Transistor (BJT)

Introduction• BJT (Bipolar Junction Transistor)• Vaccum tubes• It comes because it is most advantageous in amplification• Why it is called transistor?

Transistor = Transfer + Resistor• Why it is called BJT?• Types of BJT

Introduction(cont.)npn pnp

n p n

B

C p n pE

B

C

Cross Section

B

C

E

Schematic Symbol

B

C

E

Schematic Symbol

• Collector doping is usually ~ 109

• Base doping is slightly higher ~ 1010 – 1011

• Emitter doping is much higher ~ 1017

Junction Transistor• Sandwich structure.• Base is always in between E & C.• B is lightly doped.• E & C are heavily doped. E is more heavily doped than C and area of C

is more than E.• Two PN junctions.E

B

C

BE CB

E

B

C

BE CB

N-P-N P-N-P

Unbiased transistor• No external supply is applied.• Penetration of depletion region (less in E & C, more in B)

Transistor Biasing• Mode BE junction BC junction

• cutoff reverse biased reverse biased

• linear(active) forward biased reverse biased

• saturation forward biased forward biased

• Inverse active reverse biased forward biased

Transistor Biasing (cont.)• Transistor biasing in active region.• EB junction is forward biased and CB junction is reversed biased.

Transistor operation in active region (NPN)

E

B

CN P N

v

9

Large current

Transistor operation (cont.)• Electrons will flow from E to B.• Now electrons have three options

1. Recombine with holes (IB)2. Diffuse through base and out of the base connection.3. Remaining e- will go in C (Ic).

Transistor operation (cont.)

E

B

CN P N

Electrons emitted

Electrons collected

Recombination current

Emitter current

Collector current

IE=IB+IC

Transistor current• Emitter current, Base current, Collector current.• IE = IB + IC. (IE ≈ IC)

• IE = IPE + INE (for NPN INE for PNP IPE).• IB = IPE - IPC.• Reverse saturation current (ICBO) : It is the reverse sat. current when EB

junction is open.• IC = IPC + ICBO.

Parameters relating to current components• Emitter efficiency (ɣ) = • Transport factor (ß) = • Large signal current gain (α) = • α = =• α = ß * ɣ

Transistor as an amplifier

i i B

o L C

v R iv R i

Discussion of an amplification effect

CEBEi L

B C

vvR Ri i

B Ci iWith i ov v

50 ~ 300ov

i

vAv

E.g. for common-base configuration transistor:

Transistor construction technologies• Grown type.• Alloy type.• Electrochemically etched type.• Diffusion type• Epitaxial type.

Transistor configuration• Made one of three terminal common to i/p and o/p.• Depending on which terminal is made common. There are three

possibilities1. Common base configuration (CB).2. Common emitter configuration (CE).3. Common collector configuration (CC).

Common Base configuration (CB)

Vee Vcc

Re Rc

IE

IB

IC

Vee Vcc

Re Rc

IE

IB

IC

Common base configuration forNPN transistor

Common base configuration forPNP transistor

Common Base configuration (CB)• Input• Output• Current relations in CB configuration

1. Ic = IC(INJ) + ICBO

2. IC(INJ) (practically)3. ICBO (with emitter open)ICB= collector to base current IO = emitter is open

Common Base configuration (CB)

4. current amplification factor/current gain (αdc) αdc =IC(inj)/IE

So Ic = (αdc * IE ) + ICBO

Expression for IB: IB = (1- α)IE.

Transistor char in CB configuration1. Input char. 2. Output char. 3. Transfer char. • I/P char : graph of I/P current versus I/P voltage.• O/P char : graph of O/P current versus O/P voltage.• Transfer char: graph of O/P current versus I/P current

CB I/P char.• I/P current is emitter current(IE) and I/P voltage is emitter to base

voltage(VBE).1. Its identical to VI char of diode in FB.2. Up to cut-in V3. I/P resistance4. Effect of VCB on I/P VI char (Early effect)

VCB = 4VVCB = 8VIE (mA)

VBE

Early effect / Base width modulation• Effect on β and α.

E B C E B C

Increase VCB

Total base width = width of depletion region at CB junction + width of region which contains free charge carriers

Output char of transistor in CBIC

Active Region

Saturation RegionCutoff RegionIB = 0

IE

VCB

Operating region

1. Cutoff region2. Active region3. Saturation region

Output char of transistor in CB• Cut off region : region below the curve IE =0• Active region : IC ≈ IE (Const. current source)• Dynamic O/P resistance• Saturation region • Current controlled current source.

