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CP 208 Digital Electronics
Class Lecture 4
February 18, 2009
In This Class
We Will Discuss Following Topics:
Chap 3 Diodes
3.1 The Ideal Diode
3.7 Physical Operation of Diodes
3.9 The SPICE Diode Model
Chap 5 Bipolar Junction Diodes (BJTs)
5.10 The Basic BJT Digital Logic Inverter
Diodes
3
Diode
• The Simplest and Most Fundamental NonLinear Ckt Element
• Like Resistor, Diode has Two Terminals
• Unlike Resistor Diode has Non-Linear I-V
Characteristics
• To Understand Diode Function We Start
with a FICTITUOS Element – Ideal Diode
• Then Physical Operation which will be
Foundation for Understanding FETs BJTs
3.1 The Ideal Diode
Two Terminal Device
with The Symbol
Positive Terminal
Anode and Negative
Cathode
And I-V
Characteristics as:
When Negative
Voltage is Applied
(Reverse Biased),
No Current Flows
and Diode
behaves as Open
Circuit and Said to
be Cut Off or OFF
When Positive
Voltage is Applied
(Forward Biased),
Zero Votage Drop
Appears and Diode
behaves as Short
Circuit Circuit and
Said to be Turned
On or ON
External Circuit Must be Designed to Limit
Forward Current and Reverse Voltage to
Predetermined Safe Value
The I-V of Ideal Diode is
Highly non-linear, but it
contains two StraightLine Segments 90° to
one Another –
Piecewise Linear.
Application in Which
Signal on Device
Terminal Swing only
Along non-linear portion
then Device is Linear for
that Application
3.1.2 Diode Logic Gates
Diodes together with Resistors can be used to
implement Digital Logic
OR Gate, Y=A+B+C
AND Gate, Y=A.B.C
Example 3.2
3.7 Physical Operation of Diode
• A semiconductor diode is basically a pn junction
– p-type semiconductor material brought into
close contact with n-type material (practically n
and p regions in Si)
• Terminals -- wire connections are made through
metal (Al) contacts to n and p regions
• Besides being Diode, pn junction is the basic
element of BJTs and plays an important role in
the operation of FETs
Intrinsic Silicon
Two-dimensional Silicon Crystal, Circles represent the
silicon atoms, with +4 positive charge neutralized by the
charge of the four valence electrons. Covalent bonds are
formed by sharing of the valence electrons. At 0 K, all bonds
are intact and no free electrons are available for current
conduction.
At room temperature some
bonds are broken by thermal
ionization and some
electrons are free.
Bond breaks, Electron
leaves parent atom leaving
behind +ve charge (hole).
Electron from neighboring
atom moves to fill hole
creating another hole.
Essentially, by repetition of
the process, positive charge
(hole) is moving thru crystal
and available for conduction.
Intrinsic Silicon
Intrinsic Silicon
• Equal number of Electrons and Holes Result by
Thermal Ionization (Generation of Electron/Hole
Pair EHP)
• While moving randomly these pairs recombine and
disappear as well (Recombination)
• Recombination Rate is Proportional to number of
free Electrons and Holes, which in turn is
determined by Ionization Rate
• Generation is Strong Function of Temperature
• In Thermal Equilibrium Recombination is equal to
Generation and Concentration of e/h can be
calculated: n = p = ni and ni is free electrons or
holes concentration in Intrinsic Silicon at a given T
Intrinsic Silicon
• At an absolute temp T (in Kelvin) the
3  EG / kT
ni  BT e
Where B = 5.4x1031 (for Si), k is Boltzmann’s
constant and EG Band gap energy (1.12 eV for
Si) is minimum energy required to break a bond
for EHP generation
• At 300 K (RT) the Eq. gives ni = 1.5x1010
carriers/cm3 for Intrinsic Si
• Note that Si has about 5x1022 atoms/cm3, So at
room temp only One of every Billion atom is
ionized !!
Diffusion and Drift
• Diffusion and Drift – Two
mechanisms by which Holes and
Electrons move thru Si
• Diffusion of carriers takes place
if Concentration Gradient is
present and Gives rise to Net
Flow of Charge (Current)
• Hole Diffusion Current in xdirection: Magnitude is
Proportional to Slope of
Concentration Gradient
J p  qD
dp
p dx
Diffusion and Drift
• Similarly Electron Diffusion Current
J n  qDn
dn
dx
• In Semiconductors, Carrier also move through
another mechanism – Drift
• Carrier Drift Occurs when Electric Field is Applied
Across a Piece of Si
• Carriers are Accelerated by the E Field and
Acquire Drift Velocity (Holes drift in E direction
and Electrons opposite)
vdrift   p E
vdrift   n E
Diffusion and Drift
• Hole Drift Current through a Si with hole density p
as a result of E Field applied is:
J p  drift  qp p E
• Similarly, Electron Drift Current is:
Jndrift  qnnE
• Total Drift Current will be sum of both:
J drift  q ( p p  n n ) E
• Note that this is a form of Ohm’s law, resistively
in ohm-cm is given by:
  1 /[ q( p p  n n )]
Diffusion and Drift
• Finally, a simple relationship between diffusivity
and mobility exist, know as Einstein Relationship:
D
kT

