ISAT 436ISAT 436Micro-/Nanofabrication Micro-/Nanofabrication

and Applicationsand Applications

P-N Junction DiodesP-N Junction Diodes

David J. Lawrence

Spring 2004

N-Type SiliconN-Type Silicon Recall that phosphorus, arsenic, and antimony are

donor dopants in silicon, making it n-type. A donor atom is readily ionized, yielding a free ()

electron and leaving behind a positive ion core.

Si Si Si Si Si Si Si

Si Si Si Si Si

Si Si Si Si Si Si Si

free electron

P+ P+

P-Type SiliconP-Type Silicon Recall that boron, aluminum, and gallium are acceptor

dopants in silicon, making it p-type. An acceptor atom is readily ionized, yielding a free ()

hole and leaving behind a negative ion core. free hole

Si Si Si Si Si Si Si

Si Si Si Si Si

Si Si Si Si Si Si Si

BB

Diffusion (of electrons & holes)Diffusion (of electrons & holes) Diffusion is a process whereby particles tend to spread

out or redistribute as a result of their random thermal motion.

Particles migrate from regions of high particle concentration to regions of low particle concentration.

Electrons and holes tend to diffuse as shown in the following examples:

P-N Junction DiodeP-N Junction Diode A p-n junction consists of p-type and n-type

semiconductor material in intimate contact with one another (with no intervening material of any kind).

This structure is also called a p-n diode, or simply a diode.

The highest quality p-n junctions consist of single crystal material, part of which is p-type, the remainder being n-type.

In order to understand some of the properties of p-n junctions, we can perform a “thought experiment” in which we consider what happens when p-type and n-type material are brought together.

See Photovoltaic Fundamentals, pp. 12-16.

P-N Junction DiodeP-N Junction Diode First, recall that

P-type material contains an abundance of free holes that behave like mobile positive charges.

P-type material contains an (approximately) equal number of ionized acceptor atoms, which are immobile negative charges.

N-type material contains an abundance of free electrons, which are mobile negative charges.

N-type material contains an (approximately) equal number of ionized donor atoms, which are immobile positive charges.

Each type of material is electrically neutral overall.

P-N Junction DiodeP-N Junction Diode

Here is a legend for the upcoming diagrams:

P-N Junction DiodeP-N Junction Diode Consider the formation of a silicon p-n junction -- this is our

thought experiment. The n- and p-type silicon shown below will be brought together. See Photovoltaic Fundamentals, page 14.

Each material is electrically neutral overall.

Excess free electrons are balanced by positive donor ions.

Excess free holes are balanced by negative acceptor ions.

P-N Junction DiodeP-N Junction Diode When the n- and p- type silicon come into contact, electrons move

from the n-side to the p-side. This is because electrons on the n-side are free and tend to diffuse

from where they are abundant to where they are less plentiful. See Photovoltaic Fundamentals, page 14.

P-N Junction DiodeP-N Junction Diode This diffusion causes immobile positive charge (from ionized

donors) to build up on the n-side in the immediate vicinity of the junction.

Once on the p-side, the electrons fill the holes in the immediate vicinity of the junction. This causes immobile negative charge (from ionized acceptors)

to build up on the p-side. See Photovoltaic

Fundamentals, page 14.

P-N Junction DiodeP-N Junction Diode The buildup of immobile positive and negative charges on opposite

sides of the junction create an electric field, which eventually stops the charge transfer across the junction.

See Photovoltaic Fundamentals, page 15.

- -

- -+ +

+ +

n-side p-sideE

junction

P-N Junction DiodeP-N Junction Diode The region surrounding the junction, from which free electrons

and holes have diffused away when the junction was formed, is called the “depletion region”.

- -

- -+ +

+ +

n-side p-sideE

electric field hereneutral hereneutral here

P-N Junction DiodeP-N Junction Diode A p-n junction diode can also be described by an energy

band diagram. When a p-n junction is formed, the energy bands bend

at the junction.

conduction band

n-side

p-side

valence bandE

conduction band

valence band

Eg

P-N Junction DiodeP-N Junction Diode The electric field in the depletion region prevents more

electrons and holes from crossing the junction.

conduction band

n-side

p-side

valence bandE

Eg

depletion region

P-N Junction DiodeP-N Junction Diode Electrons behave like marbles they tend to go downhill. Holes behave like helium-filled balloons they tend to

float uphill.

conduction band

n-side

p-side

valence bandE

Eg

depletion region

P-N Junction DiodeP-N Junction Diode The bent energy bands are a barrier to electron motion. The bent energy bands are a barrier to hole motion.

conduction band

n-side

p-side

valence bandE

Eg

depletion region

P-N Junction DiodeP-N Junction Diode If the p-side is made positive and the n-side is made

negative, the barrier is reduced and electrons and holes can cross electric current flows.

This situation is called forward bias.

n-side

p-side

___

I

P-N Junction DiodeP-N Junction Diode If the p-side is made negative and the n-side is made

positive, the barrier is increased and electrons and holes cannot cross no electric current flows.

This situation is called reverse bias.

n-side

p-side

___

P-N Junction DiodeP-N Junction DiodeMetal contacts must be provided in order to

connect the diode to the outside world.

n-side p-side

depletion

region

metal contact metal contact

P-N Junction DiodeP-N Junction Diode In circuit diagrams, a diode is represented by the

following symbol. Electric current can flow in the direction of the “arrow”

in the symbol.

n-side p-side

depletion

region

metal contact metal contact

P-N Junction DiodeP-N Junction Diode The electrical characteristics of a p-n junction diode are

given by a “current-voltage” graph -- a graph of electric current through the diode as a function of applied voltage across the diode. I

Vforward bias

+

reverse bias

+

“reverse

breakdown”