NPN Transistor: What is it? (Symbol & Working Principle)

What is a NPN Transistor

What is a NPN Transistor

An NPN transistor is the most commonly used bipolar junction transistor, and is constructed by sandwiching a P-type semiconductor between two N-type semiconductors. An NPN transistor has three terminals– a collector, emitter and base. The NPN transistor behaves like two PN junctions diodes connected back to back.

These back to back PN junction diodes are known as the collector-base junction and base-emitter junction.

Regarding the three terminals of the NPN transistor, the Emitter is a region is used to supply charge carriers to the Collector via the Base region. The Collector region collects most of all charge carriers emitted from the Emitter. The Base region triggers and controls the amount of current flows through the Emitter to Collector.

The equivalent circuit of an NPN transistor is as shown in the figure below.

Equivalent Circuit of NPN Transistor
Equivalent Circuit of an NPN Transistor

As a quick reminder, an N-type semiconductor is one where a large number of free electrons are available and it acts as a majority charge carrier. Under the influence of potential difference, electrons get sufficient energy and move from the valence band to the conduction band. Because of the movement of electrons, the current will flow through the N-type semiconductor.

Conversely, in P-type semiconductors, electrons are not available and hole acts as a majority charge carrier. Because of the movement of holes current will flow through the P-type semiconductor.

Construction of NPN Transistor

As discussed above, the NPN transistor has two junctions and three terminals. The construction of the NPN transistor is as shown in the below figure.

Construction of NPN Transistor
Construction of NPN Transistor

The emitter and collector layers are wider compared to the base. The emitter is heavily doped. Therefore, it can inject a large number of charge carriers to the base.

The base is lightly doped and very thin compared to the other two regions. 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.

NPN Transistor Symbol

The symbol of the NPN transistor is as shown in the below figure. The arrowhead shows the conventional direction of Collector current (IC), Base current (IB) and Emitter current (IE).

Symbol of NPN Transistor
Symbol of NPN Transistor

How Does an NPN Transistor Work

The base-emitter junction is connected in the forward bias condition by supply voltage VEE. And the collector-base junction is connected in the reverse bias condition by supply voltage VCC.

In forward bias condition, the negative terminal of supply source (VEE) is connected to the N-type semiconductor (Emitter). Similarly, in a reverse bias condition, the positive terminal of the supply source (VCC) is connected to the N-type semiconductor (Collector).

Working of NPN Transistor
Working of NPN Transistor

The depletion region of the emitter-base region is thin compared to the depletion region of the collector-base junction (Note that the depletion region is a region where no mobile charge carriers are present and it behaves like a barrier that opposes the flow of the current).

In N-type emitter, the majority charge carrier is electrons. Therefore, electrons start flowing from N-type emitter to a P-type base. And because of electrons, the current will start flowing the emitter-base junction. This current is known as emitter current IE.

These electrons move further to the base. The base is a P-type semiconductor. Therefore, it has holes. But the base region is very thin and lightly doped. So, it has a few holes to recombine with the electrons. 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 IB. The base current is very small compared to the emitter current. Typically, it is 2-5% of the total emitter current.

Most of the electrons pass the depletion region of a collector-base junction and pass through the collector region. The current flowing by the remaining electrons is known as the collector current IC. The collector current is large compared to the base current.

NPN Transistor Circuit

The circuit of the NPN transistor is as shown in the below figure.

NPN Transistor Circuit
NPN Transistor Circuit

The voltage sources are connected to the NPN transistor as shown in the above figure. The collector is connected with the positive terminal of supply voltage VCC with the load resistance RL. The load resistance also used to reduce the maximum current flowing through the device.

The base terminal is connected to the positive terminal of base supply voltage VB with base resistance RB. The base resistance is used to limit the maximum base current.

When the transistor is switched ON, a large collector current flows through the device between the collector and the emitter terminals. But, for that small amount of base current must flowing into the base terminal of the transistor.

By KCL, the emitter current is the addition of base current and collector current.

    \[ I_E = I_B + I_C \]

Operating Mode of Transistor

The transistor operates on different modes or regions depends on the biasing of junctions. It has three modes of operation.

  • Cut-off mode
  • Saturation mode
  • Active mode

Cut-off Mode

In cur-off mode, both junctions are in reverse bias. In this mode, the transistor behaves as an open circuit. And it will not allow the current to flow through the device.

Saturation Mode

In the saturation mode of a transistor, both junctions are connected in forward bias. The transistor behaves as a close circuit and current flow from collector to emitter when the base-emitter voltage is high.

Active Mode

In this mode of a transistor, the base-emitter junction is forward bias and collector-base junction is reverse biased. In this mode, the transistor operates as a current amplifier.

The current flows between emitter and collector and the amount of current are proportional to the base current.

Mode of Operation of Transistor
Mode of Operation of Transistor

NPN Transistor Switch

The transistor operates as switched ON in saturation mode and switched OFF in cut-off mode.

When both junctions are connected in the forward bias condition and sufficient voltage is given to input voltage. In this condition, collector-emitter voltage is near to zero and the transistor operates as a short circuit.

In this condition, the current will start flowing between collector and emitter. The value of the current flowing in this circuit is,

    \[ I_C = \frac{V_C_C}{R_C} \]

Saturation Mode of Transistor
Saturation Mode of Transistor

When both junctions are connected in reverse bias, the transistor behaves as an open circuit or OFF switch. In this condition, the input voltage or the base voltage is zero.

Therefore, the entire Vcc voltage appears across the collector. But, because of the reverse bias of the collector-emitter region, the current cannot flow through the device. 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.

Cut-off Mode of Transistor
Cut-off Mode of Transistor

NPN Transistor Pinout

The transistor has three leads; collector (C), Emitter (E), and Base (B). In most of the configurations, middle lead is for Base.

To identify the emitter and collector pin, there is a dot on the surface of the SMD transistor. The pin which is exactly below this dot is a collector and the remaining pin is the emitter pin.

If the dot is not present, all pins will be placed with uneven space. Here middle pin is base. The closest pin of a middle pin is emitter and the remaining pin is a collector pin.

NPN vs PNP Transistor

The main differences when comparing NPN transistors vs PNP transistors have been summarized in the table below:

NPN Transistor PNP Transistor
StructureIt has two N-type and one P-type semiconductor.It has one N-type and two P-type semiconductors.
Direction of currentThe current will flow through the collector to the emitter.The current will flow through the emitter to the collector.
Majority charge carrierElectronHoles
Minority charge carrierHolesElectrons
Switching timeFasterSlower
Junction biasingEmitter-base junction is in forward bias and collector-base junction is in reverse bias.Emitter-base junction is in reverse bias and collector-base junction is in forward bias.
Symbol   NPN Transistor Symbol   PNP Transistor Symbol
Collector-emitter voltagePositiveNegative
Emitter arrowPointed outPointed in

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