how to create expandable text box in asp.net

<script language="javascript" type="text/javascript">

function setHeight(txtdesc) {

txtdesc.style.height = txtdesc.scrollHeight + "px";

　}

</script>

Design:

<asp:TextBox ID="txtNotes" runat="server" TextMode="MultiLine" Height="70px" Width="230px" onkeyup="setHeight(this)"></asp:TextBox>