Is there anybody can show me how the voltage distribution of this MOSFET?
In Sentaurus (T-Cad) you should change electron density to potential.

However, assuming that you have a quasi-static field you can compute the Potential from Maxwell's first law:

$$ \nabla \cdot \vec{D} = \rho $$.

Knowing that 

$$\vec{E} = - \nabla V$$

using 
$$ \vec{D} = \epsilon \cdot \vec{E}$$

and 

$$\nabla \cdot \nabla = \Delta$$

you will get the well known Laplace equation 
$$ \Delta V = - \frac{\rho}{\epsilon} $$

In your case the electron densitiy is equal the charge denistiy times e (elementary charge). So the result is:
$$ \Delta V = - \frac{e\cdot n}{\epsilon} $$