Gain is proportional to the carrier population above the threshold value. Since diode current is proportional to the carrier density, one can define a threshold current above which lasing occurs.
The problem of homojunction laser is that the threshold current density is rather large ( A/mm ) which would require a good heat sink for continuous removal of generated heat. Homojunction lasers are, therefore, used in "pulsed mode" or at temperature below 100 K.
Another disadvantage of homojunction laser is that the junction region is wide which results in a divergent beam.
2.15.2
Heterojunction Laser:
We have seen that homojunction lasers suffer from two disadvantages.
Wide beam width
Large threshold current requiring low operating temperatures.
Reduction of threshold current may be achieved by confining carriers to a small region around the junction.
above which lasing occurs. 
A/mm 
) which would require a good heat sink for continuous removal of generated heat. Homojunction lasers are, therefore, used in "pulsed mode" or at temperature below 100 K. 
