Chapter 26

Introduction to Semiconductors

Semiconductor Basics

Atoms

Protons

Neutrons

Electrons

Semiconductor Basics

Electron shells: K, L, M, N, etc.

Conductor

1 electron in outer shell (valence shell)

Insulator

8 in valence shell (outer shell full)

Semiconductor

4 in valence shell

Semiconductor Basics

Most common semiconductors

Silicon (Si)

Germanium (Ge)

Semiconductor Basics

Valence electrons have greatest energy

Electrons have discrete energy levels that correspond to orbits

Semiconductor Basics

Valence electrons have two energy levels

Valence Band

Lower energy level

Conduction Band

Higher energy level

Semiconductor Basics

Differences in energy levels provide

Insulators

Semiconductors

Conductors

Semiconductor Basics

Energy gap between Valence and Conduction Bands

Semiconductor Basics

Conductor has many “free” electrons

These are called “conduction” electrons

Energy Gap is between valence and conduction band

Semiconductor Basics

Atomic Physics

Energy expressed in electron volts (eV)

1 eV = 1.602 ´ 10^–¹⁹ joules

Energy gap

Small for conductors

Large for insulators

Semiconductor Basics

Silicon has 4 electrons in its valence shell

8 electrons fill the valence shell

Silicon forms a lattice structure and adjacent atoms “share” valence electrons

Semiconductor Basics

Electrons are shared so each valence shell is filled (8 electrons)

Valence shells full

No “free” electrons at 0 K

Conduction in Semiconductors

At temperatures > °K

Some electrons move into conduction band

Electron-Hole pairs are formed

Hole is vacancy left in lattice by an electron that moves into conduction band

Continuous recombination occurs

Conduction in Semiconductors

Electrons available for conduction

Copper ≈ 10²³

Silicon ≈ 10¹⁰(poor conductor)

Germanium ≈ 10¹² (poor conductor)

Conduction in Semiconductors

Hole: absence of an electron in the lattice structure

Electrons move from – to +

Holes (absence of electrons) move from + to –

Recombination

When an electron fills a hole

Conduction in Semiconductors

Conduction in Semiconductors

As electrons move toward + terminal

Recombine with holes from other electrons

Electron current is mass movement of electrons

Hole current is mass movement of holes created by displaced electrons

Conduction in Semiconductors

Effect of temperature

Higher energy to electrons in valence band

Creates more electrons in conduction band

Increases conductivity and reduces resistance

Semiconductors have a negative temperature coefficient (NTC)

Doping

Adding impurities to semiconductor

Creates more free electron/hole pairs

Greatly increased conductivity

Known as “doping”

Doping

Terminology

Pure semiconductor known as intrinsic

Doped semiconductor known as extrinsic

Doping

Creates n-type or p-type semiconductors

Add a few ppm (parts per million) of doping material

n-type

More free electrons than holes

p-type

More holes than free electrons

Doping

Creating n-type semiconductors

Add (dope with) atoms with 5 valence electrons

Pentavalent atoms

Phosphorous (P)

Arsenic (As)

Antimony (Sb) – Group V on periodic table

Doping

Creating n-type semiconductors

New, donor atoms become part of lattice structure

Extra electron available for conduction

Doping

Intrinsic semiconductors

Equal number of holes and electrons

Conduction equally by holes and electrons

Very poor conductors (insulators)

Doping

n-type extrinsic semiconductor

Free electrons greatly outnumber free holes

Conduction primarily by electrons

Electrons are the “majority” carriers

Doping

Conduction in an n-type semiconductor

Doping

Creating p-type semiconductors

Add (dope with) atoms with 3 valence electrons

Trivalent atoms

Boron (B)

Aluminum (Al)

