My background consists of a BS in Computer Science and an MS in Computer Science and Engineering, both from the University of Washington. My MS consisted mostly in the very high level design of a hardware sorting machine. Now I'm trying to figure out whether it would make sense to implement that machine, and I'm trying to take a look at it all the way down at the transistor level. My schematic diagram consists of all six transistors the schematic tool lets me draw.

Can anyone tell me what the essential differences are between the six transistors are?


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ If you did a quick internet search on those part numbers you would find out what kinds of transistors those are. The 2N3904, for example, is an NPN transistor. Then you can use the internet to find out what an NPN transistor is. \$\endgroup\$ – Elliot Alderson May 15 '19 at 21:36
  • \$\begingroup\$ Generally good education at UofW (born in Portland but lived in the Seattle area for a while.) And given the engineering component of the MS, I'm guessing you don't really need to ask this question here as you can work through textbooks almost as quick as anyone. But I have provided an RTL gate design discussion and also discussed TTL here and here -- all BJT circuits. It's a start. Study one, then see differences from there. \$\endgroup\$ – jonk May 15 '19 at 22:16
  • 1
    \$\begingroup\$ As an aside: implementing your design at the transistor level is unlikely to add much value unless you have something really unusual in mind. A more practical approach would be to create a gate-level implementation, then perhaps evaluate the power/area cost of that design under a standard cell library. \$\endgroup\$ – duskwuff -inactive- May 15 '19 at 23:11
  • \$\begingroup\$ If you are attempting to do hardware design down to the transistor level then you spend a lot of time reinventing the wheel, especially when you appear to have no knowledge of transistors/FETs, which makes calling it a wheel sketchy. As for the question, it is too broad to truly be answered by this forum. \$\endgroup\$ – StainlessSteelRat May 16 '19 at 11:30
  • Q1 & Q2 -- bipolar junction transistors (BJT), one NPN, one PNP.
  • M1 & M2 -- metal oxide field effect transistor (MOSFET), one N-channel, one P-channel
  • J1 & J2 -- junction field effect transisor (JFET), one N-channel, one P-channel

Each with strengths and weaknesses. The REALLY SHORT answer is that:

  • BJTs are the oldest commercial technology, they're good for linear circuits, linear power output stages, and general "knock around" design.
  • MOSFETs have really low losses at high currents when they're turned on all the way, so you see them a lot in switching power supplies and switching amplifiers.
  • JFETs tend to be specialized, and rarely used -- which makes them hard to get, and even more rarely used.

Beyond that -- please look them up on the web. The full description of each one takes a book, or more.

| improve this answer | |
  • \$\begingroup\$ Lateral mosfets are usable for linear circuits, but boutique. Also, I wonder if it is worth mentioning depletion mode vs enhancement? Seems important to me even in a short summary. \$\endgroup\$ – jonk May 15 '19 at 22:08
  • 1
    \$\begingroup\$ I hate summarizing -- no matter what makes the cut or not, in the end it's always wrong. \$\endgroup\$ – TimWescott May 15 '19 at 22:21
  • 1
    \$\begingroup\$ I suppose so. It's why I avoided this one. I couldn't do it any justice without having to sit down and organize my own thoughts -- probably for hours first before even starting to write. And I suppose I wasn't confident enough to try without spending that time. I remember a footnote by Stroustrup: "I would have preferred this paper to be either much longer or much shorter, but I did not have the time to do either." The OP should be quite competent and fast at answering his own question better than we can here, my opinion. \$\endgroup\$ – jonk May 15 '19 at 22:24

Q1 is an NPN BJT transistor
Q2 is a PNP BJT transistor
M1 is a N channel or NMOS mosfet transistor
M2 is a P channel or PMOS mosfet transistor
J1 and J2 are JFET transistors

BJT's have a voltage drop (normally 0.7V) across them, then need current to turn on
An NPN has current flowing into A that controls current flowing from B to C
A PNP has current flowing out of D that controls current flowing from E to F

What you need to know: It's easy to fry a BJT if you don't limit current with resistors, there is a voltage drop similar to a diode, but this creates loss and heat. You control a BJT through the base.

Mosfets need voltage on the gate (G and J) to switch on and off N channel mosfets need the gate voltage (G) to be a higher voltage than the Drain (I) to switch on. P channel mosfets need the gate voltage (J) to be a lower voltage than the Drain (L) to switch on.

What you need to know: Mosfets should not be installed backwards (because they have a reverse protection diode) N channels need a higher voltage on the Drain than the source. P channel mosfets need a lower voltage on the drain than the source.

Use N-channels, they are easier to wrap your head around. They are low resistance. Mosfets work best if they are fully off or fully on, you can run into thermal issues if they are in between. They are also sensitive to static (well all transistors are) but the gates are nm thick and can be blown away easily.

Jfets are simmilar to mosfets, but have lower noise and higher input impedance (two things you probably wouldn't care about unless designing an amplifier)

enter image description here
Source: https://www.allaboutcircuits.com/technical-articles/fet-vs-bjt-vs-igbt-whats-the-right-choice-for-your-power-stage-design/

enter image description here
Source: http://www.differencebetween.net/technology/difference-between-jfet-and-mosfet/

| improve this answer | |
  • 1
    \$\begingroup\$ Yeah, changing, thanks \$\endgroup\$ – Voltage Spike May 15 '19 at 21:55
  • 1
    \$\begingroup\$ To be pedantic the collector current in a BJT is a function of the base-emitter voltage. It just happens that base current is also a function of the base-emitter voltage but from an analytical point of view it is actually the voltage that is the controlling factor. The ratio between base current and collector current is a very ill-defined value. \$\endgroup\$ – Kevin White May 15 '19 at 22:45

Not the answer you're looking for? Browse other questions tagged or ask your own question.