# Dependence of transistor current gain on operating conditions

According to "Gray & Meyer: Analysis and Design of Analog integrated circuits" transistor current gain Beta (in forward-active region) depend on operating conditions of the transistor.

• Region 1 is the low-current region, where β decreases as Ic decreases,

• Region 2 is the midcurrent region, where β is approximately constant,

• Region 3 is the high-current region, where β decreases as Ic increases.

While ideally β is always constant.

My questions:

• Why does this "effect" occurs?
• If collector current increases/decreases then base current also increases/decreases for proportional amount, right? Then β should be always constant
• Is this β dependence on operating conditions significant when talking about small or large signal model?
• β being constant is an approximation, and as you've noticed, not necessarily a very good one either. Well-designed circuits should not depend on transistor β, however; even if it was constant there is significant variation between transistors as well, because it's not easily controlled. (or maybe they just don't bother to control it because it doesn't matter that much? I'm honestly not sure.) May 16, 2017 at 16:29
• Hfe is never truly constant and hFE is always less than Hfe. May 16, 2017 at 16:30
• Ad1 - For example the Early effect. AD2 - no. AD3 - We almost never include β variations in any type analysis except the temperature "effect". But we do not care much about this because we use a negative feedback to stabilize the operating point.
– G36
May 16, 2017 at 16:57
• @G36: So variation of beta with operating current isn't significant in any analysis? Especially, if negative feedback is applied to the circuit? May 16, 2017 at 17:12

"On the Variation of Junction-Transistor Current-Amplification Factor with Emitter Current," by W. M. Webster, Proc. IRE, Vol. 42, pp. 914-920, June 1954, is the seminal paper on this topic.

I've written about these effects before, here. But not directly, just as an aside. These were later (after the above paper) discussed in literature from the 1960's and 1970's typically as "Region I," "Region II," and "Region III." Region I is pretty much guided by additional factors related primarily to the base current and Region III results from changes in the collector current.

In Region I, the decline in $$\\beta\$$ is due to three components which can be ignored in the other regions but cannot be ignored with low currents involved. These are:

1. The formation of emitter-base surface channels (which can be reduced by the careful application of processing/manufacturing); and,
2. the recombination of surface carriers (which also can be reduced by the careful application of processing/manufacturing, but still remains a dominant part of the problem); and,
3. the recombination of carriers in the emitter-base space-charge layer.

All three of these have similar variations with the base-emitter voltage so that you wind up with something akin to the following typical component equations:

\begin{align*} I_{B_{channel}}&=I_{SAT_{channel}}\cdot\left(e^\frac{V_{BE}}{4\cdot V_T}-1\right)\\\\ I_{B_{surface}}&=I_{SAT_{surface}}\cdot\left(e^\frac{V_{BE}}{2\cdot V_T}-1\right)\\\\ I_{B_{space-charge}}&=I_{SAT_{space-charge}}\cdot\left(e^\frac{V_{BE}}{2\cdot V_T}-1\right) \end{align*}

Although summed exponentials are not exactly equivalent to any single resulting equivalent exponential, it is practical (and done) to combine the above into a single modeled exponential that uses $$\\eta_{_\text{EL}}\$$ values often close to 2:

\begin{align*} I_{B_{summed}}&=I_{SAT_{summed}}\cdot\left(e^\frac{V_{BE}}{\eta_{_\text{EL}}\cdot V_T}-1\right) \end{align*}

For most BJTs, the above equation can be made to approximate the reality well enough for practical purposes (and it sums into the usual current equations.)

In Region III, the injection of minority carriers into the base region starts becoming increasingly important in comparison against the majority carrier concentrations. Because the space-charge neutrality is maintained in the base, the majority concentration has to increase by the same amount.

The finding is:

\begin{align*} I_{C_{high-I_C}}&\propto e^\frac{V_{BE}}{2\cdot V_T} \end{align*}

The other factor in Region III is, of course, an 'Ohmic resistance' and is already modeled as $$\r_c\$$ so it isn't included above.

A model constant is usually applied to the above equation and the resulting term then appears in the divisor used for the usual model saturation current equation.

The upshot for Region III is:

1. The increasing importance of the injected minority carriers into the base; and,
2. Ohmic resistance.

A useful summary of the situation is taken from "Modeling the Bipolar Transistor," by Ian Getreu. (Which, although old today, can still be found at lulu.com as a September 2009 reprint of a first edition that was printed three times: March and August of 1976, and again in November of 1979 [when I received my first copy of it while working at Tektronix.])

Footnotes: Chapter on Spice BJT Modeling and Lawrence Nagel's thesis: Spice2: A Computer Program to Simulate Semiconductor Circuits and Mextram -- the most exquisite transistor model.

• Isat reffers to which current exactly? May 16, 2017 at 17:37
• @Keno I'm not sure which you mean. There are different ones mentioned. But in any case for a detailed understanding, I'd refer you to the paper I mention at the outset. Please also refer to the book mentioned at the end. Both the seminal paper and the later book address themselves to modeling questions. I want to avoid duplicating a complete and long treatise on the topic of modeling and instead focus on the identified physical explanations with only a nod towards how they are modeled.
– jonk
May 16, 2017 at 17:44
• You mentioned that Ib(summed) equation can be used to calculate practical values. That Isat(summed), is it given in data sheets (since you said "for practical purposes")? May 16, 2017 at 17:51
• @Keno Yes, I said that. It's just a description of the behavior of these three particular effects upon the BJT, but not a description of the whole BJT. To explain, in detail, how this is then incorporated into models (and there are many) would require... a book. See my response here: electronics.stackexchange.com/questions/252197/… That shows 3 equivalent but simple models. Now, go count the number of saturation currents listed there and tell me how they relate to each other and you will see my quandary in trying to answer you.
– jonk
May 16, 2017 at 17:58

The main reason current gain is never constant is because it is actually Vbe controlled Ic current on a log scale and thus current gain is nonlinear in 3 regions.

• It is specified in Region II unless curves are supplied, which are becoming less common in datasheets.

Thus hFE (DC gain) and Hfe (AC gain or incremental small signal) are approximations.

• DC current gain hFE spans a 3:1 range in Region II in production unless sorted into bins. So because of this we choose feedback ratios for Rc/Re or Rf/Rin to control voltage gain, due to wide spread of current gain.

Early Effect for recombination affects Region I

Saturation Effect reduces hFE to about 10% at Vce(sat) near rated current.

Saturation also rises with Ic but generally seen for Vce<2V where linear sinusoidal gain begins to start asymmetric peaks or %THD from reduction in current gain.