Describe the operation of a unijunction transistor (UJT).
1 Answer
A Unijunction Transistor (UJT) is a three-layered semiconductor device with two heavily doped regions (emitter and base) and a lightly doped region (intrinsic base) in between. It is characterized by its unique behavior and is primarily used as a voltage-controlled oscillator, pulse generator, and timing circuit element. The UJT's operation can be explained in the following steps:
Structure: A UJT consists of three layers: an N-type layer (emitter) and a P-type layer (base) that sandwich an N-type layer (intrinsic base). The emitter is heavily doped, while the intrinsic base is lightly doped.
Biasing: The UJT is typically biased with a positive voltage supply connected to the emitter (Ve) and a resistor (Rb) connected to the base. The other end of Rb is connected to a voltage source (Vbb) that provides a negative voltage.
Initial Conditions: In the initial state, the UJT is not conducting because the intrinsic base region is initially resistive. The emitter-base junction is forward-biased due to the Ve connection, but the UJT doesn't conduct due to the intrinsic base resistance.
Charging Period: As Ve is applied, the emitter-base junction becomes forward-biased, leading to the injection of minority carriers (holes) from the P-type base into the N-type intrinsic base. This causes the intrinsic base region to become conductive. The voltage across the intrinsic base decreases, and the intrinsic base starts to charge up. This is known as the "charging" or "build-up" period.
Peak Point: As the intrinsic base charges up, its voltage gradually decreases. At a certain point, known as the peak point (Vp), the intrinsic base voltage becomes low enough that it triggers a significant increase in emitter current (Ie). This is because the junction breakdown occurs, and a large number of holes are injected into the N-type intrinsic base, causing a sharp increase in conductivity.
Saturated Conductance: Once the UJT reaches the peak point, it enters a highly conductive state, and the current through it increases rapidly. This state is called the "saturated" or "negative resistance" region. The UJT behaves as if it has negative resistance during this phase, allowing it to produce a voltage drop across the resistor (Rb) that can be used for various applications.
Resetting: When the current through the UJT increases and reaches a certain threshold, the device enters saturation. To turn off the UJT, the emitter current must be reduced below a certain value, known as the "valley current" (Iv). This can be achieved by reducing the emitter current or by momentarily interrupting the power supply.
In summary, a Unijunction Transistor (UJT) is a semiconductor device that operates as a voltage-controlled switch. It undergoes a distinctive charging phase, peak point, and saturation phase, exhibiting a negative resistance behavior that can be exploited for various circuit applications.
Structure: A UJT consists of three layers: an N-type layer (emitter) and a P-type layer (base) that sandwich an N-type layer (intrinsic base). The emitter is heavily doped, while the intrinsic base is lightly doped.
Biasing: The UJT is typically biased with a positive voltage supply connected to the emitter (Ve) and a resistor (Rb) connected to the base. The other end of Rb is connected to a voltage source (Vbb) that provides a negative voltage.
Initial Conditions: In the initial state, the UJT is not conducting because the intrinsic base region is initially resistive. The emitter-base junction is forward-biased due to the Ve connection, but the UJT doesn't conduct due to the intrinsic base resistance.
Charging Period: As Ve is applied, the emitter-base junction becomes forward-biased, leading to the injection of minority carriers (holes) from the P-type base into the N-type intrinsic base. This causes the intrinsic base region to become conductive. The voltage across the intrinsic base decreases, and the intrinsic base starts to charge up. This is known as the "charging" or "build-up" period.
Peak Point: As the intrinsic base charges up, its voltage gradually decreases. At a certain point, known as the peak point (Vp), the intrinsic base voltage becomes low enough that it triggers a significant increase in emitter current (Ie). This is because the junction breakdown occurs, and a large number of holes are injected into the N-type intrinsic base, causing a sharp increase in conductivity.
Saturated Conductance: Once the UJT reaches the peak point, it enters a highly conductive state, and the current through it increases rapidly. This state is called the "saturated" or "negative resistance" region. The UJT behaves as if it has negative resistance during this phase, allowing it to produce a voltage drop across the resistor (Rb) that can be used for various applications.
Resetting: When the current through the UJT increases and reaches a certain threshold, the device enters saturation. To turn off the UJT, the emitter current must be reduced below a certain value, known as the "valley current" (Iv). This can be achieved by reducing the emitter current or by momentarily interrupting the power supply.
In summary, a Unijunction Transistor (UJT) is a semiconductor device that operates as a voltage-controlled switch. It undergoes a distinctive charging phase, peak point, and saturation phase, exhibiting a negative resistance behavior that can be exploited for various circuit applications.