The bjt is current controlled. The 0.65 volt b-e junction forward DROP does NOT "release" electrons. The emitter is n type silicon (for an npn device), while the base is p type. The p type base has an abundance of free holes, as they are the majority carrier. The n type emitter has an abundance of free electrons, as they are the majority carrier. The 0.65 volt b-e voltage drop is a consequence after the emitter current has transited from emitter, through the base, then arrive at collector due to attraction via collector base E field.
An external current or voltage source, can forward bias the b-e junction. When power is switched on, the b-e barrier voltage is around 25.7 mV at temp of 25 C. As soon as electrons transit through the conductors & arrive at the base & emitter terminals, they easily conduct through the base & emitter silicon material. Although the Vbe is a measly 25.7mV, holes easily move through the p type base, while electrons easily move through the n type emitter.
As the electrons emitted approach the collector, the electric field from base to collector attracts these electrons, as a result, nearly all electrons emitted transit through the base & reach the collector/ One or two electrons out of 10,000 emitted, end up recombining in the base with a hole.
The base emits holes as well. These holes cross the b-e junction & recombine with electrons headed towards the base. If 10,100 electrons enter the emitter terminal, around 99 of those electrons recombine at the edge of the b-e depletion region on the emitter side. The remaining 10,001 electrons continue & enter the base region, where 1 of them recombines in the base region with a hole, & the other 10,000 reach the collector.
The objective in fabricating a bjt is to obtain as much current gain as possible, without incurring undesirable consequences. The device described above has a forward current gain, beta, or "hfe", of 100 in value. For every 10,100 electrons entering the emitter terminal, 100 electrons exit the base terminal, & 10,000 electrons exit the collector terminal. Thus collector current is 100 times base current, for a beta of 100. Collector current is 100/101 of emitter current. This ratio is "alpha". It is desirable that alpha be as close to unity as possible.
If the base is wide, 2 undesirable things happen. First, the number of holes emitted by the base is large. An external power source (microphone, antenna, battery, photodiode, etc.) biases the b-e junction plus connected devices (resistors). The b-e junction electric field imparts motion to free electrons in emitter, & free holes in base. A large volume of base material puts more holes into conduction. The volume equals the base length times width times height. By making the width very narrow, we get a small volume of free holes being yanked into conduction by the external source E field. Also, light doping is used in the base. Heavy emitter doping puts lots of electrons in conduction towards the base & collector. Light base doping puts few holes in conduction towards the emitter.
Also, the emitted electrons from the emitter incur fewer recombinations with a narrow base region. If the electrons must travel further due to a thicker base, more recombinations occur. Both of these reduce current gain. Current from base to emitter in the form of holes is called "injection current". Current in the form of electrons recombining in the base is called "transport current". These are 2 components of the overall "base current", the injection component being much larger. To maximize current gain, we must lightly dope the base, & make it thin.
For a bjt in the common emitter or common base configuration, this base current does not power the load (load is in the collector side), so it is often regarded as "lost", a quantity that needs to be minimized. So a bjt is fabricated with base current minimization in mind, to preserve high current gain. But it is also desirable to prevent "punch through". The base-collector voltage breakdown rating needs to be sufficient for the device in the application used. If the supply can reach 30 volts, the bjt must not break down at or below that value. Some devices must withstand hundreds of volts Vce. In order to insure no break down, we need a thicker base, and/or heavier doping of acceptor atoms.
But both of these decrease current gain, which is the price paid. Examining data sheets will reveal that the higher the collector-emitter breakdown voltage, the lower the current gain. It is a classic tradeoff.
One more thing, with emitter follower configuration, the load is on the emitter side. So base current actually powers the load, & is not "lost". But, if the base is driven by a signal source with very limited current capability, it is still desirable to have high current gain. In some cases, that may not be the case. A linear voltage regulator powers the base & collector terminals from the input supply, so that high base current du to low beta is not an issue. Again, if the base is driven by a high impedance source with small current capability, high beta is needed.
Feedback/comments/questions welcome. Best regards.
Claude Abraham
