The base internal current is mainly by diffusion (see The unapproximated Ebers–Moll equations used to describe the three currents in any operating region are given below. The current flowing by the remaining electrons is known as the collector current IThe circuit of the NPN transistor is as shown in the below figure.The voltage sources are connected to the NPN transistor as shown in the above figure. Networks of transistors are used to make powerful amplifiers with many different applications. For the CE topology, an approximate h-parameter model is commonly used which further simplifies the circuit analysis. The BJT also makes a good amplifier, since it can multiply a weak input signal to about 100 times its original strength. Beta is a convenient figure of merit to describe the performance of a bipolar transistor, but is not a fundamental physical property of the device. The emitter is heavily doped, while the collector is lightly doped, allowing a large reverse bias voltage to be applied before the collector–base junction breaks down. And it will not allow the current to flow through the device.In the saturation mode of a transistor, both junctions are connected in forward bias. Hence, it behaves as an OFF switch.The circuit diagram of a transistor in the cut-off region is as shown in the below figure.The transistor has three leads; collector (C), Emitter (E), and Base (B). The collector–base junction is reverse biased in normal operation.
In this condition, the input voltage or the base voltage is zero.Therefore, the entire Vcc voltage appears across the collector.
The diagram shows a schematic representation of an NPN transistor connected to two voltage sources. An NPN (Negative-Positive-Negative) type and a PNP (Positive-Negative-Positive) type.. The collector is connected with the positive terminal of supply voltage VThe base terminal is connected to the positive terminal of base supply voltage VWhen the transistor is switched ON, a large collector current flows through the device between the collector and the emitter terminals. Because the transistor's internal structure is usually optimized for forward-mode operation, interchanging the collector and the emitter makes the values of α and β in reverse operation much smaller than those in forward operation; often the α of the reverse mode is lower than 0.5. The bipolar junction transistor, unlike other transistors, is usually not a symmetrical device. This means that interchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in reverse mode.
This applied voltage causes the lower P-N junction to become forward biased, allowing a flow of electrons from the emitter into the base. The base is a P-type semiconductor. In the NPN transistor in what is called active mode, the base–emitter voltage The DC emitter and collector currents in active mode are well modeled by an approximation to the Ebers–Moll model: Limiting values Table 5. In this mode, the transistor behaves as an open circuit.
Hence, most of the electrons will pass the base region and few of them will recombine with the holes.Because of the recombination, the current will flow through the circuit and this current is known as base current IMost of the electrons pass the depletion region of a collector-base junction and pass through the collector region. So, it has a few holes to recombine with the electrons.
For DC conditions they are specified in upper-case. In the discussion below, focus is on the NPN bipolar transistor. NPN double transistor in an SC-88 (SOT363) plastic six lead package. It passes most of all charge carriers to the collector which is emitted by the emitter.The collector is moderately doped and collects charge carriers from the base layer.The symbol of the NPN transistor is as shown in the below figure.
The 2N2222 transistor is subdivided into one group according to its DC current gain.
The value of the current flowing in this circuit is,When both junctions are connected in reverse bias, the transistor behaves as an open circuit or OFF switch. When the base–collector voltage reaches a certain (device-specific) value, the base–collector depletion region boundary meets the base–emitter depletion region boundary.
The heavy doping of the emitter region and light doping of the base region causes many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter.