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Digital Circuits I
Semiconductor Devices
University of the PhilippinesElectrical and Electronics Engineering Institute
2Joy Reyes-Madamba@2010
Semiconductor Fundamentals
Conductors solids with high conductivity and
low resistivity
Insulators solids with small or low conductivityand high resistivity
Semiconductors solids with conductivity andresistivity between conductors and insulators
Three types of materials:
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Energy Band Model
EC
EV
EG
EC = conduction energyEV = valence energy EG = energy gap
conduction band
valence band
EC
EV
EG
conduction band
valence band
ECEV
conduction band
valence band
EG
most commonly used:
Silicon (Si), Germanium (Ge)
INSULATOR
SEMICONDUCTOR
CONDUCTOR
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Types of Semiconductors
Intrinsic semiconductors semiconductors in
pure state, without chemical impurities
Extrinsic semiconductors impurities (atomsof other elements) are introduced into an intrinsic
semiconductor
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Intrinsic Semiconductors
Few charge carriers
Not good for electronic devices
At 0K, they act like insulators
Conductivity increases with temperature
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Semiconductor Structure
Semiconductor atoms form a crystalline structure
similar to that of diamond
At 0K, no free electrons (like an insulator)Si Si Si
Si
Si
Si
Si Si
Si
--
-
-
-
-
- -
-
-
--
-
-
-
-
--
--
-
-
- -
Electrons are
shared
between atoms
using covalent
bonds
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Semiconductor Structure
When an electron transfers to a hole, effect is the
hole has moved
Si Si Si
Si
Si
Si
Si Si
Si
--
-
-
-
-
- -
-
-
-
-
-
-
-
--
-
-
-
- -
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Extrinsic Semiconductors
Intrinsic semiconductors to which impurities havebeen added to increase conductivity
Process of adding impurities is called doping
Effect of doping is addition of an energy levelcloser to the conduction or valence band
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Types of impurities
N-typeor donor impurities atoms with five
valence electrons (e.g. arsenic[As], antimony[Sb],
phosphorus[P]); give rise to an n-type
semiconductor
P-typeor acceptor impurities trivalent atoms
(e.g. gallium[Ga], boron[B], indium[In]); results ina p-type semiconductor
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Extrinsic Semiconductors
Si Si Si
Si
Si
P
Si Si
Si
--
-
-
-
-
- -
-
---
-
-
-
-
--
--
-
-
- -
-
Fifth
electron has
a weaker
bond than
other
electrons
EC
EV
EG
-
- --
Free electron
from impurity
Edonor
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Si Si Si
Si
Si
B
Si Si
Si
--
-
-
-
-
- -
-
-
--
-
-
-
-
--
-
-
-
- -
Acceptor
impurity has
only three
electrons to
share; hole
is created
EC
EV
EG
Free hole
from impurity
Eacceptor+
Extrinsic Semiconductors
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Extrinsic Semiconductors
To increase the conductivity of the intrinsic
semiconductor, a small amount of impurity is
needed Doping produces semiconductors with a
predominance of one type of carrier
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Extrinsic Semiconductors
In n-type semiconductors, electrons are majoritycarriers& holes are minority carriers
In p-type semiconductors, vice versa
Doping reduces minority carriers through fasterrecombination
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Junction Diode
A two-terminal device resulting from the
combining a p-type and n-type semiconductor
P-N junction diode is the building block of allsemiconductor devices
P N
PN junction
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Diode Circuit Symbol
I
+ V
Typical V-I CurveI
V
Breakdown
region
junction diode lets the
current flow in one direction
only ideally, acts like a switch
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Physical Operation of Junction Diodes
There is a greater concentration of holes in theP-region than in the N-region, while a greater
concentration of electrons is in the N-region thanin the P-region
This results in diffusion of carriers.
P N+++
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P N+++
A depletion or space-charge region is formed, where
there are no mobile charges, only immobile ions.
These immobile ions produce an electric field which
results in a potential barrier and a drift current.
Physical Operation of Junction Diodes
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The potential barrier prevents the flow of carriers
across the junction.
An external voltage source or bias, V, isconnected to the diode to increase or decrease
the potential barrier.
Physical Operation of Junction Diodes
P-regionN-region
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Physical Operation of Junction Diodes
Forward biasing positive values of V
o Decreases the potential barrier
o A net forward current is seen at the diode
o As V is increased, the current increases
+ V -
P-regionN-region
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Physical Operation of Junction Diodes
Reverse biasing negative values of V
o Increases the potential barrier
o A small reverse or saturation current, IS, is seen atthe diode
o An increase in V does not increase the currentsignificantly, until breakdown occurs.
