CP 208 Digital Electronics Class Lecture 4 February 18, 2009

CP 208 Digital Electronics

Class Lecture 4

February 18, 2009

In This ClassIn This Class

We Will Discuss Following Topics:

Chap 3 Diodes 3.1 The Ideal Diode

3.7 Physical Operation of Diodes3.9 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 Non-Linear 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 Diode3.1 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 Straight-Line 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

3.1.23.1.2 Diode Logic GatesDiode 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 3.2Example 3.2

3.7 Physical Operation of Diode3.7 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

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.

Intrinsic SiliconIntrinsic Silicon

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.


• 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


• At an absolute temp T (in Kelvin) the

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 !!

kTEi

GeBTn /3


Diffusion and DriftDiffusion 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 x-direction: Magnitude is Proportional to Slope of Concentration Gradient

dxdp

pp qDJ

• Similarly Electron Diffusion Current

• 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)

dxdn

nn qDJ

Ev pdrift Ev ndrift



• Hole Drift Current through a Si with hole density p as a result of E Field applied is:

• Similarly, Electron Drift Current is:

• Total Drift Current will be sum of both:

• Note that this is a form of Ohm’s law, resistively in ohm-cm is given by:

EqpJ pdriftp

EqnJ ndriftn

EnpqJ npdrift )(

)](/[1 np npq


• Finally, a simple relationship between diffusivity and mobility exist, know as Einstein Relationship:

• VT is known as Thermal Voltage = 25 mV at RT

TVq

kTD

Tp

p

n

n VDD

Doped SemiconductorsDoped 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

• 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 SemiconductorsDoped Semiconductors• If the Donor Atom (P) concentration is ND in

thermal equilibrium the free electron concentration in n-type Si is nno will be:

• In Thermal equilibrium the product of electron and hole concentration remains constant, that is,

nn0pn0 = ni2

• Concentration of holes would be,

• Minority holes are function of Temp while Majority Electrons are independent of Temp.

Dno Nn

D

in N

np

2

0

Similarly When Silicon Crystal Doped with a Trivalent Impurity Boron, Each dopant atom gives rise to a hole, and the Semiconductor becomes p type.


• 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 SemiconductorsDoped Semiconductors• If the Donor Atom (B) concentration is NA in

thermal equilibrium the free Hole concentration in p-type Si is pp0 will be:

• In Thermal equilibrium the product of electron and hole concentration remains constant, that is,

np0pp0 = ni2

• Concentration of Electrons would be,

• Minority Electrons are function of Temp while Majority Holes are independent of Temp.

Ap Np 0

A

ip N

nn

2

0

The pn junction with no applied voltage (open-circuited terminals)

+ denote majority holes in p-type

- denote majority electron in n-type

Minority carriers in both sides are not shown

3.7.2 pn Junction Under Open Circuit Conditions3.7.2 pn Junction Under Open Circuit Conditions

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 RegionCharge on both sides cause an Electric Field to be established and hence a potential difference results across depletion region as shownThus 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 EquilibriumA 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:

• 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.

20 lni

DAT n

NNVV

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

• Or

D

A

p

n

N

N

x

x 0

112V

NNqxxW

DA

spndep

3.7.3 Reverse-Bias Conditions3.7.3 Reverse-Bias Conditions

Excite pn junction with constantcurrent source I in reverse direction

I < IS. I in ckt will be carried by Electrons Flowing from n to pFree Electrons leave n and Holes leave p regionResults 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 CapacitanceJust like Capacitor when V changes the Charge changesCharge on either side of junctionIs equal, so using n side:

qJ = qn = qNDxnA

In terms of depletion-layer width, Wdep

And Wdep is given by

Combing both eqs gives for non-linear qJ-VR and is plotted above

depDA

DAJ AW

NN

NNqq

)(112

0 RDA

spndep VV

NNqxxW

Depletion Layer Capacitance

This is not a linear capacitor

Using Small Signal Approx.

Depletion-capacitance is

Slope at bias point Q:

Alternatively, we can treat depletion layer as parallel plate capacitor, Cj = εA / Wdep

QR VVR

JJ dV

dqC

00

1

2 VNN

NNqAC

DA

DAsj

0

1VV

CC

R

joJ

The pn Junction in Reverse Breakdown RegionThe 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 Condition3.7.5 Forward Bias ConditionExcite pn junction with constantcurrent source I in forwarddirectionSupply 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 VThis 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 RelationshipCurrent-Voltage Relationship

Consider the current component caused by holes injected in n region.

Concentration of minority carriers at the edge of Depletion Region is:

Hole distribution in n region is:

Where

The hole diffusion current in n region at xn is:

TVVnnn epxp /

0)(

pn Lxxnnnnn epxppxp /)(

00 ])([)(

ppp DL

)1( /0 TVV

np

pdxdp

pp epL

DqqDJ

Current-Voltage RelationshipCurrent-Voltage Relationship

Similarly the Electron diffusion current in p region at - xp is:

Since both Jp and Jn are in same direction they can be added and multiplied by A to get total current;

)1( /2

TVV

An

n

Dp

pi e

NL

D

NL

DAqnI

)1( /0 TVV

pn

nn en

L

DqJ

)1( /00

TVV

pn

nn

p

p enL

Dqp

L

DqAI

)1( / TVVS eII

An

n

Dp

piS NL

D

NL

DAqnI 2

Diffusion CapacitanceDiffusion 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:

lexponentia )(under area shaded x xpAqQ np ])([ x 00 nnn pxpAq

pppp

pp II

D

LQ

2

Diffusion CapacitanceDiffusion Capacitance

Similarly for electron charge stored in p region

Total charge

Where τT is Mean Transit Time of diode

For small changes around bias point small signal diffusion capacitance Cd is

nnnn

nn II

D

LQ

2

IIIQ Tnnpp

dV

dQCd I

VC

T

Td

• 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

IV

CT

Td

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.

50

Bipolar JunctionTransistors (BJTs)

Bipolar Junction Transistors (BJTs)Bipolar Junction Transistors (BJTs)

Mode EBJ CBJ

Cutoff Reverse Reverse

Active Forward Reverse

Saturation Forward Forward

BJT as Amplifier and SwitchBJT as Amplifier and Switch

The Basic BJT Digital Logic InverterThe 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 CktsSaturated 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

Documents

CP 208 Digital Electronics Class Lecture 4 February 18, 2009