Semiconductor device

A method of manufacturing a semiconductor device, comprising:

• depositing an epitaxial layer on a semiconductor substrate, wherein the semiconductor substrate is of a first conductivity type N, and the epitaxial layer is of a second conductivity type P that is opposite to the first conductivity type, wherein depositing the epitaxial layer comprises depositing a first epi-layer of a first doping concentration, a second epi-layer of a second doping concentration and a third epi-layer of a third doping concentration, and wherein:
• the semiconductor substrate and the first epi-layer form a first P-N junction at their interface;
• the second epi-layer is arranged between the first and third epi-layers;
• and the second doping concentration is higher than each of the first doping concentration and the third doping concentration;
• forming a first doped region of the first conductivity type N at a surface of the third epi-layer, such that the first doped region and the third epi-layer form a second P-N junction at their interface, wherein the semiconductor substrate, the epitaxial layer and the first doped region form a vertical bipolar junction transistor;
• and
• forming a second doped region of the second conductivity type P at the surface of the third epi-layer, wherein the second doped region is surrounded by the first doped region, and a base of the vertical bipolar junction transistor is shorted to the second doped region.