How To Read A Transistor Datasheet? Best Guide

Image of transistor and datashet source Freepik

Before buying any electronic component, there are several things that you should look into. The first question is whether the component will be suitable for your application. Does it have enough power capacity? What about resistance? What amount of current can flow through the device without causing any damage? These are just some of the things that you will focus on before buying transistors. The surest place to find the right answers about transistors is from the datasheet.

A datasheet is a detailed document that provides all the specifications of a particular product. In our case, the product is the transistor.

The transistor family is quite big and each type is suitable for different application areas. They also come in different designs, construction and specifications. If you are not careful, you may end up buying a wring transistor.

The only way to stay safe is by conforming all the information on the transistor datasheet before purchasing one.

The good news is transistor manufacturers are required to publish all the technical specifications about transistors on the datasheet. This way, clients and customers can easily know what they are buying and whether the transistor will be perfect for their application areas.

What if I cannot find the datasheet? Well, the internet is always there for you. Simply type the name and model number of the transistor and you will get the full datasheet.

Having a datasheet is one thing and reading it is another. Would you like to know how to read a transistor datasheet? We have a perfect guide for you. In this article, we are going to look in detail at how to read transistor datasheets. We will cover all the key parameters of transistors and why you should pay full attention to the data that has been published on them with regard to the transistor performance.

Overview of transistors

Transistors are major semiconductor electronic components in the world of electronics. They are mainly used for the amplification and switching functions in different electronic applications. Transistors are highly credited for revolutionizing the world of modern electronics and technology in general.

Before you even confirm on the datasheet, you should know that there are two main types of transistors. These are the Field Effect Transistors (FET) and Bipolar Junction Transistors (BJT).

Both the transistor types have three terminals only that they differ in terms of the configurations. The three terminals in the FET transistors are the source, the gate, and the drain. On the other hand, the three terminals in the BJT are the emitter, base, and collector. These two transistor types also come in different variations. For the FET, we have the N-channel and the P-channel transistors. On the other hand, for the BJT, we have the NPN and the PNP types of transistors.

Beyond the types and configurations, there are many other specifications that you need to know about transistors. This is where the relevance of the transistor datasheet comes in. It reveals both the main and the smallest details of the transistors.

Does the datasheet cover the non-technical parameters of transistors? Yes. Details sch as physical size, design, weight, package, and materials are also revealed on the transistor datasheet. The mounting technique of the transistor is also revealed. Such details are designed to give you deeper insights on how to handle transistors in whichever application area you are in.

Specifications and parameters of transistors

Image source Freepik

Like it is the case of other electronics, one of the functions of the transistor datasheet is to cover all the specifications and parameters of the transistor that has been packaged.

Like in most electronics, other than the datasheet, specification parameters are also available on the manufacturer’s websites. In most cases, reputable electronic component distributors and suppliers will also have copies of the datasheet.

However, you should be keen not to assume that all the specifications are the same. They tend to var from one transistor manufacturer to another even for the same type and model of the transistor. This implies that you should confirm from the specific manufacturer. This is vital especially when you are trying to find the datasheet online. If you bought a transistor from a manufacturer ‘Y’, ensure that the datasheet is from that manufacturer and not any other source.

It is also prudent to confirm the parameters from second and third sources so as to be sure about the specs.

Meanwhile, here are the key parameters and specifications that you should look out for:

Transistor type number

The type number is a unique identification tag that is assigned to every type of transistor. With this number, you will be able to retrieve all the data about the transistor, including its specifications. In case you are on the manufacturer’s website, you will use the type number to get the datasheet of the transistor in question. Also, it is from the type number that you will have quick information about the performance of the transistor.

How do I read the transistor-type number? Well, there are three main international schemes that are used for denoting the type number of a transistor. These are the US JEDEC (here numbers start with 2N for transistors), the European Pro-Electron number scheme, and the Japanese number system (here numbers start with 2S)

The transistor-type numbers are more than just unique identifiers. They are also used for giving clues about the performance of the transistors. However, this mainly applies to the European Pro-Electron number scheme as it distinguishes the performance and expectations of each type of transistor. From the different types, you can easily predict the level of performance that you should expect from a transistor.

Let’s say you see the type number   BC109 on a transistor datasheet. This is a silicon transistor that is used for audio frequency and has low power. In case you find another transistor type number BFR90, it is interpreted as a low-power Radio frequency transistor.

However, the issue of transistor type number is not uniform or universal across all the datasheets. Sometimes when the PCB assembly and surface mount techniques are used, it can be quite hard for the transistor to be labeled with the full numbers. In such cases, transistor manufacturers usually deploy arbitrary condign systems whereby three unique character codes are used.

