Transistor as a Switch

A transistor is a three-terminal semiconductor that regulates the current or voltage flow and acts or operates as a switch or gate for signals. The devices in the electronics and electricals are the regulated values which allows the weak signal to regulate the large amount of flow to the nozzle that regulates the flow of current. The transistors quantifies thousands and millions of transistors which are interconnected and embedded into small integrated chip or circuit boards. A transistor switch is used for opening or closing the circuit, which means we use a transistor is used as a switch in electrical devices only for low-voltage application devices due to their low power consumption. The transistors work as same as the switch when is it in cutoff and saturation regions.

What are Transistors as a Switch?

A simple transistor is a three-terminal semiconductor which is used as an amplifier. The transistor as a switch is used for turning ON or OFF a circuit and the transistor is used as an amplifier for amplifying the current. In digital circuits, the ones and zeros are stored in the form of bits(0 or 1 ). The advantage of using the transistor as the switch is that it can control a large amount of current with a small amount of current called gain.

Types of Transistor

Construction of Transistor

The transistor is a three terminal solid state device which have two diodes in continuation. It has two PN junctions. The transistor have three terminals that are drawn out of three semiconductors materials present in it. This structure form two types of transistors PNP and NPN which means N-type material between the two P type and the other is P type between two N types.

Three terminals that are drawn from the transistor indicate the Emitter, Base and Collector terminals.

Terminals of Transistor

• Emitter : It is a moderate size and highly doped part as it’s main function is to supply majority carriers either electrons or holes. It emits electrons called emitter and denoted by letter “E”.
• Base: It is middle part of the transistor which is thin and lightly doped. It’s main function is to pass the majority carriers from the emitter to the collector and it is indicated by letter B.
• Collector : It is the right side material i the transistor and its function is to collect the carriers. It is large in size then emitter and base. It is moderately doped and it is indicated by letter C.

Terminals of Transistor

Transistor as a Switch

Transistor as a switch is the simplest application of the devices. A transistor can be used for switching operations or opening or closing of the circuit. The main or basic concept of this operation relies on it’s operation generally low voltage DC is on or off by the transistor in this mode. The both type of the PNP and NPM transistor are used as switches. The terminal transistor can be handled different from a single amplifier by biasing both the NPM and PNP bipolar transistor with “ON/OF” static switch. One of the basic and main use of the transistor is to transform a DC signal “ON” or “OFF” is solid state switch. Many devices such as LED’s, lights requires very small milliamps logic level DC voltage and can directly driven by the output of a logic. If any circuit requires Bipolar Transistor Switch then the biasing of the transistor is either PNP or NPM which are arranged for operating the transistor for both sides of the “I-V” characteristics curves.

The operating area of the transistor switch called saturation region and the cut-off Region.

Operating Region

Operating Region

In the above operating region the pink shaded area at the bottom represents at the “Cut-Off” region while the blue shaded area on the left represents the saturation region of the transistor.

Cut-off Region

In the cut-off region of the transistor, it operates under conditions of zero input base current (Ib), zero output collector current (Ic), and maximum collector voltage (Vce). This configuration creates a significant depletion layer, preventing the flow of current through the device and effectively switching it off.

Cut-off Region

• The input and base are grounded.
• Base emitter voltage Vbe<0.7v
• Base Emitter junctions is reversed.
• Base- collector junction us reversed biased.
• Transistor is “fully-Off” (cut off region)
• No collector current flows(Ic=0)
• Vout = Vcr = Vcc= 1
• Transistor operates as open switch
  The cut off region of off mod when using a bipolar transistor as a switch, both junction are reverse biased Vb<0.7 and Ic=0. For PNP transistor the emitter potential must be negative with respect to base.

Saturation Region

In this the transistor will be biased so the maximum amount of the current is applied, which results in the maximum collector current results in the minimum collector emitter voltage drop which cause in depletion layer as a small as possible and maximum current flowing through the transistor so the transistor is switched “Fully ON”.

Saturation Region

Saturation Characteristics :

• The base and input are connected to Vcc.
• Base-Emitter voltage Vbe>0.7V.
• Base-Emitter junction is forward biased.
• Base-collector junction is forward biased.
• Transistor is “fully-On”
• Max Collector current flows (Ic=Vcc/Rl)
• Vce=0 (ideal saturation)
• Vout = Vce=”0″.
• Transistor operates as a closed switch.

The saturation region or ON mode when use the bipolar transistor as a switch the junctions forward biased Vb>0.7v and
Ic = Maximum. In PNP transistor the emitter potential must be positive with respect to the Base. The easy way to the switch moderate to high amounts of power is to use the transistor with an open-collector output and the transistor sink an externally supplied voltage to ground thereby controlling the load connected to it.

Saturation Region

In the above circuit the the NPN general purpose transistor circuit the +5V is connected to Vcc and the pulse input Vin with +5V, 0V, +5V. For making the circuit working properly you we need to calculate the correct resistor values for Rb and Rc. When we use transistor as switch it is operating in digital mode that on or off mode. The transistor does not amplify the input voltage Vin in any way. For this we need to increase the base terminal current to a certain level and the transistor will go into a saturation state. This is the state where no matters how much additional current is pumped into the base terminal of the transistor and the collector current will not increase. Once the transistor attains the saturation state the it acts like SPST closed mechanical switch. When the transistor is off it attains the cuff-off state and it is either in on or off state.

