I. The Bipolar Junction Transistor A. Physical Structure:

,
,
  ,
 

• oxide-isolated, low-voltage, high-frequency design • typical of the bipolar transistor found in a BiCMOS process,

metal contact to base

p+

n

n+ buried layer

field oxide

metal contact to collector

p-type base

,, ,,, ,

,,, ,, A

n+ polysilicon contact to n+ emitter region

n+

A'

n+ buried layer

n+ - p - n sandwich (intrinsic npn transistor)

p-type substrate

(a)

,,,,,,,,, ,,,,,,,,, ,,,, , , , , ,,,, ,,,, , , ,,,,,,, ,,,,,,,, , , , (base)

A

p+

p

n + emitter area, AE (intrinsic npn transistor)

(emitter)

edge of n + buried layer

field oxide

A'

n+

(collector)

(b)

EECS 6.012 Spring 1998 Lecture 16

B. Circuit Symbol and Terminal Characteristics • Two devices that have complementary characterisitics npn transistor and the pnp transistor • The direction of the diode arrow indicates device type

pnp

npn

C B IB

+ VBE

− E

E +

IC + VCE − −IE

(a)

Normal operation: VCE positive IC positive VBE = 0.7 IB positive -IE positive

VEB −

B −IB

C

IE + VEC − −IC

(b)

Normal operation: VEC positive –IC positive VEB = 0.7 –IB positive IE positive

EECS 6.012 Spring 1998 Lecture 16

C. npn BJT Collector Characteristics • Similar test circuit as for n-channel MOSFET ... except IB is controlled instead of VBE IC = IC(IB, VCE)

+ V − CE IB

(a) IC (µA) 300

IB = 2.5 µA IB = 2 µA

250 200

IB = 1.5 µA

(saturation)

150

IB = 1 µA

(forward active)

100 IB = 500 nA 50 −3

−2

IB = 0 (cutoff)

−1 1

IB = 1 µA IB = 2 µA

2

3

4

5

6

VCE (V)

−4 (reverse active)

−8

(b)

EECS 6.012 Spring 1998 Lecture 16

D. Regions of Operation • Constant-current region is called forward active ... corresponds to MOSFET saturation region (!!!) IC = βF IB • Bipolar saturation region (modeled as a constant voltage) corresponds to MOSFET triode region V CE ≈ V CE ( sat ) = 0.1V

• Cutoff ... corresponds to MOSFET cutoff region • Reverse active ... terminal voltages for npn sandwich are flipped so that VCE is negative and VBC = 0.7 V. Only occasionally useful.

• Boundary between saturation and forward-active regions: V CE > V CE ( sat )

and

IB > 0

EECS 6.012 Spring 1998 Lecture 16

II. Bipolar Transistor Physics A. Forward Active Region of Operation

n+ polysilicon base-emitter depletion region

p-type base

n+ emitter 0

n-type collector

x base-collector depletion region

n+ buried layer outline of “core” n+pn sandwich

B. Game Plan • Understand thermal equilibrium potential and carrier concentrations. • Apply the Law of the Junction with VBE ≈ 0.7 V and VBC < 0 (typical forward-active bias point) to find the minority carrier concentrations at the depletion region edges. • Assume that the emitter and the base regions are “short” (no recombination) and find the diffusion currents.

EECS 6.012 Spring 1998 Lecture 16


, 
,

C. Thermal Equilibrium • Typically emitter is doped two orders of magnitude (at least) more heavily than the base; the collector is an order of magnitude more lightly doped than the base. • Minority carrier concentrations: pnE(x) (log)

n+ polysilicon contact

npB(x) (log)

emitter

pnC(x) (log)

base

collector

npBo = 103 cm−3

pnCo = 104 cm−3

pnEo = 10 cm−3

−WEo − xBEo

− xBEo 0

WBo

WBo + xBCo

x

(c)

