Programmable Unijunction Transistor Symbol

A Programmable Unijunction Transistor (PUT) is a three-terminal semiconductor device that behaves as a trigger or relaxation oscillator. It is similar to a conventional unijunction transistor (UJT) but with an additional programmable terminal. The PUT is primarily used in timing and triggering applications.Its largest rating is about 200 V and 1 A.The PUT is designed to control the timing of electronic circuits and can be programmed to switch on or off at specific voltage levels.

What is programmable unijunction transistor?

 A programmable unijunction transistor (PUT) is a three-lead electronic semiconductor device which is similar in its characteristics to a unijunction transistor,, except that its behavior can be controlled using external components. In a unijunction transistor, the base region is divided into two parts by the emitter. The two parts of the base form a voltage divider, which sets the operating point of the UJT. That voltage divider can be programmed with two physical resistors connected to the gate terminal of the PUT. This allows the designer some control over the  operating point of the PUT.

Construction

The programmable transistor is similar to the silicon controlled rectifier. It consists of four layers, two p-type layers and two n-type layers in equal proportion.

Basic Operation 

• The PUT operates similarly to a unijunction transistor (UJT) but includes an additional terminal for programmability.
• The PUT consists of three terminals: an anode (A), a cathode (K), and a gate (G). It has a unique structure that allows it to control the timing of switching operations. The device operates based on the principle of negative resistance and relaxation oscillation.
• The PUT operates as a voltage-controlled switch. When a positive voltage is applied to the gate terminal, it charges the capacitor CG through the resistor RG. Once the voltage across CG reaches a certain threshold, called the peak voltage (VP), the PUT enters its conducting state.
• The operation of a PUT involves the charging and discharging of the N-type region. Initially, the N-type region is depleted of charge carriers, acting as a high resistance. When a positive voltage is applied to the anode (A) terminal, it causes the N-type region to accumulate charge carriers and reduce its resistance. This process is called "priming" the PUT.
• The operation of a PUT is based on the charging and discharging of a capacitor connected between the emitter and base 2 terminals. When a positive voltage is applied to the emitter terminal, it charges the capacitor. The voltage across the capacitor gradually increases until it reaches a threshold level called the peak voltage (VP). At this point, the PUT enters a low-resistance state, and a large current flows through it from the emitter to base 1.
• Like the thyristor, its consists of 4 P-N layers.
• Has anode and cathode connected to the first and last layer and gate connected to the one of inner layer.
• Not directly interchangeable with conventional UJTS but perform a similar function.
• In a proper circuit configuration with two 'programming' resistor for setting the parameter n, they behave like a conventional UJT.
• Example: 2N2067
• The only similarity to a UJT is that the PUT can be used in the same oscillator to replace the UJT
• When we bias the PUT properly, the current can not be flow because the gate terminal is positive w.r.t cathode, when the anode voltage is increase form the cut off, the PN junction is forward bias, the PUT turn ON. The PUT remains in ON state until the anode voltage decreases below the cut off level and at that time the PUT is turn off.
• The gate terminal of PUT can be biased through voltage divider network to active the desired voltage as shown in the given diagram.
  
  
  

Behaviour of a PUT

The behavior of a PUT is determined by the voltage applied to the anode terminal (A) in relation to the voltage at the cathode terminal (K). When the anode voltage is lower than the cathode voltage, the PUT remains in a high-resistance state, acting like an open switch. When the anode voltage exceeds the cathode voltage, the PUT enters a low-resistance state, acting like a closed switch.

Structure

PUT is similar in structure to a unijunction transistor (UJT), but with an additional terminal for programming.

The structure of a PUT consists of a lightly-doped N-type silicon bar with a heavily-doped P-type material on one side and a lightly-doped P-type material on the other side. The three terminals of a PUT are called the anode (A), cathode (K), and gate (G). The anode and cathode are connected to the N-type region, while the gate is connected to the P-type region.

When a voltage is applied between the anode and cathode, the PUT remains in a high-resistance state until the voltage reaches a certain threshold level. Once the threshold voltage is reached, the PUT undergoes a negative resistance region, where the current flowing through it increases rapidly. This characteristic makes it useful for timing applications.

