Why do transistors consume less power the longer they are used?

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Why do transistors consume less power the longer they are used? [Copy link]

Transistors in electronic system chips will gradually age over time. They will slowly become old, slow to respond, have more and more problems, and even suddenly crash. However, everything has two sides. Although transistor aging is not a good thing for electronic products, its power consumption decreases over time. In this fast-paced and fast-consuming era, people are always seeking new and fast. Not only mobile phones, computers, and cars, but even ourselves want to be fashionable, new, and not outdated. If I want to say that old has its own advantages and old has its own wonders, you will definitely not agree. But this is indeed the case. Let's take a look at a few examples. We all know that cars have a running-in period. New cars should pay attention to running-in within the first 2,000 kilometers of mileage so that the various components can fit together more smoothly, so that the overall performance, service life and driving experience of the car can be optimized. In fact, it is the same for people. It is not always better to be young. We know that the knowledge people learn when they are young must go through the trials of life and need time to accumulate before it can become wisdom. So, do electronic products such as smartphones and computers have similar interesting rules? Ordinary consumers may not care about the aging of computer CPUs, smartphone memory and car automatic braking systems. But as electronic system designers or chip design engineers, we know that the transistors in these electronic system chips will gradually age. Like people and cars, they will gradually become old, react slowly, have more and more problems, and even suddenly crash and freeze. The positive effect of transistor BTI changes on the system Everything has two sides. Although the aging of transistors in chips is not good for electronic products, their power consumption decreases over time. This is the conclusion drawn by Bashir Al-Hashimi, a professor of electronic engineering at the University of Southampton in the UK, after a series of simulations and experiments. The professor and his team tested a transistor characteristic, Bias Temperature Instability (BTI), and found that changes in BTI have a positive impact on the chip and the system as a whole. What is BTI? Simply put, it is a charge accumulation effect when the transistor is in the "on" state, forming charge accumulation between the transistor channel and its gate insulating medium, which will change the voltage of the transistor's switch state change, and the transistor switch state change action will become slower and slower over time. As chip manufacturers use more high-K dielectrics and metal gate materials, this bias instability is becoming more and more obvious. Professor Bashir Al-Hashimi's team used high-performance CMOS logic transistors in the simulation experiment, and BTI aging caused the actual power consumption of these devices to decrease. The experiment showed that after simulating one month of use, the static power consumption was reduced by about 50%, and by 78% in 10 years. Static power consumption is the energy consumed when the transistor is not working, which is caused by the leakage of current on the transistor channel. In today's chip design, the transistor is in this state most of the time, so the power consumption reduction brought by BTI is quite significant. In actual chip testing, the leakage current is reduced by about 11% after 5 years of use. It is not known whether the actual power consumption reduction of electronic systems can reach the expected level, but at least the correlation between transistor aging and power consumption reduction is theoretically reasonable. Does this mean that the longer the smartphone is used, the better the battery life performance will be? If we draw such a conclusion based on this, it would be too arbitrary. After all, the battery life performance and service life of a smartphone depend on many factors, such as the material and performance of the battery itself, power management technology, operating system, installed APP software and user usage habits. Consumer demand and market competition have been driving mobile phone manufacturers and chip suppliers to continuously update and iterate, and the replacement cycle of new products is getting shorter and shorter. In such an environment, consumers and businesses will not pay attention to the aging effect of system chips. However, when we engineers design chips and smart device products, we have to consider its impact. Chris H. Kim, a professor of electrical engineering at the University of Minnesota, began to study and experiment on the impact of transistor aging on chips and electronic systems more than 10 years ago. He first proposed the concept of "chip odometer (Odometer for silicon chip)" and developed a circuit to measure transistor aging indicators that may affect chip performance. He hopes to integrate this circuit into the design of microprocessor chips to assist the microprocessor in automatically detecting operating performance and keep the chip in the best performance state by balancing several aging indicators. Professor Kim and his team have made great achievements in the research of chip odometer. The Semiconductor Research Corporation awarded them the 2016 Technology Excellence Award, which is a recognition of their research by the semiconductor industry. The chip odometer can measure three indicators of transistor aging: hot carrier injection (HCI), bias temperature instability (BTI), and time-dependent dielectric breakdown (TDDB). BTI has been explained above. HCI refers to the aging of the transistor when the state changes, and the charge is retained on the transistor gate dielectric, so that the voltage of the device switch conversion will change. BTI and HCI may not have a significant impact on the normal operation of the chip, but TDDB will cause catastrophic problems. As the transistor ages, various defects will accumulate on the gate dielectric. When the accumulation reaches a certain level, it will cause a short circuit, which will cause the chip or even the entire system to crash. This is just like a person. As the age increases, the body functions begin to age, various diseases begin to appear, and in severe cases, they may even cause cancer. The concept of "chip odometer" proposed by Professor Kim and its corresponding measurement circuit design have attracted the attention of the semiconductor industry. Chip manufacturers including Intel, TI and IBM have considered the impact of transistor aging in their chip development and are taking appropriate measures to compensate for the performance degradation of chips caused by aging. Perhaps soon, a functional module similar to the "chip odometer" will be integrated into new chips. With the development of chip design and manufacturing processes, as well as the maturity of the operating system and software of smart devices, future smart devices will still be able to work for many years under limited power supply environments, perhaps partly due to the "chip odometer". When we no longer frequently change our phones to keep up with the latest trends, we may use a phone we like for more than three years, so it is not surprising that the battery life of the phone has increased instead of decreased.

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