BJT Band diagram

Analysisتجزيه وتحليل دياگرام

باند انرژي

W

PN

The “typical” electron travels into the p-type region a distancegiven by DL

D is the diffusion coefficient (technically, the minority carrier diffusion coefficient)t is the lifetimetime (technically, the minority carrier lifetime)

W

The diffusion length L can easily by ~100 x times larger than the depletion width W

Diffusion (injection)

Recombination(excess e- combinewith holes)

How far does an electron go into the p-type region before it finds a “hole” to recombine with?

Transistors (Transfer Resistor)

Transistors

Junction-FETs (JFETS)

Field Effect TransistorsBipolar transistors

Insulated Gate FET’s

MOSFETs

NPN,PNP

N-channel, P-channel

Enhancement, DepletionN-channel, P-channel

The bipolar junction transistor (BJT)

NP

P

B

C

E

C

B

E

PN

N

B

C

E

B

C

E

B

C

E

C

B

ENPN

PNP

Arrow always points away from base and toward emitterMy pneumonic: No Point iN

Arrow always points away from emitter towards baseMy pneumonic: Points IN

Diffusion

Drift

Diffusion

Drift

At equilibrium:

BaseCollector Emitter

CB

E

+

-

+

-

PN N

+

-+

-} }

This junction isreverse-biased

This junction isforward-biased

BC E

“Quasi”-Fermi level

Since we are not at thermodynamic equilibrium, we cannotdefine a single chemical potential (Fermi level) is everywhere

A Quasi – fermi-level can be used to describe the local equilibrium ofelectrons and holes

In Use: Forward bias one p-n junction, and reverse-bias the other

PN N

+

-

+ -

BC E--

++

W (Width of depletion region)

LDiffusion

Physical thickness of base

There are three important length scales that are relevant to understandinghow a transistor operates:

DiffusionDrift

PN N

+

-

+

-

} }

This junction isreverse-biased

This junction isforward-biased

BC E

Basis of bipolar transistor operation: 1) The Base-emitter junction is forward-biased: Electrons flow from the emitter

to the base, just like in a normal forward-biased diode

2) Because the base is very thin, electrons continue to move through the baseand find themselves at the collector-base junction. Once they ‘feel’ the large electricfield at this junction, they are pushed downhill to the collector. Only a very smallfraction (typically ~ 1% - 3%) of the electrons come out through the base; theremaining 97%-99% come out through the collector.

When base is made very thin, IC>>IB--

++

VC

VB

VEIBIB

IE

VBEVCB

PN N

+

-

+ -

BC E

VC

IB

VE

IE

IB

When base is made very thin, IC>>IBand IC~IE

Bipolar transistor can be considered a current amplifier: If one can control thebase current, then this will induce a much larger change in the current in the collector and in the emitter.

a=IC/IE 1-a=IB/IE

B

C

I

I

1is the current gain of a transistor. b is commonly ~30-100

--

++

VC

VB

VEIBIB

IE

VBEVCB

If VCB constant, then as VBE is increased, current IC and IB increase exponentially

VBE

IC~IE

B

+-

+

-DVB

Small wiggle in VB, DVB, induces large change in IC. By Ohm’s Law, the voltage across RC shows a big change. So,Small DVB Big DVRE

Bipolar transistor as a voltage amplifier = Transistor + resistor(s)

VRE

Collector resistor

Field-effect Transistors

Main differences from bipolar transistors:

1) Use an electric field, established by applying a voltage to a “gate” electrode, to control current flow (voltage in Voltage out)

2) Ideally, no current flow at all into the “gate” electrode. Important: No current implies no power dissipation, at least under

certain conditions Two fundamentally different types:

1) Junction FET (J-FET)

2) IGFET (insulated-gate FET)

The MOSFET (Metal-oxide-semiconductor FET) is the most common type

Relies on a reverse-biased PN junction to prevent current flow in the gate

n

p

p

Depletion region

Depletion region

e- e- e- e- e-

+-

Source (S) Drain (D)

Gate (G)

Gate (G)

Gate forms a diode (p-n) junction with source and drain

JFET is always operated under conditions where this diode junctionis reverse-biased, so that only very little current flows from the gateto the source or the drain

