HBT : Heterojunction Biplar Transister

In normal homo junction BJT we have

In HBT the emitter is of wider band gap as shown in Fig 16.1

The electron can be easily injected from the emitter to the base but holes from p to N-emitter see a much larger energy barrier and hence the current I b due to hole back injection is reduced. Thus the base region can be made highly doped which reduces the base resistance R B . This is one of the advantage of HBT over BJT.

In abrurpt HBT, we have

Instead of using abrupt HBT we can use graded HBT made of the materials

Here the aluminium concentration is graded and hence called Graded Band Gap heterostructure.

In a Graded Band Gap Heterojunction the barrier for the hole can be made larger then electron barrier by and we have

Thus can be made even large than abrupt Heterojunction.

Here we consider an example

For As/GaAs graded hetero junction

With no Heterojunction (useless device)

And with abrupt Heterojunction

is a good material for Heterojunction

Other good Hetero junction are

The HBT structure is shown in Fig 16.2

                                                                    Fig.16.2