Peak inverse voltageor breakdown voltage
(PIV) around 75V for general- purpose diodes
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Physical Operation of Junction Diodes
Cut-inor threshold voltage forward bias voltage
needed for current to flow, typically 0.6V for silicon
diodes.
Forward voltage in normal operation, VF, typically
0.4 0.8V
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Physical Operation of Junction Diodes
Special types of diodes:
o Zener diodes typically used in the breakdown
regiono Light-emitting diodes (LED) uses materials other
than germanium and silicon; color depends on energygap of material
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Diode Circuits
Half-wave rectifier
VS Vin Vout RL
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Full-wave bridge
Diode Circuits
VSVin
VoutRL
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Vin Vout
Diode Circuits
Clipper circuit
has other variations (see book by Boylestad)
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Junction Transistor
Also called bipolar transistor
Three-element device formed from two junctions
Can function as a controlled source or switch Two types: pnpand npntransistors
Arrow of symbols indicate direction of positive
charge flow (or actual current)
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Junction Transistor
Three elements/terminals:
emitter source of mobile carriers
collector collects carriers
base controls flow of carriers from emitter tocollector
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PNP transistor
basic structure:
NPN transistor
N P N
E C
B
P N P
E C
Bsymbol:
E
C
B
E
C
B
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Junction Transistor
Convention for polarity of currents: for allterminals, currents entering the transistor arepositive
IB
IC
IEIB
IC
IE
IE = IC + IB
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Transistor Operation
Active region
Mode of operation in which transistor acts like acontrolled source
Emitter-base junction is forward-biased andcollector-base junction is reverse-biased
Cutoff region
Emitter-base junction and collector-base junctionare reverse-biased
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Transistor Operation
Saturation region
Emitter-base junction and collector-base junction areforward-biased
Reverse-active region
Emitter-base junction is reverse-biased and
collector-base junction is forward-biased
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Transistor Operation
Transistor acts as a switch when in saturationand cutoff region
When in cutoff, transistor is OFF
When in saturation, transistor is ON
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PNP Output characteristics
VEB/VBE
VCB/VBC
Forward
active Saturation
Inverted
activeCutoff
PNP/NPN regions ofoperation
Transistor Operation
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Amplifier Types
According to common terminals
common base
common emitter common collector
IE IC
P+ N P
VEB
+
_
VCB
+
_
P
N
P+
IB
IC
VEB
+
_VEC
+
_
P+
N
P
IB
IE
VCB
+
_ VEC
+
_
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Biasing a Transistor
Quiescent point point in
loadline that corresponds toquiescent conditions oftransistor; is usually found in
active region of transistor
Biasing configuring the
circuit to establish thequiescent point
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Some Transistor Circuits
Fixed bias circuit
VCC
RB RC
VBE + IBRB = VCC
IB = VCC VBERB
VBE is fixed for Si: 0.7V
VCE + ICRC = VCC
For active region, VCE > 0.2V
IC = IB
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Junction Field EffectTransistor (JFET)
A voltage-operated , unipolar transistor that
functions using only majority carriers
two types
N-channel JFET
P-channel JFET
Gate
Drain
Source
Gate
Drain
Source
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Basic JFET Structure
Cross section of basic JFET structure
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Biasing
Biasing conditions
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Transistor Operation
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Transistor Operation
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Metal Oxide Semiconductor
Field Effect Transistor (MOSFET)
Insulated gate FET
Uses a metal gate instead of a PN junction
Oxide electrically isolates metal gate fromsemiconductor channel
Two types
N-type MOSFET (NMOS)
P-type MOSFET (PMOS)
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Modes of Operation
Depletion mode conducts when zero bias is
applied to the gate
Enhancement mode conducts when positive
bias is applied to the gate
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MOSFET Symbols
Enhancement Type MOSFETs
Gate
Drain
Source
Substrate G
D
S
SS
G
D
S
G
D
SNMOS PMOS
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MOSFET Symbols
Depletion Type MOSFETs
G
D
S
SSG
D
S
SS
G
D
S
G
D
S
NMOS PMOS
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Basic MOSFET Structure
Cross section of basic MOSFET structure
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Phases of MOSFET Operation (VG > VT)
VD = 0
moderate VD
pinch-off
post pinch-off
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Digital Device Families
Different logic families are grouped according to
the major circuit element
These families are used to implement differentintegrated circuits (ICs)
ICs can be grouped by standard levels of
complexity (or no. of gates)
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Standard Levels of IC Complexity
Complexity No. of Gates Functions
Small-scaleintegration (SSI)
Fewer than 12 Gates, flip-flops
Medium-scaleintegration (MSI)
12 to 99 Registers, counters,decoders, etc.
Large-scaleintegration (LSI)
100 to 9999 RAMs and ROMs