The Devices: Diode Once Again

The Devices: Diode Once Again

Si Atomic Structure

First Energy Level: 2 Second Energy Level: 8 Third Energy Level: 4

Electron Configuration:

Doping Process

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

NN

NN PP

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

PP

Covalent Bonding;

Undoped MaterialUndoped Material Shares its 4 electrons

w/other atoms and forms a pure crystal.

Pentavalent Doping;

Donor MaterialDonor Material Impurities that have an excess of electrons. NN type Material, called

ElectronsElectrons. - charged

Trivalent Doping;

Acceptor MaterialAcceptor Material Impurities that have

missing electron, called HolesHoles or PP type

Material. + charged.

Doping: The process of adding impurities to the intrinsic material giving the material a PositivePositive or NegativeNegative characteristic.

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

NNNN

NN

NN

NN

NN

Donor Material w/an excess electron in the covalent bond w/Silicon

displays a Negative charge.Majority CarriersMajority Carriers are ElectronsElectrons..

n-type materialn-type material

I

V

PP

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISI

SISIPP

PP

PP

PP

PP PP PP

Acceptor Material has a missing electron in the covalent bond w/Silicon, displays a Positive charge.Majority CarriersMajority Carriers are HolesHoles.

p-type materialp-type material

I

V

I

Remember…

Majority Current Carriers, HolesHoles or ElectronsElectrons.

N Type MaterialN Type Material: Donor Material with an excess electron in the covalent bond in Silicon & displays a NegativeNegative charge.

Majority CarriersMajority Carriers are Electrons.Electrons.NN

P Type MaterialP Type Material: Acceptor Material has a missing electron in the covalent bond in Silicon, & displays a PositivePositive charge.

Majority CarriersMajority Carriers are HolesHoles.PP

2 Current Carriers2 Current Carriers: MajorityMajority & MinorityMinority Intrinsic impurities

inherent in silicon result in current flow in the opposite direction to Majority flow. Becomes evident in heat, leakage and break down of the device.

Minority Current carriersMinority Current carriers

The pn Junction in Si Material

At the junction, electrons fill holes so that there are no free holes or electrons there. The actual junction becomes an insulating layer. This barrier must be overcome before current can flow through the pn junction.

The pn junction is made from a single crystal with the impurities diffused into it. The n end has a surplus of negative electrons. The p end has a surplus of holes.

Depletion region


When a battery is connected as shown, the negative terminal pushes negative electrons towards the junction. The positive terminal pushes holes towards the junction. A high enough voltage will overcome the barrier and a current will flow through the pn junction.

There is a voltage across the diode of 0.7V for the silicon. The junction is said to be FORWARD BIASED. The p-type is the anode of the diode, the n-type the cathode, as shown by the diode symbol. The resistor limits the current to a safe level.

anodecathode

When the battery is connected as shown, the positive terminal of the battery attracts negative electrons away from the barrier. The negative terminal attracts holes away from the barrier. The insulating barrier widens and no current flows.

The junction is REVERSED BIASED. If the reverse voltage is made high enough, then the junction will break down and electrons will flow from anode to cathode (under normal conditions, electrons flow from cathode to anode, when forward biased).


anodecathode

+

+

+

+

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

-

-

-

---++

II

-- ++++

++

++

+

-

-

-

-

-

-

-

-

-

+

+

+

+

+

+

+

+

+

--

--

-Depletion Depletion RegionRegion

--

++

++

++

++

++

++

-

-

-

-

-

-

+

+

+

+

+

+

--

--

--

--

--

--++++++

Depletion Depletion RegionRegion

--

++

++

++

++

++

++

++

++

++

++

++

++

--

--

--

--

--

--

--

--

--

--

--

--II

++++++++

Depletion Depletion RegionRegion

REVERSED BIASED

FORWARDBIASED

Depletion Region

hole diffusionelectron diffusion

p n

hole driftelectron drift

ChargeDensity

Distancex+

-

ElectricalxField

x

PotentialV

W2-W1

(a) Current flow.

(b) Charge density.

(c) Electric field.

(d) Electrostaticpotential.

•Zero bias conditions

•p more heavily doped than n (NA > NB)

•Electric field gives rise to potential difference in the junction, known as the built-in potential

Built-in Potential

Where T is the thermal voltage

0 2

T

A D

i

N N

nln

)300(26 KatmVq

kTT

ni is the intrinsic carrier concentration for

pure Si (1.5 X 1010 cm-3 at 300K), so for

mVmV 63810*5.1

1010ln26 210

1615

0

,1

10,1

103

163

15

cmN

cmN BA

Models for Manual Analysis

VD

ID = IS(eVD/T – 1)+

–

VD

+

–

+

–VDon

ID

(a) Ideal diode model (b) First-order diode model

•Accurate

•Strongly non-linear

•Prevents fast DC bias calculations

•Conducting diode replaced by voltage source VDon=0.7V

•Good for first order approximation

Typical Diode Parameters

VD ID = IS(eVD/T – 1)

+

–

•Dn=25 cm2/sec

•Dp=10cm2/sec

•Wn=5 m

•Wp=0.7 m

•W2=0.15 m

•W1=0.03 m

Geometry, doping and material constants lumped in Is

217 /10

)(1

0

2

0

mAI

valuetypical

qAI

S

WW

nD

WW

pD

DS p

pn

n

np

Diffusion coefficientminority carrier concentration

•q=1.6*10-19 coulombs

•pn0=0.3*105/cm3

•np0=0.6*104/cm3

Secondary Effects: Breakdown

–25.0 –15.0 –5.0 5.0

VD (V)

–0.1

I D (A

)

0.1

0

0

Cannot bear too large reverse biases» Drift field in depletion region will get extremely large» Minority carriers caught in this large field will get very energetic

– Energetic carriers can knock atoms and create a new n-p pair– These carriers will get energetic, too, and so on: thus large currents!

Two types» Avalanche

breakdown– Above mechanism

» Zener breakdown– More complicated

Can damage diode

Diode SPICE Model

ID

RS

CD

+

-

VD

Required for circuit simulations» Must capture important characteristics but also remain efficient » Extra parameter in the model: n (emission coefficient, 1 n 2)

– Fixes non-ideal behavior due to broken assumptions

Additional series resistance accounts for body+contact Nonlinear capacitance includes both CD and Cj

ID IS (eVD /nT 1)

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The Devices: Diode Once Again