Power Transistors

We have heard about many devices that operate at high voltage levels. These devices have large electric potential and are the key components in many devices. The basic components of many digital devices are transistors. These transistors are used to amplify and regulate the signals and are made of high-performance semiconductor materials like silicon and germanium. These transistors can regulate and amplify a certain voltage level and can handle specific ranges of low-level voltages. To amplify and regulate high-level voltages, devices known as Power transistors were developed. In 1947, William Shockley, Walter Brattain and John Bardeen invented the first point-contact transistor.

What Are Power Transistors?

Power transistors are three-terminal devices that are composed of semiconductor materials. They feature emitter, base and collector terminals. These devices are particularly designed to control high current–voltage rating. The speciality of this device is when voltage or current is applied to one pair of terminals, it controls the voltage or current at the other pair of terminals. These transistors might be either of NPN or PNP polarity. Power transistors are available in different types with different power and switching speed ratings.

Pin Configuration:

The power transistor pin configuration with its symbol is shown below. This transistor is available with three terminals; base, emitter & collector.

Power Transistor Symbol

• Pin1 (Base Terminal): This terminal helps in biasing of transistor by turning ON/OFF this transistor
• Pin2 (Collector Terminal): The current supplies through this terminal and it is connected normally to load.
• Pin3 (Emitter): This terminal is used to drain out the flow of current and it is connected simply to the ground terminal.

Features & Specifications:

The features and specifications of the power transistor include the following.

• This transistor includes three terminals.
• It uses advanced process technology.
• The error voltage of this transistor is less
• It has a very fast switching speed.
• Its operation is full-voltage based.
• This transistor works with high current & power
• These transistors’s size is large.
• These are used in high-power applications.
• Its continuous collector current is 3A.
• Its collector-to-emitter voltage is 50 Volts.
• Its collector-to-base voltage is 50 Volts.
• Its emitter to base voltage is 4Volts.

Types Of Power Transistors

Power transistors are classified into the following types:

1. Bipolar Junction Transistors (BJTs)
2. Metal Oxide Semiconductor Field-Effect Transistor (MOSFETs)
3. Static Induction Transistor (SITs)
4. Insulated Gate Bipolar Transistor (IGBTs)

Let us study in detail about these individual types of transistors.

Bipolar Junction Transistors (BJTs)

These are the types of transistors which have the capacity to handle two polarities. Bipolar Junction Transistors can either be used as an amplifier or switch. These are mainly used to control the current, the process of controlling includes the act of amplification, switch-on, and switch-off. William Shockley in the year 1948, invented the first Bipolar Junction Transistor.

In BJT the emitter is heavily doped, the base is moderately doped and the collector is lightly doped. It features two junctions; Emitter-Base junction and Collector-Base junction. Bipolar Junction Transistors are of two types:

• NPN transistor
• PNP transistor

Power Bipolar Junction Transistors has the following characteristics:

1. Bipolar Junction Transistors are large in size and hence allow maximum current to flow.
2. Bipolar Junction Transistors have high breakdown voltage.
3. BJTs have high handling capability as well as current carrying capacity.
4. Mainly seen in high-power applications.
5. It has higher current carrying and high-power handling capability
6. It has a higher on-state voltage drop

Power Bipolar Junction Transistors Applications

• high current switching (up to 3A) loads
• Can be used as medium Power switches
• Large signal amplification
• Speed control of Motors
• Half-bridge circuits
• Inverter and other rectifier circuits
• DC motor speed control
• Lighting systems
• PWM applications
• Relay drivers
• Switch mode power supply
• Audio Amplifiers
• Signal Amplifiers

Metal Oxide Semiconductor Field-Effect Transistor (MOSFETs)

MOSFET is a type of FET transistor. Metal oxide semiconductor field-effect transistors feature three-terminals; source, base and drain. MOSFET functionality is based on the width of the channel. If the MOSFET channel is wide, the transistor works with high efficiency. Characteristics of a Metal Oxide Semiconductor Field-Effect Transistor are:

