Many types of electronic applications combine elements from two or more device classes.  For example, in semiconductor laser applications, two separate devices are commonly utilized: a semiconductor laser, and an integrated modulator circuit.  It is advantageous to combine the laser and the modulator component in a single opto-electronic package.  However, because the two component classes have different device fabrication requirements, these elements are conventionally fabricated on separate wafers.  In a typical device, both the modulator and the laser are fabricated on a semiconductor wafer with a zinc-doped indium gallium arsenide (InGaAs) cap layer and one or more layers of indium phosphate (InP) with various doping levels. 
A critical difference between the fabrication requirements of each of these device classes, however, is the depth of the p/n junction, which itself is controlled by the depth of the zinc-dopant front profile.  In particular, a semiconductor laser device requires the zinc doping to extend deeper into the substrate to reach the quantum well and confinement layer.  In contrast, the electronic devices used in the modulator require a doping front which is comparatively shallow, extending only to the second confinement layer.  In order to insure that the proper junction depth is achieved for each of these device classes, they are conventionally fabricated on separate substrates, each having a different doping profile. 
Attempts have been made to produce opto-electronic devices of different classes on the same wafer.  Conventional methods of combining such devices selectively control the junction depth by performing several masked diffusion steps, one for each different junction depth required.  Each step adds dopant only to the regions of the wafer requiring the same junction depth.  For example, a first diffusion step may be used to form the p/n junction for the semiconductor laser while the substrate regions designated to house the modulator components are protected by an oxide.  After the diffusion step, the applied oxide is removed and a second oxide layer is applied over the area for housing the laser devices while leaving the area for modulator devices uncovered.  A second diffusion step is then used to implant dopants in the modulator area at the required, different depth. 
A significant drawback to this technique is the number of steps required.  In particular, separate oxide layers must be selectively grown and then removed in order to introduce diffusion regions of differing depths.  In addition, the increased number of processing steps results in migration of earlier introduced dopants, thus resulting in a less precisely fabricated device. 
Accordingly, it would be advantageous to be able to produce a wafer having a dopant front profile which extends to different depths in different areas of the wafer without requiring several layers of oxide to be selectively placed and removed on the wafer.  It would be also be advantageous to be able to produce a combined laser and modulator device on a single wafer and as part of a single chip, rather than producing each component on a separate chip and combining the separate chips within a single package.