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

Plot of log IC and log IB

vs VBE

6.012 Spring 2009 Lecture 17 14

Common­Emitter Output Characteristics

6.012 Spring 2009 Lecture 17 15

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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