What is a Power Transistor : Types and Its Working

A transistor is a semiconductor device, which was invented in the year 1947 at Bell Lab by William Shockley, John Bardeen, and Walter Houser Brattain. It is a basic building block of any digital components. The very first transistor invented was a point contact transistor. The main function of a transistor is to amplify the weak signals and regulate them accordingly. A transistor compromises of semiconductor materials like silicon or germanium or gallium – arsenide. There are classified into two types based on their structure, BJT- bipolar junction transistor (transistors like Junction transistor, NPN transistor, PNP transistor) and FET- field-effect transistor ( transistors like junction function transistor and metal oxide transistor, N- channel MOSFET, P- channel MOSFET), and there functionality (like Small-signal transistor, Small switching transistor, Power transistor, High-frequency transistor, Phototransistor, Unijunction transistors). It consists of three main parts Emitter (E), Base (B), and Collector (C), or a Source(S), drain (D), and gate(G).

What is a Power Transistor?

The three-terminal device which is designed specifically to control high current – voltage rating and handle a large number of power levels in a device or a circuit is a power transistor. The classification of power transistor include the following.

 Bipolar Junction Transistor

A BJT is a bipolar junction transistor, which is capable of handling two polarities (holes and electrons), it can be used as a switch or as an amplifier and also known as a current control device. The following are the characteristics of a Power BJT, they are

  • It has a larger size, so that maximum current can flow through it
  • The breakdown voltage is high
  • It has higher current carrying and high-power handling capability
  • It has a higher on-state voltage drop
  • High power application.

MOSFET is a sub-classification of FET transistor, It is a three-terminal device containing source, base, and drain terminals. MOSFET functionality depends on the width of the channel. That is if the channel width is wide, it works efficiently. The following are the characteristics of a MOSFET,

  • It is also known as a voltage controller
  • No input current is needed
  • A high input impedance.

Static Induction Transistor

It is a device that has three terminals, with high power and frequency which is vertically oriented. The main advantage of the static induction transistor is that it has a higher voltage breakdown in comparison with FET- field-effect transistor. The following are the characteristics of static induction transistor,

  • The length of the channel is short
  • Noise is less
  • The turn-on and off is a few seconds
  • The terminal resistance is low.

Insulated-gate Bipolar Transistor (IGBTs)

As the name suggests an IGBT is a combination of FET and BJT transistor whose function is based on its gate, where the transistor can be turned on or off depending on the gate. They are commonly applied in power electronics devices like inverters, converters, and power supply. The following are the characteristics of Insulated-gate Bipolar transistor (IGBTs),

  • At the input of the circuit, the losses are less
  • higher power gain.

Structure of Power Transistor

The Power Transistor BJT is a vertically oriented device having a large area of cross-sectional with alternate P and N-type layers are connected together. It can be designed using P-N-P or an N-P-N transistor.


The following construction shows a P-N-P type, which consists of three terminals emitter, base, and collector. Where the emitter terminal is connected to highly doped n-type layer, below which a moderately doped p-layer of 1016 cm-3 concentration is present, and a lightly doped n- layer of 1014 cm-3 concentration, which is also named as collector drift region, where the collector drift region decides the break-over voltage of the device and at the bottom, it has an n+ layer which is highly doped n-type layer of 1019 cm-3 concentration, where the collector is etched away for user interface.


Operation of Power Transistor

Power Transistor BJT works in four regions of operation they are

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

A power transistor is said to be in a cut off mode if the n-p-n power transistor is connected in reverse bias where

case(i): The base terminal of the transistor is connected to negative and emitter terminals of the transistor is connected to positive, and

case(ii): The collector terminal of the transistor is connected to the negative and base terminal of the transistor is connected to positive that is base-emitter and collector-emitter is in reverse bias.


Hence there will be no flow of output current to the base of the transistor where IBE = 0, and also there will be no output current flowing through the collector to emitter since IC = IB = 0 which indicates transistor is in off state that is a cut off region. But a small fraction of leakage current flows throw the transistor from collector to emitter i.e, ICEO.

A transistor is said to be inactive state only when the base-emitter region is forward bias and collector-base region reverse bias. Hence there will be a flow of current IB in the base of transistor and flow of current IC through the collector to emitter of the transistor. When IB increases IC also increases.


A transistor is said to be in the quasi saturation stage if base-emitter and collector-base are connected in forwarding bias. A transistor is said to be in hard saturation if base-emitter and collector-base are connected in forwarding bias.


V-I Output Characteristics of a Power Transistor

The output characteristics can be calibrated graphically 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. Which means the device is in 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.


The advantages of power BJT 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 BJT are,

  • Thermal stability is low
  • It is noisier
  • Controlling is a bit complex.


The applications of power BJT are,

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


1). Difference between transistor and power transistor?

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.

2). Which region of transistor makes it switch faster from on to off or off to on?

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

3). What does N in NPN or PNP transistor mean?

N in NPN and PNP type transistor 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.

4). What is the unit of the transistor?

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

5). Does transistor work on ac or dc?

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 a basic component of a digital system, they are of two types based on their structure and based on their functionality. The transistor which 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.