I intend to run an underground feeder wire (approximately 150') from my home
service panel to a subpanel for a shop that I'm building in my back yard.

The question is...do I have to get a special service panel to allow for the
connection of the new wire for the shop or can I just piggy back (attach to
the same terminals) the new wires to the existing service coming in ?

Thanks,
Ray

Ray
If you want to run a set of service conductors to your shop as permitted by
"230.40 Number of Service-Entrance Conductor Sets.
Each service drop or lateral shall supply only one set of
service-entrance conductors.
Exception No. 3: A single-family dwelling unit and a separate structure
shall be permitted to have one set of service-entrance conductors run to
each from a single service drop or lateral."
You would install a meter can that will contain double barreled lugs on
the load terminations. If you run a set of conductors to the shop that
originate in the homes indoor service equipment that would be in
violation of
"230.3 One Building or Other Structure Not to Be Supplied Through Another.
Service conductors supplying a building or other structure shall not
pass through the interior of another building or other structure."
If the panel is an exterior surface mount than your only challenge will
be to obtain a panel with double barreled main lugs.

If you are actually running a feeder than it will have to be protected
by an Over Current Protective Device (OCPD) such as a set of fuses or a
circuit breaker.
--
Tom H