drift out of the active region and high doping levels might cause unnecessary free carrier absorption in layers 22 2 and 22 3 which are otherwise transparent by virtue of their high bandgap illustratively for operation at about 0 80 0 88 u03bcm the semiconductor structure 22 comprises an al x ga 1 x as buffer layer an al y ga 1 y as active layer and an al z ga 1 z as cavity length adjusting layer with y lt x z the parameter y determines the operating wavelength for example for y 0 u03bb 0 88 u03bcm while for y 0 08 u03bb 0 82 u03bcm y and z are typically 0 35 for operation at optical wavelengths of about 1 0 1 6 u03bcm the active layer typically comprises ingaasp lattice matched to inp buffer and cavity length adjusting layers a more detailed description of a crt laser utilizing these materials is given below in this example however materials dimensions and other operating parameters are provided by way of illustration only and unless otherwise expressly stated are not intended to limit the scope of the invention in this example the tube 10 was made of 1 5 inch o d glass and the target 12 comprised a 0 5 u03bcm thick n al 0 36 ga 0 64 as buffer layer 22 2 doped with sn to about 5 u00d7 10 16 cm 3 a 3 0 u03bcm thick n gaas active layer 22 1 doped with sn to about 5 u00d7 10 17 cm 3 and a 13 u03bcm.