BJT Intro and Large Signal Model

advertisement
ESE319 Introduction to Microelectronics
BJT Intro and Large Signal Model
2010 by Kenneth R. Laker, update 15Sep10 KRL
1
ESE319 Introduction to Microelectronics
VLSI Chip Manufacturing Process
2010 by Kenneth R. Laker, update 15Sep10 KRL
2
ESE319 Introduction to Microelectronics
0.35 mm SiGe BiCMOS Layout for RF
(3.5 GHz) Two-Stage Power Amplifier
Each transistor above is realized as net of
four heterojunction bipolar transistors (HBT)
2010 by Kenneth R. Laker, update 15Sep10 KRL
3
ESE319 Introduction to Microelectronics
Why BJT?
What's the competition to BJT and bipolar technologies?
What advantages does the competition have over BJT?
What advantages does BJT and bipolar technologies have
over their competition?
What circuit applications benefit from BJT and bipolar technologies?
2010 by Kenneth R. Laker, update 15Sep10 KRL
4
ESE319 Introduction to Microelectronics
Why BJT
What's the competition to BJT and bipolar technologies?
MOSFET, in particular CMOS is the leading competitor
What advantages does the competition have over BJT?
Small size (die area), low cost and low power dissipation
What advantages does BJT and bipolar technologies have over
their competition?
High frequency operation, high current drive, high reliability in
severe environmental conditions.
What circuit applications benefit from BJT and bipolar technologies?
RF analog and digital circuits,power electronics and power amplifiers, automobile electronics, radiation hardened electronics.
2010 by Kenneth R. Laker, update 15Sep10 KRL
5
ESE319 Introduction to Microelectronics
BJT Physical Configuration
NPN
PNP
Closer to actual layout
Each transistor looks like
two back-to-back diodes,
but each behaves much differently!
2010 by Kenneth R. Laker, update 15Sep10 KRL
6
ESE319 Introduction to Microelectronics
NPN
BJT Symbols and Conventions
PNP
Metal
contact
IC
IE
IE
IB
IE = IC + IB
IC
IB
Note reversal in current directions and voltage signs for PNP vs. NPN!
2010 by Kenneth R. Laker, update 15Sep10 KRL
7
ESE319 Introduction to Microelectronics
NPN BJT Modes of Operation
i E = iC + iB
vCE = vCB + vBE
vXY = VXY + vxy
large
signal
dc
bias
ac
signal
Forward-Active Mode
EBJ forward bias (VBE > 0)
CBJ reverse bias (VBC < 0)
VCB
VBE
Mode
Forward-Active
VBE
VBC
>0
<0
Reverse-Active
<0
<0
>0
>0
<0
>0
Cutoff
Saturation
2010 by Kenneth R. Laker, update 15Sep10 KRL
VBC = -VCB
Not Useful!
8
ESE319 Introduction to Microelectronics
NPN BJT Modes of Operation
VBE
Saturation
region
iC
iC
VCE
-VA
0
2010 by Kenneth R. Laker, update 15Sep10 KRL
Forward-Active
region
vBE4
vBE3
vBE2
vBE1
vCE
9
ESE319 Introduction to Microelectronics
NPN BJT Forward-Active Current Flow
Forward-biased
p
n
E
iE
iE
iE
Reverse-biased
n
Injected Injected Collected i
C
electrons electrons electrons
Injected holes (iB1)
Recombined
electrons (iB2)
iB
i B =i B1 i B2
i
vBE
VBE
2010 by Kenneth R. Laker, update 15Sep10 KRL
B
iE
iC
B
i E =i C i B
vCB
C
iC
iC
VCB
10
ESE319 Introduction to Microelectronics
NPN BJT Forward-Active Mode Basic Model
Collector-base diode
is reverse biased
V CB0
( or VBC < 0)
1
Base-emitter diode is
forward biased
saturation current
V BE ≈0.7
AE
W
NA
Dn
ni
A E q D n ni
I s=
N AW
2
Area of base-emitter junction
Width of base region
Doping concentration in base
Electron diffusion constant
Intrinsic carrier concentration = f(T)
2010 by Kenneth R. Laker, update 15Sep10 KRL
iC ≈ I S e
v BE
VT
VT=
kT
o
≈ 25 mV @ 25 C
q
iC
i B=

i E =i B i C =1i B
=common−emitter current gain
11
ESE319 Introduction to Microelectronics
NPN BJT Forward-Active Beta (β)
AE -> Area of base-emitter junction
W -> Width of base region
NA -> Doping concentration in base
ND -> Doping concentration in emitter
Dn -> Electron diffusion constant
Dp -> Hole diffusion constant
Lp -> Hole diffusion length in emitter
τb -> Minority-carrier lifetime
ni -> Intrinsic carrier concentration = f(T)
2010 by Kenneth R. Laker, update 15Sep10 KRL
1
=
Dp N A W 1 W 2

