of about 2 u00d7 10 18 cm 3 and layer thicknesses of 68 5 nm 15 0 nm 57 5 nm and 15 0 nm respectively the aluminum mole fraction within the odd numbered al x ga 1 x as layers is typically 60 percent these 48 layers 60 107 form the top reflecting stack of both the sel and rcd the use of 4 repeating layers as opposed to two in the bottom stack is to reduce the electrical resistance of these layers the top layer layer 108 is a gaas p doped layer grown with a thickness of 47 0 nm and doping levels of about 2 u00d7 10 19 cm 3 used to facilitate ohmic contact to the structure the top 51 layers 58 108 which are all p doped form the emitter region of the sel as well as the gate region of both the rcd and the hfet precise layer thicknesses are necessary to control the mode position of the fabry perot cavity formed by the two grown mirror stacks the lasing wavelength of the sel as well as the absorbing wavelength of the rcd are determined by the energy band of the 3 in y ga 1 y as quantum wells the wavelength 0 985 micrometers in free space for the embodiment disclosed then defines the required thickness in the alternating mirrors which must have a half wavelength thickness for each repeated layer sequence the layers between the two mirror stacks layers 47 through 59 must also.