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About transistors
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What was transistors?
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What types and types of transistors are there?
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Buying Criteria for Transistors – What's it all about?
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Conclusion: How to buy the right transistor
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FAQ – Frequently Asked Questions about Transistors
What was transistors?
Transistors are electronic semiconductor devices used to control or switch voltages and currents. The word transistor is derived from the word combination "transfer" and "resistor" and refers to the property of transistors to act as controllable resistors by changing the resistance of semiconductor layers. Nowadays, these semiconductor layers are mainly based on doped silicon.
Generally speaking, the term transistor refers to bipolar transistors, often with the abbreviation BJT (bipolar junction transistor). They consist of three thin semiconductor layers that lie directly on top of each other. Each of these layers has an electrical connection that is led out of the enclosure. These connections are called base, emitter and collector (B, E, C). The semiconductor path emitter-collector forms the controllable current path, which is controlled via the base connection. The application of a relatively small base current influences the conductivity of the collector-emitter path, whereby the conductivity of the transistor without base current is close to zero and increases with increasing control current. This is physically comparable to a valve that requires very little actuation force, but can control large volumes. The factor between the base current and the collector emitter current is called the current gain. If the current amplification factor of a transistor is, for example, 100, a base current of 1 mA causes a current flow of 100 mA in the collector emitter path.
What types and types of transistors are there?
Basically, depending on the order of the three semiconductor layers, transistors are referred to as NPN or PNP transistors. The difference between these two types is visible in the circuit diagram by the small arrow at the connection of the emitter. With the NPN transistor, it points outwards, with the PNP transistor inwards. Similarly, NPN type transistors are suitable for positive voltages, with PNP transistors the polarities are reversed.
Darlington transistors interconnect two normal bipolar transistors to a pre- and main amplifier circuit in a single housing. This allows a much higher current gain to be achieved without the need for additional external wiring.
So-called "pre-tensioned transistors" integrate a bias resistor in their housing. This sets a defined base voltage to set the transistor operating point to the desired range, for example for audio and AC voltage applications.
HF transistors are specially optimized for high-frequency applications. They have particularly small retroactive capacities and fast rise and fall times.
Transistors are offered in a variety of housing designs, both in SMD technology and for conventional through-hole mounting (THT). Individual transistors for smaller power ratings are usually placed in a plastic housing. Power transistors generally have to be cooled; they therefore have a well heat-conductive metal casing or a metallic base plate for coupling to a heat sink or cooling surface.
Transistor arrays contain two to seven individual transistors. They are space-saving and simplify the handling. As with many other types of housing, suitable plug sockets are available from Conrad. A later required replacement can be carried out without soldering.
Paired transistors, so-called "matched pairs," combine two identical transistors in a housing whose electrical characteristics are very close together.
Buying Criteria for Transistors – What's it all about?
If high current amplification is required, Darlington transistors are well suited. In many cases, this means that a separate pre- and main reinforcement together with additional required components can be dispensed with. For example, transistors are available as matched pairs for audio applications and multi-channel amplifier circuits. These selected transistors have small differences between each other and ensure the best possible linearity in parallel operation. RF transistors are particularly suitable for higher frequencies due to their special properties.
Transistor arrays enable the development of more compact circuits than with the exclusive use of single transistors. Especially for smaller services, they represent an advantageous and space-saving solution, which also brings advantages in manufacturing during assembly and soldering.
The names NPN and PNP must not be confused with each other in all types and designs, as both transistor types are not interchangeable. The amplification factor of the transistor is an important characteristic, it should fit as closely as possible to the intended task, and the amplification bandwidth must also be sufficient. In order to operate a transistor within the safe working area, the required information is available in the respective data sheet.
Conclusion: How to buy the right transistor
In the case of new circuit developments, an exact attention should always be paid to the respective electrical values according to the data sheet, in order to select the transistor model that is best suited to the task. The load capacity of transistors does not extend to the limit, brings advantages in terms of reliability and long-term functional reliability. Use in more demanding ambient conditions requires compliance with the permissible temperature range and may require derating in a warm environment. In these cases, select a transistor with corresponding power reserves.
If a defective transistor has to be replaced, the same type should be used as far as possible. If this is no longer available, there are comparison tables with which an electrically compatible transistor can generally be found. The housing design also deserves attention: Is the pitch correct for wired types or the position and size of the solder pads for SMD transistors? Is the housing itself compatible with the available space and any existing heat sinks and mounting holes?
Ensure that any required test marks and compliance with the required standards are observed. The transistors offered in the Conrad shop are characterized by reliability and longevity and fulfill high quality requirements.
FAQ – Frequently Asked Questions about Transistors
What was to be observed when handling and processing transistors?
Like most electronic semiconductor devices, transistors are at risk from electrostatic discharge. Removal from the packaging and all subsequent handling and processing steps must therefore be carried out in accordance with the relevant ESD guidelines to avoid damage. It is also important to maintain the maximum permissible soldering temperature and the soldering duration to prevent overheating damage.
What are the maximum operating temperatures that transistors can tolerate?
Depending on the type, a junction temperature of 150 to 200° C must not be exceeded for the silicon transistors used today. Higher temperatures either lead to immediate destruction or accelerate aging extremely. The permissible electrical values according to the data sheet usually refer to operation at° 25 C ambient temperature. Higher ambient temperatures require derating; the corresponding diagrams can be found in the data sheet.
Can transistors be overloaded electrically for a short time?
Operation for a short period of time above the specified voltage values is not permitted. Neither the permissible currents nor the maximum power loss may be exceeded. The non-controllable thermal resistance between the semiconductor plate and the housing would lead to an inadmissible temperature increase of the actual semiconductor even with good external cooling.