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Bipolar Junction Transistors (BJT)

By placing two PN junctions together we can create a bipolar junction transistor. In a PNP transistor the majority charger carriers are holes and typically germanium is favored for these devices. In NPN transistors the majority charger carriers are electrons; for this case, silicon is typically used.

PNP schematic symbol and (b) physical layout
Figure 1:  (a) PNP schematic symbol and (b) physical layout

NPN schematic symbol and physical layout
Figure 2: (a) NPN schematic symbol and (b) physical layout

The thin and lightly doped central region is known as the base (B) and has the majority charge carriers of opposite polarity to those in the surrounding material. The two outer regions are known as the emitter an (E) and the collector (C). Under the proper operating conditions the emitter will emit or inject majority charge carriers into the base region, and because the base is very thin, most will ultimately reach the collector. The emitter is highly doped to reduce resistance. The collector is lightly doped to reduce the junction capacitance of the collector-base junction. The emitter-base junction is forward biased while the collector-base junction is reversed biased. The resistance emitter-base junction is very small as compared to collector-base junction. Therefore, the forward bias applied to the emitter-base junction is generally small whereas reverse bias on the collector-base junction is much higher.

Related: The Basics of Semiconductor Physics as the Foundation of Electronics

The Transistor Operation (NPN)

The figure below shows the NPN transistor with forward bias to emitter-base junction and reverse bias to collector-base junction.

Basic connection of NPN transistor
Figure 3: Basic connection of NPN transistor

The forward bias causes the electrons in the n-type emitter to flow towards the base. This constitutes the emitter current IE. As these electrons flow through the p-type base, they tend to combine with the holes. As the base is lightly doped and very thin, therefore, only a few electrons (less than 5%) combine with the holes to constitute base current IB. In other words, the electrons which combine with holes become valence electrons. Then as valence electrons, they flow down through holes and into the external base lead. This constitutes base current IB. The remainder (i.e. more than 95%) cross over into the collector region to constitute collector current IC. In this way, almost the entire emitter current flows in the collector circuit. Hence the emitter current is the sum of collector and base currents:

IE = IB + IC

Most of the electrons from emitter continue their journey through the base to collector to form collector current because:

  • The base is lightly doped and very thin; therefore, there are few holes which find enough time combine with electrons.
  • The reverse bias on collector is quite high and exerts attractive forces on these electrons.

Transistor Operation (PNP)

The figure below shows the basic connection of a PNP transistor.

Basic connection of PNP transistor
Figure 4: Basic connection of PNP transistor

The forward bias causes the holes in the p-type emitter to flow towards the base. This constitutes the emitter current IE.

As these holes cross into the n-type base, they tend to combine with the electrons. As the base is lightly doped and very thin, hence, only a few holes (less than 5%) combine with the electrons. The remainder (more than 95%) cross into the collector region to constitute the collector current IC. As a result, almost the entire emitter current flows in the collector circuit. You may note that, current conduction within PNP transistor is by holes. However, in the external connecting wires the current is still by electrons.

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Related: Semiconductor Diodes

Bottom Line

The input circuit (emitter-base junction) has low resistance because of forward bias whereas the output circuit (collector-base junction) has high resistance due to reverse bias.

Conventional currents - Transistor
Figure 4: Conventional currents

From our discussion above, the input emitter current almost entirely flows in the collector circuit. Therefore, a transistor transfers the input signal current from a low resistance circuit to a high resistance circuit. This is an important factor responsible for the amplifying capability of the transistor

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8 responses to “Bipolar Junction Transistors (BJT)”

  1. […] You can also read: Bipolar Junction Transistors (BJT) […]

  2. […] NPN transistor is used to provide the desired current to the relay to the relay coil as the microcontroller cannot […]

  3. […] ICs are usually fabricated using either bipolar devices or MOS devices or a combination of the two. We can classify the logic families into the following […]

  4. […] transistor transistor logic devices and CMOS for complementary metal-oxide semiconductor devices. Bipolar junction transistors are the building blocks for TTL logic, whereas MOSFETs are the building blocks for CMOS logic. […]

  5. […] circuit that works in the same way as a TTL open collector output. It can be considered as an NPN bipolar transistor with a grounded emitter and an uncommitted collector as illustrated in the figure […]

  6. […] device. It has high input impedance like a MOSFET and low on-state conduction losses like a BJT. In other words, this power device combines the voltage control of the MOSFET gate with the […]

  7. […] involving the gigahertz bandwidths cannot employ conventional transistor or integrated circuit (IC) design because silicon devices suffer from stray capacitance between […]

  8. […] the development of the transistor (a semiconductor device), the electronic circuits became considerable reduced in size. This was […]

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