Small-Signal Bipolar Transistors (BJT) Information

PNP Silicon Darlington TransistorSmall signal bipolar junction transistors (BJTs) depend upon the contact between two different types of semiconductor to switch or amplify electronic signals and power. Transistors are utilized in nearly every modern electronic device, and BJTs are frequently implemented as part of an integrated circuit.


Many types of transistors are manufactured, with the most quantifiable difference being the methods of construction. The first useful transistor, the bipolar junction transistor, was constructed by Bell Laboratories in the 1950s; it was so named because the current is created by combining positive and negative charges. BJTs are composed of three 'sandwiched' pieces of semiconductor material, but all semiconductor materials are either P-type (positive) or N-type (negative). As such, there are two basic configurations for BJTs: the NPN-type and the PNP-type. The current and voltage polarities for a PNP transistor are opposite that of a NPN transistor, but the two configurations otherwise behave alike.

Selecting NPN PNP bipolar junction transistors

Image credit: Geyosoft

The most common semiconductor materials include silicon (Si) and germanium (Ge). Combinations of semiconductor materials, such gallium arsenide (GaAs), are also utilized. As of 2013, extensive research on graphene as a semiconductor material may yield a new generation of transistors.


Two-terminal devices, such as diodes and resistors, are specified with one voltage and one current value, but transistors are designated with three voltages and three currents. Each of the three terminals on a BJT are labeled and a current is named after the terminal from which it flows; the voltages are the voltages between any two terminals. Terminals are named: emitter, collector, and base. In the diagram below, which represents an NPN transistor, currents are designated as: iE, the emitter current; iC, the collector current; and iB, the base current. The voltages are labeled VBE, VCE, and VCB. In the instance of a PNP transistor, these currents and voltages have opposite polarities.

Selecting NPN transistor diagrams

Image credit: Answers


A transistor can operate in three different states: cut-off, in which the transistor has no current output; active, in which the output collector current, iC, is controlled by the base current, iB; and saturated, where the transistor collector current reaches a maximum value and an increase in the base current has no effect on the collector current.

  • Cut-off: The transistor output current is zero or otherwise negligible. When the iC is zero, the output voltage is maximum (normally equal to the source bias voltage). Therefore, in cut-off mode, VCE = VCC, and iC = 0, where VCC is the bias voltage. This mode is the opposite of saturation.
  • Saturation: The transistor will produce the maximum current for the circuit, and the value of that current is dependent upon circuit parameters. The transistor is considered saturated when the collector-emitter voltage3 is close to zero (or > .2 V). Saturation can be expressed as VCE ≈ 0 and the iC is at maximum value. Saturation is when the transistor has maximum current but minimum output voltage; the exact opposite of cut-off mode.

  • Active: In this mode, the output current, iC, is controlled by the input current, iB. Active mode is helpful when designing current and voltage amplifiers. The relationship between currents is expressed by iE = iC + iB, and iC = β · iB. By using the latter formula, we can express the emitter current as iE = (β+1) · iB. This demonstrates that the collector and emitter currents are functions of the input current iB. When a controlling current is the base current it is known as a current-controlled device.


Transistors in circuits will typically include a grounded terminal. As such, there are three ways to connect the transistor in the circuit.

1. Common base: The base terminal is grounded and becomes the common terminal between the input (emitter) and output (collector).

Image credit: All About Electronics


2. Common emitter: The emitter terminal is grounded and serves as the common terminal between the input (base) and output (collector). This is the most important grounded-terminal configuration as the circuit can produce a higher output current and voltage than the corresponding input.

Image credit: All About Electronics

3. Common collector: The collector is the common terminal to the input (base) and the output (emitter).

Image credit: Wikimedia


Most commonly, BJTs are classified by their polarity or material of fabrication.

name description symbol
NPN transistor The physical arrangement of the transistor places a base of P-type material between two layers of N-type material.

PNP transistor The arrangement of the transistors places a base of N-type material between two layers of P-type material.

Images credit: Wikimedia

Or, if nominated by material, the BJT would be called a silicon transistor, germanium transistor, etc.


The following parameters are important when considering bipolar junction transistors:

Forward transfer ratio (also known as current gain): A transistor can supply a large output, iC, with a small input, iB, thereby making it a current amplifier—one of a BJT's most essential functions. The ratio by which the iC is larger than the iB is called the DC current gain, and is represented by the Greek letter β. Though β is typically supplied by manufacturers, it can be ascertained with the formula β = iC / iB. β is the fundamental value of a BJTs ability to amplify. While ß is expected to remain a constant value, manufacturers may also provide a range of ß (also occasionally represented as hfe) to cover the applicable range of the transistor, which are usually defined as maximum, minimum, and mean. For example, one popular small-signal BJT, the 2N3903, has a ß range of 15 to 150, which varies in accordance with the collector current. ß is highest for medium collector currents, while decreasing for high and low collector currents.

Collector-to-emitter breakdown voltage (VCEO) is the maximum acceptable voltage that can be applied continuously in the reverse direction of collector junction with the emitter open.

Maximum collector current is the highest degree of current that can be used in the collector. 

Unity gain bandwidth the product of which is the frequency at which hFE is in unity. 

