While puzzling about what "EF" meant I realized that the OP was talking about emergency funds ... yikes. I agree that ER funds meant for very short term needs should not be put into investments with interest rate, credit, or other bond risks.
That said, if you have money that will not be used for a few years then it's perfectly OK to consider a bond fund matched to that duration.
So for EF funds you might need tomorrow the duration is zero and stick with good money market funds.
If your duration is maybe 2 years then here is how the Vanguard short term investment grade VFSTX fund performed over a few years back when rates went low. Note the first figure is the fund's yield at the start of the year and the second figure is the fund's total return for that year:
2002 5.1% 5.2%
2003 3.8% 4.2%
2004 3.0% 2.1%
2005 3.3% 2.3%
2006 4.5% 5.0%
For 2003-2006 the short term treasury fund underperformed this fund.