N-channel JFET

Depletion region is larger on the right-hand side because thegreen region is more positive on the right than on the left (due to VSD),so the Gate-Drain junction is reverse-biased more strongly than thegate-source junction is.

np

pDepletion region

Depletion region

e- e- e- e-

+-

S D

G

G

+

VSD

VGS

connection so both gate electrodes have the same voltage-

np

pe- e- e- e-

+-

S D

G

G

+

VSD

VGS

-

“Pinch-off”Larger (more negative) VGS

Small, negative VGS

n-channel

SYMBOL:

V”pinch-off”

IDS

V”pinch-off”

Larger (more positive) VDS

VDS

IDS

Purely resistive here (silicon actslike a resistor)

Current goes up less quicklyas depetion region narrows

Once pinch-off occurs, nofurther increase i n current

pinch-off

VDS

IDS

pinch-off

VGS~0

VGS~ -1.0 V

VGS~ -2.0 V

VGSn-channel

SYMBOL:

Everywhere switch N, PSwitch signs of all voltage sources and currents

p

n

n

Depletion region

Depletion region

+ -

Source (S) Drain (D)

Gate (G)

Gate (G)

h+ h+ h+ h+ h+

pn

nh+

+-

S D

G

G

+

VSD

VGS

- “Pinch-off”

VDS

-IDS

pinch-off

VGS~0

VGS~ +1.0 V

VGS~ +2.0 V

IGFET (Insulated-gate FET)

InsulatorMetal (G)

SiO2

Metal

S D

Semiconductor

CB

VB

Gate (G)

Body

p-Silicon

S D

n-Si n-Si

4 terminals: Source, Drain, Gate, and “Body” (sometimes called “Substrate”)

SiO2

Metal

Gate (G)

Body

p-Silicon

S D

n-Si n-Si

diode-like junctionhere (and similarly at drain)

For VG<0, the p-type silicon is in depletion or possibly accumulation. It formsresistive p-n junctions with the source and drain.

For VG>0, the p-type silicon goes into depletion.

When VG is large and positive, enough electrons are attracted to the near-surfaceregion that the region right under the SiO2 becomes inverted, and electrons canfrom from the source to the drain.

SiO2

Metal

Gate (G)

Body

p-Silicon

S D

n-Si n-Si

Inverted region

With VG=0 With VG>0

CB

VB

CB

VB

Depletion

CB

VB

Inversion

CB

Accumulation

VB

Small negative gate(for n-type sample)Surface becomes resistive, but electronsstill majority carrier

Large negative gate(for n-type sample)Surface becomesp-type, as holes become majority carrierat surface

positive gate(for n-type sample)Surface remains n-type, but becomes more conductive

“flat-band” condition

As a function of gate voltage, three different characteristic behaviors:

CB

VBe-

CB

VB

If metal has smaller work function, then when connected by a wire,Electrons move from metal to semiconductor, making semiconductor Negaitve and metal positive until their Fermi levels line up

CB

VB

CB

VB

Depletion

CB

VB

Inversion

CB

Accumulation

VB

Small positive gate(for p-type sample)Surface becomes resistive, but holesstill majority carrier

Large positive gate (for p-type sample) surface becomes n-type, as electrons become majority carrier at surface

negative gate(for p-type sample)Surface remains p-type, but becomes more conductive

“flat-band” condition

As a function of gate voltage, three different characteristic behaviors:

CB

VB

SiO2

Metal

Gate (G)

Body

p-Silicon

S D

n-Si n-Si

diode-like junctionhere (and similarly at drain)

Body is always held at potential of drain or possibly biased more negatively(to reverse bias the S and D junctions to the p-type Body)

Applying a voltage to the gate controls whether the near-surface regionis in accumulation, depletion, or inversion

SiO2

Metal

Gate (G)

Body

p-Silicon

S D

n-Si n-Si

diode-like junctionhere (and similarly at drain)

For VG<0, the p-type silicon is in depletion or possibly accumulation. It formsresistive p-n junctions with the source and drain.

For VG>0, the p-type silicon goes into depletion.

When VG is large and positive, enough electrons are attracted to the near-surfaceregion that the region right under the SiO2 becomes inverted, and electrons canfrom from the source to the drain.

SiO2

Metal

Gate (G)

Body

p-Silicon

S D

n-Si n-Si

Inverted region

With VG=0 With VG>0