Probably a dumb question - Page 1
Forum section: electronics / beginners

author: watchmaker
author_rank: Unknown
timestamp: October 25, 2023, 04:26:21 pm
content: In addition to learning ckt analysis and differential calc, I added semiconductor theory to my course of study.I am at diodes and just empirically proved to myself that indeed current does not flow below the barrier voltage and takes off like a shot after the voltage is exceeded. I do not remember EVER anyone having me do that as a student. I am talking ammeter and voltage meter measurements, curve tracers do not drive it home for me.So now I am thinking about the depletion zone and understand the donor atoms in that zone have filled valence shells and block the flow of holes and electrons across the junction.This results in one of the materials being positively charged and the other having a negative charge.Do I have that right? if so, in its quiescent state is the diode somewhat like a capacitor with a stored charge that if I had the right instruments I could measure? If not, what am I missing. I understand it cannot be considered a current/voltage source because there is no internal current flow.Thanks,Dewey

author: jwet
author_rank: Frequent Contributor
timestamp: October 25, 2023, 05:06:10 pm
content: Abolultely! Great observation. The depletion region is devoid of carriers like an insulator and P and N regions are "plates". As the bias is made more reversed, the depletion region grows and the capacitance falls. The high capacitance will be when the depletion region is unbiased or just slightly forward biased but below threshold. You can measure this capacitance if the tester uses signals small enough that it doesn't turn on the junction. Big diodes like 1N4001's have a lot of capacitance. There are a class of diodes made just for this function called varactors- they are characterized for C vs. Bias. Some of these use a PIN structure to make a larger depletion region and allow more cap range. These devices are used in VCO's often as a variable reactance- resonation against an inductor.

author: watchmaker
author_rank: Unknown
timestamp: October 25, 2023, 07:11:17 pm
content: Quote from: jwet on October 25, 2023, 05:06:10 pmWell this makes me feel good. Thank you. Varactors. Who wooda thunk it?Regards,Dewey

author: andy3055
author_rank: Super Contributor
timestamp: October 25, 2023, 07:24:06 pm
content: Google for Varycap diodes and you will find great explanations.

author: donlisms
author_rank: Frequent Contributor
timestamp: October 27, 2023, 02:04:55 pm
content: (And you -should- try to get to a point where the curve tracer "does it for you." Such voltage and current relationships, and so many other 2-value relationships, are just everywhere. It's a language you're going to need.)

author: TimFox
author_rank: Super Contributor
timestamp: October 27, 2023, 04:09:22 pm
content: If you look closely at the curve of current as a function of voltage for a PN diode, it goes through zero (the origin,I= 0 andV= 0) with a finite slope (dI/dV).To the left (reverse-bias) and right (forward-bias) of the origin, the shape of the curve is greatly different.Now, if you re-graph the same data for forward bias on a semi-log scale (logIvs linearV), whereI= 0 cannot be shown), you see that there is no actual threshold for conduction, it is just that the current increases approximately exponentially with the voltage for all positive voltage values.The common approximations of 0.6 or 0.7 V as a threshold are just the voltages for a given current (e.g., 1 mA), where the voltage for a larger current (e.g., 10 mA) is not very different.At zero and reverse-bias, the current is low and the capacitance is important: its variation with reverse bias can be exploited in varicap tuning diodes.

author: jwet
author_rank: Frequent Contributor
timestamp: October 31, 2023, 12:34:05 pm
content: Good finer point Tim. Current rises exponentially at a decade per 60 mV of voltage change. You can go through the diode exponential equation with kT/q= 26 mV and verify this yourself. Once the diode starts to conduct a little, it appears to rise very sharply- an exponential. Its behaviour is the same at low voltage changes but it doesn't amount to much until you get to about 1 mA where you can see it. In just 180mv, it would rise from 1 mA to 1amp- looks like a threshold, but its been doing the same thing for ten decades!

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 31, 2023, 02:10:26 pm
content: Quote from: jwet on October 31, 2023, 12:34:05 pmI guess it depends on the diode in question, here are the measured results for two diodes - common 1N4148 and "low leakage" BAV199 in the low forward voltage region, from 50mV to 500mV, at the diode temperature about 26C. The BAV199 is closer to the figure you quoted, at about 70mV/decade, however 1N4148 is closer to 120mV/decade. Y axis in Amps, X axis in Volts.CheersAlex

author: soldar
author_rank: Super Contributor
timestamp: October 31, 2023, 02:42:54 pm
content: Quote from: watchmaker on October 25, 2023, 04:26:21 pmPlease use descriptive titles for your threads. You will get more attention from those who can help you and save the time of those who cannot help you.If all threads were titled so vaguely we would have to open every single one to find out what each one was about.Thanks.

author: watchmaker
author_rank: Unknown
timestamp: November 02, 2023, 11:15:13 pm
content: Quote from: soldar on October 31, 2023, 02:42:54 pmUnderstood. I do not mean to be argumentative, you should note my question was answered that very same day.And in my view, I WAS concerned it was a dumb question. I wanted responders to feel free to critically evaluate my thoughts.I was obviously reaching past the regurgitation of didactic material and trying to synthesize what I thought I knew.Having said that, I will take your request to heart.In friendship,Dewey

author: jwet
author_rank: Frequent Contributor
timestamp: November 04, 2023, 01:01:55 am
content: I think your topic was fine, I saw no real ambiguity. I got it and I'm pretty thick headed. We're all on an electronics website, so the scope of the topic already excludes 90% of topics like dog names and what to do about wayward teenagers...I really applaud you for your effort to teach yourself electronics and wanted to give you some encouragement. Your post and the discourse it created was a positive in my opinion.BTW- I can't recommend Art of Electronics enough for someone in your shoes.Enjoy yourself all- this is as good as it gets...

author: JJ_023
author_rank: Frequent Contributor
timestamp: November 04, 2023, 01:15:54 am
content: Quote from: jwet on November 04, 2023, 01:01:55 amI read The Art of Electronics, There is a wealth of knowledge and information in that book. But it doesn't read easy and I found it to be cumbersome to learning new concepts from it.I have a recommendation for another book that gets fairly technical and in-depth but has a much better way of presenting the information then The Art of Electronics. At least in my opinion. It's called "Teach Yourself Electricity and Electronics" by Stan Giblisco.I think that book is a great primer for getting into more advanced books.

author: tggzzz
author_rank: Super Contributor
timestamp: November 04, 2023, 09:53:17 am
content: Quote from: watchmaker on November 02, 2023, 11:15:13 pmIt is always good to see someone thinking, realising they don't understand sufficiently, and formulating a question that contains the background and what they don't understand. Too few people manage that, including some prolific experienced posters on this forum.So good for you and welcome to the forumYou've already avoided most of the points athttps://entertaininghacks.wordpress.com/library-2/good-questions-pique-our-interest-and-dont-waste-our-time-2/and I'm glad to see you won't be falling into this trap again...Quote