22 responses to “Transistor Interview Questions”

  1. Alexandru

    Such an interview is probably disappointing. I think some of the fault for this sort of behavior lays in the way EE courses are taught. My Electronic Devices and Circuits (sort of EE101 in the area of the globe where I live) was this old-school guy nearing retirement who spent a lot of time giving us an intuitive idea of fundamental behaviors and quantities like the ones described by you. Many of my colleagues had a younger teacher who didn’t really bother to show things graphically; not having much industry experience he probably figured that you can’t really do too much graphically and you have to resort to writing the equations anyway, and besides, it’s not too practical to give datasheets at the exams.

    Of course, blaming education only gets you so far — it’s up to every student to realize he’s not well prepared and do some work/study on his/her own.

    1. Fluxor

      I absolutely agree with your point about intuition. I find that many universtiy courses often focus too much on deriving equations to the detriment of developing an intuitive feel for the subject. Both are important in trying to gain insight into a problem. As for Mr. Flop, he obviously didn’t pick it up in school. He didn’t pick it up in 5 years of working either. Too bad.

    2. Gman, the teacher

      Alexandru, you might have struck the nail squarely; many schools have failed to maintain a structured program that guides a student effectively from beginning to end. Younger professors tend to dump large quantities of metadata with no real understanding of how the parts chronologically constitute the whole. Fluxor, I participated in a debate that discussed the pertinence of a BSEET vs a BSEE a number of years ago, and the discussions were consistent with your earlier point.

  2. Chris Gammell

    Great questions Fluxor! Those really are the basis for a lot of design beginnings and I would think it’d be a good place to start with candidates. The method me and my colleagues always try to use is similar to what you mentioned forgetting to do at the beginning: Start very simply. If they easily answer those questions you can quickly ramp up into more difficult ones. If they need a bit of an ego boost from getting the easy ones right, it might put them more at ease to try and talk out what they know on other questions (I’m guessing Mr. Flop kind of started to spiral after a while…I’ve had interviews like that, even when I knew the material). And of course, if they really had no business being there and can’t answer the simplest of the questions…you can downshift into small talk and start doing a “behavioral interview” (*shudder*).

    Tell me about a specific experience and the resulting conclusion of that experience where you were in a personal conflict and how you resolved it?

  3. Cherish

    I think the perfect behavioral ‘question’ would be to stick someone in a room, ask him or her to design something, then rip into it or request a bunch of changes and see how they react. If they say, “Yeah, I can see why that would be better,” that would be a good indication that they’re going to be more concerned about the success of their project than their own ego…

    As far as your questions, since I seldom work directly with electronics, I probably couldn’t have answered your questions. On the other hand, I am fairly certain I saw the first six in my basic electronics class, so I would certainly expect someone working with ICs to be very clear on them. I’m very surprised Mr. Flop had issues.

  4. FrauTech

    So obviously I wouldn’t be able to answer any of these. But seems similar to MEs that I’ve heard have had to draw and then explain a bending-moment diagram. I could see how maybe you could forget it if you hadn’t been working in a stress/fracture based field for five years but if this guy was supposedly working in this that is surprising. I wonder if it was mostly him exaggerating or just wasn’t really put to work at his last place.

  5. J Gruszynski

    My focus was analog IC design and semiconductor device modeling in EE school so, yes, I (did/would) get all of these no problem. I still use this knowledge daily. This _is_ basic stuff that you should never get wrong when interviewing for any analog design job or any job involving semiconductors. It’s only the beginning though – but a good start.

    The “simulation stupefaction” is a funny term but also true. The truly sad part was that 30 years ago the admin at my EE school considered shutting down all undergrad labs and simply relying on SPICE for everything. Turns out the dean wanted a bigger office and taking those labs’ floor space would do the job nicely. Thankfully the real EE professors revolted at the idea but I can also imagine the bureaucratic “initiative” and precedent in place to try this hair-brained idea every year over the next 30, again and again. In some schools the morons probably won the argument since then.

    I try to tell EEs that that every SPICE output should be treated as a lie unless you can prove otherwise with pencil-and-paper or inspection of the simulation details, and even then, be dubious. Thankfully I had “analysis by inspection” drilled into me in EE school so I can look at a circuit and trivially write the loop equations directly to algebra and even do it all in my head (what is the input impedance and how does the circuit alter it in this unique topology?). It’s an essential skill for analog design IMO.

    I’ve also been in high tech sales involving semiconductors, which is probably the only place behavior interview really makes any sense to do. Everything in sales is about how you interact with others. Back in R&D, not so much. Though important, technical skill is king (and not “Book Technical Skill” but “Empirical Technical Skill and its Application” – you have to have a “feel” for it; book-smart is no where close to good enough). If you have the technical chops, then maybe go behavioral iff you have two or more equivalent candidates.

