Some fundamental questions about pn junctions

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In summary: Does this mean emitter injects electrons first irrespective of collector parameters (like biasing, doping etc) and then the collector pulls whatever electrons are injected in the base?2)If a diode is having a very thin, lightly doped p region and wide, heavily doped n region and it is forward biased, will the forward diode current be very small due to fewer recombinations in the p-region or will the current be large due to large number of majority carriers in n-region that diffuse in the p region? In other words, can the electrons exiting p-region be con
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cnh1995
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I am revising some topics in analog electronics and I am currently working on the BJT. I know the applications of these devices (diodes, transistors) and know the necessary formulae to work out problems.
But there are a few fundamental things which are still unclear to me and I am not very confident about my overall understanding of semiconductor physics.
So,
1)When we forward bias the B-E junction in a BJT, does emitter-base current start before the collector current? I mean the explanation I read in my books is like this:
"when the emitted electrons enter the base region, very few electrons recombine with holes and the rest are pulled in the collector region." Does this mean emitter injects electrons first irrespective of collector parameters (like biasing, doping etc) and then the collector pulls whatever electrons are injected in the base?

2)If a diode is having a very thin, lightly doped p region and wide, heavily doped n region and it is forward biased, will the forward diode current be very small due to fewer recombinations in the p-region or will the current be large due to large number of majority carriers in n-region that diffuse in the p region? In other words, can the electrons exiting p-region be conduction electrons (in which case the current will be large) or do they have to be only valence electrons (in which case this current will be small)?

Thanks in advance.
 
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  • #3
Thanks @Tom.G for the link , I looked up more stuff about transistor delay time and I think I have a satisfactory answer to 1).

What do you think about 2)?
I know the forward diode current Id=Ielectron+Ihole . Since the p-region is narrow and lightly doped, I believe Ihole should be very small but Ielectron should not be affected, and a large number of conduction electrons in n-region should diffuse in the p-region (although only a few of them would recombine with holes and the rest will just move through the p-region as conduction electrons). Is this correct or does this violate any law in semiconductor physics?
 
  • #4
Beats me. I posted a request for help in the Advisors Lounge thread asking for someone with Semiconductor Physics knowledge to jump in. It sometimes takes a few days, but people do tend to respond.

Cheers,
Tom
 
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  • #6
Thanks @Tom.G and @jedishrfu for your inputs. It turns out that there are two types of diode, long diode and short diode. The answer to 2) lies in the concept of short diode IMO. Need to study that tonight!

Thanks again!
 
  • #7
For question #1 I think what you're really asking is what is the base-collector transit time. Or maybe to put it another way, what is the bandwidth of the transistor. I think this handout from OCW explains it pretty well.

https://ocw.mit.edu/courses/electri...vices-spring-2007/lecture-notes/lecture37.pdf

For question #2 the answer is; it depends on the biasing, I think. To be honest I am not sure I totally understand the question. I think what you're really asking is how does change in width and doping affect the current gain in a typically derived 1-dimensional forward biased NPN. If so, see the link below. Specifically equation 5.3.11.

https://ecee.colorado.edu/~bart/book/book/chapter5/ch5_3.htm

A book on these topics, which I highly recommend as it is written for the undergraduate, is very practical, and doesn't assume a lot of prior modern physics or math background, is Principles of Semiconductor Physics by Sima Dimitrijev.
 
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cnh1995 said:
long diode and short diode
I guess I never heard diodes described as long or short before so I checked one of my favorite online books; http://ecee.colorado.edu/~bart/book/book/contentc.htm. The section on https://ecee.colorado.edu/~bart/book/book/chapter4/ch4_4.htm gives this description:

The "long" diode expression applies to p-n diodes in which recombination/generation occurs in the quasi-neutral region only. This is the case if the quasi-neutral region is much larger than the carrier diffusion length. The "short" diode expression applies to p-n diodes in which recombination/generation occurs at the contacts only.

Thanks Electrical, Computer & Energy Engineering University of Colorado Boulder.

edit: @eq1 mentioned ecee first but a different chapter. :)
 
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cnh1995 said:
I am revising some topics in analog electronics and I am currently working on the BJT. I know the applications of these devices (diodes, transistors) and know the necessary formulae to work out problems.
But there are a few fundamental things which are still unclear to me and I am not very confident about my overall understanding of semiconductor physics.
So,
1)When we forward bias the B-E junction in a BJT, does emitter-base current start before the collector current? I mean the explanation I read in my books is like this:
"when the emitted electrons enter the base region, very few electrons recombine with holes and the rest are pulled in the collector region." Does this mean emitter injects electrons first irrespective of collector parameters (like biasing, doping etc) and then the collector pulls whatever electrons are injected in the base?

