A short and very basic guide to selecting a transistor

- Categories : Products

Are you struggling in selecting a transistor for your upcoming project? Does the thought of choosing the right transistor makes you feel nervous? If yes, then you’re at the right place!

In this post, we will walk you through the process of selecting the right transistor according to your application. Whether you’re planning to use the transistor as a switch or an amplifier, we’ve got everything covered!

What Is a Transistor?

Before jumping into the process of selecting a transistor, lets first understand what a transistor is. There are mainly two types of transistors – BJTs (Bipolar Junction Transistors) and FETs (Field Effect Transistors). Transistors serve the purpose of either amplification or switching in most electronic circuits. The voltages applied to its terminals determines the mode of operation of a transistor. 

Transistors consist of two types of regions – p-type and n-type. These regions are made by adding impurities to the semiconductor (usually silicon), and the process is called doping. To form a p-type region, Boron is used as a doping material. Since Boron has three electrons in its outermost shell, it pairs up with three electrons of Silicon, leaving a ‘hole’ in place of the fourth electron. This is how holes are formed and they produce a positive charge hence, the region is called a “p-type” region.

Similarly, to form an n-type region, Phosphorus (having five valence electrons) is used. Four of its electrons pair up with the four electrons of silicon and one electron remains free to move around. This creates an overall negative charge and hence, the region is called an “n-type” region.

A BJT is a semiconductor device made up of two p-n junctions connected back-to-back. It can have two types of configurations – PNP or NPN, depending on the doping concentrations. Usually, Silicon is used as a substrate inside a BJT and it is doped according to the voltage and current requirements. A BJT has three terminals – base, emitter and collector. If it is a PNP transistor, the base terminal is connected to the n-type region while the collector and emitter terminals are connected to each of the two p-type regions.  

FETs also have three terminals like BJTs but they are made using just one type of material as the main substrate i.e. either p-type or n-type. The three terminals are called gate, drain and source. The gate is connected to the main substrate while the source and drain are connected to the heavily doped p-type or n-type regions.

How Do Transistors Work?

When working as an amplifier, a transistor converts a low input current into a large output current, giving an amplified current at the output. When working as a switch, the transistor takes a small current as an input and uses it to drive a larger current at another place hence, the smaller input current switches on the larger current.

To understand how current flows through a transistor, consider two p-n junctions connected back-to-back. The majority carriers in the n-type regions are electrons while the majority carriers in the p-type region are holes. Considering we have an NPN transistor and we apply a negative voltage on the n-type region (emitter), the electrons flow away from the negative voltage, into the p-type region (base). We understand that the emitter-base region is forward biased.

The electrons which have entered the p-type region, few of them recombine with holes present in the base while others continue to flow towards the collector to constitute the collector current. The number of electrons flowing into the collector region can be varied by controlling the base. The collector-base junction is reverse biased because the collector is supplied with a positive voltage.

Now we know that transistors work when electrons flow from the emitter to the collector through the base, and by varying the doping concentrations and applied voltages at each of the three terminals, the mode of operation of the transistor can be controlled.

How to Connect Transistors?

Before you apply any voltage to your transistor, make sure that you refer to its datasheet and figure out which of its legs is the base, which one is the emitter and which one is the collector. Once you’re clear about that, then you can supply power to it. If you connect your transistor incorrectly, chances are that you might end up with a grilled transistor and a burning smell!

Usually, when connecting the transistor as an amplifier, the base-emitter junction is forward biased and the base-collector region is reverse biased. For instance, if you’re using an NPN transistor, then you must connect the positive voltage supply to the p-type region (base) and the negative terminal to the emitter which is made up of the n-type material. This makes the base-emitter junction as forward biased. Likewise, to reverse bias the collector-base junction, you must apply a positive voltage at the collector and a negative voltage at the base. The input to the amplifier is applied across the emitter-base junction and the output is obtained from the collector.

When connecting the transistor as a switch, the usual practice is to ground the emitter and apply the switching signal as an input at the base. The output load is connected at the collector which the transistor will switch on and off using the signal applied at the base. The transistor operates in the “saturation” and “cut-off” regions when it is switched ON and OFF respectively.

What Are the Key Characteristics of Transistors?

Here are some of the key characteristics of transistors which you must understand before buying a transistor for your upcoming project.

Collector Current

The maximum collector current for normal transistors is in milliamps while that of power transistors in in amperes. The maximum value of collector current mentioned on the transistor’s datasheet must not be exceeded.

Saturation Voltage

For a transistor to work in saturation mode, a specific voltage must be applied between the collector and emitter. You can easily find this voltage mentioned as VCE in the datasheet of the transistor. This voltage must be present between the collector and emitter to allow the transistor to enter saturation mode.

Breakdown Voltages

Two breakdown voltages – collector to base breakdown voltage and collector to emitter breakdown voltage are important characteristics of transistors. These values must not be exceeded during operation because the excess voltage would damage your transistor.

Current Gain

Another important characteristic is the forward current gain of the transistor, abbreviated as β. A small input current at the base is used to drive a larger current at the collector. The current at the base amplifies according to the value of β.

This characteristic is used in transistor-based amplifiers commonly found in RF circuits and other audio amplification circuits. Different applications require different current gains hence, it is important to check the value of β while selecting a transistor.

Material

Usually, transistors are made of silicon as the major semiconductor substrate. This is because silicon has excellent properties and offers a junction voltage of about 0.6 Volts. Other semiconductor materials are also used to manufacture transistors but they offer different properties and have a different junction voltage.

Polarity

As explained in the previous sections, transistors can either be PNP or NPN. This affects the polarity of the output voltage. Usually, we require a positive output voltage hence, NPN transistors are commonly used in many applications.

How to Choose a Transistor for Your Project?

While choosing a transistor for your project, you must be sure of the source voltage, power dissipation and operating currents that would be used in the project. This will allow you to decide which transistor to choose based on the above-mentioned parameters – saturation voltage, breakdown voltage, collector current, current gain. You can find these parameters in the manufacturer’s manual that accompanies the transistor. Moreover, you need to see whether you require a positive polarity on the output or a negative one as explained above.

Make sure the current and voltage values do not exceed the maximum values mentioned by the manufacturer else you would end up destroying your transistor. 

Share this content