Yes, you are right, you also can build n-FET - without the IP of GT. My thinking was: It will hardly be possible to convert the digital info to optics without both, n and p-FET, as the light in the VCSEL (the carrier) has to be modulated by the HEMT. I guess I have to dig in more deeply there.

In addition it seems that the high mobility of electrons and holes both are important for the yield of photons in the POET VCSEL. But OK, these are the limits of my knoledge in physics, and I guess not many people worldwide have the tremendous background of GT.