//去除掉密码
 $db = new UserModel();
 $result = $db->field('password',true)->where("username='{$params['username']}' AND password='{$params['password']}'")->find();

或者指定要的字段的数组 

 $db = new UserModel();
 $result = $db->field(['username','create_time'])->where("username='{$params['username']}' AND password='{$params['password']}'")->find();