheat. If the SoC is throttled while performing a high-intensity task, the amount of data that the SoC can process is reduced, – thus slowing down the perceived speed for the user. According to Electronics , throttling on SoCs can result in a 40% performance loss. Moreover, if thermal throttling is not enough to cool the SoC, the internal structures of the SoC may be altered, resulting in sudden failure. When an SoC reaches 160 Celsius, resistance within the SoC is lowered, and more electrical current is allowed to pass through the circuit, making the chip even hotter. Such high heat may destroy the microcircuits inside an SoC and shorten its lifespan or cause complete failure.

The Next Steps for SoCs

Three-Dimensional Semiconductors

Today’s SoCs are being produced in a two-dimensional (2D) structure, where chips are designed in flat form. However, 2D SoCs face a challenge, as enlarging the size of a 2D SoC is impossible because of the limited space of portable devices. According to Professor Han Tae-hee of Sungkyunkwan University, three-dimensional (3D) processors could be the key to overcoming the limitations of miniaturization and have now become the focal point of semiconductor research. To realize 3D SoCs, high-speed interconnection between the different layers is essential, so through-silicon via (TSV) technology is being actively developed. TSV is an interconnection technology that utilizes copper metal to connect the different layers through a hole in the semiconductor layers. TSV connects each layer through a micro-bump located on each layer, to form a 3D SoC. According to semiconductor design company Synopsis, 3D SoCs with TSV technology could raise the degree of integration by ten times, and also electrical efficiency by five times compared to 2D SoCs. This method can reduce design costs as well as increase SoC design flexibility.

3D Stacking Technology (news.samsung.com)

Increasing Stability and Better Cooling Technologies

To overcome the instability and overheating problems of SoCs, developing more accurate temperature sensors and implementing small cooling technologies could be the solution. According to a 2020 report by semiconductor producer Texas Instruments, a more accurate throttling of the processor could be achieved by simply measuring the temperatures more accurately. More accurate throttling will lessen instability and enable SoCs to maintain more stable performance levels. In terms of cooling, vapor chambers are already in use with many smartphones. They use the rapid evaporation and condensation of liquid to move heat away from SoCs. As SoCs are outputting more and more heat as the performance increases, enlarging the size of such chambers could assist in dissipating additional heat. While some are concerned that 3D SoCs may overheat more frequently, according to Professor Kim Seong-dong of the Seoul National University of Science and Technology, TSV is great at evenly spreading heat throughout the chip. Future SoCs should be designed with the characteristics of 3D SoC and TSV in mind, and employ larger vapor chambers to provide a more pleasant experience for the user.

SoCs have influenced modern society without our knowledge. They are used everywhere from smartphones to televisions, and it is expected that their influence will expand. However, there are challenges disturbing the development of SoCs such as the limitations to miniaturization and overheating. As the development of SoCs correlate with improvements in modern life, how about looking forward to the technologies to overcome these challenges?

노규원  tdcby@g.skku.edu

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