D n N D L p 2 D n b
Large β =>
NA -> small
ND -> large
W -> small
12
ESE319 Introduction to Microelectronics
Note that the iC equation looks like that of a forward-biased
diode collector-base. Is it?
Using Eqs. iE = iB + iC and iC = βiB we can answer this question, i.e.
1
1
i E =i B i C = 1i C =
iC


=common−emitter current gain
=common−base current gain
and write:
v
v
iC
1
1

V
V
iE=
I S e −1= I S e −1=
=
where
1



BE
BE
T
T
AhHa! This iE equation describes a forward-biased emitter-base
“diode”.
2010 by Kenneth R. Laker, update 15Sep10 KRL
13
ESE319 Introduction to Microelectronics
So the new set of
equations are:
v BE
VT
IS
i E = e −1

v BE
VT
i C =I S e −1= i E
iC
i B=

A E q D n ni 2
I s=
N AW
2010 by Kenneth R. Laker, update 15Sep10 KRL
Where:

=
1
Typically:
50200 => 0.9800.995
10−18 I S 10−12 A.
Is is strongly temperaturedependent, doubling for a
5 degree Celsius increase
in ambient temperature!
14
ESE319 Introduction to Microelectronics
Two equivalent large signal circuit models for
the forward-active mode NPN BJT:
v BE
VT
iC = I S e −1≈ I S e
Nonlinear VCCS
v BE
VT
Nonlinear CCCS
Key Eqs.
v BE
VT
= F
i C ≈ I S e = F i E
IS
i E≈ e
F
I
v BE
VT
1
i B =i E −i C = i C

SE
2010 by Kenneth R. Laker, update 15Sep10 KRL
15
ESE319 Introduction to Microelectronics
Yet another NPN BJT large signal model
i C = i B ≈ I S e
v BE
VT
IS
⇒ iB≈ e

v BE
VT
This “looks like” a diode
between base and emitter
and the equivalent circuit
becomes:
iB
iC
iB
v BE
IS V
i B= e

