Beta of BJT in Hspice!??

hspice beta lv10 bjt

Hi all,

I've just tried to run few simple BJT circuits, fixed L, W and Temperature for all the circuits. However when I see the output of the DC , I got the different value of Beta, anyone knows what's the problems?

Anyone knows how to ".print" the Beta?

Thanks in advance.

Regards,

if beta increases will ic decrease

Hi all,

No body can help me!?

FYI, i'm using IBM's process. In theory, Beta should be constant with same process and Area.

Why am I getting the different value of Beta from different circuit!?

Really hope someone can shed me some light........

THanks in advance

how does beta of bjt change with temperature

Hi smart

As I know,when Ic is different,the value of Beta will change lightly.Have you make sure that the Ic is exactly same?

flyankh

bjt beta

flyankh said:

Hi smart

As I know,when Ic is different,the value of Beta will change lightly.Have you make sure that the Ic is exactly same?

flyankh

Click to expand...

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Hi flyankh,

Thanks for reply.

In the process manual it stated that if L=0.44u, W=3.0u, T=25"c, the Beta should be 100. But in some circuit, i got Beta=15. It's huge different.

Furthermore, if Beta is not constant with Process, it's impossible for me to know the IC-IB relationship.

Back to your suggestion, I have tried 2 circuit with almost same IC, but one i get Beta=15 the other i got Beta=320, any idea what's wrong!?

thanks in advance

Hi smart

What's the parameter "L" "W" stand for?In Hspice I always can see the parameter "area",it is the area of emitter.

flyankh

To print the Beta value of Q1 with hspice, use the following statement in hspice netlist file:
.PRINT LV10(Q1)

flyankh said:

Hi smart

What's the parameter "L" "W" stand for?In Hspice I always can see the parameter "area",it is the area of emitter.

flyankh

Click to expand...

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Hi flyankh,

L = Emitter Length, W= Emitter Width. According to the manual, Effective Area = (L-0.02) (W-0.02).

In your simulation, do you get different Beta with fixed Area and Temperature in different cicuits? I got different Beta value when I vary the RB or RC value, why is it like that!??

Thanks in advance.

Hi smart

Can you post the circuit you have simulated?Your simulation files is needed too

flyankh

Hi all,

I noticed that not only Beta will vary from the different circuit, VBE will be different too..why!??

FYI, I tried to change RB value in a circuit, what i get is the Beta will keep changing.....

For same Emitter Area, apply to different circuit, the Beta can be huge diffence, like one 10 the other 300, why!?


Thnks in advnce

Are you sure that your BJT is always biased in the active region (not in saturation) when beta decreases ?

Hi Smart,

Beta will change with the value of Ic, the higher the current, the smaller the beta. Also, if you put the transistor in saturation, it can have small beta. At high Ic, the drop across the internal Rc can put the transistor in saturation.
You can do the following simulation: connect the transistor with base shorted to the collector and emitter connected to ground. Connect a current sourse between the common base-collector node and Vcc. Do a DC simulation sweeping the value of the current source - let's say from 10uA to 10mA (the higher value depends on the size of the transistor). Plot the ratio Ic/Ib. That should give you an idea about the variation of beta with Ic and the transistor will operate in active region - well for most of the simulation. Beta will depend also on the area of the emitter.
If you have high frequency operation, betta will drop for high frequencies, too.

look the pic

sunking said:

look the pic

Click to expand...

I think the trend of this curve is correct. at the beginning ,the Beta of BJT is limited by the surface impurity, when the current goes higher and higher, the surface impurity's effect can be ignored. And the Beta increasing.

At some point , however, the current become so large that it can make Beta decrease. But i am NOT sure that the BETA will decrease so fast ?

cheer
Jax

It is logarithm coordinate (Ic is log)

Hi, Smart.

The beta of a BJT is meaningful in the active region. And, I think the beta is dependent on three factors. One is the collector current. At the relatively higher Ic, the charge flowing to the collector affects the effective base width. So, the beta will be decreased. At the relatively lower Ic, the recombination effect in the base region becomes apparent, thus, the beta will be also decreased. Another factor is the Early Effect. When the voltage applied to the collector-base junction is increased, the depletion layer of that junction is increased, which causes to reduce the effective base width and the beta will be increased. The final factor is the avalanche effect. When the collector-base voltage is very high, the momentum of an electron passing that junction is very high, thus, when this high-momentum electron collides with an atom, the electrons in the atom will be scattered out of the atom, which will cause the collector current to increase and the beta will be increased.

Finally, the emitter-base voltage and the emitter current obeys the diode current-voltage equation.

Bye~~

u can find its information in hspice manual ,like .probe LX(?)