Avalanche transistors are a special kind of bipolar junction transistors. They are made to work in the avalanche breakdown area. This area is known for the avalanche breakdown effect, which is also seen in gases like in the Townsend discharge.
This operation mode, known as avalanche mode, lets avalanche transistors handle very high currents quickly. They can do this in less than a nanosecond.
Even transistors not meant for avalanche operation sometimes show similar properties. For example, during a 12-year period, 82% of the 15V high-speed switch 2N2369 transistors could work in an avalanche breakdown state. They did this using a 90V power and produced fast pulses in less than 350 picoseconds.
Key Takeaways
- Avalanche transistors are designed to operate in the avalanche breakdown region, allowing them to switch very high currents with sub-nanosecond transition times.
- The avalanche breakdown phenomenon is similar to Townsend discharge in gases and is accompanied by negative differential resistance.
- Even non-specialized transistors can exhibit consistent avalanche properties, as evidenced by the 82% of 2N2369 transistors that could generate 350 ps avalanche breakdown pulses.
- Understanding avalanche breakdown in transistors is crucial for their application in high-speed switching circuits and linear amplifiers.
- Ongoing research is exploring compound semiconductor avalanche devices that can switch even higher currents faster than traditional avalanche transistors.
What is Avalanche Breakdown in Transistors?
Avalanche breakdown can happen in materials that insulate or semi-conduct. It’s a way for electric current to grow really quick. This happens in places where a lot of current wouldn’t normally go through. When carriers get enough energy, they create more free pairs through collisions. This grows the current massively.
Avalanche Effect in Semiconductors
In semiconductors, the avalanche effect is key to understanding breakdown. The electric field speeds up charge carriers so much that they make more pairs by hitting other particles. This starts a chain reaction that quickly increases the current flow.
Impact Ionization and Current Multiplication
Impact ionization is at the heart of the avalanche effect in semiconductors. Strong electric fields cause charge carriers to hit bound electrons. This gives off enough energy to make more pairs. This quick process means a lot more current can pass through the device.
Reverse-Biased p-n Junctions and High Electric Fields
In reverse-biased p-n junctions, high electric fields can lead to avalanche breakdown. This is used in things like avalanche transistors and Zener diodes for their functions. Here, the breakdown region helps with certain tasks.
History of Avalanche Transistors
In 1955, Ebers and Miller wrote a big paper on avalanche transistors. They looked at how alloy-junction transistors work in the “avalanche breakdown” area. Before, the first transistor models faced speed and breakdown voltage problems in older computer circuits.
So, the first use of avalanche transistors was in switching and multivibrator circuits. They applied Miller’s formula for avalanche multiplication. This was a key step in understanding how transistors work in the avalanche breakdown region.
Ebers & Miller’s 1955 Paper
The paper by Ebers and Miller, published in 1955, marked a big step in avalanche transistors. It explained using alloy-junction transistors in the “avalanche breakdown” area. This helped get past the speed and voltage issues older transistor models faced in early computer circuits.
Thanks to this paper, avalanche transistors found their first real-world use in switching and multivibrator circuits.
Early Applications in Switching Circuits and Multivibrators
After Ebers and Miller, from the 1960s to the early 1970s, many circuit ideas came up. These focused on making use of the special features of avalanche transistors. For instance, they could switch very high currents with sub-nanosecond rise and fall times.
These early applications played a big role in the development of fast-switching circuits and multivibrators. They made room for even more complex uses of avalanche transistors over the years.
Theory of Avalanche Breakdown
We look at how an avalanche transistor works when not moving. We focus on an NPN transistor for simplicity. But, this study’s findings apply to PNP types too, just with adjustments for opposite signs of voltages and currents. The study is based on William D. Roehr’s work in 1963.
Static Avalanche Region Characteristics
The physical rule of a bipolar junction transistor shows how the collector current and collector-to-emitter voltage mix during avalanche breakdown. This region acts strangely, with current dropping when voltage rises. This is due to special effects caused by the reverse-biased collector-base part of the transistor. These effects can even make the current multiply.
Miller’s Avalanche Multiplication Coefficient
Miller’s avalanche multiplication coefficient, or the factor M, tells us how fast the avalanche breakdown can get. M shows the relationship between the total collector current and the initial collector current. It depends on the collector-base voltage and the transistor’s built. The actual M value varies from 1 (no avalanche happening) to infinity (complete avalanche). Accurate M modeling is key to understanding and foreseeing how avalanche transistors act.
Explaining Avalanche Breakdown in Transistors
The differential dynamical mode, or small signal model, is key to understanding avalanche transistors. We leave out external parts purposely. This way, we can focus on how avalanche transistors work without extra clutter.
Differential Dynamical Model
The differential dynamical model helps us recognize the behavior of avalanche transistors in circuits. It concentrates on the device’s internal features. This means we look at the transistor itself, not things like the package around it.
Equivalent Circuit for Avalanche Transistor
The equivalent circuit of an avalanche transistor involves key parameters. These represent the device’s unique behavior during avalanche breakdown. Using these parameters ensures our circuit designs are dependable.