Gallium (Ga) – Group III on periodic table

Doping

Creating p-type semiconductors

New, acceptor atoms become part of lattice structure

Extra hole available for conduction

Doping

p-type extrinsic semiconductor

Free holes greatly outnumber free electrons

Conduction primarily by holes

Holes are the “majority” carriers

Electrons are the “minority” carriers

The p-n Junction

Abrupt transition from p-type to n-type material

Creation

Must maintain lattice structure

Use molten or diffusion process

The p-n Junction

Example

Heat n-type material to high temperature

Boron gas diffuses into material

Only upper layer becomes p-type

p-n junction created without disturbing lattice structure

The p-n Junction

Joined p-type and n-type semiconductors

+++++++

+++++++

------------

------------

Diffusion across junction creates barrier potential ++-+++-++

-++-++-++-

+--+--+--+

---+----+---

The p-n Junction

Depletion region

Barrier voltage, V_B

Silicon

V_B ≈ 0.7 volts at 25°C

The p-n Junction

Germanium

V_B ≈ 0.3 volts at 25°C

V_B must be overcome for conduction

External source must be used

The Biased p-n Junction

Basis of semiconductor devices

Diode

Unidirectional current

Forward bias (overcome V_B) – conducts easily

Reverse bias – virtually no current

p-type end is anode (A)

The Biased p-n Junction

Diode

n-type end is cathode (K)

Anode and cathode are from vacuum tube terminology

The Biased p-n Junction

Diode symbol

Arrow indicates direction of conventional current for condition of forward bias (A +, K -)

External voltage source required

External resistance required to limit current

The Biased p-n Junction

Holes are majority carriers in p-type

Electrons are majority carriers in n-type

The Biased p-n Junction

Reverse biased junction

Positive (+) terminal draws n-type majority carriers away from junction

Negative (–) terminal draws p-type majority carriers away from junction

No majority carriers attracted toward junction

Depletion region widens

The Biased p-n Junction

Electrons are minority carriers in p-type

Holes are minority carriers in n-type

Reverse biased junction

Minority carriers drawn across junction

Very few minority carriers

The Biased p-n Junction

Reverse biased current

Saturation current, I_S

Nanoamp-to-microamp range for signal diodes

The Biased p-n Junction

Reverse biased junction

Positive terminal of source connected to cathode (n-type material)

The Biased p-n Junction

The Biased p-n Junction

p-type

Holes are majority carriers

n-type

Electrons are majority carriers

The Biased p-n Junction

Forward biased junction

+ terminal draws n-type majority carriers toward junction

– terminal draws p-type majority carriers toward junction

Minority carriers attracted away from junction

Depletion region narrows

The Biased p-n Junction

Forward biased junction

Majority carriers drawn across junction

Current in n-type material is electron current

Current in p-type material is hole current

Current is referred to as I_majorityor I_F (for forward current)

The Biased p-n Junction

Voltage across Forward biased diode ≈ V_B

Often referred to as V_F (for forward voltage)

V_B ≈ 0.7 for Silicon and 0.3 for Germanium

Forward biased current

Majority and Minority current

Minority current negligible

The Biased p-n Junction

Forward biased junction

Positive terminal of source connected to Anode (p-type material)

The Biased p-n Junction

Forward biased junction

Conducts when E exceeds V_B

For E < V_B very little current flows

Total current = majority + minority current

Diode current, I_F ≈ majority current

V_F ≈ 0.7 volts for a silicon diode

Other Considerations

Junction Breakdown

Caused by large reverse voltage

Result is high reverse current

Possible damage to diode

Two mechanisms

Avalanche Breakdown

Zener Breakdown

Other Considerations

Avalanche Breakdown

Minority carriers reach high velocity

Knock electrons free

Create additional electron-hole pairs

Created pairs accelerated

Creates more electrons

“Avalanche” effect can damage diode

Other Considerations

Peak Inverse Voltage (PIV) or Peak Reverse Voltage (PRV) rating of diode

Other Considerations

Zener Breakdown

Heavily doped n-type and p-type materials in diode

Narrows depletion region

Increases electric field at junction

Electrons torn from orbit

Occurs at the Zener Voltage, V_Z

Other Considerations

Zener Diodes

Designed to use this effect

An important type of diode

Other Considerations

Diode junction

+++++++

+++++++

------------

------------

Other Considerations

Like a capacitor

Thickness of depletion region changes with applied voltage

Capacitance dependent on distance between plates


Electron shells: K, L, M, N, etc.
	Conductor
		1 electron in outer shell (valence shell)
	Insulator
		8 in valence shell (outer shell full)
	Semiconductor
		4 in valence shell


	Valence electrons have greatest energy
	Electrons have discrete energy levels that correspond to orbits


