What to look for in a BJT transistor datasheet

In a previous article, we discussed the fundamental concepts of a bipolar junction transistor (BJT), which are the building blocks of electronics. We covered the two types of BJTs—NPN and PNP—including their construction, I-V characteristics, circuit configurations, applications, and types.

Typically, transistors are used in a circuit or a circuit stage as a switch or an amplifier. Connecting a transistor in a circuit allows a large current between a collector and an emitter to be controlled by regulating a small amount of current at its base. Despite having a basic understanding of transistors, selecting an ideal model for a given circuit can be challenging.

After determining the intended application (switch or amplifier) and configuration (common emitter/common base/common collector) of the transistor in a circuit stage, it’s important to consider the specific circuit parameters and find a transistor that meets those specifications.

There are many transistor models available. Most are surface-mount devices, and some are still in leaded packages. A transistor model typically has the exact specifications irrespective of its packaging. However, each model will have specifications that differ in terms of heat dissipation, lead inductance, and physical dimensions.

These specifications are listed on the transistor’s datasheet. Each specification relates to an aspect of the transistor performance in different circuit scenarios. Some of these specifications are non-electrical but should also influence whether that transistor is ideal for a circuit.

In this article, we’ll discuss the key specifications of a bipolar junction transistor and compare the transistor specifications with the circuit requirements for use with a switch and an amplifier. Knowing which specifications to look for in a transistor datasheet is essential for selecting the right one for a given circuit or circuit stage.

Why specifications are required
The technical specifications of a transistor or any electronic/electrical component are part of its standardization. These specifications are the outcomes or measurements obtained through the manufacturer’s testing of the component in standard conditions and define how a particular model behaves in different circuit conditions.

Additionally, manufacturers must ascertain the specifications for each transistor model so it’s predictable and can be properly selected for a specific application or circuit. The datasheet compiles the specifications and serves as a guide for the component, its conduct in various conditions, and its possible applications.

Start with the part number
Consider these two situations. In one, you’re designing a circuit and know the required circuit parameters, but you do not know which transistor to use. In the other situation, you already have a transistor but must select other components for the circuit or modify the circuit conditions. Here, the circuit must be tuned to the transistor’s known parameters to serve the application properly.

In both cases, the transistor’s part number should be the starting point. This number can provide a shortlist of suitable transistor models if you’re looking for the ideal transistor model for a circuit design with known conditions or parameters.

If you already have a transistor, its part number is the only way you can access its datasheet or technical specifications. That number is assigned by the manufacturer and identifies specific details about the transistor like its type, polarity, package, and I-V characteristics.

The part number is a unique identifier assigned to a component by the manufacturer. Although there’s no standardized, global format for this, manufacturers generally follow a regional scheme for standardization.

There are three main international schemes for assigning commercial part numbers: the European Pro-Electron scheme, US JEDEC, and Japanese JIS. However, several legacy components do not follow any specific scheme.

The components manufactured for military and aerospace applications have strict part numbering schemes. The part number is the unique identifier of the component and is the only way to access its technical specifications. With this number, you can search for the datasheet of the component. Many transistor code reference resources and online vendors like Mouser, DigiKey, and Farnell categorize transistors by their part numbers or by their functionalities.

Most transistor models follow a regional scheme for their commercial part numbers. The European Pro-Electron scheme mainly focuses on assigning part numbers to signal diodes, bipolar junction transistors, and field-effect transistors. In this scheme, the commercial part number starts with a two-letter prefix followed by a number. For transistors, the first letter indicates the transistor material.

For example, ‘B’ indicates silicon transistor, ‘A’ indicates Germanium, and ‘C’ indicates Gallium Arsenide. The second letter indicates the type of transistor, like ‘X indicates diode, ‘A’ indicates NPN transistor, ‘B’ indicates PNP transistor, and ‘F’ indicates N-channel FET. The number provides additional details, which may not be standardized or determined by the manufacturer. This scheme is convenient for sorting transistors by their type and applications.

The JEDEC scheme is primarily used in the United States. The part numbers begin with a number or a letter followed by a number. There’s no single specific scheme in JEDEC. Rather, it outlines general guidelines for part numbers. The Japanese system is more complex and assigns an alphanumeric code depending on the manufacturer and transistor type. The specifications of a component identified by JIS must be checked by the datasheet as the identifier provides minimum clues about the component features.

Key specifications of a transistor
The key specifications of a bipolar junction transistor in a datasheet are as follows.

Polarity: A bipolar transistor can be an NPN or PNP type. The polarity in the European Pro-Electron scheme is indicated in the commercial part number. However, if that’s not the case, the polarity must be verified by the transistor’s datasheet.

