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<<< JFETs         Amplifier >>>

You can find the source code and all files used by this Java-app at the column download.
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Experiment guidance:

Experiment 1:

Don't touch the adjustments. Simply press the button "Start".

Comment:

There are two P/N junctions inside of the JFET. Without being attached to a voltage source, the movable charges inside are diffusing non directional through the crystal lattice. If an extra electron hits a hole, both charges vanish during a recombination process, leaving immobile charges at the places of the impurity atoms behind (see simulation of a diode for details). The two zones of the gate get more and more negatively charged, the N-doped zone of the crystal becomes more positively charged. The electric field caused by the charges inhibits holes from moving away from the gate area respectively electrons from moving into the P-doped region. Some minutes pass by until the state of equilibrium is reached
Stop the simulation and have a look at the distribution of charges at the 10 different zones.

Experiment 2:

Adjust the Drain voltage to 100% and start the simulation once again.

Comment:

In this simulation, the attached voltage leads to the injection of electrons at the negative terminal (source, left side), until those zone is 10 times negatively charged (100%). At the positive terminal (drain, right side), the same number of electrons is removed until the right zone is 10 times positively charged (the electrons have to move to the positive terminal before they are extracted). Those disequilibrium of charges generates an electric field which pulls electrons to the right, holes to the left side of the crystal. Hence the depletion layer is weakened at the left of the gate areas, while it is widened at the right side. For a short span of time you can observe recombination processes, by what the gate area gets more negatively charged which enforces the depletion layer, until the swap of charges is inhibited once again.
Electrons are drifting from the source to the drain terminal through the narrow channel between the two p-doped gate areas. The transistor is conductive without a voltage applied to the gate.

Experiment 3:

Adjust the gate voltage to 100%, leave the drain voltage at 100%.

Comment:

By attaching a positive voltage between source and gate, additional electrons are injected into the gate area. Those electrons recombine with holes near the gate terminal. To keep the total charge of the crystal on a electrically neutral, the same number of electrons is extracted at the source terminal. In sum the number of movable charges is reduced. The gate area becomes more negatively charged, while the n-doped area becomes more positively charged. The pn junctions are reverse biased, hence the depletion layer around the gate areas grows until no electrons can move from source to drain. The current running through the transistor is turned off.
Only for a short span of time electrons are injected into the p-doped gate area. As soon as an equilibrium is established you can't observe any recombination processes.
Lower the gate voltage slowly, until electrons can move from source to drain once again.

Experiment 4:

Click at the button "P-channel" and redo experiment 1-3

Comment:

P-channel JFETS consist of a p-doped matrix with an n-doped gate area. The functionality is identical to those of an n-channel JFET, however the polarity of the voltages attached to gate respectively drain has to be altered. A negative voltage drop has to be applied between source and drain and a positive voltage drop between source and gate, to be able to turn off the transistor.
The space between the two n-doped areas is widened at the simulation. The mobility of holes is lower than those of the extra electrons, hence the depletion layer is rising deeper into the p-doped region. Only by widening the channel, the virtual JFET is conductive without a voltage attached to the gate.

<<< JFETs         Amplifier >>>


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