Vertical field effect transistors having improved breakdown voltage capability and low on-state resistance

A field effect transistor, comprising:

• semiconductor substrate having first and second opposing faces:
• source region of first conductivity type in said substrate, adjacent the first face;
• a drain region of first conductivity type in said substrate, adjacent the second face;
• a drift region of first conductivity type in said substrate, said drift region extending between said drain region and said source region and having a graded first conductivity type doping concentration therein which decreases in a direction from said drain region to said source region;
• a channel region of second conductivity type in said substrate, said channel region extending between said source region and said drift region and forming first and second P-N junctions therewith, respectively;
• a trench in said substrate at the first face, said trench having a sidewall extending adjacent said drift region and said channel region;
• and
• an insulated gate electrode in said trench, said insulated gate electrode comprising a gate insulating region on the trench sidewall and an electrically conductive gate on the gate insulating region, opposite the trench sidewall, said gate insulating region including a first insulating region of first thickness extending between said channel region and said electrically conductive gate and a second insulating region of second thickness extending between said drift region and said electrically conductive gate, and wherein the second thickness is greater than the first thickness;
• and
• wherein said drift region has a first conductivity type doping concentration therein which decreases monotonically from a level greater than about 1.times.10.sup.17 cm.sup.-3 to less than about 5.times.10.sup.16 cm.sup.-3 in a direction from said drain region to said channel region.

A field effect transistor, comprising:

• a semiconductor substrate having first and second opposing faces;
• a source region of first conductivity type in said substrate, adjacent;
• the first face;
• a drain region of first conductivity type in said substrate, adjacent the second face;
• a drift region of first conductivity type in said substrate, said drift region extending between said drain region and said source region and having a graded first conductivity type doping concentration therein which decreases in a direction from said drain region to said source region;
• a channel region of second conductivity type in said substrate, said channel region extending between said source region and said drift region and forming first and second P-N junctions therewith, respectively;
• first and second trenches in said substrate at the first face, said trenches defining a mesa therebetween containing said source and channel regions and having respective facing sidewalls which extend adjacent said drift, channel and source regions;
• and
• an insulated gate electrode in said first trench, said insulated gate electrode comprising a rate insulating region on the first trench sidewalls and an electrically conductive gate on the gate insulating region, opposite the first trench sidewall, said gate insulating region including a first insulating region of first thickness extending between said channel region and said electrically conductive gate and a second insulating region of second thickness extending between said drift region and said electrically/conductive gate, and wherein the second thickness is greater than the first thickness;
• wherein the first conductivity type doping concentration of said drift region is less than about 2.times.10.sup.16 cm.sup.-3 at the second P-N junction;
• wherein said drift region forms a non-rectifying junction with said drain region and wherein the first conductivity type doping concentration of said drift region is no less than about 1.times.10.sup.17 cm.sup.-3 at the non-rectifying junction;
• and
• wherein a product of a distance between the facing sidewalls of said first and second trenches and the first conductivity type doping concentration of said drift region at the non-rectifying junction is between 1.times.10.sup.13 atoms cm.sup.-2 and 2.times.10.sup.13 atoms cm.sup.-2.