SPST mechanical switch

PNP as a Transistor

In general the logic and microcontroller are able to drive only the small loads by them but sometimes the load need to switched the requires more current then the required current controlling component which is supplied. In general case the transistors are used switch to reach the required current and voltage for amplification is achieved. When the PNP is used the microcontroller have only sink the base current. The emitter and collector path of the PNP transistor becomes high impedance or conductive.

In this when two P-type material are dopped into the single N-type then the transistor is known as PNP transistor.

Pnp Transistor

In this the emitter potential needs to be positive with respect to base. The transistor functions as solid-state SPST (single pole single throw ) switch when a zero signal is applied to the bas and no collector current flows. The PNP transistor works similar to the NPN in terms of switching but the current flows from base. The negative configuration use the kink of the switching.

The base terminal of the PNP is generally negative with respect to emitter.

• The base current is regulate currents both the emitter and collector.
• The current enter through base terminal in PNP. The base terminal is negative bias with respect to emitter.
• The current between the emitter and collector will conduct when we add the little negative supply at the base of the PNP.

Darlington Transistor Switch

Darlington transistor consists of two bipolar transistors is connected in such a way that the current amplified by the first transistor is amplified further by the second one. This type of transistor which use multiple switching transistor because when the DC gain of the one bi-polar transistor is very low for switching the load voltage or current. In construction or configuration a small input BJT(Bipolar Junction Transistor) is involved in switching on and off a bigger current holding output BJT transistor.

Two transistor are connected in Darlington Transistor for attaining the maximum signal gain which have two PNP or NPN bipolar transistor interconnected in such a way that current gain of the transistor 1 is multiplied by the current gain of the transistor 2, with a device that behaves like single transistor with a higher current gain when a small base current is applied.

Darlington Transistor Switch

The total current gain pf the is β

_TOTAL

=β1×β2

In NPN Darlington Transistor Switch, the collectors of the both the transistor are connected while the base of the second transistor is connected with the emitter of the first transistor.

Configuration Darlington Transistor Switch

In Darlington Transistor Switch two NPN or PNP are connected together in a wat that the emitter current of the first transistor T1 becomes the base of the second transistor T2. So the T1 is connected as an emitter and the T2 is common emitter amplifier.

Configuration Darlington Transistor Switch

Digital Logic Transistor Switch

A digital transistor switch in electronics use the transistor for controlling the current through a circuit. These types of switches are important as they enable the creation of the logic gates, processors.

The transistor switch is the fundamental block which is used for creating the circuits which performs the logical operations. The transistors are the semiconductor devices which acts switches.

The most common types of transistor used in digital circuit are BJTs (Bipolar Junction transistor) and MOSFETs (Metal-Oxide-Semiconductor Field0Effect Transistor).

BJTs (Bipolar Junction Transistor)

• BJT have three terminals C,B,E (collector, Base, Emitter).
• It work on basis of flow of current between C and E terminal which is controlled by base terminal.
• It is basically used in saturation or cut-off mode.
• When there is no voltage is applied to the base , then the flow of current is blocked and the switch is turned OFF.
• When voltage is supplied to base, then it allows the flow of current from the collector to the emitter and turn on the switch.

(MOSFET) Metal-Oxide-Semiconductor Field-Effect Transistor

• It is three terminal G,S,D (Gate, source, drain).
• It works when the voltage is applied to the gate which is controlees the flow of the current between source and drain.
• It is used in enhancement or depletion mode.
• When the power voltage is supplied to the gate terminal it allows the flow of current in between source and drain terminal, which turns the switch on.
• when there is no voltage supplied to the gate terminal, then the flow of current is blocked and the switch is turned off.

Darlington Transistor Switch

Darlington Transistor switch is directly used as switch for the load current or voltage. Generally the DC current gain of the bi-polar transistor is very low. For overcoming this or maximizing the gain both the transistor are connected with Darlington configuration where the amplification factor is the product of the two individual transistor. The Darlington Transistor switch simply contains NPN or PNP are connected together so that the current gain of the first transistor is multiplied to the current gain of the second transistor to produce the device which work as a single switch.

The product gain of the transistor device β _TOTAL =β1×β2 .

Darlington Transistor switch with high value of β and high collector current are possible as compared to a single transistor switch.

Darlington Switch

Applications of Transistor as a Switch

• The transistor as a switch is the main application in the LED’s.
• The operation on relay are managed by creating the necessary circuit changes in order to connect and control the external devices.
• The DC motors can also be controlled and monitored. It is used for starting the engine on or off. The speed of the motors are modified by changing the transistor frequency values.
• The light bulb is the simple example of this switches. It can the turn on if the setting is bright and turn off depending on the dark surrounding for which we use LDR.
• The thermistors are controlled using the switches which detects the ambient temperature sensed low and the resistance decreased when the temperature is high.

Conclusion

In this Article, we have gone through basics of the Transistor, we have also looked at the Construction of the Transistor, then we have gone Through the Topic Transistor as a Switch, In this we have seen the operation Region(Cut-off and Saturation Region),then we have seen PNP Transistor, Darlington Switch with its Configuration, In Configuration of Darlington Switch we have gone through Digital Logic Transistor Switch and Darlington Transistor Switch, at last we have seen some Applications of Transistor as a Switch.

FAQs on Transistor as a Switch

1. Which region of the transistor is very highly doped?

The base of the transistor is very highly doped.

2. What connections exist between transistor currents?

The emitter current equals the combined base current and collector current.

3. How is the diode in a transistor typically biased?

The emitter-base diode is forward-biased, whereas the collector-base diode is reverse-biased.