Power dissipation is the overall power consumption of the BJT, often expressed in watts or milliwatts. When a transistor conducts current between the collect and emitter, it also drops voltage between those two points. The power dissipated is equal to the product of the collector current and collector-emitter voltage. Transistors are rated in accordance with their ability to dissipate wattage without damage. High temperature is destructive to all semiconductor devices, and BJTs are especially susceptible to thermal damage. Power ratings account for the ambient temperature of the operating environment. If a transistor is to be used in a warm setting, the power rating can be derated to improve service life.

Transistor Packages

The accompanying list provides the most common packages of transistors.




TO-3 is a transistor outline (TO) package.


TO-8 is a transistor outline (TO) package.


TO-39 is a transistor outline (TO) package.


TO-92 is a single in-line, transistor outline (TO) package that is often used for low power devices. One of the oldest power packages, TO-92 is suitable for applications in office and communication equipment.


TO-202 is a transistor outline (TO) package.


TO-220 is a transistor outline (TO) package that is suitable for high power, medium current, and fast-switching power devices. TO-220 is used in home appliances, office and industrial equipment, and personal and consumer electronics. A package variant, TO-220 Full Pack, includes a fully encapsulated heat sink that does not require extra hardware for electrical isolation. TO-220 Full Pack has the same footprint as TO-220, provides electrical isolation up to 5 kV, and is often used in motor drive applications and power supplies.


TO-223 is a transistor outline (TO) package.


TO-237 is a transistor outline (TO) package.


TO-247 is a large, through-hole, transistor outline (TO) package. TO-247 provides excellent power dissipation and is ideal for metal oxide semiconductor field effect transistors (MOSFETs), high power bipolar transistors, and insulated gate bipolar transistors (IGBTs).


TO-263 is the surface-mount version of the TO-220 package. TO-263 is a transistor outline (TO) package with 2, 3, 5, 6, or 7 leads. TO-263 can accommodate large dice because of its large pad design. It is suitable for high power applications due to its low resistance. Typical applications for TO-263 include home appliances and personal computers.


SO-8 is a small outline (SO) package.


SOT3 is a small outline transistor (SOT) package with three leads.


SOT23 is a rectangular, surface mounted, small outline transistor (SOT) package with three or more gull wing leads. SOT23 features a very small footprint and is optimized for the highest possible current. Because of its low cost and low profile, SOT23 is used in home appliances, office and industrial equipment, personal computers, printers, and communication equipment.


SOT25 is a surface mounted, small outline transistor (SOT) package with three leads.


SOT26 is a plastic, surface mounted, small outline transistor (SOT) package with three leads.


SOT82 is a rectangular, surface mounted, small outline transistor (SOT) package with three leads. SOT82 is larger than DPAK and smaller than TO-220, but still comparable to TO-220 in performance. SOT82 lead frames are available in full nickel or selective silver plating and allow wire bonding with gold or aluminum wire. Often, SOT82 is used in over-voltage protection devices in telecommunications equipment. SOT82 is also used to package high power rectifiers, DC to AC converters (DACs) and frequency converters.


SOT89 is a plastic, surface mounted, small outline transistor (SOT) package with three leads and a collector pad for good heat transfer. Unlike other packages, SOT89 lead posts are up-set and not down-set. SOT89 is designed for medium power and high-speed switching applications. It is also used in applications that feature very low RDS (on), no secondary breakdown, and direct interface to complementary metal oxide semiconductor (CMOS) and transistor-transistor logic (TTL).


SOT123 is a flanged, ceramic, surface mounted, small outline transistor (SOT) package with two mounting holes and four leads.


SOT143 is a plastic, surface mounted, small outline transistor (SOT) package with four leads.


SOT223 is a plastic, surface mounted, small outline transistor (SOT) package with four leads and a heat sink. During soldering, the formed leads absorb thermal stress and eliminate the possibility of damage to the die. The encapsulation material enhances device reliability, allowing SOT223 to provide excellent performance in environments with high temperatures and humidity levels. SOT223 provides power dissipation of 1W to about 1.5W.


SOT323 is a plastic, surface mounted, small outline transistor (SOT) package with three leads.


TO-251 and TO-252 are low to medium power packages that feature a transistor outline (TO). TO-251 uses through-hole technology (THT). TO-252 uses surface mount technology (SMT). Both packages provide several lead frame choices and are commonly used in home appliances, personal computers, lighting, and automotive systems.


Flat package (FPAK).


Transistors are used in many electrical and electronic applications. Transistors are the building blocks of all electronic devices and systems, from smart phones to the space shuttle. There is no other component in the history of electronics that has more use than the transistor.


Some key industrial standards to refer to when considering BJTs include:

BS IEC 60747-7 — Semiconductor devices: bipolar transistors

IEC 60747-7 — Semiconductor devices – Discrete devices – Part 7: Bipolar transistors

JEDEC JESD 24-6 — Thermal Impedance Measurements for Insulated Gate Bipolar Transistors


Encyclobeamia—Bipolar Junction Transistor (BJT)

HomoFaciens—Bipolar junction transistors

Wikipedia—Bipolar junction transistors; transistor

Image credit:

Infineon Technologies AG