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 level of voltage and can handle specific ranges of low-level voltages. In order 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.

In this article, let us study in detail about the power transistors, their characteristics, structure, types and applications.

What Are Power Transistors?

Power transistors are three terminal devices which 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.

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.

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

Static Induction Transistor (SITs)

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, high frequency with short multichannel. It is purposeful in getting higher breakdown voltages than a field emitter transistors. Properties of static induction transistor are:

  • Short channel length
  • Less noise
  • Offers low terminal resistance.

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 loss is negligible at the input
  • High power gain

Structure of Power Transistor

As we know that bi-polar junction transistor is a vertically oriented device with comparatively large area of cross-section that accommodates alternate P and N-type layers connected together. A bi-polar junction transistor is composed of 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.

Power Transistors Image 1

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: 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.

Power Transistors Image 2

In this condition, the output current to the base of the transistor IBE = 0, and output current flowing through the collector to 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 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 forward bias pattern. Hard saturation condition is achieved when the collector-base and base-emitter are connected in 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.

Power Transistors Image 3

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:

  • Switch-mode power supplies (SMPS)
  • Relays
  • Converters
  • Power amplifiers
  • DC to AC converters
  • Power supply
  • Power control circuits
  • Inverters

Read more about the formula of capacitance.

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Frequently Asked Questions on Power Transistors

1. What are the types of power transistors?

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

2. What are the regions of operations in BJT?

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

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

Electrons and holes.

4. Transistor works on AC or DC?

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

5. Who invented BJT?

BJT was invented by William Shockley.

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