BIASED TRANSISTOR & CURRENTS IN NPN TRANSISTOR

BIASED TRANSISTOR:

The two junctions of a transistor can be biased in four different ways. Accordingly the transistor will operate in four different regions as given below. For normal operation of a BJT it is operated under active region.


Case Emitter Base Junction Collector Base Junction Region of Operation











With a small forward bias applied to the emitter-base junction, and a larger reverse bias applied to the collector-base junction, the transistor works under active region. As we will see, these are the normal operating conditions of this device.
Since we already know how a single-junction device, the diode, behaves, we would normally expect the base voltage to be about 0.65 to 0.7 volt positive with respect to the emitter, and to have electrons move from emitter to base, and leave the device at that point. With the collector junction reverse biased, we would expect no current to flow through that junction.
But an exciting thing happens inside the base region. The forward bias on this junction does indeed attract electrons from the emitter into the base, but there the forward momentum of the electrons carries them across most of the base region and into the depletion region around the collector junction. From there, the higher positive collector voltage attracts these electrons across the collector junction and into the collector region. (Remember that the electrons are minority current carriers within the P-type pase region, and can therefore cross the reverse-biased junction as a leakage current.)
A small amount of current does still leave the device through the base contact, but most of the current is diverted through the collector instead. In this way, the small base bias current controls the much larger collector current. If a small varying current is applied to the base along with the bias, the collector current will vary to a much greater degree. Thus, this device can not only be used to control a varying signal; it can amplify that signal as well.
Because of the way this device operates to transfer current (and its internal resistances) from one conduction path to another, it's name is a combination of the words "transfer" and "resistor:" transistor.
Emitter injection ratio (γ): It is the ratio of the electron current to the total emitter current. It is also called emitter efficiency. Generally, γ = 0.995. this means that only 0.5% of the emitter current consist of hole passing from base to emitter.
Base transportation factor (β/): The ratio of the electron arriving at the collector to the number of emitted electron is called Base transportation factor. β/ also has a value equal to 0.995.


CURRENTS IN NPN TRANSISTOR:


The emitter emits electron to the base and holes from base recombines with electron available in the emitter region. Thus the emitter region is short of electron temporarily and this shortage is immediately made-up by Vee. The –ve terminal of Vee supplies electron to the emitter region. Consider 100 electrons are supplied by –ve terminal of Vee. Then 100 electrons contribute the IE along with the holes in the emitter region. Conventional current flows in a direction opposite to the electron flow. Thus IE comes out of emitter terminal. The majority of these say 98 electrons are injected in to the collector through the base region. Out of the 100 electron emitted suppose that one recombined with hole that diffused from the base region to emitter and the other recombined with hole in the base. So the base region losses only two holes by recombination. The loss of hole is made-up by generation of two fresh holes in the base region near the base terminal. In this process simultaneously two electrons are also generated and they flow out of the base terminal which constitutes the base current IB.
The conventional current in this case is in to the transistor. Ie. IB flows in to the base terminal as shown. The 98 electron reaching collector experiences an attractive force due to the battery Vcc. They travel out of the collector terminal and reaches +ve terminal of Vcc. Conventional current IC flows in to the transistor. The –ve terminal of Vcc gives out as much as electron received from the +ve terminal of Vcc itself. These 98 electron from –ve terminal of Vcc and 2 electron from base terminal combined to make-up 100 electrons which reaches the +ve terminal of Vee. Thus the circuit is completed. Analyzing the direction of current we get, IE = IB + IC.