Device operation of InGaN heterojunction bipolar transistors with a graded emitter-base design

The device operation of InGaN heterojunction bipolar transistors with a graded InGaN emitter-base design grown by metal organic chemical vapor deposition on sapphire substrates is demonstrated. The Gummel plot, current gain, and common-emitter current-voltage characteristics of the device are presented. The dc common-emitter current gain of a 25×25 μ m2 (emitter size) device increases with collector current and the current gain reaches a high value of 13 at IC =10 mA with a base width of 100 nm and a hole concentration of p=2× 1018 cm-3. The improved device performance is attributed to the graded emitter-base design as well as the high quality of the material made possible by the indium composition grading that enables the epitaxial growth of InGaN layers with a low density of pits or defects. The results demonstrate the potential of the graded InGaN emitter-base junction design in nitride heterojunction bipolar transistors.