Try this code.
<!DOCTYPE HTML>
<html>
<head>
<title>PhoneGap</title>
<script type="text/javascript" charset="utf-8" src="cordova-1.5.0.js"></script>
<script type="text/javascript" charset="utf-8">
function onLoad()
{
document.addEventListener("deviceready", onDeviceReady, true);
}
function exitFromApp()
{
navigator.app.exitApp();
}
</script>
</head>
<body onload="onLoad();">
<button name="buttonClick" onclick="exitFromApp()">Click Me!</button>
</body>
</html>
Replace src="cordova-1.5.0.js" with your phonegap js .