Heterojunction Bipolar Transistor

Heterojunction for emitter-base

Abrupt Emitter-Base Junction

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X and Y are material with different bandgap

to increase gamma & beta ; need to reduce Ipe (holes diffusion current from Base to Emitter

Inverted base doping

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Ipe is directly proportional to pno.

pno has exponential relationship with Eg

to get low pno, Eg need to be increased

to reduce Ipe, pno need to be reduced

Nb >> Ne

indicate that BJT is HBT

advantages

low resistivity of B --> less current crowding

higher early voltage, Va

avoid high Ne --> avoid bandgap narrowing

abrupt - cannot achieve desired gain

without spike

with spike

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beta is lower due to Eg = Ev

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beta is higher due to Eg = Ec +Ev

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smoother junction

abrupt junction - presence of spike & notch

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spike - additional barrier for electrons to diffuse from E to C

eliminate spike : graded junction

Graded Emitter-Base Junction

modify mole fraction to modify electron affinity

relate to electron affinity

as the junction is gradually changing, the grading distance, Lgrade is wider

dont want sudden changes in mole fraction of Emitter material to Base material

gradually change mole fraction, electron affinity gradually changed

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