Selection Guide for Single Bipolar Transistors

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What is a bipolar junction transistor?

Discrete Bipolar Junction Transistors (BJTs) are commonly used to construct analog signal amplification functions in audio, radio, and other applications. One of the first semiconductor devices to be mass produced, their characteristics are less favorable than those of other device types for applications involving high frequency switching and operation with high currents or voltages, but they remain a technology of choice for applications requiring analog signal reproduction with minimal added noise and distortion.

BJTs come in two varieties - NPN and PNP - which refer to the sequence of semiconductor layers that make up the transistor. NPN transistors consist of a thin P-type semiconductor between two N-type materials, while PNP transistors have an N-type semiconductor between two P-types. This gives the two types opposite polarity operation. NPN transistors sink current, while PNP transistors source current.

Image showing the variety of single bipolar transistors offered by DigiKey.
Image showing the variety of single bipolar transistors offered by DigiKey.2068×1185 252 KB

An engineer’s perspective

Click to expand

Technology changes over time. The single-packaged BJT was the pinnacle of 20th century technology. There were many specialized transistors for a wide variety of applications including signal amplification, high frequency radio, digital logic, and power. As technology and manufacturing processes improved, many of these applications transitioned to advanced MOSFETs, IGBTs, and highly specialized integrated circuits. The BJT remains relevant for general purpose work and for niche application such as audio amplification.

One unfortunate result is that BJT replacements can be difficult to locate. For example, suppose you are attempting to repair a classic Marantz 2270 receiver. It used complementary Motorola output transistors type SJ2517 (NPN) and SJ2518 (PNP). It’s been 50 years since the receiver was manufactured, and those transistor types are no longer available. Consequently, you must locate suitable replacements using tools such as DigiKey’s parametric search engine. In this particular instance, the MJ21194G (NPN) and MJ21193G (PNP) are considered suitable replacements.

Must-know facts about bipolar transistors

  • Selecting a suitable replacement transistor is one of DigiKey customer’s most frequently asked questions. It’s not a trivial question as it takes time to carefully compare the original specification to a potential candidate. In rank order, we must verify package, pinout, design maximum parameters (VCE, IC, PD), and then more subtle parameters such as gain or saturation voltage. The process is further complicated as the end product may be dependent on the specific characteristic of the older and perhaps less capable transistor. Consequently, no substitution is complete without testing in the finished product.

  • Complementary Pairs: Many transistors are designed in complementary pairs with matching properties for the NPN and PNP devices. A classic example is the audio output section of an amplifier such as the previously mentioned the MJ21194G (NPN) and MJ21193G (PNP). However, the complementary devices are available for small signal devices such as the KSC1845FTA (NPN) and KSA992FBTA (PNP).

  • Heatsink: No conversation about transistors is complete without exploring the concept of a heatsink. I remember my first experiments with a 2N3055 transistor where I attempted to dim a 55 W automotive headlamp. While the transistor is more than capable of handling the current, it got very hot. A large aluminum plate took care of the situation. Today, I would recommend a switching power supply. However, we all need to start somewhere.

  • Darlington: A student’s first encounter with a transistor is typically associated with a microcontroller. Here, the transistor is used as a buffer between the microcontroller and a load such as a relay or motor. There is more to this “simple” application than most people realize as the transistor must be fully turned on with a base current about 1/10 of the collector current (forced beta condition). At the same time, we need to be careful not to overload the microcontroller’s I/O pin. The Darlington transistor can improve the situation as the high gain device is easy to drive. Instead of the microcontroller supplying a 1/10 base, the Darlington transistor requires 1/100 of the collector current.

  • Circuit design: Transistor based audio amplifiers remain popular and can be a rewarding hobby. To learn more, please review the works of Douglas Self including “Audio Power Amplifier Design Handbook” and “Small Signal Audio Design.” I learn something new every time I read the material. Be sure to visit Elliott Sound Products and PassDiy. Feel free to share other resources in the comments section.

Looking forward to continuing the conversation on this forum.

Best wishes,

Aaron Dahlen
LCDR USCG (Ret.), MSEE
DigiKey Applications Engineer

P.S. The history of the transistor is captured by Jack Ward at The Transistor Museum. The oral history narratives are fascinating.

Introduction to DigiKey’s Product Selection Guide

This page is one of many in DigiKey’s Product Selection Guide (PSG). This particular page is focused on single bipolar transistors. It provides a description of the individual parameters used to characterize the part. This information will allow you to better understand and navigate DigiKey’s parametric search engine. Click here for a case study showing how to use the search tools.

How are bipolar transistors categorized?

  • Transistor Type: Enumerates product characteristics such as polarity, number of devices incorporated in a package, internal connections among devices, etc.

  • Current - Collector (Ic) (Max): The maximum current value the device can sustain at its collector without risking damage.

  • Voltage - Collector Emitter Breakdown (Max): The maximum voltage value the device can sustain at its collector without risking damage.

  • Vce Saturation (Max) @ Ib, Ic: Characterizes the voltage drop between a device’s collector and emitter terminals when a device is operated in saturation. Measured with the base and collector current values shown.

  • Current - Collector Cutoff (Max): Characterizes current flow through a device’s collector terminal when the device is in an “off” state.

  • DC Current Gain (hFE) (Min) @ Ic, Vce: Characterizes the ratio of collector current to base current for a bipolar transistor, under DC test conditions including the indicated values of collector current and collector-emitter voltage. Varies significantly among devices and with operating conditions.

  • Power - Max: Typically this represents the maximum power rating to which a device will meet its given specifications. Exceeding this power rating may damage the device and other system components.

  • Frequency - Transition: Characterizes a BJT’s frequency response, indicating the frequency at which a device’s current gain falls to a value of one.

  • Operating Temperature: Recommended operating temperature, typically given in a range or as a maximum. Exceeding these temperatures may affect performance or damage the device and other system components.

  • Mounting Type: Indicates how the device is attached.

  • Package / Case: Indicates the type of protective case used on an electronic component to allow easy handling, installation, and protection. This selection was determined to be the closest industry standard applicable to the supplier device packaging. Typically, it is best to check the actual dimensions from the datasheet rather than depend on this terminology when designing your circuit.

  • Supplier Device Package: This is what the manufacturer calls the package of this device. They are manufacturer specific. It is typically best to use the actual dimensions from the datasheets rather than to go by this terminology when designing your circuit.

Examples of single bipolar transistors

  • The onsemi brand PN2222ABU is a through-hole transistor in a TO-92 package. This is a good substitution for the classic 2N2222 transistor. The modern TO-92 package is less expensive than the original TO-18 metal can. Datasheet

Image of the onsemi PN2222ABU transistor in a TO-92 package.
Image of the onsemi PN2222ABU transistor in a TO-92 package.640×640 41.8 KB


Image of the Comchip MMBT3906-G PNP transistor in a SOT23-3 package
Image of the Comchip MMBT3906-G PNP transistor in a SOT23-3 package640×640 17 KB


  • The STMicroelectronics brand TIP125 is a through-hole Darlington PNP transistor in a TO-220 package. The TIP120 is NPN complement. Together, the complementary pair may be used in push-pull amplifier circuits. Datasheet

Image of the STMicroelectronics TIP125 PNP Darington transistor in a TO-220 package.
Image of the STMicroelectronics TIP125 PNP Darington transistor in a TO-220 package.640×640 22.8 KB


Schematic symbol

The schematic symbol for the PNP and NPN transistors are shown below.

PNP

image

NPN

image

Schematic symbols courtesy of Scheme-it.

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