Text Box onkeydown client side script ; Not getting text box enterd value after press TAB key

<sbnt:SBNTTextBox ID="txtEmpNO"  AutoPostBack ="true"   runat="server"  önkeydown="return KeyDown(event)" >            

   <script type="text/javascript" id="telerikClientEvents1">

           var textBox = $get("<%= txtEmpNO.ClientID %>_text"); 
            function KeyDown(e) {
                if (e.keyCode == 9) // tab key
                {
                   alert(textBox.get_textBoxValue()); //Not getting text box entered value after //press TAB key

                    var manager = $find("<%= radAjaxManager.ClientID %>");
                    if (!manager) return;
                   
                    manager.ajaxRequest('EmpNoSearch');
                }
                document.title = textBoxValue.toLowerCase();
            } 
  </script>