Transcript The Design of SiGe pnp HBTs

The Design of SiGe pnp HBTs
At present, SiGe technology development is almost
exclusively centered on npn SiGe HBTs.
However, for high-speed analog and mixed-signal
circuit applications, a complementary (npn+pnp)
bipolar technology offers signification performance
advantages over an npn-only technology .
for example : Push-pull circuits
Push-pull circuits
+VCC
Class B output stage
vI
vO
RL
-VCC
NPN and PNP of Si BJTs
Performance :
npn Si BJTs > pnp Si BJTs
due to μ
n
ele
(P - type base of an npn Si BJT)
 μ hole (N  type base of a pnp Si BJT)
NPN SiGe HBT
Valance band offset in SiGe strained layers translates
into an induced conduction band offset
Base
Enhance minorty electron
transport
Fermi-level 被拉平
PNP SiGe HBT
The valance band offset directly results in a valance
band barrier , even at low injection.
Base
strongly degrades minority hole transport and limits
the frequece response.
Simplistic hypothetical npn and
pnp SiGe profiles
With const. E,B,C doping, and a Ge content not subject
to thermodynamic stability constraint.
This artificial assumption on
constant doping yields ac
performance numbers (eg. fT)
that are lower than what would
be expected for a real
complementary.
Profile Optimization Issues
Npn without any Ge retrograding into the collector(i.e.
an abrupt transition from the peak Ge content to zero
Ge content in the CB junction).
Ge profile
E
Abrupt transition
B
C
C-B junction
Profile Optimization Issues
pnp without any Ge retrograding into the
collector
An obvious valence
band barrier even
for low Ge content
Valence band barrier in pnp
acts to block minority holes transiting the base.
The pileup of accumulated holes produces a retarding
electric field in the base jc, which compensates the Gegrading-induced drift field.
 Jc

Decreasing  
 f
 T
this effect worsens
as the current density increases
Retrograding Ge into the collector
retrograding of the Ge edge into the collector can
“smooth” this valence band offset in the pnp SiGe HBT,
although at the expense of film stability.
For an increase of the Ge retrograde from 0 to 40nm
 yieldingroughly a 2  increase in peak fT
over the pnp Si BJT performance at equal doping.
FT and Current gain 
Ge retrograding in PNP “smooth”
valence band barrier
40-50 nm of Ge retrograding
in the pnp SiGe HBT is sufficient to
“smooth” the valence band barrier
The box Ge retrograde (back side)
on PNP
The box Ge retrograde is not effective in improving
the pnp SiGe HBT performance , since it does not
smooth the Ge barrier, but rather only pushes it deeper
into collector
The effects of Ge retrograding on
NPN
The effects of Ge retrograde on the npn SiGe HBT
performance ------- minor, while the film stability is
worse due to the additional Ge content.
So, we know using one Ge profile design for
both npn and pnp SiGe HBTs is not optimum for high
peak Ge content values.
Stability Constraints in PNP SiGe
HBTs
The total amount of Ge that can be put into a given
SiGe HBT -----limited by the thermodynamic stability.
Above the critical thickness
the strain in
the SiGe film relaxes
generating defects.
The empirical critical thickness of a SiGe multilayer
with a top-layer Si cap
----- approximately 4x the theoretical
stability result of Matthews and Blakeslee
Stability Constraints in PNP SiGe
HBTs
The peak Ge content
The Ge retrograde distance
trade off
The best design point in PNP
Similar exercise for the npn
For npn SiGe HBT , the ac performance is not
sensitive to the SiGe profile shapes used.
So the same Ge profile may be used
for both pnp and npn SiGe HBTs.
being advantageous from a
fabrication viewpoint.
Low-Injection Theory
Minority carrier transport in an npn SiGe HBT
JC 
q [(e qVBE / kT )  (e qVBE / kT )]
Wb
Pb ( x)dx
0 Dnb ( x)nib2 ( x)
The minority carrier base transit time  B
----- determined by the net force acting on electron
resulting from the induced electric field.
The net force on the electrons
Two components :
(1). The quasi-electric field due to the gradient
induced by conduction band offset.
(2). The built-in electric field.
The hole current density with a nonuniform
bandgap :
dEV
dp
J p  qD p  p p
dx
dx
The net force on the electrons
dEV
dx

actual valence band

q
d
dx

potential
(doping 不均勻所造成 )
Applying the Webster approximat ion (J
d [ EV ]
dx

p

SiGe 引 起 的
valence band offset
 0), the built  in
d
D
kT
electric field( ε   dx ) , and the classical Einstein relation( μ  q ).