Valence electrons have two energy levels
	Valence Band
		Lower energy level
	Conduction Band
		Higher energy level


	Differences in energy levels provide
		Insulators
		Semiconductors
		Conductors


	Conductor has many “free” electrons
	These are called “conduction” electrons
	Energy Gap is between valence and conduction band


	Atomic Physics
		Energy expressed in electron volts (eV)
		1 eV = 1.602 ´ 10^–¹⁹ joules
	Energy gap
		Small for conductors
		Large for insulators


	Silicon has 4 electrons in its valence shell
	8 electrons fill the valence shell
	Silicon forms a lattice structure and adjacent atoms “share” valence electrons


	Electrons are shared so each valence shell is filled (8 electrons)
	Valence shells full
		No “free” electrons at 0 K


	At temperatures > °K
		Some electrons move into conduction band
	Electron-Hole pairs are formed
		Hole is vacancy left in lattice by an electron that moves into conduction band
		Continuous recombination occurs


	Electrons available for conduction
		Copper ≈ 10²³
		Silicon ≈ 10¹⁰(poor conductor)
		Germanium ≈ 10¹² (poor conductor)


Hole: absence of an electron in the lattice structure
	Electrons move from – to +
	Holes (absence of electrons) move from + to –
	Recombination
		When an electron fills a hole


	As electrons move toward + terminal
		Recombine with holes from other electrons
		Electron current is mass movement of electrons
		Hole current is mass movement of holes created by displaced electrons


	Effect of temperature
		Higher energy to electrons in valence band
		Creates more electrons in conduction band
		Increases conductivity and reduces resistance
		Semiconductors have a negative temperature coefficient (NTC)


	Adding impurities to semiconductor
		Creates more free electron/hole pairs
		Greatly increased conductivity
		Known as “doping”


	Terminology
		Pure semiconductor known as intrinsic
		Doped semiconductor known as extrinsic


Creates n-type or p-type semiconductors
	Add a few ppm (parts per million) of doping material
	n-type
		More free electrons than holes
	p-type
		More holes than free electrons


Creating n-type semiconductors
	Add (dope with) atoms with 5 valence electrons
	Pentavalent atoms
		Phosphorous (P)
		Arsenic (As)
		Antimony (Sb) – Group V on periodic table


	Creating n-type semiconductors
		New, donor atoms become part of lattice structure
		Extra electron available for conduction


	Intrinsic semiconductors
		Equal number of holes and electrons
		Conduction equally by holes and electrons
		Very poor conductors (insulators)


	n-type extrinsic semiconductor
		Free electrons greatly outnumber free holes
		Conduction primarily by electrons
		Electrons are the “majority” carriers


Creating p-type semiconductors
	Add (dope with) atoms with 3 valence electrons
	Trivalent atoms
		Boron (B)
		Aluminum (Al)
		Gallium (Ga) – Group III on periodic table


	Creating p-type semiconductors
		New, acceptor atoms become part of lattice structure
		Extra hole available for conduction


	p-type extrinsic semiconductor
		Free holes greatly outnumber free electrons
		Conduction primarily by holes
		Holes are the “majority” carriers
		Electrons are the “minority” carriers


	Abrupt transition from p-type to n-type material
	Creation
		Must maintain lattice structure
		Use molten or diffusion process


	Example
		Heat n-type material to high temperature
		Boron gas diffuses into material
		Only upper layer becomes p-type
		p-n junction created without disturbing lattice structure


	Joined p-type and n-type semiconductors

	+++++++
	+++++++
	------------
	------------
	Diffusion across junction creates barrier potential ++-+++-++
	-++-++-++-

	+--+--+--+
	---+----+---


	Depletion region
	Barrier voltage, V_B
	Silicon
		V_B ≈ 0.7 volts at 25°C


	Germanium
		V_B ≈ 0.3 volts at 25°C
	V_B must be overcome for conduction
	External source must be used


	Basis of semiconductor devices
	Diode
		Unidirectional current
		Forward bias (overcome V_B) – conducts easily
		Reverse bias – virtually no current
		p-type end is anode (A)


	Diode
		n-type end is cathode (K)
		Anode and cathode are from vacuum tube terminology