the problem about result on DEVICE

the used example was the p-n diode provided by lumerical.
i changed the region of “nepi” and “pwell”, which made the emitter and the base were not in the region, as shown in picture below.

in my opinion, the changed region should be closed to reality, but result are so different compared with unchanged case.the results are shown in figure below.

please explain the phenomenon.
thank you !

Hi. Looking at you screenshots I agree that the device should behave similarly to the default design in the example. Looking at the I-V curve however I can see that your device is basically OFF for some reason. One thing that we always recommend is to extend the doping regions into the metal contacts. This is to ensure that the doping gets applied at the interface which sometimes do not happen if you just terminate the doping object right at the interface. I would recommend extending the two doping objects at the top and bottom into the metal and see if that solves the problem. You can also enable the “band” monitor so check if the band profile looks like a pn junction. If you can not figure out the source of the problem then please share the file here and I will take a look.

appreciate for you reply.
i till have some questions, the phenomenon just appeared in p-n diode case, i had test other solar cells case, in which the phenomenon did not show.
meanwhile, i used DEVICE to simulate the case in page 133 of the book, ‘solar cell device physics(second edition)’, the phenomenon also appeared, and i found that when the region of doping got applied at the interface, the short circuit current is right, but the open circuit voltage is wrong, instead, when the region of doping did not get applied at the interface, the short circuit current is wrong, but the open circuit voltage is right.

Hi. One reason behind the I-V curve looking like the top one in your post could be that the series resistance of the diode is really large for some reason. This could be due to doping not getting applied at the surface so you have a potential barrier at the contact or may be you have defined a small surface recombination velocity at the metal-semiconductor interface that is introducing a large resistance at the contact or may be you have a large series resistance at the electrical contact that is restricting the current in the diode. If none of these reasons are valid in your case then I may need to take a look at your simulation file to provide more feedback. The same goes for your solar cell simulation as well.

PN_book.ldev (618.7 KB)

the phenomenon still exist.i have uploaded my case.
please help me check it.
thank you

Looking at your file, I noticed that the doping was not getting applied at the bottom (substrate-base interface). I simply extended the doping regions slightly inside the metal regions and the I-V looked find in forward bias (I swept from 0 to 1 V in 6 steps).

One other point I want to mention is that in the boundary condition, I noticed that you are sweeping the voltage from -1 to 1 V. This would mean that the solver will take a long time to get the result at the beginning for -1 V since the initial guess it starts with is calculated for zero bias. To make the simulation faster you can run two simulations instead, one from 0 to 1 V and another from 0 to -1 V. You can then easily combine the result using the script.

Finally, the number of bias points is really large. If you are interested in having more points where the diode turns on, you can use the “values” option rather than the “range” option and simply use a large step size at lower voltage and smaller step size as you get close to 0.7 V. This way you can get a smoother I-V with fewer bias points.

p.s. You can make the simulation even faster by making the x span smaller since the device is uniform in x direction.

the parameter of the case is from the book, ‘solar cell device physics(second edition)’.
In the book, the open circuit voltage is about 0.4V. if the region of doping included the metal electrode, the open circuit voltage was wrong, but, as mentioned before, when region of doping did not include the metal electrode, the open circuit voltage was right, but the current I would be quite small.

Unfortunately I do not have the book available to me. However, if you can share the .ldev file with me then I can definitely take a look and check if there are any issues with your simulation setup.

Maybe my expression was not clear. i had sent the .ldev file to you, in which, the parameter is from the book, and i described the phenomenon that was also of the .ldev file in the last question.

Sorry. My mistake. I didn’t realize that the file was from the book (although the name should have been a hint!!). So the I-V curve you get with the doping extended to cover the interface gives you the right short circuit current but wrong open circuit voltage, is that right?

I would not be too surprised then. The short circuit current basically depends on one thing, the generation rate. Since the bulk generation rate in DEVICE is probably using the same analytic equations as the book, the short circuit current should be a match.

The open circuit voltage however depends on the reverse saturation current, Is of the diode. This current is sensitive to the non-idealities in the device such a carrier lifetime, bandgap narrowing, etc. These effects are turned on the the silicon model you are using but should be neglected in a text book. The reverse saturation current in your diode is thus different from the books and this will give you a different open circuit voltage. If the book assumes an ideal diode then you need to turn off all non-idealities in your silicon model if you want to match the book’s results.