Bjt configuration

In common base configuration,we know that alpha is almost 1 which means that the base current is almost zero.But what is the physics behind this base current almost equal to 0?And similarly in common emitter the base current is more than that in common base configuration,but why?I mean i want to know the underlying physics behind these concepts.

• Consider to spell out acronyms. – Qmechanic Sep 20 '18 at 7:57
• In a common base circuit, the base is connected to ground. You aren't injecting current into the base, you are not manipulating the minority carrier population there (which controls transport across the base), so basically all the emitter current goes to the collector and the base isn't adding anything. – Jon Custer Sep 20 '18 at 13:43
• @JonCuster $\alpha=\frac {\beta} {\beta+1}$ – V.F. Sep 20 '18 at 14:21
• Is this the reason why early effect in CE is more pronounced than that in CB? – sourav guha Sep 20 '18 at 16:27
• @V.F. - I'm well aware of how $\alpha$ and $\beta$ are related. And they are related through minority carriers in the base which controls how bipolar junction transistors work. I might suggest starting with Andy Grove's book on Physics and Technology of Semiconductor Devices (or an equivalent). – Jon Custer Sep 20 '18 at 19:20

The physics behind the operation of a bipolar transistor is pretty complicated, so I won't even attempt to give you a detailed explanation, but here is a possible, simplified, way to think about it.

In either configuration, common emitter or common base, the base current is a small fraction of the emitter or collector current.

The base-emitter junction is forward biased, so the current can flow between the emitter and the base. The base-collector junction is reverse biased, so the current cannot flow between the collector and the base.

The reason the current flows between the emitter and the collector is because the collector intercepts charges passing from the emitter to the base.

Let's consider a simplified example of an npn transistor.

When a sufficient forward voltage is applied to the base-emitter junction, the conduction band electrons will flow from the emitter (n-type) to the base (p-type), but as they proceed toward the base terminal, they get into a strong electric field across the reverse biased base-collector junction and get pulled toward the collector.

On their way, some electrons will recombine with the holes of the base and won't be able to get to the collector: they will make it all the way to the base terminal, contributing to the base current. In order to reduce the percentage of electrons recombining with the holes (or in order to increase $$\beta$$, the ratio between the collector and base currents), the base of the transistor is made very narrow, so that the electrons flowing toward the base terminal are getting very close to the base-collector junction and are likely to be swept away by the base-collector field before they recombine with holes.

In summary, high $$\beta$$ is achieved due to a special geometry of the base (as well as appropriate doping levels), designed to minimize the recombination rate in the base region and thus to maximize the percentage of electrons crossing the base-collector junction and contributing to the collector current.

• So now can anyone tell me then why the effect of Early effect is more pronounced in CE mode than CB? – sourav guha Sep 22 '18 at 15:59
• @souravguha If you look at CE output characteristics, they have a noticeable slope, which says that the collector current grows a little with the collector voltage, which is an indication of Early effect. This is because, for each curve, we maintain the base current, so, when the base shrinks due to increased Vbc and the beta increases, the collector current increases as well. – V.F. Sep 22 '18 at 21:14
• @souravguha When you look at CB characteristics, there is not much of a slope, which indicates less of Early effect. This is because, for each curve, we maintain emitter current, so when Vbc increases and the beta increases, we do not allow the emitter current to grow, so collector current does not grow either. If we did, the Early effect would be similar to CE configuration. – V.F. Sep 22 '18 at 21:16
• Can i look at the reason in this way?we know beta increases with increase of collector current..but alpha does not increase much even if beta drastically increases..so CE suffers more from Early Effect..is this logic ok? – sourav guha Sep 23 '18 at 3:05
• @souravguha "...CE suffers more from Early Effect..." I suppose we can say that - under the regime, when the emitter current is held constant. Fundamentally, though, the two configurations are similar, since in both the increase of the collector voltage has similar effect on the width of the base. – V.F. Sep 23 '18 at 11:47