Chapter 1

Semiconductor PN Junction Theory

Contents of this chapter

1. Semiconductor materials

2. Introduction of p-n junction

3. Forward-biased and Reversed-biased p-n junction

4. IV characteristics of forward-biased and reversed-biased p-n junctionbiased p-n junction

Learning Outcome

At the end of this topic, you should be able to:1. Describe the types of semiconductor materials

2. Explain how the p-n junction is formed

3. Explain the operation of forward-biased and reversed bias of p-n junctions bias of p-n junctions

4. Extract the information from i-v characteristic of p-n junction

Revision on chemistry…

• Atomic structure

When protons = electrons, the atom is electrically neutral; otherwise it is an ionand has a net positive or negative charge.

Note: One electron contains -1.6 x10-19 Coulomb of charges

(Proton contains +1.6x10-19 Coulomb.)

The mass of proton = 1.67 x10-27 kg (≈ neutron).

The mass of electron = 9.1x10-31kg

The number of protons → determines the type of atoms

Example: Atomic structure of Carbon and Silicon (Group IV)

Carbon has 6 electrons. → 4 valence electrons

Silicon has 14 electrons. → 4 valence electrons

• The number of valence electrons → determines how the atom combines with one another

– Atoms with one or two outer electrons + atoms with six or seven outer electrons → ionic bonding

– Atoms with four outer electrons combines by sharing – Atoms with four outer electrons combines by sharing them with other atoms → covalent bonding

• The type of bonding → electrical characteristics of the material

Energy Levels

• Electrons are always in motion.

• They move around the nucleus within their specific shells. Each within the path has a specific amount of energy.amount of energy.

• To jump from one path to another, an electron needs to receive specific amount of energy measured in electron volt (eV).

1 eV = 1.6 x 10-19 Joules

Energy Band

• When these atoms combines, the electron of atoms interact with one another → energy level for single atom form band of energy or energy band.

• The amount of energy separating the bands determines the electrical characteristics of the determines the electrical characteristics of the material.

Insulator Semiconductor Conductor

• Three bands of energy levels form

i) Valence Band – most of the electrons are here

ii) Conduction Band – electrons here give the ii) Conduction Band – electrons here give the material electrical conductivity

iii) Forbidden Band – electrons must jump this band to get from the valence to the conduction band

• When materials have energy bands that overlap, electron requires very little energy to move from one level to another. →Conductors

• When materials have large gap on energy bands, electron requires very high energy to move to another band. → Insulators→ Insulators

• When materials have allowable gap on energy bands as such at room temperature, some electrons have enough energy to cross the gap. → Semiconductor

(example: the energy gaps for Si is 1.1eV, Ge is 0.67eV and GaAs is 1.43eV for at room temperature, 25°C. )

Trivial Questions

• Gallium has 31 protons. How many electrons does it have in each shell?

[Hint: 2n2]

• If the energy gap of element A and element B is 1.1 eV and 6 eV respectively, which of these elements will be a better conductor? Why?

INTRODUCTION

Semiconductor

• Most important material in the study of electronics.

• The ability to conduct electricity is intermediate. At room temperature -> not a good conductor. At room temperature -> not a good conductor. As temperature ↑, the conductivity ↑ as the resistivity ↓.

e.g. Si (resistivity = 1.69x10-6 Ω/cm), Ge

(All these tetravalent material – i.e. they have 4 valence electrons -> covalent bond between atoms)

GaAs

Intrinsic Semiconductor

• For pure semiconductor, very few unattached electrons available at room temperature.

• As temperature ↑, kinetic energy ↑ and number of free electrons ↑. Resistivity ↓.

Semiconductor has a negative temperature Semiconductor has a negative temperature coefficient.

Doping process

• Doping is process of adding an impurity (concentration: 1 part/10 million) to the semiconductor as such it will be a better conductor.

→form extrinsic semiconductor

• Impurity material is known as dopant. 2 types:

– Acceptor to form P type material

– Donor to form N type material

N-type material

• When pentavalent atoms (e.g. phosphorus, arsenic and antimony) are used as dopants, there will be many free electrons available for conductivity.

→ this dopant is known as donor

∴This extrinsic semiconductor is known as N-type because it contains many electrons which are negatively charged.

Majority carriers are electrons. Minority carriers are holes.

P-type material

• When trivalent atoms (e.g. boron, aluminum and gallium) are used as dopants, there will be many holes available for conductivity by attracting free electron.