 VT

q
Dn
n

Dp
p
 VT
• VT is known as Thermal Voltage = 25 mV at RT
Doped Semiconductors
• In Intrinsic Si the Holes and Electrons
have equal Concentration and strongly
dependent on temperature
• While in Doped Semiconductors Carriers
of one kind (either hole or electron) are
predominant
• Doped silicon in which majority of charge
carriers are negatively charged electrons
is called n-type
• With majority of +ve holes is p-type
Doped Semiconductors
• Doping of Si to make it n or p type is
achieved by introducing small number of
impurity atoms
• Introducing impurity atoms of pentavalent
element such as P results in n-type Si
• Each P atom replacing Si atom is donating
a free electron to Si Crystal
• Note that for free electron no hole is
generated, hence majority of charge
carriers in P doped silicon are electrons
Silicon Crystal Doped By a Pentavalent Element. Each Dopant
Atom Donates a Free Electron and is Thus Called a Donor.
The Doped Semiconductor Becomes n Type.
Doped Semiconductors
• If the Donor Atom (P) concentration is ND in
thermal equilibrium the free electron
concentration in n-type Si is nno will be:
nno  N D
• In Thermal equilibrium the product of electron
and hole concentration remains constant, that is,
nn0pn0 = ni2
• Concentration of holes would be,
2
i
n
pn 0 
ND
• Minority holes are function of Temp while Majority
Electrons are independent of Temp.
Doped Semiconductors
Similarly When Silicon Crystal Doped with a Trivalent
Impurity Boron, Each dopant atom gives rise to a hole, and the
Semiconductor becomes p type.
Doped Semiconductors
• Introducing impurity atoms of Trivalent
element such as B results in p-type Si
• Each B atom replacing Si atom is
accepting a free electron from Si Crystal to
form a covalent bond, thus each B atom
gives rise to Hole
• Note that for Hole no electron is
generated, hence majority of charge
carriers in B doped silicon are Holes
Doped Semiconductors
• If the Donor Atom (B) concentration is NA in
thermal equilibrium the free Hole concentration in
p-type Si is pp0 will be:
p p0  N A
• In Thermal equilibrium the product of electron
and hole concentration remains constant, that is,
np0pp0 = ni2
• Concentration of Electrons would be,
np0
2
i
n