Breakdown voltage and punch-through effect• Increasing VCB causes CB junction to breakdown.• Reach through / Punch through effect

VCB

IC

E B C

Potential variation through transistor

Without biasingWith external bias

Transfer characteristic• Linear rela.tionship IC

IE

Common Emitter (CE) configuration

VccVcc

VBEVCE

RBRC

VBB VBBVBE

VCE

RC

RB

Common emitter config. for NPN transistor

Common emitter config. for PNP transistor

CE Configuration• Input• Output• Current relation

IE = IB + IC IC = α * IE + ICBO

IC = IB (α/1- α) + ICBO / (1- α)But (α/1- α) = β

So IC = IB * β + (β+1) ICBO

IC = IB * β + ICEO

CE Configuration• Reverse leakage current in CE configuration (ICEO)• Thermal instability, so thermal stabilizing circuit is required.• Relation between α and β. α = β/(1+ β) and β = α /(1- α)

CE characteristics (Input)• Same as conventional PN junction diode• Dynamic i/p resistance• Base current reduces as VCE

increases.VCE = 8V

VCE = 4VIB (μA)

VBE

CE characteristics (output)

VCE (V)

IC(mA)

IB = 50 A

IB = 0

30

5 10 15 20 0

0

IB = 100 A

IB = 150 A

IB = 200 A

22.5

15

7.5

Saturation Region

Active Region

Cutoff Region

1. Cut off region2. Active region3. Saturation region4.Dynamic O/P resistance5. Definition of β6. Maximum VCE and breakdown

Transfer characteristic• Why CE o/p char is more sloping than CB o/p char???

IC

IB

VCE = 2V

VCE = 5V

Typical transistor junction voltage values• Cutoff region• Short-circuited base• Open-circuited base• Cut-in voltage• Saturation voltage

Voltage Si transistor Ge transistorVBE

(Cutoff)0 -0.1

VBE (Cut in)

0.5 0.1

VBE (active)

0.7 0.2

VBE (sat.) 0.8 0.3

VCE (sat.) 0.2 0.1

Standard test for regions• Saturation region

1. find IC, IB then check IB >= IC/ β 2. measure VCB, positive for PNP and negative for NPN.

• Active regionmeasure VBE = 0.7 and measure VCB is negative (reverse biased)

• CutoffVBE is < 0.5 and VCB is negative

Common collector configuration

VccVcc

VBCVEC

RBRE

VBB VBBVBC

VECs

RERB

Practical way to draw CC config.VCC

O/P voltage

I/P voltage

Current relation• IE = IB + IC IC = α * IE + ICBO

IE = (β+1) * IB

Current gain γ = IE /IBMaximum use of CC is for impedance matching (I/P is high and O/P is low).

I/P char. And O/P char.

VEC = 2VVEC = 1VIB (μA)

VBC

IE

Active Region

Saturation RegionCutoff RegionIB = 0

IB

VEC

Transfer char.IE

IB

VCE = 2V

VCE = 5V

Comparison of configurationsparameter CB CE CC

Common terminal between i/p and o/p

Base Emitter Collector

I/P current IE IB IB

O/P current IC IC IE

Current gain α = IC/IE β= IC/IB Γ= IE/IB

I/P voltage VEB VBE VBC

O/P voltage VCB VCE VEC

Analytic expression for transistor char.• IC = -αN * IE – ICO ()

subscript N to α is transistor is being used in normal mode.For the inverted mode of operation

IE = -αI * IC – IEO ()

Base spreading resistance

E

B

C

VCE

VEBVCB

VCB = VC + rbb * IBVE VC

Ebers – moll model

VE VC

IE

IB

IC

αI IC αN IE

Ebers – moll model (cont.)• I = IC + αN IE

IC = - αN IE + I(I is diode current)

VC

αN IE

IC

I

I = I0 ()

Now Take, I0 = -ICO , V = VC and n=1

So I = -ICO ()

Why can’t we construct a transistor by connecting back to back diodes?

VE VC

Photo transistor

Dark current

20 mW/(cm* cm)

40 mW/(cm* cm)

60 mW/(cm* cm)

80 mW/(cm* cm)

Photo transistor (cont.)• Advantages:

• Photo current multiplied by β• High sensitivity• Good switching speed• No memory effect.

Phototransistor (cont.)• Disadvantages

• Not so fast as conventional transistor because of photo-conducting material

• Poor linearity• Temperature sensitive device• External voltage source is needed for operation