Doping objects


#1

Hi Lumerical team,

I was experimenting with constant doping objects using the project downloaded from the
p-n Junction Diode example page on https://kb.lumerical.com/en/other_application_gs_pn_diode.html. I made some small changes to the project as follows:

  1. modified the x span to 220 nm for all the objects in the geometry group and the simulation region to avoid creating regions with undefined material parameters
  2. I also disabled the nepi doping object and created 2 further constant doping objects (p and n) without adding any heavily doped regions (Type A) - disabled the objects created in 3 below.
  3. I also added heavily doped regions (p++ and n++ of 2 microns), called this Type B.

When I simulated the two cases, I got completely different results for the I-V charactersitics, with the differences pronounced at low doping levels (and of course short Lp and Ln lengths). At Lp = Ln = 5 microns, there is almost a 5x difference between the two cases. This seems odd as the heavily doped regions are far away enough from the junction to influence it significantly. Is there something we are not doing right? Is this due to the absence of the background doping, which I disabled. The results do not seem to support the physics (the depletion region width is of the order of a micron in the doping range we are considering).

I have attached the simulation file and the two schematic for Type A and Type B, plus some representative results.

Type_A

Thanks for your support.
Simeon.


#2

Hi @simeon1,

Your results make total sense to me. The type B device is longer in length and thus has additional resistance compared to type A. This additional resistance lowers the amount of current passing through the device for the same amount of applied voltage. When the doping is lower, the additional resistance is bigger and the difference is more noticeable. As you increase the doping, the resistance becomes lower and the difference between the two types becomes smaller. I bet if you increase the doping any further, the difference becomes even smaller.


#3

Hi @mmahpeykar,

Thanks for your response to the earlier query about doping objects. We did some further tests to understand the issues better, but we are still not close to getting a convincing solution. While it is true that Type B device is longer due to the extra 2 microns each of n++ and p++ regions, these are doped at 1e20cm-3 and there contribution to the total resistance of the device is negligible, especially at low levels of of the p/n regions as shown in the table below:

image

The resistance of the n++/p++ regions (~1.74 Ohms) is 4 orders of magnitude smaller than the resistance of the n/p regions for a doping level of 1e16 and 2 orders smaller in the case of 1e18. The concern is that with this small contribution to the resistance, the currents should differ by 5x. In fact a simple application of the potential divider rule shows that the effect should therefore be more pronounced as the doping level of the p/n regions approaches the doping of the n++/p++ since the contribution to the resistance increases. This is not what we are seeing and also against what you postulated in your response.

Could there be a different explanation? Relating the impact on the junction? How far should the n++/p++ regions be before the effect of the n++/p++ regions becomes negligible? I uploaded the project last time for you to check. Is there anything we are not right in setting up the doping objects?

Looking forward to hearing from you.
Best regards.


#4

@simeon1,

Thanks for the details. In my previous response I was under impression that the doping values you mentioned in your first post were for n++/p++ regions, otherwise I think you are right and the difference should not be that much. Can you share your project file in the topic so I can have a look at your simulation setup? I don’t see your file being posted in the topic at the moment.


#5

Hi @mmahpeykar,

Please find attached the project file. You can edit it to get the Type A setup from it. I have disabled monitors.

pluspn_diode_10um.ldev (5.6 MB)

Thanks,
Simeon.


#6

Hi @simeon1,

Sorry for the late reply. I was working on your simulation to understand the reason behind the behavior and it took me some time to figure it out. I think the reason lies in the band structure of the junction which is different between type A and type B devices.
Type A looks like a normal pn junction (this is @ 1V forward bias):
image

Type B however shows some discontinuity (potential barrier) at the same voltage:
image

This potential barrier which is created mainly due to the difference in doping and also doping-dependent bandgap narrowing in Si, opposes the carrier flow across the junction and that’s why type B shows lower current. However, as you reduce the difference in doping (by increasing the n,p doping), this barrier becomes smaller and the difference in current becomes smaller too.