NA
• Minority Electrons are function of Temp while
Majority Holes are independent of Temp.
3.7.2 pn Junction Under Open Circuit Conditions
The pn junction with no
applied voltage (opencircuited terminals)
+ denote majority holes in
p-type
- denote majority electron
in n-type
Minority carriers in both
sides are not shown
Diffusion Current ID:
Because Hole concentration is high in p
region and low in n region, holes diffuse to n
region, likewise, electrons diffuse to p
region, giving rise to Diffusion Current ID
from p to n side
Depletion Region:
The Electrons that diffuse from n region to p region
leave behind + charged donor atoms in n region
near junction.
In p region electron recombine with holes and
create – charge on Acceptor Atoms in p region
near junction.
Thus the area near junction becomes depleted of
free electrons and holes on both sides and a net
bound charges are established.
The Carrier Depletion Region or
Depletion Region is also called
Space Charge Region
Charge on both sides cause an
Electric Field to be established
and hence a potential difference
results across depletion region
as shown
Thus the Electric Field opposes
any further diffusion of carriers,
in fact, voltage drop V0 across
depletion region acts as barrier
for carriers. Larger the barrier
smaller number of carrier will be
able to diffuse. ID strongly
depends on V0.
Drift Current Is and Equilibrium
A current component due to
minority-carrier drift also exist
across the junction.
Some thermally generated holes
in n region diffuse to the edge of
depletion region where they
experience E Field and are swept
across junction to p region. Same
for electrons in p region.
These two add together to form
drift current Is from n to p side
Is is independent of V0 and
strongly dependent on Temp
• Under open-circuit no external current
exists, thus, two opposite currents across
junction should be equal, ID = IS.
• This Equilibrium condition is maintained by
V0
• If ID exceeds IS it will result creating more
bound charges on both sides, widening
the depletion region, and increase V0. This
in turn will cause ID to decrease until
equilibrium is reached
The Junction Built-In Voltage
• The V0 across pn junction is given by:
 N AND 

V0  VT ln 
2
 ni 
• Typically for Si at RT, V0 is in the range of 0.6 V to
0.8 V
• The voltage measured between pn junction
terminals is zero, this is because the contact
voltages existing at the metal-semiconductor
junctions at diode terminals exactly balance the
barrier voltage.
Width of Depletion Region
• Usually the doping levels on both sides are not
equal hence the depletion region is not same on
both sides
• In order to cover same amount of charge the
deletion region will extend deeper in lightly doped
material
• If xp is width of depletion region in p side and xn is
in n side for equal charges on both sides,
qxpANA = qxnAND
2 s  1
1 
• Or x
N
n
xp

A
ND
Wdep  xn  x p 

V0

q  N A ND 
3.7.3 Reverse-Bias Conditions
Excite pn junction with constant
current source I in reverse direction
I < IS. I in ckt will be carried by
Electrons Flowing from n to p
Free Electrons leave n and Holes leave p region
Results in more – and + charges to build at junction
– increase width of depletion region and voltage
across it, causing ID to reduce, IS constant
In Steady state (equilibrium) IS – ID = I
The increase in Voltage above V0 will appear at
diode terminals as Reverse Voltage VR
Depletion Layer Capacitance
Just like Capacitor when V
changes the Charge changes
Charge on either side of junction
Is equal, so using n side:
qJ = qn = qNDxnA
In terms of depletion-layer width, Wdep
NAND
qJ  q
AWdep
N A  ND
And Wdep is given by
2 s  1
Wdep  xn  x p 
1 

(V0  VR )

q  N A ND 
Combing both eqs gives for non-linear qJ-VR and is
plotted above
Depletion Layer Capacitance
This is not a linear capacitor
Using Small Signal Approx.
Depletion-capacitance is
Slope at bias point Q:
dqJ
CJ 
dVR
VR VQ
Alternatively, we can treat depletion layer as parallel
plate capacitor, Cj = εA / Wdep
CJ 
C jo
VR
1
V0
  s q  N A N D  1 
 
C j0  A 

 2  N A  N D  V0 
The pn Junction in Reverse Breakdown Region
The pn junction excited by a reverse-current source I >
IS. The junction breaks down, and a voltage VZ , with the
polarity indicated, develops across the junction.
3.7.5 Forward Bias Condition
Excite pn junction with constant
current source I in forward
direction
Supply Free Electrons to n and
Holes to p region. Results:
Neutralize – and + charges at junction – decrease
width of depletion region and voltage across it, causing
ID to increase, IS constant
In Steady state (equilibrium) ID – IS = I
The Voltage V0 decrease by Forward Voltage V
This cause Holes injection in n and Electrons Injection
in p regions
Minority-carrier distribution in a forward-biased pn junction
(NA > ND). In Steady state excess minority carrier profile
remains constant. This distribution gives rise to increase of
Diffusion current ID above IS as these carriers diffuse and
disappear by recombination and equal number is replenished
by external circuit
Current-Voltage Relationship
Consider the current component caused by holes
injected in n region.
Concentration of minority carriers at the edge of
V / VT
Depletion Region is:
p n ( xn )  p n 0 e
Hole distribution in n region is:
pn ( x)  pn0  [ pn ( xn )  pn0 ]e
Where
 ( x  xn ) / L p
L p  D p p
The hole diffusion current in n region at xn is:
J p   qD
dp
p dx
q
Dp
Lp
pn 0 (eV / VT  1)
Current-Voltage Relationship
Similarly the Electron diffusion current in p region
at - xp is:
D
Jn  q
n
Ln
V / VT
n p 0 (e
 1)
Since both Jp and Jn are in same direction they can
be added and multiplied by A to get total current;
 Dp
 V /V
D
n
I  A q
pn 0  q
n p 0 (e T  1)
 L

L
p
n


 Dp
 V /V
D
n
(e T  1)
I  Aqn 

L N

L
N
p
D
n
A


2
i
I  I S (e
V / VT
 1)
 Dp

D
n

I S  Aqn 

L N

L
N
p
D
n
A


2
i
Diffusion Capacitance
• FB junction in steady state has certain amount of
excess-carrier charge stored in each p and n bulk
region
• If terminal voltage changes this charge has to
change before new steady state is achieved
• This gives rise to another capacitive effect
• The charge stored due to excess minority-carrier
holes in n region:
Q p  Aq x shaded area under pn ( x) exponentia l
 Aq x [ pn0 ( xn )  pn 0 ]
Qp 
2
p
L
Dp
I p  pI p
Diffusion Capacitance
Similarly for electron charge stored in p region
2
n
L
Qn 
In   nIn
Dn
Total charge
Q   p I p  nIn  T I
Where τT is Mean Transit Time of diode
For small changes around bias point small signal
diffusion capacitance Cd is
dQ
Cd 
dV
 T
Cd  
 VT

 I

 T
Cd  
 VT

 I

• The Cd is directly proportional to I and is
negligibly small when diode is reverse biased
• To keep Cd Small τT must be small an
important requirement for diodes intended for
high speed and high frequency operation
The SPICE Diode Model
The Static Behavior is modeled by exponential i-v
Dynamic Behavior by the non-linear CD which is sum of
the Diffusion cap Cd and junction cap Cj
Series RS represent total R of n and p regions. It is
ideally zero but is few ohms for small-signal diodes
The SPICE Diode Model
For small Signal SPICE Uses incremental resistance rd
and incremental values of Cd and Cj
Table 3.3 provide partial parameters list used by SPICE
For discrete diodes parameter values can be determined
from diode data sheets. PSPICE includes param. in lib.
Bipolar Junction
Transistors (BJTs)
50
Bipolar Junction Transistors (BJTs)
Mode
EBJ
CBJ
Cutoff
Reverse
Reverse
Active
Forward
Reverse
Saturation
Forward
Forward
BJT as Amplifier and Switch
The Basic BJT Digital Logic Inverter
We learned in Chap 1 that
Logic Inverter is Most
Fundamental Component of
Digital System
We will use this BJT Ckt to
realize Logic inverter
Makes use of Cutoff and
Saturation modes of BJT to
work as Logic Inverter
In these modes the power
dissipation is low
For
RB=10 kΩ, RC=1k
Ω, β = 50, and
VCC=5V
In the ckt the VTC
is Shown
Saturated vs Nonsaturated BJT Digital Ckts
• Inverter we just discussed belongs to
saturated variety of BJT Digital ckts – TTL
• Some TTL versions are in use, Generally
saturated bipolar digital ckts are no more
technology of choice in Digital System
Design
• The reason being the speed of operation
• Long time delay to turn off a saturated BJT
• Minority carrier distribution
in base region of saturated
BJT: Blue triangle gradient
gives the diffusion current
across base
• Grey rectangle causes
transistor to be driven
deeper into saturation
• To achieve high speeds the
BJT should not saturate –
Current Mode Logic or ECL
(Chap 11)
Home Work No. 3
1. Problem 3. 6
2. Problem 3.10
3. Problem 5.168
4. Problem 5.171
In Next Class
We Will Discuss:
Chap 4 MOS Field-Effect Transistors
4.1 Device Structure and Physical
Operation
4.2 Current-Voltage Characteristics