Checkout Indium Phosphide HEMTs (High electron mobility transistors): http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=4419013
Mark Rodwell's group in UCSB (http://www.ece.ucsb.edu/Faculty/rodwell/rodwell_info/rodwell...) has been working on these transistors for a while. I think they're pushing 2 THz currently.