T
iE
Note that in this model, the diode current is represented in
terms of the base current. In the previous ones, it was represented in terms of the emitter current.
2010 by Kenneth R. Laker, update 15Sep10 KRL
16
ESE319 Introduction to Microelectronics
NPN BJT Operating in the Reverse-Active Mode
Recall for NPN Reverse-Active Mode VBE < 0 & VBC > 0
Weak transistor action if we:
●
Forward bias the base-collector junction and
Reverse bias the base-emitter junction
Collector and emitter reverse roles
●
●
●
The physical construction of the transistor results
●
Weak reverse-active performance
●
●
●
Small values of β on the order of 0.01 to 1
Correspondingly smaller values of α, e. g.
R
0.1
 R=
≈
≈0.091
 R1 1.1
2010 by Kenneth R. Laker, update 15Sep10 KRL
for
 R =0.1
17
ESE319 Introduction to Microelectronics
The equivalent large signal circuit model for
the reverse-active mode NPN BJT: FWD
iB
Active
iC collector and emitter reverse roles
RVRS
Active
iC
iB
iC
iB
iE
iE
Note that the directions of
the reverse-active currents
are the reverse of the forward-active currents; hence
the “minus” signs.
iE
v BE
VT
i C ≈ I S e = F i E
Key Eqs.
−I S
iC ≈
e
R
v BC
VT
i E ≈−I S e
2010 by Kenneth R. Laker, update 15Sep10 KRL
=−I SC e
v BC
VT
v BC
VT
BJT is non-symmetrical
 R ≪ F
 R ≪ F
= R i C
18
ESE319 Introduction to Microelectronics
The Ebers-Moll Large Signal Model
The E-M model combines
the FWD & RVRS Active
equivalent circuits:
Note that the lower left
diode and the upper right
controlled current source
form the forward-active
mode model, while the upper left diode and the lower
right source represent the
reverse-active mode model.
v BE
i C = F i DE −i DC
i E =i DE − R i DC
i B =1− F  I DE 1− R  I DC
2010 by Kenneth R. Laker, update 15Sep10 KRL
IS V
i DE = e −1
F
T
v BC
IS V
i DC = e −1
R
T
19
ESE319 Introduction to Microelectronics
Operation in the Saturation Mode
Recall for Saturation Mode vBE > 0 & vBC > 0 (or vCB < 0)
Consider the E-M model for collector current.
i C = F i DE −i DC
v BE
VT
v BC
IS V
i C = I S e −1− e −1
R
T
The first term is the forward mode collector current:
v BE
VT
 F i DE = I S e −1
The second is the reverse mode collector current:
v
IS V
i DC = e −1
R
BC
T
2010 by Kenneth R. Laker, update 15Sep10 KRL
20
ESE319 Introduction to Microelectronics
Combining terms:
Using typical values:
v BE
VT
v BC
IS V
iC = I S e −1− e −1
R
[
i C = e
v BE
VT
 R1
−1−
e
R
We obtain:
i C =[ e
40 v BE
 R =0.1
T
−v CB
VT
1 R 1
=
R
R
−40 v CB
−1−11e
]
−1 I S
I S =10−14 A
V T =0.025V
−1 ] 10−14
Let's plot iC vs. vBC (or vCB) with vBE = 0.7 V
2010 by Kenneth R. Laker, update 15Sep10 KRL
21
ESE319 Introduction to Microelectronics
Scilab Saturation Mode Calculation
//Calculate and plot npn BJT collector
//current in saturation mode
vBE=0.7;
VsubT=0.025;
VTinv=1/VsubT;
v BE
v BC
betaR=0.1;
I
V
S
V
IsubS=1E-14;
i C =I S e T −1− e T −1
R
alphaR=betaR/(betaR+1);
alphaInv=1/alphaR;
ForwardExp=exp(VTinv*vBE)-1;
vCB=-0.7:0.001:-0.1;
vBC=-vCB;
ReverseExp=alphaInv*(exp(VTinv*vBC)-1);
iC=(ForwardExp-ReverseExp)*IsubS;
signiC=sign(iC);
iCplus=(iC+signiC.*iC)/2; //Zero negative values
plot(vCB,1000*iCplus); //Current in mA.
2010 by Kenneth R. Laker, update 15Sep10 KRL
22
ESE319 Introduction to Microelectronics
Saturation Mode Plot
iC(ma)
Note: forward-active NPN operation continues for negative vCB
down to - 0.5V; i.e. vCB > - 0.5V
Forward-active
Recall for Sat.
Mode
vBE > 0
&
vBC > 0
(or vCB < 0)
Saturation
A
vBE = 0.7V
vCB(V)
2010 by Kenneth R. Laker, update 15Sep10 KRL
23
ESE319 Introduction to Microelectronics
Scilab Plot of NPN Characteristic
(iC vs. vCE and vBE)
//Calculate and plot npn BJT collector
//characteristic using Ebers-Moll model
VsubT=0.025;
VTinv=1/VsubT;
betaR=0.1;
v BE
v BC
alphaInv=(betaR+1)/betaR;
I S VT
VT
IsubS=1E-14;
i C =I S e −1− e −1
R
for vBE=0.6:0.02:0.68
ForwardExp=exp(VTinv*vBE)-1;
vCE=-0:0.001:10;
vCE = vCB + vBE
vBC=vBE-vCE;
ReverseExp=alphaInv*(exp(VTinv*vBC)-1);
iC=(ForwardExp-ReverseExp)*IsubS;
signiC=sign(iC);
iCplus=(iC+signiC.*iC)/2; //Zero negative vals
plot(vCE,1000*iCplus); //Current in mA.
end
2010 by Kenneth R. Laker, update 15Sep10 KRL
24
ESE319 Introduction to Microelectronics
Plot Output
iC (ma)
saturation mode
forward-active mode
v BE =0.68 V
Early effect
not included.
v BE =0.66 V
vCE = vCB + vBE
v BE =0.64 V
v BE =0.62 V
vCE (V)
@ start of saturation V CEs at =vCE =v CB v B E≈−0.4 V 0.7 V =0.3 V
2010 by Kenneth R. Laker, update 15Sep10 KRL
25
ESE319 Introduction to Microelectronics
More on NPN Saturation
The base-collector diode has much larger
area than the base-emitter one.
●
●
Therefore, with the same applied voltage, it will
conduct a much larger forward current than will
the base-emitter diode.
v BE
VT
v BC
VT
IS
i C = I S e −1− e −1
R
●
where αR << 1
When the collector-emitter voltage drops below
the base-emitter voltage, the base-collector
diode is forward biased and conducts heavily.
v CB =v CE −v BE
2010 by Kenneth R. Laker, update 15Sep10 KRL
=>
v CE ≈V CEsat =v CB v BE ≈0.3 V
26
ESE319 Introduction to Microelectronics
More on NPN Saturation - cont.
In saturation the forward-biased the current
through the collector-base junction increases iB
and decreases iC as vBC increases.
●
iC
 sat = forced =  ≤ 
i B sat
Test for saturation mode operation
●
●
or
●
vCE = VCEsat = 0.1 to 0.3 V => collector-base junction is
forward biased
Current ratio
forward biased
iC
  ≤  => collector-base junction is
i B sat
2010 by Kenneth R. Laker, update 15Sep10 KRL
27
ESE319 Introduction to Microelectronics
iC Expansion Around Zero vCE
forward-active mode
iC (mA)
iC = 0
vBE = 0.68 V,
vCE = 0.25 V,
vBC= vBE - vCE= 0.43 V
Diode forward voltage
vCE (V)
Note that the collector current is zero at about vCE = 0.06 V, not 0 V !
Also note the large reverse collector-base current for vCE < 0.06 V.
2010 by Kenneth R. Laker, update 15Sep10 KRL
28
ESE319 Introduction to Microelectronics
Voltage
at
Zero
Collector
Current
v
v
IS V
i C = I S e −1− e −1
R
BE
BC
VT
T
 R e
v BE
VT
−1=e
−
 R 1−e
v BE
VT
 v BE −vCE 
VT
=e
−
v CE
VT
V BE
≈40∗0.7⇒ e
VT
−
iC = 0 =>
v
v
IS V
V
e −1= I S e −1
R
 R e
BC
BE
T
T
v BE
VT
−1=e
v BC
VT
−1
−
−e
v BE
VT
v BE
VT

