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Heterojunction Bipolar Transistor - Coggle Diagram
Heterojunction Bipolar Transistor
Heterojunction for emitter-base
X and Y are material with different bandgap
to increase gamma & beta ; need to reduce Ipe (holes diffusion current from Base to Emitter
Ipe is directly proportional to pno.
to reduce Ipe, pno need to be reduced
pno has exponential relationship with Eg
to get low pno, Eg need to be increased
Inverted base doping
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 Emitter-Base Junction
abrupt - cannot achieve desired gain
relate to electron affinity
without spike
beta is higher due to Eg = Ec +Ev
smoother junction
with spike
beta is lower due to Eg = Ev
abrupt junction - presence of spike & notch
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
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
