isat 436 micro-/nanofabrication and applications p-n junction diodes david j. lawrence spring 2004
TRANSCRIPT
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”