I. the Bipolar Junction Transistor A. Physical Structure

Home , Depletion region

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,

n+ polysilicon contact metal contact to base to n+ emitter region metal contact to collector

p-type base

+ + A p n n A' n+ buried layer n+ buried layer

p-type substrate field oxide n+ - p - n sandwich (intrinsic npn transistor) (a)

(emitter)

(base) edge of n+ buried layer

field oxide

A A'

+ p p n+

+ n emitter area, AE

(intrinsic npn transistor) (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

npn pnp

C E IC IE + + + VEB B V B − V + CE EC − IB VBE − IB − − −I −I E E C C (a) (b)

Normal operation: Normal operation: I positive V positive C CE ÐIC positive VEC positive I positive V = 0.7 B BE ÐIB positive VEB = 0.7 -IE 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

(a)

I C = µ (µA) IB 2.5 A 300 = µ IB 2 A 250

200 I = 1.5 µA (saturation) B

150 = µ (forward active) IB 1 A 100 = IB 500 nA 50 I = 0 (cutoff) −3 −2 −1 B

1 2 3 4 5 6 VCE (V) = µ −4 IB 1 A (reverse active) −8 = µ IB 2 A

(b)

EECS 6.012 Spring 1998 Lecture 16

D. Regions of Operation ¥ Constant-current region is called forward active ... corresponds to MOSFET saturation region (!!!)

β IC = F IB ¥ Bipolar saturation region (modeled as a constant voltage) corresponds to MOSFET triode region

≈ VCE VCE() 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:

> > VCE VCE() 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 n+ emitter

p-type base 0

n-type collector x

n+ buried layer base-collector outline of “core” depletion region 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) npB(x) pnC(x) (log) (log) (log)

emitter base collector n+ polysilicon p = 104 cm−3 contact nCo = 3 −3 npBo 10 cm = −3 pnEo 10 cm

− − − + x WEo xBEo xBEo 0 WBo WBo xBCo (c) ¥ Electrostatic potential:

φ (x) (mV) n+ emitter: o φ = 550 mV 500 o n collector: φ = 360 mV 250 o

− − − + x WEo xBEo xBEo 0 WBo WBo xBCo

−250 p base: − φ = − 500 o 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

1.5 = VCE 2 V 1 thermal equilibrium 0.5

0 − − − + WE xBE xBE = WB WB xBC x VBE 0.7 V −0.5

(a)

pnE(x) npB(x) pnC(x)

npB(0)

emitter base collector

n+ polysilicon − contact pnE( xBE) pnCo

npBo npB(WB) p (W + x ) pnEo nC B BC

− − − + x WE xBE xBE 0 WB WB xBC (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

hole diffusion flux n+ emitter

majority electrons majority hole flux from base contact electron p-type base diffusion

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

p (x) pnE(x)pnE(x) npBn(pBx)(x) pnCnC(x)

polysiliconpolysilicon (emitter)emitter (base)base (collector)collector contactcontact

- W−W - x − x - x −x 0 W W + x x E E BE BE BE BE0 WB B WB +B xBC BC x

A. Collector current:

¥ Electron diffusion current density X emitter area

 V ⁄ V V ⁄ V   BC th BE th  n  e Ð e  n ()W Ðn()0 pBo  diff pB B pB   J ==qD ------qD ------nB nW n W B B

V⁄ V diff qDnnpBoAE BE th I==ÐJA ------e CnB E  WB

EECS 6.012 Spring 1998 Lecture 16 B. Base current ¥ Reverse-injected hole diffusion current density X emitter area

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

V⁄ V diff qDppnEoAE BE th I ==ÐJ A ------eÐ 1 B pE E  WE

C. Emitter current

¥ Sum of IB and IC according to KCL (negative ... reference is positive-in)

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

EECS 6.012 Spring 1998 Lecture 16 IV. Forward-Active Current Gains α A. Alpha-F - F ¥ The ratio of collector current to the magnitude of the emitter current qD n A ------n pBo E- I W ------C ==------B α F IE qDppnEoAE qDnnpBoAE ------+------ WE WB

1 α= ------FDNW paB B 1+------D N W n dE E

α α ¥ F --> 1 ... typically, F = 0.99. β B. Beta Forward Current Gains - F ¥ The ratio of collector current to base current α I 1 Ð F I ===Ð I Ð I ------C- Ð I I ------B E C α C C α F F

α F I==------IβI. CαBFB 1ÐF

β ¥ A typical value is F = 100

EECS 6.012 Spring 1998 Lecture 16