NPN transistors are more common as most of the charge carriers are electrons. As electrons have higher mobility than holes, NPN transistors are more efficient. For these transistors, the negative supply line (i.e., ground) is the common point between the input and the output stage. This makes the circuit design simpler than PNP transistors, which require a positive supply line for the common point between the input and the output stage. This is one reason NPN transistors are the preferred choice for digital and dc circuits, including many analog circuits.

Transistor material: In the European Pro-Electron scheme, the transistor material is indicated in the commercial part number, but this is not always the case in other schemes. In all cases, it’s important to consider the transistor material. For instance, silicon transistors are commonly used in electronic circuits and have a higher turn-on voltage (0.7V) than Germanium transistors (0.3V). Silicon is more abundant, offers better thermal stability, and its oxidation provides better stability. The manufacturing technology of silicon transistors is more mature than that of other materials.

Terminal voltages and currents: Each transistor model has a maximum limit of terminal voltages and currents. The maximum voltage that can be applied between the collector and the base of the transistor is indicated as VCB (collector-base voltage). The maximum voltage that can be applied between the collector and the emitter is indicated as VCE (collector-emitter voltage). The maximum voltage that can be applied between the emitter and the base of the transistor is indicated as VEB (emitter-base voltage). The maximum current that can flow through the collector is indicated as IC (collector current). The maximum current that can flow through the base is shown in the IB (base current). These voltages and currents should be well above those that might be applied to the transistor in the circuit.

The collector current is a crucial factor in switching and amplifier applications. If the current in the circuit exceeds the collector current, there’s a strong chance the transistor will get damaged after connecting with the circuit. For small-signal transistors, the value of the maximum collector current is typically in the order of milliamperes. For power transistors, this value could be in amperes. The maximum base current plays an important role in the biasing of the transistor and in determining the efficiency of its current gain.

Collector-emitter breakdown voltage (BVceo): This is the maximum voltage that can be applied between the collector and the emitter terminals before a transistor breaks down. This voltage is measured by keeping the base terminal open. The operating voltage of the circuit should not exceed 50 to 60% of this value. If inductors are used in a circuit, its voltage can go twice as high as its rail voltage. So, it’s important that all posible voltage spike and fluctuation considerations are made to ensure they do not exceed the breakdown voltage.

Collector-base breakdown voltage (BV_cbo): This is the maximum voltage that can be applied between the collector and the base terminal before a transistor breaks down. This voltage is measured by keeping the emitter terminal open. The BV_cbo is always greater than BV_ceo. A voltage applied higher than this rating (between the collector and the base of the transistor) can leave the base terminal damaged.

Base-emitter breakdown voltage (BV_ebo): This is the maximum voltage that can be applied between the emitter and the base terminal before a transistor breaks down. This voltage is measured by keeping the collector terminal open.

Dc current gain (hFE): This is the ratio of change in the collector current compared to the change in the base current for constant collector-emitter voltages. In some datasheets, the dc current gain is specified for a specified collector current and, in others, it’s for a range of values. A higher value of hFE means a bigger change in the collector current and a small change in the base current. This is useful for amplifier applications. The gain is calculated for common-emitter configuration. There are two versions of the specification — one is for the dc signals (indicated as hFE), and the other is for the ac signals (shown as hfe).

Collector-emitter saturation voltage (VCEsat): This is the voltage between the collector and the emitter of the transistor when the transistor is fully on. When the transistor is on, the voltage between the collector and emitter is smaller than between the base and the emitter. In a datasheet, the collector-emitter saturation voltage is given for specific base and collector currents. A lower value of collector-emitter saturation voltage indicates a lower power dissipation, faster switching, and linearity in amplifier operation.

Base-emitter saturation voltage (VBEsat): This is the voltage between the base and the collector of the transistor when the transistor is fully on. A lower value of base-emitter saturation current indicates lower power dissipation and faster switching time.

Power dissipation (P_Dmax): This is the maximum power transistor can dissipate without overheating or getting damaged. The value is often quoted for ambient temperature. The actual power dissipation of the transistor is the product of voltage across the transistor and the collector current flowing through it.

Transition frequency (fT): This is the frequency at which the current gain of the transistor falls to ‘1.’ The transition frequency is significant in the design of high-frequency amplifiers, as it’s the maximum frequency at which the transistor can effectively amplify an input signal. It sets the upper limit of the bandwidth of the circuit.

Operating temperature range: This is the range of the ambient temperature in which the transistor can operate without getting damaged or breaking down.