• Electrostatic potential: n+ emitter: φo = 550 mV

−WEo − xBEo

−xBEo

φo(x) (mV) 500

n collector: φo = 360 mV

250

0

WBo

WBo + xBCo

x

−250 p base: −500 φo = −420 mV

EECS 6.012 Spring 1998 Lecture 16

D. Carrier Concentrations under Forward Active Bias • Boundary conditions at the edges of the depletion region are: Emitter-Base: exp[VBE / Vth] >> 0......Base-Collector: exp[VBC / Vth] = 0 • Ohmic contacts return carrier concentration to equilibrium φ(x)(V) 2.5 2 1.5 VCE = 2 V 1

thermal equilibrium

0.5


,
,  −WE − xBE

−xBE

W VBE = 0.7 V B

x

(a) pnC(x)

npB(x) npB(0)

emitter

collector

base

pnE(−xBE) pnCo npBo

npB(WB)

pnEo

−WE − xBE

WB + xBC

−0.5

pnE(x)

n+ polysilicon contact

0

−xBE 0

WB

pnC(WB+ xBC)

WB + xBC

x

(b)

EECS 6.012 Spring 1998 Lecture 16



,, 

E. The Flux Picture - Forward Active Bias

• Rather than current densities, we use the concept of flux [# per cm2 per second] • The width of the electron flux “stream” is greater than the hole flux stream. n+ polysilicon

n+ emitter

,,,,,,,,,,,, ,,,,,,,,,,,, ,,,,,,,,,,,, ,,,,,,,,,,,, ,,,,,,,,,,,, ,,,,,,,,,,,, , , , , , ,

hole diffusion flux

majority electrons

majority hole flux from base contact

electron diffusion

p-type base

n-type collector

n+ buried layer

majority electron flux to coll. contact (minimum resistance path is through the n+ buried layer)

(b)

• The electrons are supplied by the emitter contact and diffuse across the base • Electric field in the collector depletion region sweeps electrons into the collector • n+ buried layer provides a low resistance path to the collector contact • The holes are supplied by the base contact and diffuse across the emitter • The reverse injected holes are recombined at the polysilicon ohmic contact

EECS 6.012 Spring 1998 Lecture 16


, 
,

III. Forward-Active Terminal Currents pnE(x)pnE(x)

emitter polysilicon (emitter) polysilicon contact contact

− xBE - W−W E -ExBE

pnCpnC (x)(x)

npBn(x) pB(x)

collector (collector)

base (base)

- xBE −xBE 0 0

WB WB

x WBW+Bx+ BC BC

x

x

A. Collector current: • Electron diffusion current density X emitter area

 V  BC e n pBo  

⁄V

th

V

BE

⁄V



th 

–e   n pB(W B) – n pB(0)  diff  = qD  -----------------------------------------------  = qD -----------------------------------------------------------------------------------J nB n n W W  B B

 qD n n pBo A E  BE diff I C = – J nB A E =  -------------------------------  e WB   V

⁄V

th

EECS 6.012 Spring 1998 Lecture 16

B. Base current • Reverse-injected hole diffusion current density X emitter area

 qD p p nEo  V BE ⁄ V th diff J = –  -------------------------  ( e –1) pE W   E

 qD p p nEo A E   BE diff I B = – J pE A E =  -------------------------------   e WE   V

⁄V

th

 – 1 

C. Emitter current • Sum of IB and IC according to KCL (negative ... reference is positive-in)

 qD p p nEo A E   qD n n pBo A E  V BE ⁄ V th diff   diff I = J +J A = –  ----------------------------------  +  ---------------------------------  e E nB pE  E W W     E B

EECS 6.012 Spring 1998 Lecture 16

IV. Forward-Active Current Gains A. Alpha-F - αF • The ratio of collector current to the magnitude of the emitter current  qD n n pBo A E   -------------------------------  WB IC   -------- = ----------------------------------------------------------------------------------- = α F IE  qD p p nEo A E   qD n n pBo A E   -------------------------------  +  -------------------------------  WE WB    

α

1 = ---------------------------------------------F  D p N aB W B  1 +  -----------------------------   D n N dE W E 

• αF --> 1 ... typically, αF = 0.99.

B. Beta Forward Current Gains - βF • The ratio of collector current to base current IC  1 – αF I B = – I E – I C = ------- – I C = I C  ----------------  αF  αF   αF  I C =  ----------------  I B = β F I B .  1 – αF • A typical value is βF = 100

EECS 6.012 Spring 1998 Lecture 16