The buried layer is formed with a high density dose implant of n-type dopant, usually Phosphorous Some lateral diffusion of dopants takes place during the implant Dopant is also introduced into the masking oxide.
After the masking oxide is removed etched , the result is a highly doped buried layer region in the original silicon substrate. Using epitaxial deposition similar to CVD , a layer of very high quality crystalline silicon is deposited on the surface Some diffusion of dopants occurs from the highly doped buried layer into the more lightly doped epitaxial layer.
The resulting epitaxial layer will form the collector region in the BJT, with the highly doped low R buried layer forming an equipotential region under the device Note that the buried layer is now completely enclosed in silicon material. Layers of masking oxide layer and PR are formed on the surface The PR is exposed using an isolation implant mask mask 2.
After a long enough implant so that the isolation regions reach the underlying substrate, the masking oxide is removed. Connection will be made to the buried layer collector using a sinker, an implantation of high concentration and significant depth Oxide and PR layers are exposed using a sinker mask mask 3. Once a window has been etched in the oxide, an n-type dopant implantation is performed to create the sinker.
Depending on the implantation conditions and the thickness of the epi region, the sinker may or may not reach right down to the buried layer.
A p-type dopant usually Boron is implanted into the epi layer through the oxide window to form the p-type base region. After implantation is complete, a p-type counterdoped base region has been formed in the n-type collector epi.
A heavy n-type dopant usually As is implanted through the oxide window opening to form the emitter Control of this implant is critical to the BJTs operation. Oxide is deposited which will isolate metal connections PR is exposed using contact cut mask mask 6. Etching of contact openings requires selectivity so etching does not remove emitter, therefore wet etchant used.
After etching, openings have been created in the oxide which will allow access to the collector sinker, base and emitter regions. After metal is deposited and PR formed on surface, exposure is performed using a metal mask mask 7. Once the metal has been etched to form interconnections between contacts, the structure is complete Most modern processes would use more than one level of metal.
Top view layout shows the various masks used to realize the basic structure Cross section is duplicated to show correspondence Buried layer and isolation masks can by generated automatically based on size of base region and position of collector. Many other devices possible - example shows single emitter and collector with two base contacts Large current devices have many interdigitated emitter and base regions.
In the integrated BJT structure, the principle current flow for normal operation is through the sinker, buried layer and up through the vertical npn structure The actual active area of the device is a relatively small portion of the overall structure.
For initial analysis, isolate the active region between the emitter and buried layer and assume 1D operation as in diode, with uniform doping approximations Neglects collector resistances associated with sinker and buried layer and base resistance associated with external regions, but can add those later Use 1D area given by product of emitter width and length.
Calculate the equilibrium minority concentrations, diffusion coefficients and the widths of the collector, base and emitter neutral regions for the structure given below at the biases shown.
Example Lecture Summary Looked in detail at fabrication of double diffused structure Current flow path illustrates importance of conductive epi buried layer 1D approximation uses emitter area AE Three regions require slightly more complex naming scheme for dopings, etc.
Depletion widths calculated based on pn junction equation applied to each junction Base neutral width is region of base not accounted for by depletion regions. Open navigation menu. Close suggestions Search Search. User Settings. Skip carousel. Carousel Previous. Carousel Next. What is Scribd? Explore Ebooks.
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Uploaded by jayalaxmi H. Document Information click to expand document information Description: ppt. Original Title Bj t Fabrication. Did you find this document useful? Is this content inappropriate? Report this Document. Description: ppt. Flag for inappropriate content. Download now. Original Title: Bj t Fabrication.
Related titles. Carousel Previous Carousel Next. Jump to Page. Search inside document. Used for poly-Si, oxide, nitride. Common-collector It is called the common-collector configuration because both the signal source and the load share the collector lead as a common connection point. Common-base This configuration is more complex than the other two, and is less common due to its strange operating characteristics. The pnp Transistor Current flow in an pnp transistor biased to operate in the active mode.
The pnp Transistor Two large-signal models for the pnp transistor operating in the active mode. Documents Similar To Bj t Fabrication.
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