Two settings in the Simulation parameters panel decide how much of the doping you typed is actually electrically active — and, when you want it, let you set the carrier density directly instead.
01Temperature
Temperature is in kelvin and starts at 300. It feeds everything that depends on thermal energy: the carrier distributions, the intrinsic concentration, and — unless you override it with the next setting — the fraction of your dopants that are ionised.
Set it deliberately rather than leaving it at room temperature out of habit. A structure that behaves one way at 300 K can behave quite differently at 77 K, and the difference is rarely subtle.
02FullyIonised
This checkbox is off by default, so Poisson uses partial ionisation unless you tell it otherwise.
- Unticked — each donor or acceptor is occupied according to Fermi–Dirac statistics and its binding energy from the nearest band edge. A fraction of your dopants stay neutral and contribute no charge.
- Ticked — every dopant is assumed ionised. The doping you typed is the charge you get.
03How much difference it makes
Cooling a structure is where it shows up first. Acceptors in GaAs, at a moderate 1017 cm−3:
300 K 99.0 % ionised
150 K 91.9 %
77 K 46.9 %
50 K 13.1 %The other axis is how deep the dopant sits. At room temperature and 1019 cm−3:
Si acceptor 0.01 eV 67.5 % ionised
GaAs acceptor 0.03 eV 51.9 %
GaN acceptor 0.23 eV 3.9 %So at room temperature and moderate doping the two settings agree closely, and the default rarely causes trouble. It starts to matter when you go cold, or when the dopant is deep — magnesium in GaN being the standard example.
04The doping column as a carrier concentration
Ticking the box has a second use that has nothing to do with accuracy. It makes the doping column a direct control on the free-carrier density: what you type is what the solver gets, net of any other factors. The column stops describing a dopant population and starts describing the carriers themselves.
That is what you want when you are working backwards from a measurement rather than forwards from a growth recipe. To extract a carrier concentration from a Hall measurement: put the measured mobility in the layer’s Mobility column, tick FullyIonised, and adjust the doping figure until the calculated sheet resistance matches the measured sheet resistivity. Because the solution accounts for the depleted regions at the surface and the interfaces, the figure you land on is the concentration in the part of the layer that is actually conducting — which is what a Hall measurement alone cannot separate from the thickness it is conducting through.
05Dopant depth
The layer table‘s Dopant depth (eV) column overrides the material’s own binding energy for that layer. Leave it blank and the database value is used.
Two things to know about it. The value you enter is applied to the donor or the acceptor, dependent on which dopant column you have used for that layer. And it is ignored entirely when FullyIonised is ticked, since nothing then depends on where the level sits.
The setting applies to the whole structure, not to one layer. That is awkward when a device mixes a degenerate cap with a lightly doped active region, because the two want opposite treatment. The partial-ionisation calculation has no mechanism for the impurity band that forms at degenerate densities and keeps those dopants ionised in practice, so it under-ionises the cap — hence GaAs showing only half ionised at 1019 cm−3 above, which is not what a real n⁺ contact does. Forcing full ionisation fixes the cap and overstates the active region.
You have to pick one. At room temperature, forcing it is usually the better trade: from the figures above it costs under a percent up to 1017 cm−3. Cold is the other way round, since freeze-out is then the very effect you are trying to capture.
Software Utilities 