jfet
Considering the output characteristic of the n channel JFET, can we somehow obtain the same characteristic only for the p-channel type of this transistor? 
In R.Jaeger's book about microelectronics circuit design, only a vague description was given on how only the polarities are changed, and everything else is left untouched. I suppose that means, that we will either be graphing the same curve in the first quadrant, or in the third also, only using the Vsg voltage.
It's the direction of the arrow that tells you:
N-Channel
P-Channel
The arrow points from P to N (like a diode) so you can tell which is which channel wise. For example in the top diagram, the arrow points to the channel, so the channel is N-type. In the bottom diagram it points away, so the channel is P-type (i.e. from P to N)
To clarify, this also applies to the symbol that has the gate connection drawn in the middle:
As far as I am aware, this should be used only when the source/drain are interchangeable, but is often used in error (many JFETs are not symmetrical so cannot be used either way round)
If a JFET is symmetrical, it can be used either way round and work just the same. In this case I think it is just convention to call one terminal source and the other drain even though it doesn't matter in practice.
However, some JFETs are designed so the channel is not the same thickness all the way along, and gate-drain capacitance differs from gate-source capacitance. This will matter little for many applications and the device can still be used either way round, but may be important in some (e.g. high frequency) applications.
I couldn't find any good references that go into detail on the subject quickly but Google for "Asymmetrical JFET" and have a read of the various patents. Also "Art of electronics" has a brief mention of this.
The JFET is acting as a diode. So why not just use a diode? The main reason for using a JFET as a diode is that it has low leakage compared to ordinary diodes.
Best Answer
It's not practical because the manufacturing tolerances for any specific N channel JFET are so wide to make it a worthless exercise. See below from the 2N5486 data sheet: -
The above describes the channel resistance against temperature WHEN the voltage across gate and source is zero.
If you picked out one device that had Vgs(off) = -1V then you'd use the top curve but who knows what device you have grabbed from the handy bag of JFETs. It could easily be a device whose Vgs(off) was -8V
Because of this, Fairchild (and other suppliers) have basically created three part names for the same device because the tolerances are so wide. If you read the data sheet there are: -
Irrespective of all that, they are still anticipating that some may have a characteristic that is -8.0 volts! It's not unknown on Fairchild data sheets to have made a typo and actually meant -6.0V.
So, trying to characterize a P ch JFET from an N ch device is fruitless