in fig4 a along with the double mirror enhanced tl characteristics of fig4 b the threshold current is reduced from 1 b 23 to 19 ma and the tl voltage range significantly extended u2266 2 5 v it is the reduced narrow line signal level at reduced threshold higher q lower current operation that permits higher bias voltage increased voltage range before the bias and direct gap photon assisted tunneling franz keldysh effect cause breakdown u03b1 u2192 1 constrained by i e i b i c 0 the tl threshold boundary along the lower and right edge of the shaded region of fig3 b is seen to follow and be relatively smooth along the current i b i thres const to u02dc 2 5 v and then turn upward crossing i b i thres at slope change u201c breakdown u201d corners at progressively higher i b values at these corners the tl switches from stimulated to spontaneous recombination from high coherent optical field to lower incoherent u201c signal u201d that is to broad spontaneous recombination radiation lower effective field and smaller slope of the breakdown curve this behavior is illustrated in fig5 with the tl biased at the relatively high current i b 27 ma the high field mode 1003 nm at 2 3 v switches off at slightly higher bias 2 42 v.