Why the direction of current is right to left in common base configuration(NPN transistor)?
Answers
Answer:
We know that in transistors and diodes, electric current is carried by both free electrons and holes. Free electrons and holes travel in opposite directions. For example, if free electrons flow from left to right then the holes will flow from right to left.
However, the direction of holes is assumed as same as the direction of the current.
When Benjamin Franklin started doing experiments on electricity, he believed that something is moving through the electrical wires. He named these moving things as charge. At that time, people did not know about electrons and protons.
Franklin assumed that there was only one type of charge and this charge will always flow from higher concentration region (excess charge carriers) to the lower concentration region (fewer charge carriers). He called the higher concentration region as positive and the lower concentration region as negative.
So according to Franklin, the charge always flows from positive to negative. We know that electric current means the flow of charge. So the electric current direction is from positive to negative.
However, after the discovery of electrons and protons, scientists realized that the conventional current direction is wrong. The electric current is actually carried by the free electrons which flow from negative to positive. Thus, the actual current direction is from negative to positive. However, in honor of Franklin’s discovery, we still following his conventional current direction (I.e. from positive to negative).
The conventional current direction of npn and pnp transistor is shown in the below figure.
Conventional current direction in pnp transistor
Consider the pnp transistor as shown in the below figure. The emitter-base junction (p-n junction) or emitter junction is forward biased and the base-collector junction (n-p junction) or collector junction is reverse biased.
The emitter current (IE) direction which is represented by an arrow shows that the emitter current is flowing into the transistor. On the other hand, the base current (IB) and collector current (IC) are flowing outwards the transistor.
Conventional current direction in npn transistor
Consider the npn transistor as shown in the below figure. The emitter-base junction (n-p junction) is forward biased and the base-collector junction (p-n junction) is reverse biased.
The emitter current IE direction which is represented by an arrow shows that the emitter current is flowing outwards the transistor. On the other hand, the base current IB and collector current IC are flowing into the transistor.
Transistor current components
The various current components in the pnp transistor which flow across the forward biased emitter junction JE and the reverse biased collector junction JC are shown in the below figure.
The emitter current IE consists of hole current IpE (majority carriers holes crossing from emitter into base) and electron current InE (majority carriers electrons crossing from base into emitter).
Therefore the total emitter current IE is the sum of hole current IpE and electron current InE
IE = IpE + InE
The ratio of hole to electron currents, IpE / InE, crossing the emitter junction is proportional to the ratio of the conductivity of the p material to that of the n material. We know that conductivity is directly proportional to the doping level. If doping level is more, conductivity is more similarly if doping level is less, conductivity is less.
In a commercial transistor, the doping of the emitter is made much larger than the doping of the base. Hence, in a pnp transistor the emitter current consists almost entirely of holes.
The holes crossing the emitter junction JE and reaching the collector junction JC constitutes hole current IpC in collector.
Not all the holes crossing the emitter junction JE reaches the collector junction JC, because some of them combine with the electrons in the n-type base.
We know that base is very thin and lightly doped. So only a small number of holes combine with the electrons in the n-type base, constituting the base current IPE - IPC. The remaining large number of holes cross the base region and enters into the collector region, constituting the hole current IpC in collector region.
Explanation:
Explanation:
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