Next-Generation Nanoelectronics: A decade of progress, coming advances
Dr Abhay Jere
Traditional silicon-based integrated circuits are found in many applications, from large data servers to cars to cell phones. Their widespread integration is due in part to the semiconductor industry’s ability to continue to deliver reliable and scalable performance for decades.
However, while silicon-based circuits continue to shrink in size in the relentless pursuit of Moore’s Law — the prediction that the number of transistors that can fit on an integrated circuit doubles every two years — power consumption is rising rapidly. In addition, conventional silicon electronics do not function well in extreme environments such as high temperatures or radiation.
In an effort to sustain the advance of these devices while curbing power consumption, diverse research communities are looking for hybrid or alternative technologies.
Nanoelectromechanical (NEM) switch technology is one option that shows great promise. “NEM switches consist of a nanostructure (such as a carbon nanotube or nanowire) that deflects mechanically under electrostatic forces to make or break contact with an electrode,” said Horacio Espinosa, James N and Nancy J Farley Professor in Manufacturing and Entrepreneurship at the McCormick School of Engineering at Northwestern University.
NEM switches, which can be designed to function like a silicon transistor, could be used either in standalone or hybrid NEM-silicon devices. They offer both ultra-low power consumption and a strong tolerance of high temperatures and radiation exposure.
Given their potential, the past decade has seen significant attention to the development of both hybrid and standalone NEM devices. This decade of progress is reviewed by Espinosa’s group in the current issue of journal Nature Nanotechnology. Their review provides a comprehensive discussion of the potential of these technologies, as well as the primary challenges associated with adopting them.
For example, one longstanding challenge has been to create arrays of millions of the nanostructures, such as carbon nanotubes, that are used to make these NEM devices. (For perspective, modern silicon electronics can have billions of transistors on a single chip.) The researchers’ review describes the methods demonstrated to date to create these arrays, and how they may provide a path to realising hybrid NEM-CMOS devices on a mass scale.
Similarly, while individual NEM devices show extremely high performance, it has proven difficult so far to make them operate reliably for millions of cycles, which is necessary if they are to be used in consumer electronics. The review details the various modes of failure and describes promising methods for overcoming them.
An example of the advances that facilitate improved robustness of NEM switch technologies is reported in the current issue of Advanced Materials. Here Espinosa and his group show how novel material selection can greatly improve the robustness of both hybrid NEM-CMOS and standalone NEM devices.
“NEM devices with commonly-used metal electrodes often fail by one of a variety of failure modes after only a few actuation cycles,” said Owen Loh, a PhD student at Northwestern University and co-author of the paper, currently at Intel.
Simply by replacing the metal electrodes with electrodes made from conductive diamond-like carbon films, the group was able to dramatically improve the number of cycles these devices endure. Switches that originally failed after fewer than 10 cycles now operated for 1 million cycles without failure. This facile yet effective advance may provide a key step toward realising the NEM devices whose potential is outlined in the recent review.
The work reported in Advanced Materials was a joint collaboration between Northwestern University, the Center for Integrated Nanotechnologies at Sandia National Laboratories, and the Center for Nanoscale Materials at Argonne National Laboratories. Funding was provided by the National Science Foundation, the Army Research Office, The US Department of Energy, and the Office of Naval Research.
“Ultimately, realizing next-generation hybrid NEM-CMOS devices will enable continued scaling of the electronics that power numerous systems we encounter on a daily basis,” Espinosa said. “At the same time, it will require continued push from the engineering, basic sciences, and materials science communities.”
(Source: Northwestern University)
(The writer can be contacted at [email protected])
Clinician researchers in China have developed a vaccine that acts simultaneously against HIV-1 and M tuberculosis (Mtb). An estimated 14 million people worldwide are coinfected with the two pathogens. The research is published in the May 2012 issue of Clinical and Vaccine Immunology.
The vaccine is composed of antigens from both pathogens. The team, led by SidongXiong of Fudan University, Shanghai, incorporated four Mtb epitopes (the part of an antigen that is recognised by the immune system) into a backbone composed of HIV-1 p24 protein, a protein that is known to produce protective immunity against HIV-1. The logic of this construction: many epitopes are short peptides, with poor immunogenicity unless they are introduced into a carrier protein—which in this case was the p24 protein.
The vaccine induced cellular immune responses to both pathogens, in which immune system cells including macrophages search out and destroy pathogens; and humoral immune response against HIV-1, in which the immune system produces antibodies against the pathogen. The vaccine was tested in a mouse model.
Tuberculosis is one of the leading causes of death worldwide, third, after hepatitis C and then HIV/AIDS among infectious diseases, according to the World Health Organization (WHO). An estimated 2 billion—28 per cent of the world’s population—are infected with M tuberculosis, but most of these infections are latent. However, HIV infection is the strongest risk factor for the progression of latent tuberculosis infection to active TB. And TB is the direct cause of death in about one quarter of all deaths among people with HIV/AIDS, according to the WHO.
(Source: American Society for Microbiology)
Male obesity linked to low testosterone levels
Obesity, a condition linked to heart disease and diabetes, now appears to be associated with another health problem, but one that affects men only — low testosterone levels.
Testosterone is a steroid hormone found in men and it plays a key role in the development of male reproductive tissues such as the testis and prostate as well as promoting secondary sexual characteristics such as increased muscle, bone mass, and the growth of body hair.
Results of a study published in the journal Diabetes Care, conducted by University at Buffalo endocrinologists, showed that 40 per cent of obese study participants had lower-than-normal testosterone readings.
The percentage rose to 50 percent among obese men with diabetes. Results also revealed that as body mass index (BMI)—a relationship of weight–to-height—increased, testosterone levels fell.
“The effect of diabetes on lowering testosterone levels was similar to that of a weight gain of approximately 20 pounds,” says Sandeep Dhindsa, MD, an endocrinology specialist in the UB Department of Medicine and first author on the study.
“We hypothesized that obese men are more likely to have low testosterone than non-obese men, and that we would find more low testosterone levels in men with diabetes than in men without diabetes, both obese and non-obese.”
Results confirmed these hypotheses, showing a 40 per cent higher prevalence of low testosterone in obese men compared to the non-obese participants. Men with diabetes, whether obese or not, showed lower levels of testosterone than non-diabetic men across all weight categories. Testosterone levels decreased significantly in both diabetic and non-diabetic men as BMI increased.
“In view of the increasing prevalence of obesity, even in younger populations, it would be important to conduct a similar study in the men at the prime of their reproductive years,” Dr Dhindsa says.
This is the largest analysis of the association between obesity and low testosterone, and the first to compare prevalence of low testosterone with obesity and diabetes separately and together. The study shows that obesity and diabetes may exert independent influences on testosterone concentrations.
“The Endocrine Society now recommends that all men with type 2 diabetes should have their testosterone levels measured. Our new study shows that obese men also have a very high prevalence of low testosterone levels, so physicians should consider screening obese non-diabetic men, as well, for low testosterone.”
(Source: University at Buffalo)