Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some global infrastructure stocks to your portfolio, the SPDR FTSE/Macquarie Global Infrastructure 100 ETF (NYSEMKT:GII) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The SPDR ETF's expense ratio -- its annual fee -- is 0.59%, and it recently yielded 3.2%. The fund is fairly small, too, so if you're thinking of buying, beware of possibly large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

This ETF has underperformed in recent years, lagging the world market over the past three and five years. As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

With a low turnover rate of 10%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

Why global infrastructure?
Our global economic slump won't last forever, and there are already signs of life here and there. Thus, companies specializing in materials and utilities are poised to prosper as construction and infrastructure projects get under way and manufacturing kicks into a higher gear.

More than a handful of global infrastructure companies had strong performances over the past year. Utility company PPL (NYSE:PPL), for example, advanced 16%. Yielding 4.8%, it recently hit a 52-week high, but its debt has been growing, too, along with capital spending. The company has been focusing more on regulated generation, which tends to be less risky. Bulls like its adoption of greener smart grid technology, and management is optimistic about 2013.

National Grid (NYSE:NGG) gained 15%, and offers a hefty yield. With extensive operations in the U.S. and the U.K., it's investing in clean energies, which is promising, but its growth has stalled in recent years, and some fear a dividend cut. Others have been dismayed by the company buying back shares at relatively rich values.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Natural gas specialist Spectra Energy (NYSE:SE) shed 3%, and yields 4.2%. Growth initiatives have included opening a new natural-gas processing plant in British Columbia, and expanding its pipeline to deliver more natural-gas capacity to the New York-New Jersey region. It's also been inking some promising partnerships. Its fourth-quarter earnings were pressured by lower commodity prices.

Exelon (NYSE:EXC), the nation's largest nuclear-power company, lost 12%, and recently slashed its dividend by 41%. The company has been hurt by the relatively high cost of nuclear energy in an environment of very low gas prices, but the current situation won't last forever, and Exelon is involved in other energy-generation businesses, too. It carries a lighter debt load than many peers, and is expanding into solar and wind power.

The big picture
A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.