Pspice Modeling of BJT

[viren]

pspice bjt

I want to model a real BJT IC Vs Vce Characteristics using student version pspice.
I have used a Pspice BJT Model and tried to model a real BJT using .Model parameters but it is not matching the real bjt characterisitcs. Can u please provide some advice?

 

[flatulent]

bf equation in pspice model

It is difficult to cut and try the parameters to do the model. Some companies have specialized programs that show you the curves in real time as you vary the parameters.

This is no job for a beginner in device physics. It is so complex that you will need to get a book on modeling.

in general, each parameter in the model can be found by plotting two external device characteristics and finding the slope and intercept of the line.

One thing I notice on your graph that the beta increases with current. This means that you are still operating in the region where leakage currents affect beta and high level injection does not. It seems unlikely that such a transistor would be put in an IC package. The heat dissipation of the package would come into play before the device reached its limits. This would be uneconomical.

 

[krashkealoha]

how to change beta in pspice

Try playing with these parameters:

VAF - Will change the slope of the collector current. In another words it will change Ic with changing Vce.

RC - It will determine where Vce(sat) will occur. Increasing RC will increase Vce(sat) and lowering RC will lower Vce(sat). Vce(sat) is the "knee" of the collector current, which is where collector current goes from a linear characteristic to a saturated curve (no change in Ic with increasing Vce, except if VA is low).

IS - Will get you close to the appropriate Ic value for a given Ib.

BETA - Will vary the spacing between each Ib curve.

 

[viren]

gummel poon parameter spice

I have read a book on Modeling of semiconductor devices using Pspice. I changed the ISE and NE parameters as there is drop in BF at low currents. But I don't know how to calculate the value of ISE and NE . The book shows a ln(Ic) , ln(Ib) Vs Vbe graph and my graph is as shown in the attachment . I need to extrapolate the region for Ib curve at low current levels so that the value of ISE (i.e C2 ) can be determined. But I am getting a negative value when i extrapolate which is wrong so I need to know if I am doing it right or is there some other way.

 

[krashkealoha]

ideal bjt pspice

Viren,

What are you trying to fit? Your original message has output characteristics curve, but now you are trying to play with parameters for a Gummel curve. NE is just a fitting parameter for Ib at low Vbe to model the increase in base current. It will usually be anywhere between 1.0 (ideal) to 2.0. ISE is the current when Vbe=0 or where Ib intercepts the y-axis when extropolated.

I have attached a document "Gummel-Poon Bipolar Model : Model Description & Parameter Extraction." I downloaded this from the web a long time ago (but I don't remember where), so someone maybe able to find it and provide the link, if they don't want to use any points.

Page 5 has the equations for IB related to ISE and NE.

As Flatulent had said before, modelling a transistor is a "craft" which requires understanding of the measurement and extraction methodology, plus understanding of the device physics of a bjt.

On page 13 of this document, has a recommended procedure to extract bipolar G-P parameters. There are a lot of figures showing how each parameter is measured, which is helpful.

 

[viren]

pspice vce sat

Hi krashkealoha
I cannot open the file so please send me an email with the file
My email is joshiviren@gmail.com

Added after 4 hours 16 minutes:

krashkealoha,
I am trying to fit the DC characteristics of the real BJT which I had attached in my earlier email using a pspice BJT model. I am using the gummel poon model parameters to model the real BJT . I don't know which parameters to change . Since there is drop of gain at low currents I have to figure out how to model it in pspice model of BJt the same loss of gain BF . Since the equation for IB in the book had Ib (nonideal)= C2 Is(0) (e^(q*Vbe/nelkT) -1) I changed the value of C2 and nel to describe the nonideal component of Ib which is dominant at low currents.So let me know if i am going wrong and please refer me to someone who knows about this.

 

[viren]

graph bjt characteristics pspice

I can now open the file. Thanks

Added after 1 hours 47 minutes:

How do I use these equations and do parameter measurements in Pspice .

 

[krashkealoha]

gummel poon spice parameter extraction

Viren,

Do you have measured Gummel curve data (Ic & Ib vs. Vbe)? If you do... Extract IS, BETA, ISE (set NE=2 for now) from that curve. The document that I uploaded will tell you how to do it. Once you have those parameters, you can go your output characteristics data and optimize VAF & RC to fit the Ic vs. Vce curves better.

If you don't have measured Gummel curve data, then pretty much have to change IS until your Ic vs. Vce curves come close to the appropriate Ib, then change BETA to fit the high current beta, then change ISE to fit the low current BETA. This is probably the best you can do without the Gummel curve data.

 

[viren]

changing transistor beta value in pspice

Hi,
My value for NE=4 ISE=3.9e-3 BF=15 IS=39.34e-7
I have got my low current curves for IC fitting with the real Ic Vs Vce curve but when Ichange ISE for large IC then it increases the max IC current to 6 A .

 

[krashkealoha]

gp model for bipolar transistors

Figure out at what collector current the transistor will be used at and make sure you have the best fit for those curves. Your BF parameter seems a little low for a npn device, unless it is a pnp device.

It is normal not to have a good fit at the higher currents because this is due to self-heating effects, which the G-P model doesn't model. This is why more current BJT models such as VBIC, HICUM have become popular because they have improved accuracy at the higher current because they have implemented self-heating, and better accuracy for high-injection regime, plus other improvements.

So make sure you model fits the curves where one would be using the transistor, and don't worry if you can't fit the high current curves because that is one of the drawbacks of the G-P model.