Issue with IE 11

I am working on upgrading an IE 8 compatible application to IE 11 compatible. I could see the request and response xml are creating issues. ex.

var xmlHttp=new ActiveXObject("Microsoft.XMLHTTP")
xmlHttp.open("POST", "ASPinclude/HttpRequest.asp", false)
xmlHttp.send(xmlDoc)

Here var xmlHTTP is throwing error in IE 11 but working as expected in IE 8. Please suggest.