The coding system works perfectly on the surface mount transistor packages. However, this coding system is not that straightforward as compared to the type number system. You will have to refer to various codebooks and number codes that give details about different types of transistors and their numbering codes.

Transistor material

In as much as it may not appear to be an electronic parameter, it has a direct impact on the overall performance and functionality of the transistor. This is why it is also added on the transistor datasheet.

As we have already mentioned, transistors are made of semiconductor materials. However, there are different types of semiconductor materials and each type has a different level of performance. This is why transistor manufacturers indicate on the datasheet the exact type of semiconductor material that has been used on their product.

Some of these materials include silicon, gallium arsenide, and germanium. Most transistors in the market are made of silicon semiconductor material, thanks to its ease of availability and low price tag.

So, which transistor material should I choose? It will be prudent to take time to get all the relevant information about each material that is used in each type of transistor.

Other than being the cheapest semiconductor material silicon also comes with reliability and delivers satisfactory performance. For example, its base-emitter junction voltage is 0.6 volts, a figure that is way higher than that of germanium which tilts between 0.2-o.3 volts.

The polarity of the transistor

The data sheet will also give details about the polarity of the transistors. When it comes to polarity, you should know whether it is an NPN or PNP transistor. The configurations of these two transistor types usually result in different polarities when included in the electric circuits. NPN transistors are quite popular and this is mainly because they have electrons as their majority charge carriers.

By knowing the types of transistors, you will be able to tell its polarity and as well as configuration.

How can I tell the polarity based on the information printed  on the datasheet? Different numbers are used to show the polarity. “BC557” or “2N3906” reveal that the transistor is of PNP type. On the other hand, the number codes “2N3904” and “BC547” indicate that it is an NPN transistor type.

Other than the numbers, the datasheet will give more details about the polarities of the transistors.

Collector to base breakdown voltage VCBO

Your transistor datasheet should also have the collector to base breakdown voltage (VCBO). This is the maximum voltage coming from the collector to the base of the transistor when the emitter is open. The VCBO should not exceed the operation voltage of a circuit.

You should pay close attention to this parameter because in most cases, there will be some leakage electric current that will flow between the collector and the base terminals of the transistor. This current will eventually generate some heat which will damage the collector-base junction. To avoid any possibility of such damage, the collector-base breakdown voltage should not be within the rated margin. Additionally, the VCBO is always higher than the collector-emitter breakdown voltage VCEO.

Collector to emitter breakdown voltage  VCEO

The transistor datasheet will also specify the collector-emitter breakdown voltage which is usually indicated as VCEO. This is the maximum voltage between the collector and the emitter of the transistor when the base terminal is open.

Exceeding the rated voltage will also result in physical and electrical damage to the regions between the collector and the emitter. However, it is allowed to go up to 60% of the rated collector to emitter breakdown voltage. Again, this is usually indicated on the transistor datasheet.

Collector current (IC)

On your datasheet you will also see a defined collector current specification. It is also symbolized by the Ic and is calibrated in amps or milliamps. Like all other major specifications in the transistor, the collector current should not go beyond the specified value.

Collector emitter saturation voltage VCEsat

This is the rated saturation voltage across the collector-emitter terminals when the transistor s connected to the circuit and there is a flow of electric current through it. This value is usually defined by the current values of both the collector and the base. Usually, the voltage between the collector and the emitter is lower than that between the base and the emitter junction. In most transistors, the saturation voltage tilts within 0.2 volts.

Frequency transition (FT)

The Frequency transition (fT) specifies the frequency at which the transistor changes or transitions from operating as a low-frequency transistor to a high-frequency transistor. This specification is usually vital when you intend to use the transistor for the amplification purpose. In case you intend to use a bipolar junction transistor for the radio frequency application, then you should pay attention to its fT

Forward current gain (hFE

On a transistor datasheet, this value refers to the general current characteristics of the transistor.  The “h” stands for the hybrid parameter while the “e” represents the emitter configuration of the transistor.

Forward current gain is obtained by dividing the collector current by the base current.

Transistor power dissipation Ptot

The datasheet should also state the amount of power that is dissipated from the transistor. This is usually quoted in terms of the total power dissipated in a given ambient temperature which is 25C. The power dissipated is calculated by taking the current flowing through the collector multiplied by the voltage across the transistor.

Transistor package type

This is another parameter or specification that you will find on a transistor datasheet. The packaging is usually denoted by the letters TO (transistor outline) followed by  numbers.

For example, TO5, TO18, SOT54, TO92 numbers for the leaded transistor. On the other hand, surface mount transistors have numbers such as SOT-23 and SOT-223 outlines.

At this point, you should be able to explain the details of all the information on the transistor datasheet. Also, when buying transistors, confirm with the supplier about the datasheet.

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