IMPORTANT QUES AND ANS in BJT

QUES: why BJT is BIPOLAR?
ANS:   Bipolar transistors can have both minority and majority carriers flowing, whereas FET's only have majority carriers flowing. The fact that bipolar transistors have two types of carriers flowing simultaneously results in the name 'bi-polar'. The minority carrier flow is responsible for collector conduction modulation and transistor storage time.

QUES: Why is a bipolar transistor current driven?

ANS  : A bipolar transistor has the base electrically connected and needs base current to turn on.

QUES:What does the SOA define ?

ANS  :The SOA (Safe Operating Area) defines the permissible region of operation for linear applications. The circuit designer must make sure the transistor is never used outside the SOA boundaries.

QUES:Can the base be driven negatively ?

ANS  :Yes, but there is a maximum level that should not be exceeded. The maximum negative base-emitter voltage can be found in the maximum ratings table. Exceeding this value will not immediately destroy the transistor if base current is held to a moderate value, but repeated avalanche base-emitter reverse breakdown may lead to shifts of dopants altering the characteristics of the transistor.

QUES:What is fT ?

ANS  : The transistion frequency is where the current gain of the BJT is reduced to unity. This means that it is the highest frequency at which there is amplifying action. Practical circuits will use the transistor at frequencies well below fT.

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