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How can I model a photodiode (ActivePixelSensor) in HSpice??

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Hi there!

I am a new member and I stabled upon this very useful forum while looking for info about modeling photodiodes in HSpice!

I have a project about an analog to digital Active Pixel Sensor (Fowler et al.) and I could really use some examples of active pixel sensors and photodiodes in HSpice.

Thanks!
 

Re: How can I model a photodiode (ActivePixelSensor) in HSpi

i have seen some of my colleges use hspice in active pixel sensor design , and simulation , but i don't know y

but they used it , and they simulated the dark current

khouly
 

Re: How can I model a photodiode (ActivePixelSensor) in HSpi

The concept of the project is to first simulate the design of the ADG Active Pixel Sensor in HSpice then make a mask for it using Microwind, extract the Netlist from Microwind and then compare it with the previous results from HSpice.

If it was up to me I would use different tools, but this is an obligatory project with a standard procedure so I don't have any other options than this.
 

Hi,hspice is for circuitry simulation,it is not fit to simulate PD in APS.If you do want to simulate it with hspice,you can use a current source to instead PD,but I don't think you can get precise result to decribe the phycial process in APS,but just approximated result and it's done by transistor level simulation.Maye you should use TCAD to instead.

For me,I'm also doing related research in CIS,you can contact me if you want:leohart2000(a t)gmail.com
 

Re: How can I model a photodiode (ActivePixelSensor) in HSpi

Thanks for the clarification!

The whole concept of the project is to prove that we can work with these two tools so an approximate simulation with HSpice is what I really need right now. Can you suggest a source for a photodiode model I can use in HSpice (current source, capacities etc.)? The photodiode in the real circuit is a p+ region diffused in an nwell.
 

Just use a dc current source parallel a capacitor,for detail value of these two components,refer to some papers,normally,the photo current is less than a pA and the capacitance is several tens of femto F.

I am NOT sure whether the simulation using hpice will make reasonable result ,I would like to help with it or PLS share ur result with me if it is possiable

Regards
 

Re: How can I model a photodiode (ActivePixelSensor) in HSpi

I've found a paper by T. N. Swe and K. S. Yeo with a photodiode model for Spice but it's a bit too complicated for what I need.

I asked my supervisor and he agreed that a current source and a capacitor will be enough.

I'll contact you when I finish the circuit!

Thanks again!
 

Our company designed many APS CMOS image sensors.

If the photodiode is made from, for example, n+ in the substrate or nwell in the substrate, use the diode model that the foundry supplies for that structure. Make the diode w and L the size of your PD.

Use a current source to simulate the light (it will be picoamps to nanoamps usually for normal light levels).
 

Re: How can I model a photodiode (ActivePixelSensor) in HSpi

Can you tell me which would be normal-average dimensions for a p+ photodiode in an n-well?

I am trying to create a mask in Microwind and the papers I have for my project do not include this information.

Thank you!
 

Re: How can I model a photodiode (ActivePixelSensor) in HSpi

Using the n-well as photodiode:
This arrangement usually uses a connection to the n-well - as one terminal - with the p substrate on the bottom and the p+ diffusion on the top as the other photodiode terminal (connected together by overlapping p+ over the n-well). This way, there are two junctions (and two diodes in parallel). This has some advantages of sensitivity and dark current, but since it's larger it isn't used much in modern CMOS image sensors (since around 0.5 micron technology and 8 micron pixel size).
The size of the n-well will be determined by the size of your pixel and the spacing rules for n-well to n-well in the technology that you are using.
Hope that helps.
 

Re: How can I model a photodiode (ActivePixelSensor) in HSpi

The photodide is basically a capacitor that would hold charge.
Calculate the capacitance of the junction of the light collecting area. The capacitance value is given in the model files.
Next while simulating do the following
make Gmin as 1e-18 rat5her than the default value which the simulators use of 1e-12.
This is for the reason that the capacitance that you would get would be in femto farad range and I would not like for simulator to get a solution for the matrix it put a 1pA current source to ground from the cvapacitor plate.
Also I hope you have the layout for it done to get the value of the fill factor.

Hope it helps
 

hi ambereesh:

1)what is Gmin and why we should reduce it to 1e-18?
2)Can you elaborate more on "I would not like for simulator to get a solution for the matrix it put a 1pA current source to ground from the cvapacitor plate. "

THX alot
 

Re: How can I model a photodiode (ActivePixelSensor) in HSpi

simulator always put resistor 1/Gmin parallel to capacitor
helps convergence.
reversed diode has small capacitance, Gmin discharges it even if no other current sink.
less Gmin for less error current. set KCLTEST=on too helps.
 

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