Bipolar Junction Transistors and types

Bipolar Junction Transistors BJT are the types of transistors that have two (2) charges carriers that is; electron and hole. Likewise, related links are provided below in this post after you might have read this article “Bipolar Junction Transistors and types” to the end.


Bipolar Junction Transistor come in two (2) different types. That is:

  1. NPN Bipolar Junction Transistor (NPN BJT)
  2. PNP Bipolar Junction Transistor (PNP BJT)

NPN Bipolar Junction Transistor (BJT) which is one of the two types of Bipolar Junction Transistors (BJTs) is a type that has three regions layers semiconductors where P-type being the Base (B) layer is doped between the two other N-types. That is; Emitter (E) and Collector (C) layers forming the NPN layers. By applying positive potential difference (Pd) across the base (B) of the transistor to its emitter (E) (making the base higher than the emitter) and also across the collector to the emitter at the same time, the transistor becomes active.  When a current enters the base of the transistor, is then amplified thereby causing a larger collector and emitter current to flow.

The circuit symbol for NPN BJT.


NPN BJT circuit symbol

Below pictures illustrates NPN type of bipolar junction transistor (BJT). Connections of two PN-junction diodes back – to – back make them share the P-type which is the common.

NPN PN junction


In PNP BJT, an N-type [as its base (B)] semiconductor is doped between the two other P-types layers forming the PNP layers. This transistor is the reverse type of the NPN. When current is leaving the base (B) it will be amplified at the collector (C) output. That is to say connecting the base of the transistor to the ground turns ON the transistor. Moreover, in PNP BJT the emitter is always connect to the positive potential difference while the output is between the Collector (C) and the ground or negative potential difference.

Below is the circuit symbol for PNP Bipolar Junction Transistor (BJT)

PNP BJT circuit symbol

An illustration of PNP Bipolar Junction Transistor (BJT) by connecting two PN-junction diodes face – to – face so that the N-type is the junction shared between the two other P-types.

PNP PN junction

Moreover, Bipolar Junction Transistors (BJTs) are dope in three (3) different semiconductor regions.  In NPN, the regions are N-type; P-type; and the other N-type region. The P-type is the Junction region which the two N-types share in common thus the “NPN” name. In the PNP, the three regions are; P-type; N-type; and the other P-type region. The N-type region is the common junction for the two P-types regions thus the name “PNP.” These three (3) regions in both of the BJT types are call; Emitter (E), Base (B), and Collector (C); the base (B) being the common layer region of the both two BJT types.


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