retain value of password filed when postback

use this code:

If(IsPostBack)
{
this.txtFedTaxID.Attributes["value"] = txtFedTaxID.Text;
this.txtFedTaxID2.Attributes["value"] = txtFedTaxID2.Text;
}
"txtFedTaxID.Text = txtFedTaxID.Text" won't work for the password field.