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Many schematic symbols exist for transistors (such as the examples shown below), but each seems to be specific to the transistor structure and polarity.
Is there any suitable symbol that would represent a transistor of any structure (rather than specifying it as a BJT or a FET), for applications in which the specific transistor structure is not important?
BJT NPN:
JFET N-channel:
In their book "Feedback Amplifiers - Theory and Design" (Kluwer, 2002), Gaetano Palumbo and Salvatore Pennisi describe their circuits by making use of a generic transistor that represents BJTs, HBTs, Mosfets and Mesfets.
The generic device of a given polarity is represented by this symbol (actually two, to consider the possibility of a substrate)
and is used in conjunction with "a generic small-signal model applicable to a variety of different transistor types operating in the active region" that is this one:
This 'unified' device is introduced
"to ensure that all analytical results derived herein are applicable to feedback configurations realised with both BJT and MOSFET technologies –including heterostructure bipolar transistor (HBT) and III-V compound metal-semiconductor field effect transistor (MESFET) technologies."
and
"is identified by four terminals denoted as X, Y, Z and B.
Specifically, X, Y, and Z respectively representing the emitter, base, and collector terminals for BJTs (and HBTs) or the source, gate, and drain of MOSFETs (and MESFETs). The fourth terminal B represents the substrate or bulk node, it is almost always biased at a fixed potential and conducts a negligible static current. Symbols X, Y and Z were chosen to remind us of the functional equivalence between our generic device and the negative second generation Current Conveyor"
The first appearance ought to be in the paper
G. Palumbo, J. Choma Jr.,
“An Overview of Analog Feedback Part I: Basic Theory,”
Analog Integrated Circuits and Signal Processing,
Vol. 17, No. 3, pp. 175-194, Nov. 1998.
Is there any suitable symbol that would represent a transistor of any structure (rather than specifying it as a BJT or a FET)?
No.
Nothing is more critical to the depiction of a transistor on a schematic than its type. The vast vast vast majority of circuits will not function at all with the wrong type of transistor (s). If you think about the kind of information a schematic has to convey to be useful, a square box with the word "transistor" inside it is just about useless.
There is not any generic transistors that I have seen. Part of the reason is transistors operate on different mechanisms and are not readily interchangeable in most applications.
For example: It is very important to know the difference between PNP and NPN, which determines the direction of the current in the transistor and how it operates. Using one in place of the other could create a smoked part.
Another example: Field Effect Transistors operate off of voltage and a gate, using an Bipolar Junction transistor would produce vastly different results because the 'gate' (BJT's don't even have gates, they have bases) of a BJT would pull down because it operates on current. Also a BJT has a votlage drop simmilar to a diode where FETs do not
Regular switch symbols could work, perhaps with a circle to denote those with an active low input (like a PMOS). It neither very pretty nor very useful though.
simulate this circuit – Schematic created using CircuitLab
Transistors are not generic devices despite how they have been cataloged, modeled and share common concepts. At best a common symbol is used for technology (BJT vs FET) but even that is a simplification that has marginal utility.
It is important to understand, ahead of time, what your work product is trying to communicate, an electrical design consists of many different documents and deliverables. A schematic as a design asset needs to be specific as possible. It is simultaneously defining the implementation and serves as a documentation for other designers. In extreme cases some professional shops go as far as define a symbol for every part number. A simplified schematic has a different use case (publication in academic paper) than a production worthy schematic.
If you are making a system or architecture viewpoint diagram, sure use whatever is the functional equivalent for the discussion at hand, e.g. a switch for logic diagram, high impedance input for an interface diagram etc.
The closest thing to a generic or ideal transistor would be a current conveyor, although they're not commonly used. But if you're looking for a symbol that works like an ideal op amp or ideal diode, there isn't one. Transistors are not taught, understood, or used as idealized devices.
If you want an amplifier symbol, there's a symbol for that, or you could use an ideal op amp symbol.
If you want a switch symbol, there's already one:
