New York (CNN Business) Corporate America has been giving more and more cash back to investors through dividends and stock buybacks. Companies are also using some of their excess cash on mergers.

But many big companies are still sitting on a LOT of cash -- even though it earns next to nothing on their balance sheets, because interest rates are so low.

Apple AAPL Alphabet GOOGL Microsoft MSFT Amazon AMZN Facebook FB Tech giantsandhave about a half trillion in cash combined, for example. Apple alone has $206 billion. That's down from a year ago, though, and market experts are hopeful that more of this cash will find its way into the pockets of investors.

Apple bought more than $18 billion worth of its own stock in its most recent quarter. Buybacks reward investors by reducing the amount of stock that's outstanding. That boosts earnings per share. Apple also paid out $3.5 billion in dividends last quarter.

Changes to the US corporate tax code in 2017 have allowed companies to bring back cash they had sitting overseas at a lower tax rate of 15.5% compared to the previous rate of 35%. So it's no surprise that many big companies are rushing to bring the cash back and dole out some of it to shareholders -- although many companies are also taking advantage of low rates to borrow more and use those proceeds for dividends and buybacks as well.

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