TRANSISTOR DESIGN How to design a transistor?

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Transistor design is dependent upon multiple factors like area, application, operating temperature, doping level, frequency response, and many more. Here is a comprehensive guide to understanding factors crucial for transistor circuit design and its implementation.

6 steps for transistor design

1. Estimate the area

The area of the desired transistor must be considered before the initiation of the design process. A transistor could either be a discrete device or embedded inside a microprocessor chip. The typical size of a transistor device is a few millimeters while a transistor fabricated on a chip is extremely small in nanometers! Each integrated circuit (IC) contains billions and trillions of transistors. Such a large number of transistors are embedded in the IC on a very high scale. The design process dealing with fabricating transistors on an IC is known as VLSI design (Very-large-scale integration).

2. Decide the application: most important

The transistor design majorly depends on the application it is going to serve. If a transistor needs to perform fast switching, a MOSFET should be designed. BJT must be designed to switch heavy currents and power transistors to serve industrial applications with high-power ratings. For obtaining a high current gain, a Darlington transistor design must be considered.

3. Specify the operating temperatures

The range of operating temperatures is an important factor for transistor design. Consumer electronics have a maximum operating range from 0° C up to 55 °C and 70 °C. While industrial operating temperatures fall in the range from -40 °C to 85 °C. The transistor design for military applications needs to operate efficiently from -55 °C to 125 °C. Such transistors need to possess exceptional mechanical strength and operate under tough conditions without any faults.

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4. Determine doping levels

Doping concentration is another important factor for transistor design. The amount of doping is one of the factors to decide the flow of current in a transistor. For BJTs, the doping profile of each terminal varies. In the case of FETs, the source and drain have the same doping concentration. An additional lightly doped N- layer is added in most power semiconductor devices to improve current rating and offer conductivity modulation.

5. Predict the frequency response

Predicting the frequency response of a transistor is a crucial step in transistor amplifier design. Each transistor responds differently to a variety of frequencies. Junction capacitances form internal RC circuits in BJT amplifiers that affect the frequency response. At higher frequencies, the amplifier gain starts to reduce because of internal capacitances. Understanding the response enables us to predict the operating frequency range of an amplifier for which it provides a constant gain. Moreover, frequency response is the base of understanding the operation of RF amplifiers.

6. Heat sink against power dissipation

Power transistors are kept in an insulating material to protect them from overheating. A heat sink is used to protect a power transistor in high power dissipation and provide excellent thermal conductivity and corrosion resistance.

5 steps for transistor circuit design

Post-transistor design implementation, the circuit should be tested to obtain desired results.
Watch this video to learn more about transistor circuit design:

1. Define the requirements

It is important to define the values for supply voltage, resistances, capacitances, and other electronic components. Faulty components must be replaced with functional ones.

2. Decide the configuration

Transistors work in different topologies and configurations. For example, NPN or PNP BJT. Furthermore, there are many topologies common emitter, common base or common collector are important for transistor circuit design. Similarly, enhancement-type or depletion-type MOSFET configurations serve multiple applications.

3. Set up biasing

Set up conditions for forward and reverse bias in the transistor circuit. The biasing conditions decide the operation of the transistor.

4. Realize the circuit

It is essential to perform DC analysis and small signal analysis of the transistor circuit. Kirchhoff’s voltage and current law along with hybrid models enable calculating transistor parameters and plotting the characteristics.

5. Test circuit

When the transistor is successfully designed and ready for operation, it should be tested on a basic breadboard. Multiple tests may predict its behavior on a wider level. A digital circuit simulation using a variety of software tools like LTspice, PSpice, Proteus Design Suite, CircuitLogicX, etc, can provide quick transistor design solutions.

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