→ this dopant is known as acceptor→ this dopant is known as acceptor

∴This extrinsic semiconductor is known as P-type because it contains many holes which are ‘positively’ charged.

Majority carriers are holes. Minority carriers are electrons.

How the electron flows?

In Si crystal, electrons (and holes) move through these mechanism:– Diffusion

Random motion due to thermal agitation

May happen when one part has more free electrons that the other May happen when one part has more free electrons that the other part

Gives rise to diffusion current

– DriftOccurs when an electric field is applied across a piece of Si

Gives rise to drift current

Drift current and applied electric field -> represents one form of Ohm’s Law

Formation of P-N junction

Free electrons diffuse across the junction to combine with holes that are nearby.

depletion region → reduction of electrons and holes in the region

potential difference (due to +ve and –ve ions created at this junction) -> depends on the material

Vbarrier = 0.7 V (silicon) Vbarrier = 0.3 V (germanium)

This P-N structure is called semiconductor diode.

Formation of P-N junction: When

P-type and N-type are brought closer...

• There is a natural tendency for electrons to move across to the P-type region. This diffusion causes recombination of holes and electrons on each side of the junction.

• Each electron that leaves the N-type region and recombines with a hole on the p-type region creates two recombines with a hole on the p-type region creates two ions:

– a negative ion on the p-type

– a positive ion on the n-type

• Diffusion continues until the electrostatic field (known as barrier potential) created by the junction ions cancels the forces driving the diffusion process.

Reverse-biased P-N junction

The holes (majority carriers) within the p-type material are attracted to the negative terminal of the voltage source.

The electrons (majority carriers) within the n-type material are attracted towards the positive terminal.

This action causes it to act like a high resistance -> barrier potential ≈applied voltage

no majority carriers flow (will flow through it)

there is small leakage current (a.k.a. reverse saturation current, IS) due to minority carriers.

Current-Voltage Plot – (Reverse)

• Wider depletion region -> barrier potential ≈applied voltage

• there is small leakage current, IS which is of approximately constant approximately constant value until voltage reaches breakdown voltage, VBR.

• V<-VBR, existing covalent bond will breakdown -> creating additional free carriers.

Note: Diode Rating (based on peak inverse voltage, PIV – the highest reverse bias voltage that could be applied to the diode, else it may be badly damaged.) -> VBR

Forward-biased P-N junction

The positive terminal of the battery repels holes within the p-type material towards p-n junction. While the negative terminal of the battery repels electrons within the n-type material.

When a hole combines with the electron or vice-versa, an electron-pair bond will breakdown -> free electrons move towards the positive terminal (hence there will be an electron flow from negative-to-positive flow).

Initially, the depletion region will shrink as the voltage increases. Once the voltage value reaches the barrier potential, an appreciable current will flow through the p-n junction.

Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

E.g. Silicon diode

Current-Voltage Plot – (Forward)

• The initial part of voltage -> reducing the depletion region

• Once V= Vk -> driving the majority carriers

• V>Vk, I is increases rapidly (being limited only by the small resistance).

• Relationship between forward current and voltage drop across the p-n structure is nonlinear -> Ohm’s Law is not applicable except for certain condition.

Vk

Vk = 0.7V for Silicon, 0.3 V for Germanium

I-V characteristic of P-N junction

Also known as transconductance curve.

Shockley’s Equation

This equation is used to describe the p-n junction characteristic.

( )1−= TD nVV

SD eIII = Current through diode (i.e. P-N junction) ID = Current through diode (i.e. P-N junction)

IS = Saturation current

n = emission coefficient (determined by diode construction, varies with I), assume it to be 1 unless otherwise stated or to be determined

VD = Voltage drop across diode (i.e. P-N junction)

VT = Thermal equivalent voltage, given by this;

q

kTVT =

k = Boltzmann’s coefficient, 1.38 x 10-23 J/K

q = electron charge (1.6 x 10-19 C)

T = temperature in Kelvin (i.e. ºC +273)

Reference

• Abraham Pallas, Electronic Devices and Circuit Analysis, Delmar Publishers, 1986

• Robert L. Boylestad and Louis Nashelsky, Electronic Devices and Circuit Theory, 9th Electronic Devices and Circuit Theory, 9th Edition, Prentice Hall, 2006

• Denton J. Dailey, Electronic Devices and Circuits: Discrete and Integrated, Prentice Hall, 2001