The BEST explanation of PNP/NPN junction and electron band gaps/electricity  - Page 1
Forum section: electronics / beginners

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 05, 2017, 05:03:23 pm
content: https://youtu.be/wPHG0DCWcC0Both technical, but also you can understand it without a background in physics or chemistry.

author: Ratch
author_rank: Regular Contributor
timestamp: February 05, 2017, 09:19:48 pm
content: Like many presentations on You Tube, there is no review to check for correctness. Toward the end, the presenter states that the holes of the BJT make a circuit proscribed by the base-emitter junction, battery, and wires. Not so. Holes are only present in the semiconductor. There are no holes in the wires and battery. The sea of electrons present in metal conductors would instantly annihilate any holes present.He also give the impression that the base current controls the collector current. The collector current is controlled by the base voltage vbe. That is why a transistor is a transconductance device (voltage controls current), not a current controlled device. The base current is aindicatorof collector current, but does not control Ic.One more thing. I don't recall him stating that the current in a junction diode is caused by diffusion. As the depletion region builds up, the back voltage causes the diffusion current to slow and eventually stop. Applying a forward voltage across the diode lowers the back voltage and allows the charge carriers to flow again by the diffusion process. Diffusion also explains why the collector current is an exponential function of the vbe voltage.Once the charge carriers are in the base region, the voltage difference between the base and collector causes the charge carriers to be attracted to the collector by the electric field associated with the voltage difference. No matter what the voltage difference between the base and collector, the current is limited by the amount of charge carriers in the base. Since the current does not vary much with collector voltage, the collector acts like a current source controlled by the base voltage. To summarize, diffusion controls collector current and the base voltage controls the the diffusion. The base current is a waste current that has to be taken into consideration and removed. So, the BJT by itself is a transconductance amplifier.Ratch

author: retrolefty
author_rank: Super Contributor
timestamp: February 05, 2017, 10:12:48 pm
content: QuoteYour description is not what I was taught. BJT Transistor is a current controlled device, not a voltage controlled device. Ic = Ib x beta. Can you provide a source for your opinion?

author: radiogeek381
author_rank: Regular Contributor
timestamp: February 05, 2017, 10:27:42 pm
content: In general it was quite entertaining. But I agree with Ratch -- and in this case, we are the peer review.However, his assertion that modern computers are built on bipolar transistors is somewhat out ofdate. He'd have been right in 1960, 1970, 1980 (maybe) but by the mid 80's CMOS dominated andby 1990 there were few new ECL or TTL computer designs in progress. Nowadays, of course,you'd be hard pressed to find a bipolar processor in anything but the most exotic computing devices.As for Ratch's comment on transistors being voltage controlled current sources, vs. current controlledcurrent sources, certainly most of the computational models implement this view. However, in reasoningabout bipolar circuits the CCCS model, simple as it is, can often explain behaviors much more readily,provided we don't get caught up in assuming that the results of our back-of-the-envelope modeling willbe anything more accurate than a guide.For those following along, the Hybrid-pi model treats the transistor as a VCCS -- a transconductancedevice. The Gummel-Poon and Ebers-Moll models treat it as a VCCS, though there are two CCCS'sin the typical drawing of the latter model. The h-parameter model, that most of us were taught as the"goto" tool in BJT circuits does treat the collector as a CCCS.I almost always resort to the simple, yet effective, h-parameter model that assumes that vbe is constant as a way of gettinga first-order handle on Ie, which we then assume is equal to Ic (not a bad assumption as long as beta islarge). Again, not spice quality, but sufficient for lots of casual analysis, and almost always useful for determiningbias networks for class A amplifiers.

author: Ratch
author_rank: Regular Contributor
timestamp: February 05, 2017, 10:57:38 pm
content: Quote from: retrolefty on February 05, 2017, 10:12:48 pmCertainly.https://www.google.com/search?q=vbe+vs+Ic&rlz=1C1GIWA_enUS690US690&tbm=isch&imgil=ASKt1PWPvsPmuM%253A%253BdjL93FNjQj2bsM%253Bhttps%25253A%25252F%25252Ftechdocs.altium.com%25252Fdisplay%25252FAMSE%25252FSPICE%2525252BModel%2525252BCreation%2525252Bfrom%2525252BUser%2525252BData&source=iu&pf=m&fir=ASKt1PWPvsPmuM%253A%252CdjL93FNjQj2bsM%252C_&usg=__Q_4NJ4SWhn7-rcqeJwy2yp1569k%3D&biw=1280&bih=649&ved=0ahUKEwjEjtXchPrRAhVIxoMKHYLnBkAQyjcILg&ei=bquXWMSfGMiMjwSCz5uABA#imgrc=hgS6EFslRBAiIM:The above link shows several vbe vs Ic curves. As you can see, they are nonlinear due the diffusion mechanism that governs BJTs. If you drive a BJT with a current generator, or put a large amount of resistance in the base and emitter circuits, then you will see a linear relationship between ib and ic. But, then you have a current amplifyingcircuit, not a "naked" BJT transistor. The BJT will still be controlled by the vbe, however.Ratch