• MOSFET is known as a voltage controller
• It does not need input current
• Offers a high input impedance

So, in general, there are 4 different types of MOSFETs

• N-Channel Depletion mode MOSFET
• P-Channel Depletion mode MOSFET
• N-Channel Enhancement mode MOSFET
• P-Channel Enhancement mode MOSFET

N-channel MOSFETs are called NMOS

P-Channel MOSFETs are called PMOS

Applications

• 
  
• Motor Control
• Audio Amplifiers
• Switched Converters
• Control Circuit
• Pulse Amplifiers
• Motor Control
• DC-DCconverter
• Power switch
• solenoid drive
• Audio Amplifiers

Static Induction Transistor (SITs)

A static induction transistor is a vertically oriented device that contains three terminals. When compared with field-effect transistors, static induction transistors provide higher voltage breakdown. This type of transistor has high power, and high frequency with short multichannel. It is purposeful in getting higher breakdown voltages than field emitter transistors. The properties of static induction transistors are:

The characteristics of SITs include the following.

• The length of the channel is short.
• Its noise and distortion are less.
• It provides low terminal resistance.
• It has less gate series resistance and less capacitance from gate to source.
• It has less thermal resistance.
• It has a maximum capacity of audio frequency power.
• Its short turn-on & off time is normally 0.25 μs (few seconds)

Insulated Gate Bipolar Transistor (IGBTs)

It is a device composed of semiconductor material and features three terminals. Insulated gate bipolar transistors are mainly utilized for switching purposes. Insulated gate bipolar transistors consist of (P–N–P–N) four alternating layers that are controlled by a metal–oxide–semiconductor (MOS) gate structure. Properties of IGBT are:

The characteristics of IGBT include the following.

• The loss is insignificant at the input.
• Its power gain is high.
• Its efficiency is high.
• Its size & cost is low.
• It has fewer switching losses.

Structure of Power Transistor

As we know that bi-polar junction transistor is a vertically oriented device with a comparatively large area of cross-section that accommodates alternate P and N-type layers connected together. A bi-polar junction transistor is composed of a PNP or an NPN transistor. The power transistor BJT features three terminals: emitter, base, and collector. The PNP and NPN-type transistor is shown in the figure below.

Operation of Power Transistor

Based on the forward and reverse bias condition of the power transistor, it operates in four regions.

• Cut off region
• Active region
• Quasi saturation region
• Hard saturation region

When an NPN power transistor is connected in reverse bias condition, a power transistor will enter cut-off mode condition.

Case 1: The power transistor base is connected to the negative terminal and the positive terminal is connected to the emitter.

Case 2: The collector is connected to the negative terminal and the positive terminal is connected to the base terminal of the transistor.

In this condition, the output current to the base of the transistor IBE = 0, and output current flowing through the collector to the emitter IC = IB = 0. Small fraction of leakage current flows through the transistor from collector to emitter.

When the collector-base region reverse bias and the base-emitter region is forward biased, the transistor is said to be in an inactive state. When IB increases, IC also increases.

In the quasi-saturation stage of a transistor, the collector-base and base-emitter are connected in a forward bias pattern. A hard saturation condition is achieved when the collector-base and base-emitter are connected in a forward bias pattern.

V-I Output Characteristics of a Power Transistor

The output characteristics are as shown below, where the x-axis represents VCE and the y-axis represents IC.

• The below graph represents various regions like the cut-off region, active region, hard saturation region, quasi saturation region.
• For different values of VBE, there are different current values IB0, IB1, IB2, IB3, IB4, IB5, IB6.
• Whenever there is no current flow, it means the transistor is off. But few current flows which are ICEO.
• For increased value of IB = 0, 1,2, 3, 4, 5. Where IB0 is the minimum value and IB6 is the maximum value. When VCE increases ICE also increases slightly. Where IC = ßIB, hence the device is known as a current control device. This means the device is in an active region, which exists for a particular period.
• Once the IC has reached to maximum the transistor switches to the saturation region.
• Where it has two saturation regions quasi saturation region and hard saturation region.
• A transistor is said to be in a quasi-saturation region if and only if the switching speed from on to off or off to on is fast. This type of saturation is observed in the medium-frequency application.
• Whereas in a hard saturation region the transistor requires a certain amount of time to switch from on to off or off to on state. This type of saturation is observed in the low-frequency applications.

