ISAT 436 Micro-/Nanofabrication and Applications P-N Junction Diodes David J. Lawrence Spring 2004

N-Type Silicon H 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 free electron P+

P-Type Silicon H 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 BB BB

Diffusion (of electrons & holes) H Diffusion is a process whereby particles tend to spread out or redistribute as a result of their random thermal motion. H Particles migrate from regions of high particle concentration to regions of low particle concentration. H Electrons and holes tend to diffuse as shown in the following examples:

P-N Junction Diode H 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). H This structure is also called a p-n diode, or simply a diode. H The highest quality p-n junctions consist of single crystal material, part of which is p-type, the remainder being n-type. H 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. H See Photovoltaic Fundamentals, pp

P-N Junction Diode H First, recall that vP-type material contains an abundance of free holes that behave like mobile positive charges. vP-type material contains an (approximately) equal number of ionized acceptor atoms, which are immobile negative charges. vN-type material contains an abundance of free electrons, which are mobile negative charges. vN-type material contains an (approximately) equal number of ionized donor atoms, which are immobile positive charges. vEach type of material is electrically neutral overall.

P-N Junction Diode H Here is a legend for the upcoming diagrams:

P-N Junction Diode H Consider the formation of a silicon p-n junction -- this is our thought experiment. H The n- and p-type silicon shown below will be brought together. H 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 Diode H When the n- and p- type silicon come into contact, electrons move from the n-side to the p-side. H 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. H See Photovoltaic Fundamentals, page 14.

P-N Junction Diode H This diffusion causes immobile positive charge (from ionized donors) to build up on the n-side in the immediate vicinity of the junction. H 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. H See Photovoltaic Fundamentals, page 14.

P-N Junction Diode H 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. H See Photovoltaic Fundamentals, page n-side p-side E junction

P-N Junction Diode H 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-side E electric field here neutral here

P-N Junction Diode H A p-n junction diode can also be described by an energy band diagram. H When a p-n junction is formed, the energy bands bend at the junction. conduction band n-side p-side valence band E          conduction band valence band EgEg

P-N Junction Diode H The electric field in the depletion region prevents more electrons and holes from crossing the junction. conduction band n-side p-side valence band E          EgEg depletion region

P-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 band E          EgEg depletion region

P-N Junction Diode H The bent energy bands are a barrier to electron motion. H The bent energy bands are a barrier to hole motion. conduction band n-side p-side valence band E          EgEg depletion region

P-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. H This situation is called forward bias. n-side     p-side       ______ I

P-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. H This situation is called reverse bias. n-side     p-side      ______ 

P-N Junction Diode H Metal contacts must be provided in order to connect the diode to the outside world. n-side p-side       depletion region metal contact

P-N Junction Diode H In circuit diagrams, a diode is represented by the following symbol. H Electric current can flow in the direction of the “arrow” in the symbol. n-side p-side       depletion region metal contact

P-N Junction Diode H 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 V forward bias  + reverse bias +  “ reverse breakdown ”