半导体物理与器件 第10章

iB
6. the Modes of Operation
① cutoff mode: Vbe 0, Vbc0. Vbe0→ iE1= 0 Vbc0 → iC = 0
② forward-active mode: V be0, Vbc 0;
cutoff
Vbe0 → iE1 > 0 →iC >0
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3. The Basic Principle of Operation
Qualitative
analysis in thermal equilibrium state energy-band diagram
N ++
P+
N
distribution of doping impurity concentration
VBE iC=ISexp( pn(── xV ) ) T
pn0
iE
iC
p n( x )
iC is controlled by the VBE
np0
pn0
x=0
x=xB
② emitter Current – iE
N+ iE1 iE
pn(x) pn0
P
np(x)
N iC
pn0
iE1: e→b electron flow under
junction current
→iBarecombination
current of minority carrier and hole in base, hole currents generated by the external hole supplement.
Vbe0, Vbc 0
IE
N+
VEB
IC p IB VBC
an npn bipolar transistor under forward-active mode
N
Distribution of space charge region
① Forward active mode
distribution of space charge density;
distribution of space charge density;
▓
N+ VEB
IC p IB VBC N
Distribution of space charge region under inverse active mode ND - N A WE WB WC
+
distribution of electric field; ▓ energy band diagram under reverse-active mode.
Assumptions of the ideal transistor condition
Forward active mode
① The uniform distribution of impurity, the electric field in neutral area =0; ② pn junction is the ideal plane junction, carrier for one-dimensional motion; ③ lateral dimensions >> width of the base region; ④ width of the base region << the diffusion length of minority carrier; ⑤ emitter junction area = collector junction area; ⑥ low-injection.
forward-bias;
VBE iE1=iC=ISexp( ── ) VT
iE2: b→e hole flow under
forward-bias;
iE2
np0
pn(x)
VBE iE2=IS2exp( ── ) VT
iB
VBE iE= iE1+iE2= ISEexp( ──) VT
The total emitter current iE
▓
Distribution of space charge region under cutoff mode ND - N A WE WB WC
+
x
E
x
④ Saturation mode
IE
Vbe 0, Vbc 0
distribution of space charge density;
N+ P N ABE—the cross-sectional area of the B-E junction; nB0—the thermal equilibrium electron concentration in the base. nB0=ni2/Na
The diffusion of electrons is inn the +x direction p(x)
“short diode”
an npn bipolar transistor under forward-active mode
Simplified Transistor Current Relations
VBE is a Short-diode, e→b at the ideal situation minority carrier concentration ) np(0) = n p 0 exp( electron flowof the base width (≈no recombination). linear function VT N+ P
10.1 The Basic Principle of Operation -- Bipolar Junction Transistor--BJT
1. introduction
Forward current of PN junction: majority carrier diffusiongreat; Reverse current of PN junction : minority carrier drift small.
→iBb exp(VBE/VT)
iB= iBa+iBb exp(VBE/VT)
N+
iE1
iE pn(x) pn0
P
np(x)
N
Define: b ≡iC/iB the common-emitter current gain. iC>> iB b >>1.
iC
pn0 pn(x)
iE2
np0
iBa iBb
2. The basic structure
Three separately doped regions; two pn junctions; emitter, base, collector the base width << Minority carrier diffusion length
E n++ P+ n
+ ND - N A WE

WB
WC

x E
distribution of electric field in depletion region
x
▓
operating mode
Forward
active saturation Reverse active Cutoff.
① Forward active mode
If provide a large number of minority carrier for a reverse biased
PN junction What will happen?
How to provide a large number of minority carrier for a reverse biased PN junction? How to control the number of minority carrier for a reverse biased PN junction?
▓
N+ VEB
IC
p IB VBC N
distribution of electric field; ▓ energy band diagram under saturetion mode.
▓
ND - N A WE WB WC
+
x E
x
5. Simplified Transistor Current Relations
iC and iE are all functions of exp(VBE/ VT) iC/iE=constant.
Define: a≡iC/iE --the common-base current gain.
iE>iC→a<1
③ Base Current --iB
▓ iBa—b-e
Considering the recombination in base
▓
x
E
x
③ Cutoff mode
IE
Vbe 0, Vbc 0
distribution of space charge density;
▓
N+ VEB
IC p IB VBC N
distribution of electric field; ▓ energy band diagram under cutoff mode.
▓
ND - N A WE WB WC
+
distribution of electric field; ▓ energy band diagram under forwerd-active mode bias.
▓
x E x
eVBE
eVBC
② inverse active mode
IE
Vbe 0, Vbc 0
E
N++
(Nd-Na)
C
P+
N
N++
N
B
P+
Idealized doping profile of a uniformly doped npn bipolar transistor
The actual device structure diagram*
Buried layer: reduce series resistance; isolation: the PN junction
Chap10 The Bipolar Transistor The main content 10.1 The bipolar transistor action 10.2 Distribution of minority carriers; 10.3 Low-frequency common-base current gain; 10.4 Non-ideal effects; 10.5 Equivalent circuit models; 10.6 Frequency limitation. “Bipolar” refers to what? What is an active device? What are the main application?
C
E p++ n+ p
C
e
B
c
e
B
c
b ++: heavy doping, Doping concentration: emitter>base>collector
b
The basic structure* Typical impurity doping concentrations: emitter: ~1019; base: ~1017~1018; collector:1015/cm3.
Excess minority carrier conce N ntration at the edge
iE1 iE
pn (x)
np ( x ) pn0 np0
iE2
pn0
iC
p n( x )
x=0
b→e hole flow
iB
x=xB
Minority carrier distribution and current direction under forward-active bias
① Collector Current--iC
Assuming the ideal linear electron distribution in the base the collector current is a diffusion current
dn ( x ) n B ( 0) - 0 = eDABE [ iC = eDA BE ] dx 0 - xB eDA BE VBE =· nB0 exp( ) xB VT