Characteristics and Working of Avalanche Transistor Circuit

A transistor is a device that is used to regulate current and voltage flow in a circuit. It acts as a switch or gate for electronic signals. A transistor consists of three layers of semiconductor material like silicon or germanium from three terminals. When a current or voltage is applied to one pair of transistor terminal it controls the current through the other pair of terminals. A transistor is a basic unit in an IC.

A Bipolar Junction Transistor (BJT) is a type of transistor that uses electron and hole charge carrier while Field Effect Transistor (FET) uses only one type of charge carrier. BJT uses two junctions formed between the p-type and n-type semiconductors for its operation. These are available in NPN and PNP types. BJTs are used as amplifiers and switches in electronic circuits.

What is an Avalanche Transistor?

An Avalanche Transistor is a bipolar junction transistor. This operates in the region of its collector current or collector-to-emitter voltage characteristics beyond the collector-to-emitter breakdown voltage, called an avalanche breakdown region. This region is characterized by the avalanche breakdown phenomenon.

Avalanche Breakdown

When a p-type and n-type semiconductor comes in contact, a depletion region is formed around the p-n junction. The width of the depletion region decreases with the increase in voltage of forwarding bias, while the depletion region increases in reverse bias condition. The below figure shows the I-V characteristics of a p-n junction in forwarding bias and reverse bias condition.

Here the figure demonstrates that the current through the semiconductor increases with an increase in voltage level in forwarding bias. Further, there is a certain minimum current flowing through the p-n junction under reverse bias. This current is called reverse saturation current (Is).

At the initial stage reverse saturation current Is is independent of the applied voltage, but on reaching a particular point the junction breaks down leading to the heavy flow of reverse current through the device. This is because as the reverse voltage increases the kinetic energy of the minority charge carrier also increases. These fast-moving electrons collide with the other atoms to knock off some more electrons from them.

The electrons so released further release much more electrons from the atoms by breaking the covalent bond. This process is known as carrier multiplication and this leads to a considerable increase in the flow of current through the p-n junction. This phenomenon is called Avalanche breakdown and the voltage is called Avalanche breakdown voltage (VBR).

Avalanche breakdown occurs in the lightly doped p-n junction when the reverse voltage increases beyond 5V. Further, it is difficult to control this phenomenon as the number of charge carriers generated cannot be directly controlled. Moreover, the avalanche breakdown voltage has a positive temperature coefficient which means the avalanche breakdown voltage increases with the increase in the junction temperature.

Avalanche Transistor Pulse Generator

The pulse generator is capable of generating a pulse of around 300ps rise time. Therefore, it is very helpful in measuring bandwidth and also used in projects which require a pulse with fast rise time. A pulse generator can be used to calculate the bandwidth of an oscilloscope. An advantage of the avalanche transistor pulse generator is, it is a much cheaper way than using the 3D method which needs a high-frequency function generator.

The above circuit is a schematic for the avalanche transistor pulse generator. This is a sensitive and high-frequency circuit with LT1073 chip and 2N2369 transistor. This circuit makes use of the breakdown property of the transistor.

Normal chips like 555 timer chip or logic gates cannot produce pulses with fast-rising time. But an avalanche transistor helps to produce such pulses. An avalanche transistor needs a 90V converter which is supported by LT1073 circuitry. The 90V is fed to the 1M resistor connecting the 2N2369 transistor.

The transistor-based is connected to 10K resistor, so 90V cannot pass through it directly. The current is then stored in the 2pf capacitor. The transistor has a breakdown voltage of 40V while it is fed with 90V DC. Therefore the transistor will break down and the current from the capacitor will discharge into the base-collector. This creates a pulse with a very fast rise time. This does not last long. The transistor recovers very quickly and becomes non-conductive. The capacitor will build up charge again, and the cycle repeats.

Monostable Multivibrator

A monostable multivibrator has one stable and a quasi-stable state. When an external trigger is applied to the circuit the multivibrator will jump from a stable state to quasi-state. After a period of time, it will automatically set back to a stable state without any external trigger. The time period required to return to the stable state depends on the passive elements like resistors and capacitors used in the circuit.

Circuit Operation

When there is no external trigger to the circuit, one transistor Q2 will be in saturation state and other transistor Q1 will be in the cutoff state. Q1 is put at negative potential until the external trigger operates. Once the external trigger to the input is fed, Q1 will turn on and when Q1 reaches the saturation the capacitor which is connected to the collector of Q1 and the base of Q2 will make transistor Q2 to turn off. This is a state of the turn off Q2 transistor is called astable or quasi-state.

When the capacitor charges from Vcc, the Q2 will turn on again, and automatically Q1 is turned off. So, the time taken by the capacitor for charging through the resistor is directly proportional to the astable state of the multivibrator when an external trigger is applied.

Characteristics of Avalanche Transistor

Avalanche transistor has characteristics of breakdown when operated in reverse bias, this helps in switching between the circuits.

Applications of Avalanche Transistor

• Avalanche transistor is used as a switch, linear amplifier in electronic circuits.
• The major application of avalanche transistors is to generate pulses with very fast rise times, which is used to generate the sampling pulse in a commercial sampling oscilloscope.
• One interesting possibility is an application as a class C amplifier. This involves switching the operation of an avalanche transistor and should utilize the full collector voltage range rather than just a small part of it.

Thus, this is all about Avalanche transistor characteristics and its applications. We hope that you have got a better understanding of this concept. Furthermore, any doubts regarding this concept or to implement electronics projects please, give your valuable suggestions by commenting in the comment section below. Here is a question for you, What is an Avalanche Transistor?