prestashop 1.4.1 ajax调用

I'm trying to call a function of my new module in ajax but I can't. I receive a "500 Internal Server Error", however my url is good.

my php file (module.php)

class test extends Module{

    function ajax_test(){
        #do some stuff
    }
}

my ajax request

$.ajax({
      type: "GET",
      contentType: "application/json; charset=utf-8",
      url: "module.php",
      dataType: "json",
      data : {},
      success: 
         function(data){

         }
});

Any ideas?