Is there a simple circuit which would behave similarly to a diode, but with a higher cut-in voltage?
How important is the exponential part of the I-V?
As @nidhin suggested, 2 LED's in series could work.
or a 5V zener.. or a 2 2.5V zeners in series.
The low voltage zeners are somewhat exponential,
but not as good as diodes. 


  EDIT: Actually, what is important for me is that diode to have a
  smoother knee in the I-V characteristic. The more evidently
  exponential the thing the better.


OK then try 2 LED's.