author: Ratch
author_rank: Regular Contributor
timestamp: February 05, 2017, 11:18:31 pm
content: Quote from: radiogeek381 on February 05, 2017, 10:27:42 pmToo late. All the bad info has been dissseminated.QuoteModels tell you what a device does, not how it works. Not unless the device models take into consideration the physics of the device instead of just the empirical behavior. Most models do not go into that detail.Ratch

author: radiogeek381
author_rank: Regular Contributor
timestamp: February 06, 2017, 03:54:56 am
content: The suggestion that the physical reality compels us to model a BJTas a voltage controlled current source is not as certain as it mayseem. In fact, neither the CCCS nor the VCCS model account for allthe situations that a circuit designer will face.In particular, while many simulators use the voltage controlled modelas their basis, the fundamental operation of the device depends on theinjection of minority carriers into the base region. Thischarge-controlled model correctly explains two important phenomena:1. In many optocouplers, there is no base terminal in the outputtransistor. The base region is exposed to the photo diode where thelight impinging on the base region creates free minority carriers inthe base. These carriers create the path for the collector-emittercurrent.2. In bulk CMOS processes the N well diffusion (created to hold theP-FET) combined with the P doped substrate, and a nearby N diffusionfor a source/drain terminal for a neighboring N-FET can form aparasitic NPN transistor. It is possible to turn this transistor on(and create a short between Vdd and Vss!) by injecting electrons intothe sliver of P material between the well and the source. This chargecan come from a number of sources that include:Current pumped out of a nearby on-chip decoupling capacitor. (Weused to make these out of N devices where the gate was connected toVdd and and the source and drain both connected to Vss. Works greatunless the poly gate is too big and acts as a charge pump...)Current dumped into the substrate when an I/O pin falls belowground or rises above Vdd.The VCCS model doesn't do a good job of accounting for either effect.I'm not arguing that there are no models that can legitimately relateIc to Vbe. In fact, they're pretty useful even on theback-of-an-envelope if you want to deal with q/kT variation withtemperature.The Wikipedia article makes your point in support of the VCCSmodel. But it presents without support the contention that (in theirwords) "to accurately and reliably design production BJT circuits, thevoltage-control (for example, Ebers–Moll) model is required." Yes,SPICE and other simulators use models that often have VCCS components,but that doesn't mean their models mimic the physics -- they justaccount for the effects sufficiently to justify their costs. In fact,I think it goes too far. I don't believe that such models arerequired for the design synthesis process, but they are warranted as avalidation/verification step.The graph cited above presents a <relation> between Vbe and Ic. Thereare many such relations. The length of my morning commute is relatedto the number of pizzas sold in a given year. The existence of therelation does not prove causality. Further, the graph correctly showsan exponential relation between Ic and Vbe, but if you had chosen thegraph of Ic vs. Ib, the plot would have been largely linear. Whichwould you rather reason with?My biggest problem with the Ic vs. Vbe graph is that it suggests Ic isthe "output" from some function that takes Vbe as an input. Thisnotion of a function (as a thing that operates on an independentvariable and produces a value of a dependent variable) is a trap.These expressions are <relations> that the physics tells us must holdtrue.As an example, consider a diode connected to a stiff voltage source.Ramp the source from 0 to 0.7 V and measure the current. Clearly Idwill be proportional to exp(Vd * q / kT). So, we have a case where Idis determined by Vd.Now let's put a current source across the diode. Ramp the currentfrom 0 to 5mA. What do you see? Of course Vd = k * ln(Id/Is) So wehave a case where Vd is determined by Id.Both models are useful. Both reflect physics as well as we cared to.Most of our readers are working to develop an intuitive understandingof how circuits work. I've found that the h-parameter model relatingIc to Ib has been the most useful over the years. The CCCS h-parametermodel is the clear choice when thinking about the classiccommon-emitter or common-collector configurations.But the VCCS model is pretty good at explaining how a current mirrorworks. (and the CCCS model is singularly unhelpful for that). Whilethe transconductance model is at the heart of most simulation models,it cannot account for charge injection modes. Further its awkwarddependence on the exponential relation makes it inconvenient as areasoning mechanism in initial design creation. The awkwardness issuch that the model is often linearized to make it useful.On the other hand, the h-parameter model can tempt designers intoan unholy faith in hfe, and is useless for non-linear design likeoscillators (it can't predict the limiting amplitude, for instance)or for predicting harmonic content of an amplifier output. (Thoughit should be noted that the VCCS model handles this with the aidof Mr. Bessel. For a real treat, take a look at the first chapterof "Communication Circuits: Analysis and Design" by Clarke and Hess.Their presentation of the non-linear analysis is a joy to read.)Neither model does a very good job of handling the extremes.Transconductance, alpha, and beta vary with temperature, frequency,and current. The VCCS model probably comes out ahead in large signalanalysis, but datasheets are often pretty sparse when it comes tothings like gm.In the end, the most powerful argument for the h-parameter model isthat there's a chance that you can find the parameters for "normal"devices. The transconductance model suffers here. What is the gm fora 2n3904? 2n2222? 2n5179? All specify at least their hfe, and I canfind specs for all four parameters for the first two devices.So, I would suggest that there is no valid categorical claim that aBJT must be modeled exclusively a transconductance device or that itis a current controlled current source. If I am controlling terminalvoltages, then I think of it in terms of a VCCS. If I controlterminal currents, then my best bet is to treat it as a CCCS,recognizing that the terminal voltages can become hard to treat if Ipush the base too hard. If I'm ready to sharpen a pencil and crankthrough the Bessel series approximations, I'm back to Ebers-Moll.Finally, if my only control is over charge injection, I'm in the soup.(Though there are simulators that deal with those effects.)In any case, this has been a stimulating discussion. For the reader,the points on both sides are non-trivial. These choices matter.If there is a takeaway here, it is that all we have are "models" of thereality, and our choice of models is influenced by where we've beenand where we are, and what we're give to work with.For the vast majority of the problems that our fellow readers encounter,Ic = hfe * Ib and Vbe = 0.6V will allow them to calculate the permissiblerange of base resistors for that 2n3904 that is driving a LED, orthe emitter resistor to get 5x gain out of a single amplifier stage.With luck they'll develop a style and habits that ensure the stuff workswhen it gets cold, or when the batch of 1000 2n3904s from theSemiconductor and Screen Door Company of Greater Guangzhouturns out to have a beta of 20.'73

