Transistor diode model

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In a diode model two diodes are connected back-to-back to make a PNP or NPN bipolar junction transistor (BJT) equivalent. This model is theoretical and qualitative.

Examples

Structure of a transistor is quite similar to a pair-of diode connected head-on

PNP transistor

To make a PNP transistor, the cathodes of both diodes are back-to-back connected to form a large N type base region.

NPN transistor

To make an NPN transistor, the anodes of both diodes are back-to-back connected to form a large P type base region.

Base biasing

As the base region is a combination of two anodes or two cathodes, and is not lightly doped, more base biasing is required for making this model operational.

References