I-V Characteristics of BJT
Common-Emitter Output Characteristics
iB B C E C i CE v B C E iB C i EC v
To illustrate the IC-VCE characteristics, we use an enlarged βR 0 5 10 -5 -1 0 1 2 VCE (V) Reverse-Active
Region SaturationRegion Cutoff IB= 100 µA IB = 80 µA IB = 60 µA IB= 40 µA IB = 20 µA IB= 0 µA Forward Active Region Saturation Region βF = 25; βR = 5 Collector Curr en t (mA) vCE ≥ vBE iC = βFiB vCE ≤ vBE vCE ≤ vBE iC = –(βR + 1)iB vCE ≤ vBE ≤ 0
Common Base Output Characteristics iE B C E v CB C i B C E v BC C i iE
Forward-Active Region IE = 0 mA IE = 0.2 mA IE = 0.4 mA IE = 0.6 mA IE = 0.8 mA IE = 1.0 mA βF = 25; βR = 5
v
CBor v
BC(V)
-2 0 2 4 6 8 10 0 0.5 1.0C
o
llector
Curr
ent
(mA)
Common-Emitter Transfer Characteristic iC - vBE
. BE voltage changes as -1.8 mV/oC - this is its temperature coef-ficient (recall from diodes).
vBC = 0
C
o
llector
Curr
ent
I
C(m
A)
-2 0 4 6 8 10 2 60 mV/decadeBase-Emitter Voltage (V)
0.0 0.2 0.4 0.6 0.8 1.010 -11 10-9 10-8 10-6 10-4 10-2 Log (I C )Common-Emitter Transfer Characteristic iC - vBE (p. 180)
. BE voltage changes as -1.8 mV/oC - this is its temperature coef-ficient (recall from diodes).
IC IS vBE VT --- 1 – exp = vBC = 0
C
o
llector
Curr
ent
I
C(m
A)
-2 0 4 6 8 10 2 60 mV/decadeBase-Emitter Voltage (V)
0.0 0.2 0.4 0.6 0.8 1.010 -11 10-9 10-8 10-6 10-4 10-2 Log (I C )Junction Breakdown - BJT has two diodes back-to-back. Each diode has a breakdown. The diode (BE) with higher doping concentrations has the lower breakdown voltage (5 to 10 V).
In forward active region, BC junction is reverse biased. In cut-off region, BE and BC are both reverse biased.
Junction Breakdown - BJT has two diodes back-to-back. Each diode has a breakdown. The diode (BE) with higher doping concentrations has the lower breakdown voltage (5 to 10 V).
In forward active region, BC junction is reverse biased. In cut-off region, BE and BC are both reverse biased.
Junction Breakdown - BJT has two diodes back-to-back. Each diode has a breakdown. The diode (BE) with higher doping concentrations has the lower breakdown voltage (5 to 10 V).
In forward active region, BC junction is reverse biased. In cut-off region, BE and BC are both reverse biased.
Minority Carrier Transport in Base Region Inj. Elec. recombined electrons Coll. Elec. iT IF/βF IR/βR IREC N P N
Emitter Base Collector
Space Charge regions (pno, npo) + - - + iE iB iC vBE vBC n(x) x n(WB) WB iT qADndn dx ---= n(0) 0 (pno, npo) Electron conc. in base (neglects recombination) Inj. Holes IREC n 0( ) nbo vBE VT --- exp =
Transport current iT results from diffusion of minority carriers (holes in npn) across base region.
Base current iB is composed of holes injected back into E and C and IREC
needed to replenish holes lost to recombination with electrons in B. The minority carrier concentrations at two ends of base are
and where is the
equilib-rium electron density in the base region.
The junction voltages establish a minority carrier concentration gradient at ends of base region. For a narrow base, we get
is the B width; is the cross-sectional area of B region. The saturation current is
nbo
Transport current iT results from diffusion of minority carriers (electrons in npn) across base region.
Base current iB is composed of holes injected back into E and C and IREC
needed to replenish holes lost to recombination with electrons in B. The minority carrier concentrations at two ends of base are
and where is the
equilib-rium electron density in the base region.
The junction voltages establish a minority carrier concentration gradient at ends of base region. For a narrow base, we get
is the B width; is the cross-sectional area of B region. The saturation current is
n 0( ) nbo vBE VT --- exp = n W( B) nbo vBC VT --- exp = nbo WB A
Transport current iT results from diffusion of minority carriers (holes in npn) across base region.
Base current iB is composed of holes injected back into E and C and IREC
needed to replenish holes lost to recombination with electrons in B. The minority carrier concentrations at two ends of base are
and where is the
equilib-rium electron density in the base region.
The junction voltages establish a minority carrier concentration gradient at ends of base region. For a narrow base, we get
.
is the B width; is the cross-sectional area of B region. The saturation current is
n 0( ) nbo vBE VT --- exp = n W( B) nbo vBC VT --- exp = nbo iT qADndn dx --- qADnnbo WB --- vBE VT --- vBC VT --- exp – exp – = = WB A
Transport current iT results from diffusion of minority carriers (holes in npn) across base region.
Base current iB is composed of holes injected back into E and C and IREC
needed to replenish holes lost to recombination with electrons in B. The minority carrier concentrations at two ends of base are
and where is the
equilib-rium electron density in the base region.
The junction voltages establish a minority carrier concentration gradient at ends of base region. For a narrow base, we get
.
is the B width; is the cross-sectional area of B region.
The saturation current is .
n 0( ) nbo vBE VT --- exp = n W( B) nbo vBC VT --- exp = nbo iT qADndn dx --- qADnnbo WB --- vBE VT --- vBC VT --- exp – exp – = = WB A IS qADnnbo W --- qADn ni 2 N W ---= =
Base Transit Time
Forward transit time is time associated with storing charge Q in Base region and it is with . Using we get τF Q iT ----= Q qA n 0[ ( ) n– bo] WB 2 ---= Q qAnbo vBE VT --- 1 – exp WB 2 ---=
Using we get and . Q
n(x)
x
nbo n(0) n(WB) = nbo 0 WB Q = excess minority charge in Base Q qAnbo vBE VT --- 1 – exp WB 2 ---= iT qADn WB ---nbo vBE VT --- 1 – exp = τF WB 2 2Dn --- WB 2 2VTµ n ---= =This defines an upper limit on frequency f 1 . 2πτF ---≤ Q
n(x)
x
nbo n(0) n(WB) = nbo 0 WB Q = excess minority charge in BasePSPICE EXAMPLE
*Libraries:
* Local Libraries :
.LIB ".\example10.lib"
* From [PSPICE NETLIST] section of C:\Program Files\OrcadLite\PSpice\PSpice.ini file: .lib "nom.lib"
*Analysis directives: .DC LIN V_V1 0 5 0.05 + LIN I_I1 10u 100u 10u
.PROBE V(*) I(*) W(*) D(*) NOISE(*) .INC ".\example10-SCHEMATIC1.net" **** INCLUDING example10-SCHEMATIC1.net **** Q1 V1 0Vdc BF=100 I1 0Adc IS=1.0e-15 VAF=80
