Upload
walter-houston
View
221
Download
0
Tags:
Embed Size (px)
Citation preview
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 i
v R i
Discussion of an amplification effect
CEBEi L
B C
vvR R
i i
B Ci iWith i ov v
50 ~ 300ov
i
vA
v
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 configuration
1. 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 = 4V
VCB = 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 CB
IC
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 biasing
With 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 transistor
VBE (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 region1. 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 = 2V
VEC = 1VIB (μA)
VBC
IE
Active Region
Saturation RegionCutoff RegionIB = 0
IB
VEC
Transfer char.
IE
IB
VCE = 2V
VCE = 5V
Comparison of configurations
parameter 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