transistors-not how to hookup but curious how do they work?

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salty joe

Joined Dec 14, 2010
43
Transistors are too small for me to take apart and figure out. All I can find is a tiny current from base to emitter allows a much greater current to flow from collector to emitter.

Just wondering what physical change occurs inside the transistor when a tiny current is introduced to the base?
 

dl324

Joined Mar 30, 2015
9,600
Transistors are too small for me to take apart and figure out
If you find some in metal cans, you can take the top off. You're still not going to see much. Just an emitter and base connection; the collector is on the bottom.

All I can find is a tiny current from base to emitter allows a much greater current to flow from collector to emitter.
The current amplification (gain) is called beta. It varies with temperature, base current, collector-emitter voltage.

This seems to be a reasonable explanation:
https://www.electronics-notes.com/articles/electronic_components/transistor/how-does-a-transistors-works-basics-tutorial.php
 

MrChips

Joined Oct 2, 2009
19,923
This is not something with a simple explanation. You need to study semiconductor physics, valency band, impurity doping, carrier concentration, electron-hole pair generation and recombination, etc. It is not as simple as understanding how a butterfly valve or a water faucet works. Even an electromechanical relay or a thermionic valve (tube) are relatively easy things to understand.

Here is as simple an analogy as I can make it. Imagine a swampy marshland that a platoon of soldiers are trying to cross. They are bogged down and mired in mud. A small group of enterprising reservists comes along the marsh and lays down wooden planks that makes the crossing easier. Now the troops can cross the swamp more easily.

If you want a better explanation try Wikipedia.

https://en.wikipedia.org/wiki/Bipolar_junction_transistor

1576072766043.png
 

Papabravo

Joined Feb 24, 2006
12,767
Right. Base current makes the potential barrier (swampy mud) lower (shallower) so the electrons (platoon of soldiers) can cross the base region (swamp) easier.
 

crutschow

Joined Mar 14, 2008
24,101
Transistors are too small for me to take apart and figure out.
All you would see would be a small chip of semiconductor with three tiny wire connections.

A BJT (Bipolar Junction Transistor) chip contains three doped areas with the center doped area oppositely doped to the two end dopings.
The center is the base connection, with one end being the collector and the other end being the emitter junctions.

The transistor collector-base junction is normally reversed-biased, thus it looks like a reverse-biased diode, and no current flows when the base current is zero.

When sufficient voltage is applied to the forward-biased base emitter junction causing a small current to flow (about 0.6V for silicon transistors), some of the carriers generated at that junction by this voltage, diffuse into the collector-base area of the junction, allowing collector current to start to flow also, which flows into the emitter connection.

The junctions are designed such that the resulting collector current is much larger than the base current, typically by a factor of 20-150, which is the gain of the transistor (Beta).

A FET (Field-Effect-Transistor) such as a MOSFET, works on a different principle, with its source current depending on the gate-source voltage, (the gate drawis no current).
 

dl324

Joined Mar 30, 2015
9,600
Transistors are too small for me to take apart and figure out.
I happened to be going through some old stuff last night and found this 2N2222A that I opened when I was a technician and was curious about what things looked like.

This was just in a bag of stuff with no protection. The top bond wire is still there after 40+ years, but the bottom one got knocked off. The collector connection is from the substrate to the can.
2N2222A-topOff.jpg

My boss (back in the late 70's) had some die that had transistors, resistors, and capacitors that we could use to make small "integrated" circuits. We just had to mark the bond wires on a diagram and a tech in the lab wired them up.

I de-lidded a power transistor and found that the junctions acted like tiny solar cells; before solar cells were generally available as I recall.

I first got interested in what integrated circuits looked like while I was in junior college. One of my instructors said that some people could identify devices and gates by looking at the top metal layer. He was sort of right. You can only see the top metal but, back in those days, they only used one metal layer.

Now dimensions are so small that you can only identify functional blocks. Individual wires are too small to see without a microscope.
 
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dl324

Joined Mar 30, 2015
9,600
In this die photo of a CD4049, you can make out the input protection diodes (on the right) and three inverters in series (even though the schematic in the datasheet shows a single inverter):
1576084574316.png
The first inverter is small to avoid loading the driver. The next inverter drives the large output inverter.

The devices on the bottom are P channel MOSFETs and the top are N channel MOSFETs. NMOS devices back then had 1.5 to 2 times the drive of PMOS, so the PMOS had to be larger. That trend continued until they started straining the silicon of PMOS devices and drive strength equalized.

Here's the full die if you're interested:
CD4049-2.jpg
 

MrChips

Joined Oct 2, 2009
19,923
Right. Base current makes the potential barrier (swampy mud) lower (shallower) so the electrons (platoon of soldiers) can cross the base region (swamp) easier.
Hey, I'm starting to like this analogy.

The swamp is the base region.
Troops are the electrons trying to cross the swamp but they can't get through.
Some electrons (reservists) enter the base and lay their bodies down on the swamp.
Now the troops can cross the swamp.

(A field-effect transistor FET is a different story. An analogy of the FET is water flowing down a tubing (channel). When you pinch the tubing it stops the flow of water.)
 
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