What is a unijunction transistor (UJT)?
1 Answer
A unijunction transistor (UJT) is a type of semiconductor device that is used in electronics as a simple switching device or as a relaxation oscillator. It is a three-terminal device, meaning it has three leads or connections. The UJT is composed of a single junction with three layers of doped semiconductor material.
The three terminals of a UJT are called:
Emitter (E): This terminal is heavily doped with an N-type material.
Base 1 (B1): This terminal is lightly doped with an N-type material.
Base 2 (B2): This terminal is lightly doped with a P-type material.
The physical structure of a UJT consists of two distinct regions of N-type material (the emitter and one side of the base) separated by a P-type material region. The P-type region acts as a "barrier" region between the two N-type regions. Due to its construction, the UJT has a unique characteristic known as negative resistance behavior.
When a small voltage is applied between the emitter and B2 terminals, it creates a forward bias across the emitter-base 2 junction. However, the UJT does not conduct much current at this stage due to the barrier region. As the voltage increases, at a certain point, it reaches a peak voltage called the "peak point voltage" (Vp). At this voltage, the UJT experiences a phenomenon called "intrinsic standoff ratio" or "η," where the emitter current starts to increase significantly.
The negative resistance characteristic means that as the emitter current increases, the voltage drop across the device decreases, leading to a negative resistance slope in the device's current-voltage characteristic. This behavior is unique and allows UJTs to be used as relaxation oscillators or timing circuits.
UJTs were commonly used in electronic circuits in the past, but with advancements in semiconductor technology, other types of transistors like bipolar junction transistors (BJTs) and field-effect transistors (FETs) have become more popular and replaced UJTs in many applications.
The three terminals of a UJT are called:
Emitter (E): This terminal is heavily doped with an N-type material.
Base 1 (B1): This terminal is lightly doped with an N-type material.
Base 2 (B2): This terminal is lightly doped with a P-type material.
The physical structure of a UJT consists of two distinct regions of N-type material (the emitter and one side of the base) separated by a P-type material region. The P-type region acts as a "barrier" region between the two N-type regions. Due to its construction, the UJT has a unique characteristic known as negative resistance behavior.
When a small voltage is applied between the emitter and B2 terminals, it creates a forward bias across the emitter-base 2 junction. However, the UJT does not conduct much current at this stage due to the barrier region. As the voltage increases, at a certain point, it reaches a peak voltage called the "peak point voltage" (Vp). At this voltage, the UJT experiences a phenomenon called "intrinsic standoff ratio" or "η," where the emitter current starts to increase significantly.
The negative resistance characteristic means that as the emitter current increases, the voltage drop across the device decreases, leading to a negative resistance slope in the device's current-voltage characteristic. This behavior is unique and allows UJTs to be used as relaxation oscillators or timing circuits.
UJTs were commonly used in electronic circuits in the past, but with advancements in semiconductor technology, other types of transistors like bipolar junction transistors (BJTs) and field-effect transistors (FETs) have become more popular and replaced UJTs in many applications.