ABSTRACT
There are a number of problems with the homojunction laser. The current required for the laser to reach threshold, the point where it just lases, is very high, about 5 x 104 A cm-2. This is partly due to the fact that we are not confining the carriers to a small volume; they are able to diffuse away from the junction region without constraint. Another problem is that we are not able to confine photons to the junction region either. The refractive indices of the material on either side of the junction are similar (it's the same material but with a different doping concentration) and this means we are unable to confine the photons. For a more efficient laser, i .e . one with a lower threshold current but capable of giving the same light output we desire the following: (a) photon confinement, (b) carrier (electron and hole) confinement. Why does this help? Confining the carriers to a small volume pushes up the probability of their radiative recombination. Confining the photons to the same small volume pushes up the stimulated emission rate which is what we want for lasing. We are extremely fortunate that, again, Nature has decided to play along with us and allows us to have both conditions together in the double heterojunction laser diode (DHLD) .
