Complex sentence: This photoconductive response in the semi-insulating substrate material was a result of photon emission around the edge of the mesa of the tunnel diode, however, the photons were infrared, which cannot be seen by the human eye.
Split into simpler sentences: This photoconductive response in the semi-insulating substrate material was a result of photon emission around the edge of the mesa of the tunnel diode. The photons were infrared, which cannot be seen by the human eye.