Investors have piled into ETFs indexed to speculative-grade corporate bonds as they stretch for yield and take on more risk in the process.

For example, iShares iBoxx High Yield Corporate Bond (NYSEArca: HYG) and SPDR Barclays High Yield Bond (NYSEArca: JNK) are among the five best-selling ETFs the past month. Each high-yield ETF has raked in more than $500 million during the past four weeks, according to

“The market for junk bonds, risky corporate debt that pays high interest rates, is red hot,” DealBook reports.

HYG and JNK have posted total returns of more than 6% year to date. They also boast 30-day SEC yields of more than 6%.

However, investor appetite for junk bonds has pushed yields to record lows.

“Junk bond funds are on a pace to take in a record amount of money this year. Companies with less than stellar credit are issuing hundreds of billions of dollars of bonds,” DealBook reports.

“Fueling this frenzy are investors of all stripes — including individuals, mutual funds and state pensions — who are desperate for returns in their bond portfolios and willing to take more risk to get them,” it notes. “Demand is insatiable, even as analysts warn that the market has become overheated and is ripe for a fall.”

Other junk bond ETFs include PowerShares Fundamental High Yield Corp Bond (NYSEArca: PHB) and PIMCO 0-5 Year US High Yield Corporate Bond Index Fund (NYSEArca: HYS). [High-Yield ETFs for a Low-Rate Market]