6.012 Microelectronic Devices and Circuits, Lecture 17
Transcript of 6.012 Microelectronic Devices and Circuits, Lecture 17
Lecture 17The Bipolar Junction Transistor (I)
Forward Active Regime
Outline
• The Bipolar Junction Transistor (BJT): – structure and basic operation
• IV characteristics in forward active regime
Reading Assignment: Howe and Sodini; Chapter 7, Sections 7.1, 7.2
6.012 Spring 2009 Lecture 17 1
1. BJT: structure and basic operation
n+ polysilicon contact metal contact to base to n+ emitter region
\ /
A'
I 'field oxide n+-p - n sandwich
(intrinsic npn transistor)
(a)
edge of n+ buried layer
Bipolar Junction Transistor: excellent for analog and fkont-end communications applications.
6.012Spring 2009 Lecture17'
NPN BJT Collector Characteristics
Similar to test circuit as for an nchannel MOSFET …except IB is the control variable rather than VBE
VCE
IC
= IC
(IB, V
CE)
(a)
1
50
100
150
200
250
300
IC�
(µA)
VCE
(V)6
(b)
−1−2−3
−4
−8
IB = 1 µA
IB = 2 µA
(reverse�
active)
(forward active)
(saturation)
IB = 0 (cutoff)
IB = 500 nA
IB = 1 µA
IB = 1.5 µA
IB = 2 µA
IB = 2.5 µA
IB
+
−
5432
6.012 Spring 2009 Lecture 17 3
• Close enough for minority carriers to interact – ⇒ can diffuse quickly through the base
• Far apart enough for depletion regions not to interact – ⇒ prevent “punchthrough”
Simplified onedimensional model of intrinsic device: Emitter Base Collector
IE
IB
IC
WB-XBE WB+XBC WB+XBC+WC-WE-XBE
VBE VBC +
-
+
-
x 0
Regions of operation:
saturation
reverse cut-off
forward�
active
VBC
VBC
VCE
VBE
VBE
B
C
E
+
-
+
- +
-
VCE = VBEVBC
n p n
NaBNdE NdC
6.012 Spring 2009 Lecture 17 4
BJT=two neighboring pn junctions back-to-back
Basic Operation: forwardactive regime
VBE>0 ⇒ injection of electrons from the Emitter to the Base injection of holes from the Base to the Emitter
VBC<0 ⇒ extraction of electrons from the Base to the Collector extraction of holes from the Collector to the Base
n-Emitter p-Base n-Collector
IE<0
IB>0
IC>0
VBE > 0 VBC < 0
6.012 Spring 2009 Lecture 17 5
Basic Operation: forwardactive regime • Carrier profiles in thermal equilibrium:
• Carrier profiles in forwardactive regime:
ln po, no
po
pono
no
NdE
NdC
ni2
NdE ni2
NaB
ni2
NdC
0
NaB
WB-XBE WB+XBC -WE-XBE WB+XBC+WC
x
ln p, n
n
NdE
NdC
ni2
NdE ni2
NaB
ni2
NdC
0
NaB
WB-XBE WB+XBC -WE-XBE
p
WB+XBC+WC
x
1019cm3 1017cm3
1016cm3
6.012 Spring 2009 Lecture 17 6
Basic Operation: forwardactive regime
Dominant current paths in forward active regime:
n-Emitter p-Base n-Collector
IE<0
IB>0
IC>0
VBE > 0 VBC < 0
IC: electron injection from Emitter to Base and collection by Collector
IB: hole injection from Base to Emitter IE: IE = (IC+IB)
6.012 Spring 2009 Lecture 17 7
The width of the electron flux "stream" is greater than the hole flux stream.
The electrons are supplied by the emitter contact injected across the base-emitter SCR and difhse across the base Electric field in the base-collector SCR extracts electrons into the collector. Holes are supplied by the base contact and diffuse across the emitter. The reverse injected holes recombine at the emitter
nic contact. -........................................ -..............-........................................-..............-........................................ -..............-............... ..............K::::::::::::::"................+ po1ysiliwn:iii:i;i::iii:
6.012Spring 2000 Lecture I?
2. IV characteristics in forwardactive regime
npB(0) = npBoe VBE Vth[ ]
, npB (WB ) = 0
Collector current: focus on electron diffusion in base
Electron profile:
Boundary conditions:
npB(x) = npB (0) 1− x WB
n
x 0
npB(0)
JnB
npB(WB)=0
npB(x)
WB
ni2
NaB
6.012 Spring 2009 Lecture 17 9
Electron current density:
Collector current scales with area of baseemitter junction AE:
IC = −JnB AE = qAE Dn WB
npBo • e VBE Vth[ ]
Collector terminal current:
JnB = qDn dnpB
dx = −qDn
npB(0)
WB
or
IC = IS e VBE Vth[ ]
IS ≡ transistor saturation current
�
Ic AE
B E
n p
n
6.012 Spring 2009 Lecture 17 10
Base current: focus on hole injection and recombination at emitter contact.
Boundary conditions:
Hole profile:
pnE(−xBE) = pnEo e VBE Vth[ ]
; pnE(−WE − xBE) = pnEo
pnE(x) = pnE(−xBE ) − pnEo[ ]• 1 + x + xBE WE
+ pnEo
p
x-xBE
pnE(x)
pnE(-xBE)
-WE-xBE
ni2
NdE
pnE(-WE-xBE)= ni2
NdE
6.012 Spring 2009 Lecture 17 11
Hole current density:
Base current scales with area of baseemitter junction AE:
IB = −J pE AE = qAE Dp
WE pnEo • e
VBE Vth[ ]−1
IB ≈ qAE Dp
WE pnEo • e
VBE Vth[ ]
Base terminal current:
J pE = −qDp dpnE dx
= −qDp pnE (−xBE ) − pnEo
WE
�
IB AE
B E
p
n
Emitter current: (IB + IC)
IE = − qAE Dp
WE pnEo
+ qAE
Dn WB
npBo
• e
VBE Vth[ ]
6.012 Spring 2009 Lecture 17 12
Forward Active Region: Current gain
ββββF = IC IB
=
npBo • Dn WB
pnEo • Dp
WE
= NdEDnWE NaBDpWB
To maximize βF:
• NdE >> NaB
• WE >> WB
• want npn, rather than pnp design because Dn > D p
ααααF = IC IE
= 1
1 + NaBDpWB
NdE DnWE
Want αF close to unity> typically αF = 0.99
IB = −IE − IC = IC ααααF
− IC = IC 1 − ααααF
ααααF
ββββF = IC IB
= ααααF 1 − ααααF
6.012 Spring 2009 Lecture 17 13
What did we learn today?
• Emitter “injects” electrons into Base, Collector “collects” electrons from Base – IC controlled by VBE, independent of VBC
– (transistor effect)
• Base: injects holes into Emitter ⇒ IB
npn BJT in forward active regime:
IC ∝ e VBE Vth[ ]
IC ∝ IB
n-Emitter p-Base n-Collector
IE<0
IB>0
IC>0
VBE > 0 VBC < 0
Summary of Key Concepts
6.012 Spring 2009 Lecture 17 16
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6.012 Microelectronic Devices and Circuits Spring 2009
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