Avalanche Differential Resistance and Capacitance
Avalanche transistors stand out from regular transistors with specific differential resistance and capacitance values. These are crucial for understanding how the transistor acts in high-speed, high-current conditions. Here, the avalanche effect is very important.
Second Breakdown Avalanche Mode
When the collector current exceeds the standard limit, a new issue arises: the second breakdown. It happens when certain parts (hot spots) in the transistor’s base-emitter zone get too hot. This makes current through them skyrocket.
This surge in current leads to more heat, creating a cycle of increasing temperatures. This is known as ‘positive thermal feedback’.
Hot Spots and Thermal Feedback
Second breakdown starts when the collector current goes over its limit. This leads to a fast drop in collector voltage. Meanwhile, current through the collector spikes due to the hot spots.
Transistors need hot spots for second breakdown. They’re part of how we refine silicon. The potential between base and emitter (VBE) drops as it gets hotter. This drop can cause hot spots to occur during use. And if not managed, these hot spots can destroy the device.
Enhancing Current and Voltage Limits
Scientists have worked on boosting the current and voltage capacities of bipolar junction transistors during avalanche breakdown. By using newer materials and designs, they’ve made devices that switch high currents very quickly. Much faster than before.
The SPICE model for these transistors includes forty parameters. But sometimes, you won’t need all of them. If you leave some out, the model just fills them in with simpler values, like zero for missing resistances.
Zener Breakdown vs Avalanche Breakdown
The main difference between Zener and avalanche breakdown is how they happen. Zener breakdown is found in diodes with a 5-8 volt range. It happens when a strong electric field moves electrons from the p-type to the n-type material’s bands.
Avalanche breakdown occurs in diodes needing over 8 volts. This happens when a high voltage makes more free electrons. This leads to more electric current in semiconductors and insulating materials.
Mechanism of Zener Breakdown
Zener breakdown happens in diodes that are heavily doped. It occurs when the electric field’s power reaches a certain critical point. Then, there’s a sudden increase in current while the voltage stays mostly the same. Zener breakdown is reversible. This means the diode can go back to normal if the voltage goes down.
Mechanism of Avalanche Breakdown
Avalanche breakdown occurs in diodes that are either lightly doped or have high doping concentrations. It happens when carriers move at very high speeds. This causes the current to quickly and exponentially rise with voltage. Avalanche breakdown can permanently damage the diode. It’s not reversible like Zener breakdown.
Differences Between Zener and Avalanche Breakdown
Zener and avalanche breakdowns differ in how they react to temperature and voltage, their current-voltage curves, and the level of doping in diodes. Zener breakdown’s breakdown voltage decreases with higher temperature. For avalanche breakdown, the voltage actually goes up with more heat.
Zener breakdown shows a distinct curve on a graph, while avalanche breakdown’s curve is not as defined. Highly doped diodes face Zener breakdown, while lightly doped diodes see avalanche breakdown. Sometimes, both can occur in the same semiconductor device.
Voltage-Current Characteristics
In semiconductor devices, a large voltage drop can create much heat. If a diode faces too much current during avalanche breakdown due to a transistor breakdown, it might get ruined. Luckily, avalanche breakdown by itself doesn’t have to harm the crystal.
Avalanche diodes like high voltage Zener diodes break down at a fixed voltage to avoid issues. They handle a fair bit of current well, thanks to the avalanche effect and current multiplication.
Voltage-Current Curves for Avalanche Breakdown
The breakdown voltage marks where avalanche breakdown begins. There’s a neat feature too. Even if the voltage lowers, the material will keep conducting past the breakdown voltage. This happens when a lightly doped p-n junction faces a reverse voltage over 5V.
Applications of Avalanche Transistors
Avalanche transistors have unique features that make them very useful. They can handle high currents quickly. This makes them great for electronic circuits. They became popular from the 1960s to the 1970s. Many circuits using these transistors were created during this time.
Switching Circuits
These transistors are perfect for fast switching needs. They work well in digital logic and pulse generators. For instance, pulse generators using avalanche transistors can create quick pulses in about 300 picoseconds. This speed is crucial for various projects.
Linear Amplifiers (CATT and IMPISTOR)
Apart from switching, avalanche transistors can also amplify signals. The first one made for this, the CATT, came about in 1976. There’s also the IMPISTOR, which combines the transistor with an IMPATT diode. This new development uses the best of avalanche transistor’s features. Creating amplifiers with these transistors is still a hot topic in research.
Source Links
- https://en.wikipedia.org/wiki/Avalanche_transistor
- https://byjus.com/physics/difference-between-zener-breakdown-and-avalanche-breakdown/
- https://en.wikipedia.org/wiki/Avalanche_breakdown
- https://northcoastsynthesis.com/news/transistors-for-the-perplexed/
- https://en.wikipedia.org/wiki/Avalanche_diode
- https://ttu-ir.tdl.org/bitstreams/e163c278-b0ca-4a54-b25c-eb16fbffd1f8/download
- https://www.physicsforums.com/threads/help-with-the-key-points-of-zener-breakdown-and-avalanche-breakdown.1012752/
- https://www.vedantu.com/jee-main/physics-difference-between-zener-breakdown-and-avalanche-breakdown
- https://www.elprocus.com/avalanche-transistor-circuit-working-characteristics/
