Semiconductor laser device

A semiconductor laser device comprising:

• a substrate including a semiconductor crystal of a first conductivity type;
• a first clad layer formed on said substrate and including a semiconductor crystal of the first conductivity type, said first clad layer having a band gap of Eg.sub.cl and a refractive index of n.sub.cl ;
• an active layer formed in contact with an upper surface of said first clad layer and including a semiconductor crystal of one of first and second conductivity types, said active layer having a band gap of Eg.sub.a and a refractive index of n.sub.a, and Eg.sub.cl being larger than Eg.sub.a and n.sub.cl being smaller than n.sub.a ;
• a second clad layer formed in contact with an upper surface of said active layer and including a semiconductor crystal of the second conductivity type, said second clad layer having a band gap of Eg.sub.c2 and a refractive index of n.sub.c2, and Eg.sub.c2 being larger than Eg.sub.a and n.sub.c2 being smaller than n.sub.a ;
• a current blocking layer formed on said second clad layer and including a semiconductor crystal of the first conductivity type, said current blocking layer having a band gap of Eg.sub.cb ;
• a slit-shaped current injection opening formed in said current blocking layer;
• a cap layer exposed to said current injection opening, formed on said second clad layer and said current blocking layer, and including a semiconductor crystal of the second conductivity type;
• an upper electrode formed on said cap layer and including metal;
• and
• a lower electrode formed on an undersurface of said substrate,
• wherein said current blocking layer has a thickness and a carrier concentration such that a depletion layer of a reversed junction formed in said current blocking layer alongside said second clad layer causes a punch through in said current blocking layer when said semiconductor laser device is driven at a current which is one to ten times as large as a maximum rated value of a DC driving current.