The gate terminal is used to control the triggering voltage of the PUT. By varying the voltage applied to the gate, the threshold voltage can be adjusted. This programmability feature allows for customization and fine-tuning of the switching behavior according to specific circuit requirements.

V-I characteristics of a PUT

In a PUT, G is always biased positive with respect to cathode. When anode voltage exceeds the gate voltage by about 0.7 V, junction J1 gets forward biased and PUT turns on. When anode voltage becomes less than gate voltage, PUT is turned off.

Advantages and Disadvantages of PUT

Advantages

1. Versatile Triggering: PUTs can be triggered using various voltage levels, making them highly versatile in different circuit designs. They can be easily programmed to respond to specific trigger voltages, allowing for customization and flexibility.
2. Simple Circuitry: PUTs have a relatively simple structure and can be easily integrated into circuits due to their minimal external components. This simplicity leads to cost-effective designs and ease of implementation.
3. Low Power Consumption: PUTs typically have low power requirements, making them suitable for applications where power efficiency is essential. They can operate effectively with low voltage levels and consume minimal power during standby or off states.
4. High Reliability: PUTs are known for their high reliability and robustness. They can withstand harsh environments, temperature variations, and voltage fluctuations, making them suitable for industrial and automotive applications.
5. Adjustable Output Characteristics: The programmable nature of PUTs allows users to adjust and control various output characteristics, such as trigger voltage, current, and timing. This adjustability makes them useful in applications that require precise control over switching or timing events.
6. Pulse Generation: PUTs can generate precise and repeatable pulses, which find applications in timing circuits, pulse generators, and oscillators. They offer stable and controlled output waveforms, enabling accurate timing and synchronization in electronic systems.
7. Switching Capabilities: PUTs can function as switches, making them valuable in applications such as relay drivers, motor control, and power control circuits. They can handle moderate current levels and provide reliable switching performance.
8. Low Cost: PUTs are typically economical compared to other types of solid-state devices, such as thyristors or integrated circuits. Their simple design and ease of manufacturing contribute to their cost-effective.

Disadvantages & Limitations

1. Limited applications: PUTs are primarily used for timing and triggering purposes, making them less versatile compared to other types of transistors. They are not suitable for high-power or high-frequency applications, limiting their use in certain electronic circuits.
2. Limited voltage handling capability: PUTs typically have a relatively low voltage handling capability compared to other transistors. They are designed to operate at low to moderate voltages, and exceeding their voltage ratings can lead to device failure or malfunction.
3. Limited current handling capability: Similarly, PUTs have limited current handling capability. They are best suited for low to moderate current applications and may not be suitable for high-current circuits.
4. Non-ideal characteristics: PUTs exhibit non-ideal characteristics such as temperature sensitivity and parameter variations. Temperature changes can affect their performance, leading to inconsistent behavior. Additionally, manufacturing variations can result in variations in device parameters, making it challenging to achieve precise and consistent circuit operation.
5. Availability: Compared to other more common types of transistors like BJTs (bipolar junction transistors) or MOSFETs (metal-oxide-semiconductor field-effect transistors), PUTs may be less readily available. This limited availability can make them harder to source or may result in higher costs compared to more commonly used transistors.

Advantages of PUT over UJT

1. The switching voltage is easily yarned by changing Vg through the potential divider.
2. PUT can operate at lower voltages then IC’s. 
3. Peak current is lower UJT.

PUT as relaxation oscillator

A PUT can be used as relaxation oscillator. The gate voltage VG is maintained from the supply by the potential divider R1 and R2 and determines the peak point voltage Vp In the case of the UJT, VP is fixed for a device by the dc supply voltage. But VP of a PUT can be varied by varying the potential divider it and its the anode voltage VA is less than the gate voltage Vg the device will remains in its off state.

Application

1. Logic and SCR trigger Circuit 
2. Relaxation oscillator
3. Time-delay circuits
4. Oscillator circuits
5. Switching circuits
6. Pulse generators
7. Sawtooth waveform generators
8. Voltage-controlled switches
9. Motor control circuits
10. Voltage regulators
11. Low-power applications