The Introduction of The BJT Transistor

A bipolar junction transistor (BJT) is a type of transistor used for amplification and/or switching purposes. It consists of three layers of semiconductor material, namely the emitter, base, and collector. The BJT operates by controlling the flow of current between the emitter and collector terminals, which is regulated by the current flowing into the base terminal. 

There are two main types of BJT transistors: NPN (negative-positive-negative) and PNP (positive-negative-positive). The primary difference between NPN and PNP transistors lies in how power is allocated to their terminal pins for them to provide amplification or switching. 

What are PNP and NPN Transistors?

Transistors are created by mixing two different types of semiconductors: n-type and p-type. N-type semiconductors carry electron donor atoms. The electron acceptor atoms are carried by p-type semiconductors (holes).

NPN Transistor

NPN transistors are made of p-type semiconductor material with low doping levels. The emitter is doped with much higher donor impurities than the collector, while the collector is doped with much lower levels than the emitter.

Electrons have a higher mobility than holes and are the main charge carriers of the NPN type. Therefore, the response time of NPN transistors is faster than that of PNP transistors. Therefore, NPN-type transistors are the most commonly used in high-frequency related devices, and they are simpler to manufacture than PNP-type transistors, making them the more commonly used of the two types.

PNP Transistor

PNP transistors are made of n-type semiconductor material with low donor impurity doping concentration. The emitter is doped with a higher impurity concentration than the collector, and the collector is doped with a lower impurity concentration than the emitter.

NPN and PNP Transistors

The BE junction is forward-biased by applying a lower potential to the base, while the BC junction is reverse-biased by applying a considerably lower voltage to the collector. PNP transistors can be used as switches or amplifiers in this form.

PNP and NPN transistors behave similarly when used in circuits. However, the polarity of the voltage source connections and the direction of current flow are different. In most cases, NPN transistors can be replaced with PNP transistors and vice versa, but the supply polarity must be changed.

PNP And NPN Transistors Differ in What Ways?

NPN and PNP transistors differ in their structure, operation, and applications. One of the significant differences is that in NPN transistors, when a positive supply is applied to the base, the current flows from the collector to the emitter, whereas in a PNP transistor, when a negative supply is applied to the base, the charge carriers flow from the emitter to the collector. The following table shows a comparison of the two BJTs under different parameters.

NPN VS PNP: C-connection Differences for Sensors

Transistor Effect

Transistors are semiconductors that initialize micro-relays and are used in discrete device on/off sensor applications in transistor electronics. They amplify very small signals, such as the position-sensing component of a proximity switch, in order to turn a larger signal on or off. This increased signal can be sent to a DI point, current, or any other device with the appropriate current rating. Transistors are divided into two types: PNP (source) and NPN (sink).

Switching Using a Transistor 

Transistors are commonly used in two types of applications: switching type operation and signal amplification. Solid-state switches can be created using transistors. The transistor acts as a closing switch when operating in the saturation region and as an off switch when operating in the cutoff region. Both PNP and NPN transistors can be used as switches. A transistor conducts current in the collector-emitter path only when a voltage is applied to the base. When no base voltage is present, the switch is closed. When the reference voltage is present, the switch opens.

NPN Transistor Switching Circuit

To use an NPN transistor as a switch, the voltage applied to the base terminal is varied. When enough voltage is applied between the base and emitter (usually greater than 0.7V), the collector-to-emitter voltage is approximately equal to 0, which means the transistor acts as a short circuit or closed switch. Similarly, when no voltage or zero voltage is applied to the input, the transistor operates in the cutoff region and acts as an open circuit.

PNP Transistor Switching Circuit

In a PNP-type transistor, the base terminal is always negatively biased with respect to the emitter terminal, so the current always flows from the base. When a voltage is applied at the base, the transistor acts as a closed switch and when zero voltage is applied at the base terminal, the transistor acts as an open switch.

Applications of NPN And PNP Transistors

NPN Transistor Applications

While PNP and NPN sensors perform the same basic function, you may wonder why one is more popular than the other. There are certain differences, and NPN transistors are preferred in most circuit design applications. This is because the “N” substrate can transport positrons and holes much faster than the “P” substrate. This provides significant advantages for high-speed switching and amplifier circuit applications. In addition to this advantage, NPN transistors are easier to manufacture than PNP transistors and, therefore, cheaper to manufacture.

PNP Transistor Applications

If you’re starting with these components, industrial sensors can throw a wrench into your knowledge if you’re not careful. As we all know, PNP and NPN sensors are equipped with positive and negative power leads, which then output a signal to indicate an “on” state. In the “on” state, the PNP sensor produces a positive signal to the industrial control input, while the NPN sensor produces a negative signal. If you learned to use sensors before learning about transistors, you might mistakenly believe that positive voltages control PNP transistors.