2)If a diode is having a very thin, lightly doped p region and wide, heavily doped n region and it is forward biased, will the forward diode current be very small due to fewer recombinations in the p-region or will the current be large due to large number of majority carriers in n-region that diffuse in the p region? In other words, can the electrons exiting p-region be conduction electrons (in which case the current will be large) or do they have to be only valence electrons (in which case this current will be small)?

Thanks in advance.

Minority carrier injection in on both sides of any PN junction and only requires base junction bias. This is a "fundamental" property of the PN junction itself. In the case of metal-semi Schottky junctions, you get injection on both sides also but the metal's carrier concentration is so much higher than any semiconductor that the injection distance is practically zero (or maybe on or two atoms only). So you are getting emitter injection into the base regardless of the collector bias.

And you are getting injection into the emitter from the base as well! This is a key aspect of "emitter efficiency" which is tuned up with a heterojunction BJT: the reverse injection of a heterojunction is pretty much zero so the injection efficiency is very high. Consider that the reverse injection is canceling some of the forward injection for normal forward-linear operation.

These "decay" exponentially with distance as long as their is a current path to suck up the recombination current that must be supplied to enable recombination. This is also related to base transit and base width: BJTs ONLY work when the depletion thickness is wider than the physical base width (based on including the base-collector depletion layer width from the metallurgical junction).

It's this minority carrier injection that leads to the extra diffusion capacitor seen in BJTs - if you pulse the base-emitter voltage and measure the base current, you'll get a different answer for the collector being open vs. being shorted to the base or emitter because of diffusion capacitance being caused by injected minority carriers NOT being instantly recombined. When you short the collector, you have a current path for that recombination current.

Per the diode depletion layer equation, the answer for this is pretty obvious. Intuitively you get a smaller depletion layer with higher doping because you "cancel" the charge in a shorter distance. The current you see in either case (light or heavy doping) changes the Is (saturation current) value though it's as a ratio with the intrinsic doping.

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

There's never a current that is valance band electrons. All the electrons (or holes) that conduct current are 100% from dopant atoms near the conduction (or valance) band, respectively that have been thermalized into the conduction (or valance band) by heat. This is why semiconductors don't work at cryogenic temperatures: there are no longer free carriers available - for silicon it becomes like diamond or silicon carbide - an apparent insulator. This is also why, with enough heat, you can make the latter two into semiconductors.
 
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  • #11
Can you explain light and heavy doping, I always thought it was based solely on the electron configuration of silicon minus or plus an electron in the crystal lattice.
 

Related to Some fundamental questions about pn junctions

What is a pn junction?

A pn junction is a type of semiconductor junction that is formed when a p-type semiconductor and an n-type semiconductor are brought into contact with each other. This creates a depletion region, where there is a lack of free charge carriers, and allows for the flow of current in one direction.

How does a pn junction work?

A pn junction works by utilizing the properties of p-type and n-type semiconductors. The p-type semiconductor has an excess of positively charged holes, while the n-type semiconductor has an excess of negatively charged electrons. When the two are brought into contact, the electrons from the n-type semiconductor diffuse into the p-type semiconductor, creating a depletion region. This creates a potential barrier that allows for current to flow in one direction, from the p-side to the n-side.

What is the difference between forward and reverse bias in a pn junction?

Forward bias is when the positive terminal of a voltage source is connected to the p-side of the pn junction, and the negative terminal is connected to the n-side. This decreases the potential barrier and allows for current to flow through the junction. Reverse bias is when the positive terminal is connected to the n-side and the negative terminal is connected to the p-side. This increases the potential barrier and prevents current from flowing through the junction.

What are some applications of pn junctions?

Pn junctions have many applications in electronic devices. They are commonly used in diodes, transistors, solar cells, and integrated circuits. These devices utilize the properties of pn junctions to control the flow of current and create electronic signals.

How do temperature and doping affect a pn junction?

Temperature and doping can both affect the behavior of a pn junction. Higher temperatures can cause an increase in the number of free charge carriers, which can decrease the potential barrier and increase the flow of current. Doping, or intentionally adding impurities to the semiconductor material, can also affect the properties of a pn junction. Doping with different materials can change the conductivity and potential barrier of the junction, allowing for more control over its behavior.

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