Transistors as Switches

The transistor as a switch is the genesis of many digital circuits. Transistors consist of three layers and have three semiconductor terminal devices used for amplification and switching operation. The operating modes of a transistor are active, saturation and cut-off mode. 

Transistors are used both in analogue and digital domains with diverse applications such as amplifier circuits, voltage regulators, digital logic circuits, oscillator circuits, embedded systems, power supply circuits and control systems. The transistor is one of the finest devices to study and understand. This article explains the operation of the transistor in the circuit and its usages. 

Transistors are of two types – Bipolar Junction Transistors (BJT) and Field Effect Transistors (FETs). The BJT consists of two junction semiconductor devices, three layers and three terminals. It has two PN junctions sandwiched with a middle layer. When it comes to learning about transistors, it is referred to as the Bipolar Junction Transistors.

Transistor

Basically, the transistor has three terminals, i.e., base, emitter and collector. The function of the emitter is to emit the electrons to the base as the emitter has a highly doped terminal. 

The function of the base as a lightly doped terminal is to make way for the emitter-injected electrons to the collector.

Further, the collector  has moderately doped properties and grabs the electrons from the base. Compared to other terminals, the collector is large in size and at times starts generating less heat for the operation’s working.

Types of transistors

The Bipolar Junction Transistor  has two types i.e.  PNP and NPN transistor. The PNP and NPN have similar functions but vary based on power supply polarity and biassing. Let us learn a little more about the PNP and NPN transistors. 

In NPN transistors, the P-type material comes in between the other two N-type materials. And in the case of PNP transistors, the N-type comes in between the other two P-type materials.

 In short, you can understand in this way that the transistor is nothing but a current controlling device where the output current is managed completely by the input current. The Bipolar Junction Transistor carries two charges: the electrons, the negative charge carriers and the positive charge carrier holes. 

Operating modes of transistor in different regions

The cut-off zone area and the saturation zone are together referred to as the transistor switch’s working regions. Juggling between the switching absolute off and top-off (saturation), the use of a transistor is to initialise the Q-Point and voltage divider circuit for amplification.

Cut-off region

In this region, the emitter-base junction and the collector-base junction are put in a reverse way. The PN junction in both terminals is put in a reverse-bias mode where the passage of the current is near about zero or nil except for minor leakage of the currents (in some pico amps or nanoamps). The transistor in this region is switched off and acts as an open circuit.

Cut-off region characteristics

Some of the characteristics of the cut-off region are mentioned below:

•   The Bipolar Junction Transistor or the transistor act as an ‘open switch’
•  The input and the bases are highlighted as (0V)
•  The emission voltage of the base is  VBE > 0.7 V
•  The emitter in this region is put in reversed order
•  The cut-off area (full-off) bipolar junction transistor or simply transistor (collector flow = 0)  VOUT = VCC = “1′′”
•  Current flow in the collector is zero (IC = 0)

The cut-off region and off mode is in reverse bias with Vb < 0.7 V and IC = 0, when putting a bipolar transistor as a switch.

Saturation region

In the saturation region, the base bears the highest current. The collector current acquires the average voltage, and the leakage is comparatively less and maximum current runs in the transistor. This is the reason behind the Fully On transistor being activated.

The saturation field, also known as the On step, can be described as  VW > 0.7 V and IK = complete when using the transistor as a switch.

Let’s take the case of the base-bias transistor in CE pathway and put the Kirchhoff rule of the voltage in the input and the output of the circuit can be mentioned as

VBB = IBRB + VBE … (1)

VCE = VCC – ICRC … (2)

VBB is the input voltage (Vi) and VCE  is the output Voltage DC (Vo). That’s the reason we get the following output:

Vi = IBRB + VBE

Vo = VCC – ICRC

The transistor is in active mode when Vi goes past 0.6 V. IC >0 and Vo is dropping (as ICRC is increasing). In the current scenario, with the rising Vi, IC increases approximately linearly. Vo also drops approximately linearly till its value decreases below 1 V.  

Applications of transistor as a switch

The transistor as a switch has the following applications:

1.  In LEDs, such applications are used as transistor connections.
2.  DC motors and light-bulb transistors as switch applications are used in a broadway.

Conclusion

Two things are concluded in these study material notes on transistors as switches. In case Vi is low, the bipolar junction transistor is not able to put in forward-bias, Vo is high (= VCC).

In case Vi is high, Vo is very low (~ 0) in the transistor.

In case the transistor cannot reflect, it shows as switched off.  The above description gives the complete idea of the saturation and the cut-off transistors. Suppose we holistically consider the scenario, the small input switches turn off the transistor whereas the high input turns on the transistor. This is how a Bipolar Junction Transistor or transistor as a switch works.