Temperature coefficients of key parameters such as hFE, V_BE, and V_CEsat: The temperature coefficients indicate how a parameter changes with respect to temperature. The dc current gain of a bipolar junction transistor has a negative temperature coefficient. It decreases as the temperature increases. The temperature coefficient of the dc current gain varies based on the manufacturer and the transistor type. The base-emitter voltage also has a negative temperature coefficient and it decreases with the increase in temperature. The collector-emitter saturation voltage has a positive temperature coefficient. It increases with a rise in temperature. As the collector-emitter saturation voltage is important in switching applications, the effect of the temperature coefficient must be considered in a circuit design.

Package type: Bipolar junction transistors come in various packages, including leaded and SMD. In an JEDEC scheme, the package identifier for leaded transistors begins with the letters ‘TO,’ followed by a number that’s a maximum of three digits. The package identifiers for SMD transistors begin with the letters ‘SOT,’ followed by a number, which can be a maximum of three digits. Most commercial transistors come in SMD packages, as they’re compliant with the automated PCB assembly techniques.

Noise figure: The noise in bipolar junction transistors is expressed as Equivalent Noise Resistance (Rn) or Noise Figure (NF). Equivalent Noise Resistance is a theoretical resistance expressed in ohms representing the BJT transistor’s thermal noise. The transistors designed for audio applications instead have noise expressed as Noise Figure. It is the logarithmic ratio of the signal-to-noise ratio at the input of the transistor to the signal-to-noise ratio at the output of the transistor. The Noise Figure is expressed in decibels. A transistor’s Noise Figure or Equivalent Noise resistance should be as low as possible.

Linearity: Linearity is essential for minimizing signal distortion in an audio amplifier. Two main parameters influence the linearity of a transistor: transconductance (gm) and collector output capacitance (Cob). Transconductance is the change in collector current for a small change in base-emitter voltage. For better linearity, the transconductance value should be as high as possible. The collector output capacitance is the capacitance between the collector and the base terminal of the transistor. The Cob should be as low as possible for better linearity.

Most transistor specifications are independent of the transistor type (i.e., NPN or PNP type), except for a few parameters.

For instance:

The dc current gain is generally higher for NPN transistors than PNP transistors.
The main difference between an NPN and a PNP transistor is their biasing.
NPN transistors require their base-emitter junction to be forward-biased and the base-collector junction to be reverse-biased to operate in a circuit.
PNP transistors require their base-emitter junction to be reverse-biased and the base-collector junction to be forward-biased.

Specifications checklist for a switching transistor
When looking for a transistor to use in a switching application, first determine the circuit conditions. This should include the load current, supply voltage, and control voltage. The load current will be the maximum current required by the transistor in the circuit to switch on or off. The supply voltage is the voltage powering the circuit. The control voltage could be the voltage signal from a microcontroller or a logic circuit applied at the input of the BJT transistor to switch it on or off. If the control voltage is positive with the ground, an NPN transistor will be required for switching. If the control voltage is negative with the ground, a PNP transistor is needed for switching.

Next, tally the transistor specifications with the circuit requirements. The transistor’s maximum collector current should always be greater than or equal to the load current. The maximum collector-emitter voltage of the transistor should also be greater than or equal to the circuit’s supply voltage. The maximum base-emitter voltage of the transistor should be equivalent to the control voltage in the circuit. The maximum power dissipation of the transistor should be enough to handle the load current and supply voltage. It’s ideal to use a switching transistor with sufficient dc current gain so that driving the transistor in the circuit would become easy. If it’s an ac circuit, the transition frequency of the transistor should be sufficient on the higher side to handle high-frequency signals.

Specifications checklist for a transistor amplifier
If you’re looking for a BJT transistor to use as an amplifier, such as an audio amplifier, you must confirm the circuit requirements. First, identify the amplifier stage (i.e., input stage, voltage gain stage, or output stage) where the transistor will be used. Also, determine the nature of amplification, such as whether the circuit is a power or a small signal amplifier, as well as the required current and voltage gain in the circuit.

Once you’re sure of the circuit requirements of the amplifier, compare the transistor specifications with those of the amplifier. The maximum collector current of the transistor should be greater than or equal to the current needed for the amplifier’s output stage load. The maximum collector-emitter voltage should be greater than the voltage variation expected between the BJT transistor’s collector and emitter . The noise figure of the transistor should be as low as possible. The greater the transistor’s current gain, the greater the voltage gain delivered in the amplifier circuit.

If it’s an ac circuit, the transition frequency of the transistor should always be more significant than the signal’s highest frequency component. In amplifier circuits, the linearity of the transistor plays an essential role in eliminating any signal distortion. As discussed above, the linearity of a BJT transistor depends on its transconductance and collector output capacitance. The transistor’s power dissipation must match the circuit’s power dissipation. A heat sink can be used to manage the transistor’s power dissipation.

Conclusion
A bipolar junction transistor is characterized by several specifications. Whether you are looking for a transistor to use as a switch or an amplifier, the transistor specifications must match the circuit requirements.