E3 238 Analog VLSI Circuits
Lecture 7: BJTs
Gaurab Banerjee
Department of Electrical Communication Engineering,
Indian Institute of Science, Bangalore
banerjee@ece.iisc.ernet.in
Bipolar Junction Transistors (BJTs)
NPN
PNP
-> Key thing to remember: emitter “emits” the carriers, collector “collects”.
-> The current flow is determined by the type of transported carrier.
Transition Frequency in BJTs
(MOS)
(BJT/HBT)
-> fT varies as the inverse square of the effective dimension of transport
-> Scaling has resulted in CMOS fT values previously possible in BJTs/HBTs
-> Bipolar (e.g. HBTs in BiCMOS) still “trailblazes” for CMOS to follow….
Bipolar Junction Transistors (BJTs)
B
n
E
p
Depletion
Region
n
n+
C
-> Most commonly used bias condition (for linear circuits) = “forward active” biasing
(VBE > 0, VBC <0)
-> Emitter base junction is forward biased: similar to forward biased diode
-> Carriers driven into the base diffuse through it -> transport driven by concentration
gradient : compare to MOS devices where the transport is driven by drift produced by
an electric field
-> Collector base junction is reverse biased: carriers injected from the emitter are
minority carriers in the base
-> Carriers entering the CB reverse biased region are swept to the collector under the
influence of the high electric field
-> # of minority carriers in the base making it to the collector successfully, determines
the quality of the bipolar device
-> Recombination of minority/majority carriers in the base : base current.
DC Performance Measures
Decrease
base-width
BASE TRANSPORT
FACTOR
Lower
recombination,
increase lifetime
EMITTER INJECTION
EFFICIENCY
Make this small
by making ND/NA
large
-> WB = Base width, τb = recombination lifetime of minority carrier in base
-> Dp, Dn = diffusion constants for holes, electrons
-> Lp = Diffusion length
-> NA, ND = Base, emitter doping concentrations
DC Performance Measures
So, what we need is:
-> Heavily doped emitter -> large ND -> poly-Si emitters
-> Lightly doped base -> Small NA
-> Small base width (WB)
-> Transport is diffusion based -> slower.
-> Advanced BJTs (e.g. SiGe HBTs) introduce an electric field in the
base by controlling the band-gap to supplement diffusion with drift
-> result = much faster devices! (SiGe BiCMOS)
BJT cross-section
Surround the base with
pickup taps to reduce base
spreading resistance (RB )
SiGe HBT enhancements:
•
Bandgap engineering
enables higher β , tradeoff for
higher base doping (lower rb)
•
Graded Ge-profile ->
electric field in base -> higher fT
•
Relaxed trade-offs at CB
junction -> fT vs. BVCEO
Diffused base -> lower control on WB
Epitaxially grown in modern BJTs/HBTs