author: Ratch
author_rank: Regular Contributor
timestamp: February 06, 2017, 04:11:45 am
content: radiogeek381,Are you addressing your post to me? If so, you should put my handle at the beginning like I did yours above.I did say that BJT transistors are voltage controlled. But, I never said that the BJT should be modeled that way. In design, we are interested in what a device does, not how it does it. Since it is more convenient to model the transistor circuit rather than the naked transistor, that is the way to go. If you are going to put lots of resistance in the base and emitter for various reasons, by all means use the current amplifier model to facilitate the design process. However, just don't tell me that the transistor by itself is a current constrolled device. It isn'tRatch

author: rsjsouza
author_rank: Super Contributor
timestamp: February 06, 2017, 04:17:06 am
content: There are some discussions about this around here - one that was very long with the occasional mud flinging ishere.

author: Ratch
author_rank: Regular Contributor
timestamp: February 06, 2017, 04:32:38 am
content: Quote from: rsjsouza on February 06, 2017, 04:17:06 amI skimmed some of that thread just now. It appears that your post #6 gives a good description of what is happening. I would add that the base current is waste that cannot be eliminated, but has to be considered. It has no affect on the Ic, but is a proportional indicator of Ic. I wish I was cognizant of that thread back then. I am sure I could present some points that would shorten the discussion.Ratch

author: Zero999
author_rank: Super Contributor
timestamp: February 06, 2017, 11:00:26 pm
content: Quote from: Ratch on February 06, 2017, 04:32:38 amUnfortunately that thread fell victim to some trolls. Fortunately the main perpetrator has been banned so hopefully it won't be as bad this time.