Types of Transistors

Types of Transistors

Transistor Types

There are many different types of transistors and they each vary in their characteristics and each have their own advantages and disadvantages.

Some types of transistors are used primarily for switching applications. Others can be used for both switching and amplification. Still other transistors are in a specialty group all of their own, such as phototrransistors, which respond to the amount of light shining on it to produce current flow through it.

Below we show a list of the various types of transistors; we will go over the characteristics that make them each up.

Bipolar Junction Transistors

BJT Transistor

Bipolar Junction Transistors are transistors which are made up of 3 regions, the base, the collector, and the emitter. Bipolar Junction transistors, unlike FET transistors, are current-controlled devices. A small current entering in the base region of the transistor causes a much larger current flow from the emitter to the collector region.

Bipolar junction transistors come in two main types, NPN and PNP. A NPN transistor is one in which the majority current carrier are electrons. Electron flowing from the emitter to the collector forms the base of the majority of current flow through the transistor. The other type of charge, holes, are a minority. PNP transistors are the opposite. In PNP transistors, the majority current carrier are holes.

Overall, bipolar junction transistors are the only type of transistor which is turned on by current input (input into the base). This is because BJTs have the lowest input impedance of all transistors. The low impedance (or resistance) allows current to flow through the base of the transistor. Because of this low impedance also do BJTs have the highest amplification of all transistors. The downside of BJTs is because they have low input impedance, they can cause loading in a circuit. Loading is when a device can draw significant current from a circuit, thus disturbing a circuit's power source.

Field Effect Transistors


Field Effect Transistors are transistors which are made up of 3 regions, a gate, a source, and a drain. Unlike bipolar transistors, FETs are voltage-controlled devices. A voltage placed at the gate controls current flow from the source to the drain of the transistor.

Field Effect transistors have very high input impedance, from several megohms (MΩ) of resistance to much, much larger values. This high input impedance causes them to have very little current run through them. (According to ohm's law, current is inversely affected by the value of the impedance of the circuit. If the impedance is high, the current is very low.) So FETs both draw very little current from a circuit's power source. Thus, this is ideal because they don't disturb the original circuit's power elements to which they are connected to. They won't cause the power source to be loaded down. The drawback of FETs is that they won't provide the same amplification that could be gotten from bipolar transistors. Bipolar transistors are superior in the fact that they provide greater amplification, even though FETs are better in that they cause less loading, are cheaper, and easier to manufacture.

Field Effect Transistors come in 2 main types: JFETs and MOSFETs. JFETs and MOSFETs are very similar but MOSFETs have even higher input impedance values than JFETs. This causes even less loading in a circuit.

Types of Transistors By Function

Now we will go over the types of transistors by function, meaning what they do or, rather, are designed to do. Some transistors are used primarily for switching. Others more so for amplification.

We discuss this below:

Small Signal Transistors

Small Signal Transistors

Small Signal Transistors are transistors that are used primarily to amplify low-level signals but can also function well as switches.

Transistors come with a value, called the hFE values, which denotes how greatly a transistor can amplify input signals. Typical hFE values for small signal transistors range from 10 to 500, with maximum Ic (collector current) ratings from about 80 to 600mA. They come in NPN and PNP forms. Maximum operating frequencies range from about 1 to 300 MHz.

As a design note, small signal transistors are used primarily when amplifying small signals, such as a few volts and only when using milliamperes of current. When using larger voltage and current (larger power), using many volts or amperes of current, a power transistor should be used.

Small Switching Transistors

Small Switching Transistors

Small Switching Transistors are transistors that are used primarily as switches but which can also be used as amplifiers. Typical hFE values for small switching transistors range from 10 to 200, with maximum Ic ratings from about 10 to 1000mA. They come in NPN and PNP forms.

In terms of for design, small switching transistors are used primarily as switches. Though they may be used as an amplifier, their hFE value only ranges to about 200, which means they are not capable of the amplification of small signal transistors, which can have amplification of up to 500. This makes small switching transistors more useful for switching, though they may be used as basic amplifiers to provide gain. When you need more gain, small signal transistors would work better as amplifiers.

Power Transistors

Power Transistor

Power transistors are suited for applications where a lot of power is being used- current and voltage.

The collector of the transistor is connected to a metal base that acts as a heat sink to dissipate excess power.

Typical power ratings range from around 10 to 300 W, with frequency ratings from about 1 to 100 MHz. Maximum Ic values range between 1 to 100 A. Power transistors come in NPN, PNP, and Darlington (NPN or PNP) forms.

High Frequency Transistors

High Frequency Transistor

High Frequency (RF) Transistors are transistors that are used for small signals that run at high frequencies for high-speed switching applications.

These are transistors that are used for high frequency signals and must be able to switch on and off at very high speeds. High frequency transistors are used in HF, VHF, UHF, CATV, and MATV amplifier and oscillator applications. They have a maximum frequency rating of about 2000 MHz and maximum Ic currents from 10 to 600mA. They are available in both npn and pnp forms.



Phototransistors are light-sensitive transistors.

A common type of phototransistor resembles a bipolar transistor with its base lead removed and replaced with a light-sensitive area. This is why a phototransistor has only 2 terminals instead of the usual 3. When this surface area is kept dark, the device is off. Practically, no current flows from the collector to emitter region. However, when the light-sensitive region is exposed to light, a small base current is generated that controls a much larger collector-to-emitter current.

Just like regular transistors, phototransistors can be both bipolar or field effect transistors. Field-effect phototransistors (photoFETs) are light-sensitive field-effect transistors. Unlike photobipolar transistors, photoFETs use light to generate a gate voltage that is used to control a drain-source current. PhotoFETs are extremely sensitive to variations in light and are more fragile, electrically speaking, than bipolar phototransistors.

Unijunction Transistors

Unijunction Transistor

Unijunction transistors are three-lead transistors that act exclusively as electrically controlled switches; they are not used as amplifiers.

This differs from other transistors in that general transistors usually provide the ability to act as a switch and also as a an amplifier. But a unijunction transistor does not provide any decent type of amplification because of the way it is constructed. It's simply not designed to provide a sufficient voltage or current boost.

The three leads of a unijunction transistor are B1, B2, and an emitter lead, which is the lead which receives the input current. The basic operation of a UJT is relatively simple. When no potential difference (voltage) exists between its emitter and either of its base leads (B1 or B2), only a very small current flows from B2 to B1. However, if a sufficiently large positive trigger voltage- relative to its base leads- is applied to the emitter, a larger current flows from the emitter and combines with the small B2-to-B1 current, thus giving rise to large B1 output current. Unlike other transistors- where the control leads provide little additional current- the UJT is just the opposite. Its emitter current is the primary source of current for the transistor. The B2 to B1 current is only a very small amount of the total combined current. This means that unijunction transistors are not suitable for amplification purposes, but only for switching.

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Related Resources

Difference Between an NPN and PNP Transistor

BJT vs FET (Transistors)

JFET vs MOSFET (Transistors)

Transistor Schematic Symbols

How to Test a Transistor

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