02-02-2013, 10:39 AM
BJT TECHNOLOGY
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Major Processing Steps for a Junction Isolated BJT Technology
Start with a p substrate.
1. Implantation of the buried n+ layer
2. Growth of the epitaxial layer
3. p+ isolation diffusion
4. Base p-type diffusion
5. Emitter n+ diffusion
6. p+ ohmic contact
7. Contact etching
8. Metal deposition and etching
9. Passivation and bond pad opening
MODIFICATIONS TO THE STANDARD npn TECHNOLOGY
Types of Modifications
1.) Dielectric isolation - Isolation of the transistor from the substrate using an oxide layer.
2.) Double diffusion - A second, deeper n+ emitter diffusion is used to create JFETs.
3.) Ion implanted JFETs - Use of an ion implantation to create the upper gate of a p-channel JFET
4.) Superbeta transistors - Use of a very thin base width to achieve higher values of bF.
5.) Double diffused pnp BJT - Double diffusion is used to build a vertical pnp transistor whose
performance more closely approaches that of the npn BJT.
SUMMARY
The objective of these notes have been to give a physical understanding of how the npn BJT is fabricated.
• The fabrication sequence for a typical npn BJT has been illustrated
• Methods of implementing other active devices in the npn BJT technology were shown.
• Simple npn BJT technology chooses to emphasize the npn over the pnp because the npn BJT
performance is always superior to the pnp BJT performance. Thus, the philosophy in design is to use
the npn where ever possible and incorporate the pnp only where it has to be used.
We will examine the passive components that can be implemented in a typical npn BJT process later.