Transistor as a Switch: A Comprehensive Overview

Introduction

We will discuss functions of transistor as a switch in this detailed article. Transistors find their applications in major switching and amplification purposes and also in many digital circuits where we see the transistor as a switch. We have already studied three operating modes of transistors. These modes include Active, Saturation, and Cut-off Mode. How transistor as a switch is possible in both NPN and PNP variants? And also in some real-world application circuits?

As we look around we find a common application for this three-terminal semiconductor device “transistor” which is in signal amplification and switching. Transistors are one of the most important components in power electronics circuits. We see its applications in embedded systems, adapters, drives, RF Systems, wireless, communication, digital circuits, control systems, and in almost every electronic equipment in the forms of integrated circuits.

You can find the basics of transistors including their operation in our previous article regarding BJT Modes and BJT configurations. While we will try to stick to our today’s topic regarding the switching characteristic of transistors and how can we see a transistor as a switch.

1. Quick Overview of BJT

There are two main categories of transistors which include bipolar junction transistors normally we refer to as “BJT”. The other type is the field effect transistor which we refer to as the “FET”. Both types of transistors can perform the function of “transistor as a switch”.

Let’s focus on BJT, it has three terminals or regions which we denote as the collector, base, and emitter. These regions are made of N-Type and P-Type Materials which when combined, form PN junctions. Now, these PN junctions and materials arrangement tells us that there are two types NPN and PNP. Let’s have a look at below construction of BJT.

Now, depending upon these three terminals we can connect transistors in the circuit in three different ways. We call these connections as BJT configurations. These three configurations include a common base, a common collector, and the common emitter configuration. Want to know which configuration is used more in the circuits? Click here to read why common emitter configuration is used more. So, coming back to a topic that how does a transistor work as a switch? Let’s quickly overview BJT modes.

2. Overview of BJT Modes

Depending upon the biasing or supply connections with the BJT, PN junctions of BJT are either reverse-biased or forward-biased. Without going into detail about the construction of BJT and the terminal doping mechanism which you can go through here; let’s see how BJT modes work. As we discussed PN junctions are either forward-biased or reverse-biased; there are three different modes of BJT. These BJT modes include active mode, cut-off mode, and saturation mode of operation.

2.1 Active Mode

Current amplifiers mostly employ this mode of bipolar junction transistor “BJT”. There are two main junctions of BJT, the emitter-base junction and other is a collector-base junction. The emitter-base junction in this mode is forward biased and the collector-base junction operates in reverse-biased mode. This is how the maximum flow of charge carriers occurs through the emitter and the collector region. The output of the collector current is therefore controlled by the base region current.

2.2 Saturation Mode

In saturation mode of operation, both junctions emitter-base and the collector base operates in forward-biased mode. Therefore, the bipolar junction transistor “BJT” behaves like a short circuit in this mode of operation and most digital circuits employ this mode for switching.

2.3  Cut-Off Mode

In cut-off mode, the emitter base and collector base are reverse biased and hence there is practically no current flow. This mode of operation is called the cut-off mode of the bipolar junction transistor “BJT”. This mode is basically used in switching applications and many logic circuits. So, let’s discuss how can we see a transistor as a switch.

3. Transistor as a Switch

For switching applications we normally see relays functioning in the circuits and opening or closing the circuits. But when it comes to high accuracy and low current values, solid-state devices outnumber the relays. Solid-state devices like transistors provide reliability and these devices are economical to use in circuits. That’s why we see transistors in switching applications and where ratings are higher we see power transistors.

Both variants of BJT NPN and PNP can function as a switch. The methodology will be the same as discussed before in cut-off mode. Let’s explore how an NPN transistor works as a switch. 

4. NPN Transistor as a switch

Based on applied voltages on the base terminal, the transistor as a switch works in a way to open and close the circuit. When the voltages on the base terminal and the emitter terminal are larger than 0.7, the output voltages between the collector and emitter increase and give a significant amount of collector current. Resultantly the current flows from the source to the ground. So, the NPN transistor is said to be in saturation mode.

But as we reduce the voltages between the emitter and the base terminal lower than 0.7, the output voltages drop to zero and hence the collector current also drops. So, practically there is no flow of current from source to ground.  Therefore in this situation, we can say that the transistor is working as a switch. This mode of operation is said to be the cut-off mode.  

5. PNP Transistor as a Switch

The base terminal in the PNP transistor plays the important role in terms of the transistor working as a switch. The base terminal in the PNP transistor is N-type material and is biased negatively. The more the applied voltage on the base region is negative, the more current flows. The base terminal is negative with respect to the emitter terminal and this type of configuration is used more frequently in negative ground arrangements. When we apply more negative voltages, more current flows and transistor behaves as a closed switch or a short circuit. But the reverse happens when the voltage with respect to emitter is positive and therefore transistor behaves as an open circuit.

6. Examples of Transistor as a Switch

We can use transistors in LEDs where to turn them on and off we employ transistors. In practical examples, we see 2N3904, an NPN transistor connected before LED to operate it and work as a switch. In this type of circuit, the current through the base terminal flows as we turn on the switch and the transistor goes into saturation mode which resultantly keeps the LED in an ON state. The reverse happens when we turn off the switch.

We can easily operate the relays using the transistor as a switch which will energize the relay coil. The load connected to the relay against a circuit breaker is all dependent on the transistor which will function to energize and de-energize the relay. Here in these types of circuits, where we have also an inductive load connected; turning off input supply voltages will definitely resist the induced EMF.  So, to avoid damage to the circuitry, we use a parallel diode that blocks the back EMF by the inductive load in the circuit.

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