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The P-N junction is a small area within a package where heating is most prominent. Junction temperature is a key parameter we try to track when operating a diode or a transistor.

I am wondering what the so-called 'junction' really looks like in real-life, when you, say, cut a semiconductor in half. I was told the junction is more like a plane. Can someone elaborate?

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It really depends on the BJT, they can be constructed multiple ways.

enter image description here
Source: https://www.technocrazed.com/2-8-bipolar-junction-transistors

enter image description here
https://www.electricalclassroom.com/bjt-bipolar-junction-transistors/

The actual device looks like this from the top down:

enter image description here
https://twitter.com/kenshirriff/status/1428042844590071810

From the side they look like this:

(this one isn't a total BJT, but gives you an idea of what they look like) enter image description here Source: https://smartech.gatech.edu/bitstream/handle/1853/7192/nayeem_mustayeen_b_200508_mast.pdf

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I'm adding one more image, in 3D projection, because it might make it easier to visualize.

The junction is the boundary area between the two differently doped areas in the silicon wafer. In the image below the two junctions are the boundaries between blue and gray areas.

(Image credit: How a BJT works, Circuit Crush)

You could compare it to the boundary between solder and a wire: there is atomic level bonding between the two, they are not coming apart but there is difference in materials.

Unlike wire and solder, in the case of a semiconductor the material is almost pure silicon on both sides. What differs is the doping atoms, of which there are just a few per thousands or even millions of silicon atoms. Doping concentration varies between device types.

The boundary is not a flat plane, but it often thought as being 2-dimensional. In reality the transition between doping regions is not perfectly sharp, and the doping concentration changes gradually over a few nanometers.

The actual doping atoms do not make a visual difference in appearance of the silicon, so the boundary is invisible with any optical microscope. What can be seen are the metal contacts and the difference in thickness of oxide layers that are used as a mask for the doping (compare with typical BJT manufacturing steps). From a top down image, these can help determine where the transistor regions are, but they are not the actual junction itself.

Electron microscopes can show the boundaries, as they have some sensitivity to the charge present in the material. This is what is seen in the last image in Voltage Spike's answer.

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There are different methods for constructing a PN junction. There may be varying levels of doping and there may be usage of metals as well.

Here are some figures which show a PN Junction used to do some experiment.

enter image description here

Usually the junction is made of P and N type materials and the outer layer of P or N type is coated with silicon dioxide just to prevent any surface abnormalities due to oxidation of the surface.

The second diagram below shows the coating of \$SiO_2\$.

enter image description here

This figure shows also the depletion region in the third picture and approximate scale of depletion width of a semiconductor junction. Metals are used a contact surfaces of P and N materials with voltage sources.

Reference:

Individuality of Dopants in Silicon Nano-pn Junctions

Daniel MORARU 1 ∗ , Sri PURWIYANTI 1, 2 , Roland NOWAK 1, 3 , Takeshi MIZUNO 1 , Arief UDHIARTO 2 , Djoko HARTANTO 2 , Ryszard JABLONSKI 3 , Michiharu TABE 1

1 Research Institute of Electronics, Shizuoka University, Japan

2 Department of Electrical Engineering, University of Indonesia, Indonesia

3 Division of Sensors and Measuring Systems, Warsaw University of Technology, Poland

Link:

http://dx.doi.org/10.5755/j01.ms.20.2.6312

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