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Lecture 17 The Bipolar Junction Transistor (I) Forward Active Regime Outline The Bipolar Junction Transistor (BJT): structure and basic operation I-V characteristics in forward active regime Reading Assignment: Howe and Sodini; Chapter 7, Sections 7.1, 7.2 6.012 Spring 2009 Lecture 17 1
17

6.012 Microelectronic Devices and Circuits, Lecture 17

Mar 25, 2022

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Page 1: 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

Page 2: 6.012 Microelectronic Devices and Circuits, Lecture 17

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'

Page 3: 6.012 Microelectronic Devices and Circuits, Lecture 17

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

Page 4: 6.012 Microelectronic Devices and Circuits, Lecture 17

• 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

Page 5: 6.012 Microelectronic Devices and Circuits, Lecture 17

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

Page 6: 6.012 Microelectronic Devices and Circuits, Lecture 17

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

Page 7: 6.012 Microelectronic Devices and Circuits, Lecture 17

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

Page 8: 6.012 Microelectronic Devices and Circuits, Lecture 17

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?

Page 9: 6.012 Microelectronic Devices and Circuits, Lecture 17

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

Page 10: 6.012 Microelectronic Devices and Circuits, Lecture 17

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

Page 11: 6.012 Microelectronic Devices and Circuits, Lecture 17

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

Page 12: 6.012 Microelectronic Devices and Circuits, Lecture 17

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

Page 13: 6.012 Microelectronic Devices and Circuits, Lecture 17

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

Page 14: 6.012 Microelectronic Devices and Circuits, Lecture 17

Plot of log IC and log IB

vs VBE

6.012 Spring 2009 Lecture 17 14

Page 15: 6.012 Microelectronic Devices and Circuits, Lecture 17

Common­Emitter Output Characteristics

6.012 Spring 2009 Lecture 17 15

Page 16: 6.012 Microelectronic Devices and Circuits, Lecture 17

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

Page 17: 6.012 Microelectronic Devices and Circuits, Lecture 17

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6.012 Microelectronic Devices and Circuits Spring 2009

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