≪1
For βR = 0.1 => αR = 0.09 => vCE = - 0.06 V
−1
2010 by Kenneth R. Laker, update 15Sep10 KRL
29
ESE319 Introduction to Microelectronics
pnp
npn
The PNP Transistor
Mode
Forward-Active
VEB
VCB
>0
<0
Reverse-Active
<0
<0
>0
>0
<0
>0
Cutoff
Saturation
2010 by Kenneth R. Laker, update 15Sep10 KRL
VCB = -VBC
Not Useful!
30
ESE319 Introduction to Microelectronics
PNP BJT Forward-Active Mode Basic Model
Collector-base diode
is reverse biased
V BC 0
Emitter-base diode is
forward biased
V EB≈0.7
Note reversal in voltage
polarity and in current
directions!
2010 by Kenneth R. Laker, update 15Sep10 KRL
i C =I S e
iC
i B=

v EB
VT
−1
i E =i B i C =1i B
31
ESE319 Introduction to Microelectronics
PNP BJT Large Signal Model
FWD. Active
NPN
rotate 180o
v EB
VT
PNP
i C =I S e −1
iC
i B=

iE
i E =i B i C =1i B
Note reversal in current
directions!
2010 by Kenneth R. Laker, update 15Sep10 KRL
Substituting, as in the
npn case, we get:
v EB
IS V
i E = e −1

T
32
ESE319 Introduction to Microelectronics
Yet another PNP BJT large signal model
v EB
VT
v EB
VT
IS
IS
i C = i B = I S e −1⇒ i B = e −1≈ e


This “looks like” a diode
between base and emitter
and the equivalent circuit
becomes:
v EB
VT
iB
iC
Again, in this model, the diode carries only base current,
not emitter current.
2010 by Kenneth R. Laker, update 15Sep10 KRL
33
Download