Many exchange-traded funds (ETFs) give investors access to a variety of investments issued by or backed by the U.S. government. These range from short-, medium- and long-term treasury securities to treasury inflation-protected securities (TIPS) and mortgage-backed securities, among others. These types of securities are attractive because of their low-risk characteristics. Since these securities are backed by the full faith and credit of the U.S. government, the risk of default is practically zero. These investments therefore offer an extremely safe way to invest capital and make more returns than a simple money market fund. Based on assets under management (AUM), below is a list of the three largest U.S. government ETFs available to investors as of March 2016.

iShares TIPS Bond ETF

With $16.31 billion in AUM, the iShares TIPS Bond ETF (NYSEARCA: TIP) is the largest U.S. government ETF available to investors. The fund seeks protection against intermediate-term inflation by investing in government bonds whose face values rise with inflation.

The portfolio is 99.85% TIPS and 0.15% cash. The fund invests in 40 different issues across maturities from less than one year to over 20 years. The weighted average maturity of the portfolio is 8.49, while the effective duration is 7.74 years. Its weighted average yield to maturity is 1.65%, and its weighted average coupon is 0.80%. Versus the S&P 500, TIP's beta is 0.03 and the fund's standard deviation is 5.18%. The fund has an expense ratio of 0.2% and a 12-month trailing yield of 0.33%. The 52-week range on TIP is $108.98 to $115.59.

iShares 1-3 Year Treasury Bond ETF

The iShares 1-3 Year Treasury Bond ETF (NYSEARCA: SHY) is the second-largest U.S. government ETF based on AUM as of March 2016. The fund has $11.47 billion under management. It seeks to track the investment results of an index composed of U.S. Treasury bonds with remaining maturities between one and three years.

Investing in 105 different securities, the portfolio is 99.75% treasuries and 0.25% cash. Bonds that have maturities between one and two years comprise 60.62% of the fund, while 36.5% of the fund consists of bonds with maturities between two and three years. The remainder of the fund is invested in bonds that mature in less than one year or more than three years. The weighted average maturity is 1.87 years, and the effective duration is 1.83 years. The portfolio's average coupon is 1.28%, and yield-to-maturity is 0.85%. The fund has an expense ratio of 0.15% and a 12-month trailing yield of 0.57%. The 52-week range on SHY is $84.31 to $85.18.

iShares 20+ Year Treasury Bond ETF

With $10.43 billion in AUM, the iShares 20+ Year Treasury Bond ETF (NYSEARCA: TLT) is the third-largest U.S. government ETF on the market. The fund gives investors exposure to long-term U.S. treasury bonds. It seeks to track the investment results of an index composed of U.S. Treasury bonds with remaining maturities greater than 20 years.

The portfolio is 98.92% invested in treasuries and 1.08% in cash. Of the 32 different holdings in the fund, 0.61% is invested in treasuries with maturities between 15 and 20 years. The weighted average maturity of the fund is 26.6 years, and its effective duration is 17.77 years. The weighted average coupon is 3.22%, and the yield-to-maturity is 2.55%. Since this fund has a long duration, it is more sensitive to interest rate changes and thus more volatile. Its standard deviation is 11.61%, but its beta versus the S&P 500 is -0.16. The fund has an expense ratio of 0.15% and a 30-day SEC yield of 2.44%. The 52-week range on TLT is $114.88 to $135.25.

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