Heavily-doped semiconductor lasers

A theoretical study of the effect of heavily doping the active layer of a semiconductor laser shows that the minority carrier density required to reach inversion decreases with increasing doping. Unfortunately, the minority carrier lifetime also decreases since there is a component of the recombination rate that is proportional to the doping density. It is found that for a dopant with a recombination rate coefficient <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">KB, 5E-10</tex> cm 3 /s (Zn in GaAs), the inversion current density has a local minimum at zero doping, but decreases again for n-type doping above <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1E18</tex> /cm, and is one third of the zero value at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4E18</tex> /cm. The value of K <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> for other dopants and materials is unknown; however, for a dopant with a coefficient smaller than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5E-11</tex> , the inversion current would be less than one tenth the zero value at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4E18</tex> n-type, and would also decrease with the addition of p-type dopant. These results indicate that by heavily doping the active layer with the proper dopant, one might obtain both faster response and a lower threshold current, particularly with n-type dopants.