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Transistors

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A Transistor is a solid state device whose operation depends on flow of electric charge carriers within the solid. It is thus capable of amplification and is most analogous to vacuum triode. It has more advantages over the vacuum triode such as compact size, light weight, rugged construction, more resistant to shocks.

What is Transistor?

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A transistor is a semiconductor device basically made of silicon or germanium crystal. It can be either of NPN or PNP type. It comes from the word transfer of resistance that tells that transfer a low resistance into the circuit that has a high resistance. They are basically of two types
  • Bipolar junction transistor (BJT)
  • Field effect transistor (FET)

How Transistor works?

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There are basically three terminals in a transistor
  • Emitter
  • Base
  • collector
The base is lightly doped, the emitter is heavily doped and the collector is moderately doped. Lets see how it works in the NPN transistor and analyze it.

n-p-n Common Emitter Configuration

Applying KVL to the circuit, we get
$I_{E}$ = $I_{B}$ + $I_{C}$
where,
$I_{E}$ = Emitter Current
$I_{B}$ = Base Current
$I_{C}$ = Collector Current.

The supply $V_{EE}$ is forward biased emitter base junction. As the current flows the potential barrier gets exceeds after some time. The majority charge carriers diffuse from emitter to base termed as the emitter current $I_{E}$. Once the flow enters the base few electrons or hole undergo recombination process and rest diffuse through collector base junction due to potential on the collector side.

Types of Transistors

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There are two types of transistors
  • NPN
  • PNP
If the p-type is sandwiched between any two n-type, the transistor is n-p-n transistor. If n-type semiconductor between two p-type, the transistor is the p-n-p transistor.

PNP NPN Junction

Symbolically it is represented as,

Types of Transistor

Bipolar Junction Transistor

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A Bipolar (junction) transistor (BJT) is a three-terminal device that has two types of charge carriers namely electrons and holes. Field effect transistors (FET) differ from BJT and it has only one type of charge carrier either electron or hole. So it acts as current controller whereas FET act as Voltage controller. They are of two types :
  • NPN transistor
  • PNP transistor.
Here, charge flow is due to diffusion of both the electrons and holes across the junction between two regions due to doping.

Field Effect Transistor

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Field Effect Transistors is a three terminal device that has two layers of semiconductor material. Between the two layers a channel acts through which the electricity flows. The terminals are
  • Gate
  • Source
  • Drain.
Gate is the channel that modulates the conductivity. Source is a terminal where the majority carriers enter the channel. Drain is the terminal where the majority carriers leave the channel.

Field Effect Transistor

Basically FET are of two types :
  • Junction Field Effect Transistor (JFET)
  • Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

Transistor Characteristics

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To study the transistor, a transistor is either operated in a common emitter configuration or in common base configuration.
Transistor as Amplifier
The input resistance is given as
$R_{i}$ = $\frac{\delta\ V_{BE}}{\delta\ I_B}$

The output resistance is
$R_{o}$ = $\frac{\delta\ V_{CE}}{\delta\ I_C}$

The current gain is
$A_{v}$ = $\frac{I_c}{I_B}$

Transistor as a Switch

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There are basically three regions namely active region, saturation region and cut off region. Transistor used in amplifier as switch so that it operates more in active region. This range is between saturation and cutoff region. When it is set to the maximum level, it is saturation region and when it is completely off the condition is Cutoff.

Transistor Graph


Switch Off
When transistor is switched off it acts in cutoff region where base emitter junction voltage $V_{BE}$ < 0.7 V. Here base-emitter junction and base-collector junction is reverse biased. Hence no collector current flows.
Transistor as Switch Open
Switch On
When transistor is switched on it acts in saturation region where base emitter junction voltage $V_{BE}$ > 0.7 V. Here base-emitter junction and base-collector junction is forward biased. Hence maximum collector current flows.
Transistor as Switch Closed

Transistor as a Amplifier

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Amplification is the process where the amplitude of signal is increased linearly. It is not limited only to that use but acts as a great device in amplification.

NPN Common Emitter Configuration

The common-emitter amplifier is designed so that a small change in voltage ($V_{in}$) changes the small current through the base of the transistor; the transistor's current amplification combined with the properties of the circuit mean that small swings in $V_{in}$ produce large changes in $V_{out}$.