1. Fundamentals
The basic structure and operating principle of this device are covered in the Fundamentals chapter Transistors and Memory. The following sections describe fabrication in detail.
2. Fabrication of an NPN Bipolar Transistor
1. Substrate
The basis for an NPN bipolar transistor is a p-doped silicon substrate, using boron as the dopant. A thick oxide layer (e.g. 600 nm) is deposited on top of it.
2. Buried Layer Implantation
The oxide serves as an implantation mask. Antimony (Sb) is used as the dopant, since it diffuses less than phosphorus during subsequent diffusion processes. The heavily n+-doped buried collector serves as a low-resistance contact area for the collector terminal.
3. Homoepitaxy
In an epitaxial process, a high-resistance, lightly n--doped collector layer is deposited (typically 10 µm).
4. Base Implantation
The p-doped base is created using boron ions; a subsequent diffusion step enlarges the region.
5. Emitter and Collector Implantation
Using phosphorus ions, the two heavily n+-doped emitter and collector terminals are introduced.
6. Metallization and Photolithography
Aluminium is deposited in a sputtering process to contact the three terminals, and a resist layer is patterned on top of it.
7. Etching
Finally, the terminals for emitter, base, and collector are patterned in an anisotropic dry etch step.
Due to various optimizations, bipolar transistors today are built from more than three layers (npn or pnp). The collector region always consists of at least two zones with different doping levels. The terms npn and pnp refer only to the active inner region, not to the actual layer stack.