Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some bonds to your portfolio but are dismayed by the low yields of Treasuries, consider higher-yielding corporate bonds. The PowerShares Fundamental Investment Grade Corporate Bond ETF (NYSE: PFIG) could save you a lot of trouble. Instead of trying to figure out which investments are best, you can use this ETF to invest in a lot of them simultaneously. The ETF weights its holdings according to fundamental measures such as cash flow and revenue.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The PowerShares ETF's expense ratio -- its annual fee -- is a very low 0.22%.

This ETF doesn't have a track record to evaluate, though, as it's rather new. 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. The ETF is small, too, so if you're thinking of buying, beware of occasionally large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

What's in it?
Lots of promising companies offer tempting bonds. A peek inside this ETF reveals some examples, many of which have seen their shares perform well lately.

Abbott Laboratories (NYSE: ABT), whose shares are up 21%, is splitting itself in two by separating its pharmaceutical and medical products divisions. The company's best-selling product, Humira, a treatment for arthritis and psoriasis, generates nearly $8 billion annually. It just received approval to be used in Europe to treat ulcerative colitis, a condition affecting more than a million people there. Meanwhile, its pipeline offers some promise with offerings such as its drug for hepatitis C. The Abbott bond the ETF owns currently yields 22% to its 2016 maturity.

International Paper (NYSE: IP), up 12% and offering a bond yielding 3.3% to its 2018 maturity, is a giant in the paper products and packaging industry. Some are bullish on its merger with Temple-Inland, but mergers can take a while to start producing results, and in the meantime it may be facing rising timber prices, yet it has sold off much of its timberland.

Other companies didn't do as well last year but could see their fortunes change in years to come. Hartford Financial Services (NYSE: HIG), down 25% and with a bond yielding 5.3% to its 2020 maturity, surprised investors with better-than-expected earnings recently. (The stock has been pummeled over the past year or so by natural disasters, asbestos liabilities, and workers' compensation claims.) Hedge fund manager John Paulson has been advocating splitting the company's property-and-casualty insurance business from its life-insurance business, but that has yet to happen, and my colleague Dan Caplinger thinks the stock is compelling. Bulls are drawn to its seemingly low valuation and strong expected growth.

Bank of New York Mellon (NYSE: BK) shed 18% and sports a bond yielding 3.55%. Its profits plunged late last year, due in part to low interest rates and a sluggish economy, but the big custody bank has been boosting its assets under management and cutting costs, setting itself up for rising profits. It's also not as affected by banking reforms, as it's more focused on providing custodial services for other financial institutions than on serving individual consumers.

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.

Learn about four ETFs you can count on. And if you're looking for some great investments beyond ETFs, consider these five stocks growing their dividends by 20% per year.