    1. Fluxor

      I fully agree with the suspicion one should take towards simulation results. There should be an expectation of results even before the “run” button is pressed so that one does not blindly trust simulation results. This is simply a more complicated version of using a calculator — make sure you do a quick approximation in your head first so you can tell if the calculator results are in the ballpark.

      I read your story about banning labs with amusement. One suggestion by a higher-up manager here is to do away with prototypes altogether and just simulate more. Then our products shall be perfect the first time around. I’ve ranted about this in my blog numerous times before.

    2. Alexandru

      They actually managed to do this at my university, albeit for different reasons. Fast forward five years and the results are utterly disastrous. I have colleagues who hand in course projects with biasing resistors of 1021.231 ohms and see absolutely nothing wrong with that because all the potentials and currents seem to have the right values. A significant proportion aren’t able to hook up a wattmeter properly and I think I’m optimistic when I say that only about 50% know how to use a soldering gun with decent results. Most of them fail at basic circuit analysis tasks; any sort of design is completely out of their league.

      Unfortunately, only a handful of teachers were properly revolted by this. The vast majority of the faculty staff (myself included although I’m not exactly faculty staff since I’m not allowed to — I’m in my last undergrad year) are doing research in fields like computational electromagnetism and various other applications of modeling and simulation so they spend far more time writing code than burning stuff. Those who didn’t agree with this sort of change were quickly labeled as old-fashioned dinosaurs who didn’t see the benefit of the digital era.

      1. Gman, the teacher

        We are not totally powerless in controlling change. The IEEE, ASME, and ABET conduct extensive reviews that rely on expert feedback to guide future curricula. All your collective inputs are important to the education of today’ s young Engineering professionals.

    3. Gman, the teacher

      J, SPICE, and CADENCE etc. are still rapidly susrping good old classroom theory. ON the positive side, my young engineers have capitalized on Field notes from T&M manufacturers like Agilent and Anritsu who generally do a great job with baseline theory. Great comments.

  6. Charles J Gervasi

    I wonder if the candidate reads this blog.

    The questions were pretty hard IMHO. I know about how the parts work on a board but have only crude models of how they’re realized in Si.

    Hopefully you’ll do another post on flipping the transistors around. I once saw someone flip a BJT. It appeared to work in his circuit, he said. It was NPN, and he had the base-collector junction forward biased and the base emitter junction reverse biased. I can’t imagine you’d get a decent beta that way, but it was working for whatever he was doing. I gave this mode the appellation “reverse-active mode”

    I don’t see why you wouldn’t flip the source and drain of a FET, but I’ve never done it on purpose.

    1. Fluxor

      I’m curious, which questions do you think are hard? I hope not the first two.

      Yes, they do get progressively harder, but that’s what we look for in a candidates that wants to do analog IC design, even junior candidates. Candidates don’t have to get every question correct, but these questions are good in determining the level of grounding in the interviewees.

      Personally, I think these questions are challenging to those that come straight out of a Bachelor’s degree. Often, they have not done enough design work themselves to know what is important to internalize and what is knowledge that is reasonable to look up in a text book. That’s why all of the analog guys I work with have a graduate degree in analog IC design. By the time you work through a thesis doing your own design, a lot of this stuff becomes embedded in one’s knowledge. In some cases, like Mr. Flop, it never sunk in.

      As for flipping a BJT, yes, beta goes down significantly. Many text books call that the reverse mode or reverse-active mode. That question (#7) wasn’t intended to ask why one would want to do it, what rather what happens if one does do it. The follow up question would be — why does that happen?

  7. Phil

    Hi Fluxor,

    I am currently a graduate student in controls but will hopefully be transferring next fall to a different school that has an analog design program as my passion and knowledge is in that area. In the mean time, I have been applying like crazy to some local companies that are looking for analog guys. My experience is very low as I just graduated last May with my BS in electronic engineering and I was wondering if you can offer any advise to an aspiring analog designer? The interview questions you posted were great and with my lack of experience, I fear I may pull a Mr. Flop on my interview. Well assuming I hear back from any of them :) Can you offer some more interviewing tips/related questions by chance?

    Thank you kindly.

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  10. Sai N.

    Hello Mr. Fluxor,
    This post and the previous post regarding the disappointing interview tell students what to watch out for in the interview. I really can’t believe how a Master’s student could not answer some basic questions (his thesis would have covered much more complex topics).

    I came across your post because I ran a google search about how to prepare for an Circuit Designer interview. I have experience testing PCBs and some basic schematic experience but I really want to dig deep into circuit design. How can I get experience good enough to present to an interviewer like you and impress? All software that does circuit design is proprietary and not available to students. And since I am in 3rd year of Electrical Engineering, I cannot access a 4th year class’ circuit design software without actually being in the class.

    I really want to get an internship at a top company and come back to start full time employment. But I’m worried that even though I have the theoretical knowledge, I do not have the practical knowledge in design.

    Please advise.

    Really love this site, it’s been great help to many.

    Regards,
    Sai