Advantages and Disadvantages of Power Transistors

The advantages of power transistors are:

• Voltage gain is high
• The density of the current is high
• The forward voltage is low
• The gain of bandwidth is large

The disadvantages of power transistors are:

• Low thermal stability
• Controlling is a complex task
• High noise

Applications of Power Transistors

Power transistors are used in:

• Relays
• Converters
• Power amplifiers
• DC to AC converters
• Power supply
• Power control circuits
• Inverters
• It is also used in relays.
• It is used for the amplification of the power as a power amplifier in electronic devices.
• It is also used in switch-mode power supplies which are also known as SMPS.
• It is also used in audio devices.

Frequently Asked Questions on Power Transistors

Q. What are the types of power transistors?

Answer:

• Bipolar Junction Transistors (BJTs)
• Metal Oxide Semiconductor Field-Effect Transistor (MOSFETs)
• Static Induction Transistor (SITs)
• Insulated Gate Bipolar Transistor (IGBTs)

Q. What are the regions of operations in BJT?

Answer:

• Active Region
• Saturation Region
• Cut-off Region
• Reverse-active / Inverse Region

Q. What are the types of charge carriers in BJT?

Answer: Electrons and holes.

Q. Transistor works on AC or DC?

Answer: A transistor is a variable resistor that can work on both AC and DC.

Q. Who invented BJT?

Answer: BJT was invented by William Shockley.

Q. How do you identify a power transistor?

Answer: We can identify a power transistor by testing it with a multimeter.

Q. What is the difference between a transistor and a power transistor?

Answer: The major and only difference between a power transistor and a transistor is that a power transistor is larger than a normal transistor and is capable of carrying more current through it without burning or melting the transistor.

Q. What are the types of power transistors?

Answer: The type of power transistors are as follows:Bipolar Junction Transistors (BJTs)Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)Insulated Gate Bipolar Transistors (IGBTs)Static Induction Transistors (SITs)

Q. Are power transistors and BJT the same?

Answer: No, the power transistor and BJT are not the same. BJT is a type of power transistor.

Q. How many layers does a power transistor have?

Answer: A power transistor has three layers of semiconductor material.

Q. What is the difference between a transistor and a power transistor?

Answer: A transistor is a three or four-terminal electronic device, where on applying an input current to a pair of the terminals of the transistor, one can observe a change in current in another terminal of that transistor. A transistor acts like a switch or an amplifier. Whereas a power transistor acts like a heat sink, which protects the circuit from damage. It is larger in size than a normal transistor.

Q. Which region of the transistor makes it switch faster from on to off or off to on?

Answer: The power transistor when it is in quasi saturation switches faster from on to off or off to on.

Q. What does N in NPN or PNP transistor mean?

Answer: N in NPN and PNP type transistors represents the type of charge carriers used, which is in an N-type the majority charge carriers are electrons. Hence in NPN two N-type charge carriers are sandwiched with a P-type, and in PNP single N-type charge carrier is sandwiched between two P-type charge carriers.

Q. What is the unit of the transistor?

Answer: The standard units of a transistor for electrical measurement are Ampere (A), Volt (V), and Ohm (Ω) respectively.

Q. Does the transistor work on AC or DC?

Answer: A transistor is a variable resistor that can work on both AC and DC but cannot convert from AC to DC or DC to AC.

The transistor is a basic component of a digital system, they are of two types based on their structure and based on their functionality. The transistor that is used for controlling large voltage and current is a power BJT (bipolar transistor) is a power transistor. It is also known as a voltage-current control device that operates in 4 regions cut-off, active, quasi-saturation, and hard saturation based on the supplies given to the transistor. The main advantage of a power transistor is it acts as a current control device.