d
kT dp 1 d [EV ]


dx
qp dx q
dx
The net force on the electrons
The net force acting on the electrons becomes
d [ EC ]
dx


F 
n

quasi electric field
d
q
dx

built in electric field
d [ EC ]
kT dp
d [ EV ]
 q dx 
dx
dx

finally,
F
n
 
kT
dp
q
dx

d [ E G ]
dx
The net force on the electrons
So the base transit time  B is determined only by the
total band offset EG across the neutral base.
--------and is independent of its distribution
between conduction and valence bands for lowinjection operation(i.e. p=Nab).
Impact of High Injection
E 
C
 can affect thecutoff frequencyroll- off .
E 
V
To shed light on this issue, we consider the following
four representative band offset distributions(band
alignments) :
Four representative band offsets (with
the same total band offset)
EC  0 EV  Eg
(1) This case is closest to
strained SiGe on Si
EC  2Eg
EV  Eg
(3) This case is applicable to
published SiGeC bandgap predictions
EV  0 EC  Eg
(2) This case is applicable to
strained Si on relaxed SiGe
EV  2Eg
(4)
EC  Eg
Simulation
2-D dc and ac simulations were performed for a
0.5µm emitter width
npn SiGe HBT.
FT V.S. JC
( 2)
(3)
( 4)
(1)
The conduction band edge of
simulation
The explantation of the physics
EV  2Eg
EC  Eg
(4)
This case gives the highest(best) FT and the
highest(best) Jcritical , because it has the largest
valence band offset , which acts to effectively
prevent hole injection into the collector.
The explantation of the physics
EC  2Eg
EV  Eg
(3) This case is applicable to
published SiGeC bandgap predictions
This case gives the lowest FT and Jcritical, because it has the
largest conduction band offset which serves as a barrier to
electron, and thus results in excess charge storage.
The explantation of the physics
For FT and Jcritical
EC  0 EV  Eg

(2) This case is applicable to
strained Si on relaxed SiGe
(1) This case is closest to
strained SiGe on Si
Because this conduction band barrier
height resulting from the pileup of
holes
EV  0 EC  Eg

this coduction band barrier
height
SiGe HBTs under High –Current
density operation
From the viewpoint of improving the ac characteristics
of SiGe HBTs under high-current density operation, a
large positive valence band offset together with a
negative conduction band offset is the most desirable
bandgap offset distribution.(as condition (4) ).
It is worth noting that these offsets(condition (1) ) are in
fact different from those produced by strained SiGe on
Si (i.e. mostly EV, with a small EC.)
Profile Optimization Issues
One way to minimize high-injection barrier effects in
SiGe HBTs is to retrograde the mole fraction deep into
the collector.
Low-C-content SiGeC
Low-C-content SiGeC layers can provide
better thermodynamic stability than strained
SiGe
- So we can allow a higher average Ge
mole fraction for a deeper grading.
Ge-Induced Collector-Base Field
Effects
The specifics of the backside Ge profile(i.e. on the CB
side of the neutral base) strongly influence highinjection heterojunction barrier effects, which produce
premature roll-off of and FT at high current density.
Here we will show that the backside Ge profile also
alters the electric field distribution in the CB spacecharge region, and thereby indirectly affects impact
ionization in SiGe HBTs.
Influence on Impact Ionization
For a strained SiGe layer on Si, the band offset
in the SiGe film predominantly resides in the valence
band and its value is proportional to the Ge content.
- according to ΔEV =0.74x (eV), where x is Ge
fraction (i.e. 10% Ge=0.10) .
Valence band edge for Strained
SiGe on Si (NPN)
Ge profile
6nm
B-C junction
This SiGe “control” profile is
labeled “0nm Ge”.
(i.e. the location of the SiGe-Si
heterointerface is referenced to
the metallurgical CB junction)
Heterojunction-Induced QuasiElectric Field
This change in the valence band creates a heterojunctioninduced quasi-electric field
 Ge
q(0  EV ) 0.74x


, D r is the retrograde distance.
Dr
Dr
For the present SiGe control profile
 Ge  1.23  105 V/cm , for x  10 % , Dr  6nM
Which is larger than the peak field formed by the
doping-induced charge in the CB space charge region.
Heterojunction-Induced QuasiElectric Field
絕
對
值
It is clear that as the backside
Ge retrograde location moves
toward the neutral collector, the
peak electric field moves in the
same direction and the
magnitude of the peak electric
field drops.
Avalanche multiplication factor
This decrease of the peak field reduces the impact
ionization rate , as reflected in the avalanche
multiplication factor (M-1) .