Transistor Interview Short Question Answer - 1

Important Questions of Transistor

In this post, some of the important questions of transistors are given. It includes meaning of transistor, bipolar junction transistor, current amplification factor, doping level in the transistor, configuration of transistor, function of transistor, practical value of α and β, collector leakage current and active, cut off and saturation region of transistor.

Transistor

What is meaning of the transistor?

Transistor

• The transistor is a three terminal semiconductor device in which one type of semiconductor material (either P or N) is sandwich between other types of semiconductor material (either N or P) of opposite polarity.

Why the transistor is called as Bipolar junction transistor?

Bipolar junction transistor

• The current conduction is the transistor is done by holes as well as electrons therefore it is called as Bipolar junction transistor.

Describe the main function of the transistor.

• To amplify weak signal

How many PN junction the transistor consists of?

• Two

How many terminals the transistor consists of?

• Three terminals – Emitter, Base and Collector

Why the collector terminal is made larger than the base terminal of transistor?

• Because most of the heat produced across collector junction

Whether the holes or electrons are responsible for collector cut off current in the transistor?

• The collector cut off current flows due to minority charge carriers. The collector cut off current flows due to holes in the NPN transistor and electrons in the PNP transistor.

Doping level in Transistor

What are the doping levels in the transistor?

The transistor consists of three terminals.

Emitter

• The emitter is heavily doped therefore it supplies large numbers of charge carriers to the base and collector of the transistor.

Base

• The base is lightly doped and very thin, therefore only small current passes through the base.

Collector

• The collector is moderately doped therefore the most of the current passes through it which is supplied by the emitter. The emitter current ( I_E ) is sum of the base current ( I_B ) and collector current ( I_C ).

I_E = I_B + I_C

Current Amplification Factor

Explain: Current amplification factor α

Current amplification factor α

• The ratio of change in collector current to the change in emitter current at constant collector base voltage in the Common Base transistor is known as current amplification factor.

                 α  = dI_C / dI_E at constant V_CB

Explain: Current amplification factor β

Current amplification factor β

• The ratio of change in collector current to the change in base current at constant collector emitter voltage in the Common Emitter transistor is known as current amplification factor.

             β = dI_C / dI_B at constant V_CE

Explain: Current amplification factor γ  

Current amplification factor γ

• The ratio of change in emitter current to the change in base current at constant collector emitter voltage in the common collector transistor is known as current amplification factor.

              γ = dI_E / dI_B at constant V_CE

Describe the different configurations are used in the transistor.

Different configuration of the transistor

As the transistor consists of three terminals. The transistor must be one common terminal in order to supply input and get output of the transistor. 

The following three types of configurations are used in the transistor

• Common emitter (CE) configuration
• Common base (CB) configuration
• Common collector (CC) configuration

What is practical value of α and β?

• The practical value of α is in the range of 0.9 to 0.99 and β is in the range of 20 to 500. 
• The value of α is increased by making base thin and lightly doped.

Write the relation between current amplification factor α and β

Relation between current amplification factor α and β

β = α / (1 – α)

OR

α = β / (1 + β)

Bias Voltage

Explain: Bias voltage. Describe the function of bias voltage in the transistor.

Bias voltage

• The DC voltage at the input side of the transistor is called as bias voltage. 
• The emitter base circuit is always forward biased for faithful amplification. 
• The emitter base becomes reverse biased during negative half cycle of the signal without bias voltage. 
• Therefore, magnitude of the bias voltage should be such that it always keeps the emitter base circuit forward biased.

Collector leakage current

What is I_CBO?

    OR

What is meant by collector leakage current in the transistor?

Collector leakage current

• The flow of the collector current while input or emitter circuit kept open circuited in the common base transistor is called as collector leakage current. It is donated by I_CBO.

OR

• The flow of the collector current while input or base circuit is kept open circuited in the common emitter transistor is called as collector leakage current. It is donated by I_CEO.

What is zero signal collector current in the transistor?

Zero signal collector current

• It is defined as the collector current flows when the input circuit of the transistor is forward biased by only battery or direct voltage.
• OR
• It is defined as the flow of collector current when no signal is applied, the input circuit is forward biased by battery or direct voltage.

Describe the main factor affecting value of collector leakage current.

Factor affecting collector leakage current

• The value of collector leakage current is neglected in the low power transistor whereas it is in the range of micro – ampere for high powered transistor. 
• The collector leakage current depends upon the temperature and its value increases rapidly with increase in the temperature.

Cut off, Active & Saturation region

Explain the term:  Cut off region, Saturation region, Active region

Cut off region

• The transistor is operated in the cut off region when base current is equal to zero.

Saturation region

• The collector and emitter terminal of the transistor are shorted together. The transistor behaves like closed switch between collector and emitter.

Active region

• The region between cut off and saturation is called as active region of the transistor.  
• The base – emitter of the transistor always forward biased whereas the collector – base junction remains reverse biased in this region under all conditions.

Describe the working of two back-to-back diodes in the cut off, active and saturation region of the CE transistor.

Status of back-to-back diode in the transistor

• Cut off: Emitter diode OFF, Collector diode OFF
• Active: Emitter diode ON, Collector diode OFF
• Saturation: Emitter diode ON, Collector diode ON

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