EE314 Basic EE II

The Devices: Diode

EE314 Basic EE II

Chapter 10: Diodes

1.Semiconductors2.Doping concept3.n & p-type semiconductors 4.Si diode5.Forward & reversed bias6.Examples7.Diode Characteristic

http://www.amazon.com/Engineer-Training-Reference-Michael-Lindeburg/dp/0912045566Engineer-In-Training Reference Manual

EE314 Basic EE II

Outline

Motivation and Goals Semiconductor Basics Diode Structure Operation

» Static model

EE314 Basic EE II

Composed of 3 Basic particles: Protons, Electrons & Neutrons.

An Atom requires balance, an equal No. of Protons & Electrons.

When an atom has one more particle (protons or electrons) it acquires a charge:

+ Ion Ion has more Protons Protons than Electrons,- Ion Ion has more ElectronsElectrons than Protons.

Atom

EE314 Basic EE II

What do we know about an atomic structure?

EE314 Basic EE II

Semiconductor Basics I Electrons in intrinsic (pure) Silicon

» covalently bonded to atoms» “juggled” between neighbors» thermally activated: density eT

» move around the lattice, if free» leave a positively charged `hole’ behind

http://www.masstech.org/cleanenergy/solar_info/images/crystal.gif

EE314 Basic EE II

Semiconductor Basics II Two types of intrinsic carriers

» Electrons (ni) and holes (pi)

» In an intrinsic (no doping) material, ni=pi

» At 300K, ni=pi is low (1010cm-3)» Use doping to improve conductivity

EE314 Basic EE II

Semiconductor Basics III Extrinsic carriers

» Also two types of dopants (donors or acceptors)– Donors bring electron (n-type) and become ive ions– Acceptors bring holes (p-type) and become ive ions

» Substantially higher densities (1015cm-3)» Majority and minority carriers

– if n>>p (n-type) electrons majority and holes minority– Random recombination and thermal generation

EE314 Basic EE II

ConductorConductor;Has loosely bound electrons in its outer or Valence ring, they are easily displaced.

InsulatorInsulator;Has tightly bound electrons in its outer or Valence ring, they cannot be easily displaced.

SemiconductorSemiconductor;Has at least 4 electrons in the outer or Valence ring, it is neither a conductor nor an insulator. In its pure state it makes a better insulator than conductor. 4 electrons allows easy bonding w/ other materials.

Conduction

EE314 Basic EE II

Semiconductor Basics I Electrons in intrinsic (pure) Silicon

» covalently bonded to atoms» “juggled” between neighbors» thermally activated: density eT

» move around the lattice, if free» leave a positively charged `hole’ behind

EE314 Basic EE II

Semiconductor Basics II Two types of intrinsic carriers

» Electrons (ni) and holes (pi)

» In an intrinsic (no doping) material, ni=pi

» At 300K, ni=pi is low (1010cm-3)» Use doping to improve conductivity

Extrinsic carriers» Also two types of dopants (donors or acceptors)

– Donors bring electron (n-type) and become ive ions– Acceptors bring holes (p-type) and become ive ions

» Substantially higher densities (1015cm-3)» Majority and minority carriers

– if n>>p (n-type) electrons majority and holes minority– Random recombination and thermal generation

EE314 Basic EE II

The Diode

p

n

B A SiO 2Al

Cross section of pn-junction in an IC process

P-type regiondoped with acceptor impurities (boron)

N-type regiondoped with donor impurities (phosphorus, arsenic)

EE314 Basic EE II

The Diode

A

B

n

p

A

B

Al

One-dimensionalrepresentation diode symbol

The pn region is assumed to be thin (step or abrupt junction)

Different concentrations of electrons (and holes) of the p and n-type regions cause a concentration gradient at the boundary

Simplified structure

EE314 Basic EE II

•Concentration Gradient causes electrons to diffuse from n to p, and holes to diffuse from p to n•This produces immobile ions in the vicinity of the boundary•Region at the junction with the charged ions is called the depletion region or space-charge region•Charges create electric field that attracts the carriers, causing them to drift

•Drift counteracts diffusion causing equilibrium ( Idrift = -Idiffusion )

Depletion Region

hole diffusionelectron diffusion

p n

hole driftelectron drift

EE314 Basic EE II

Depletion Regionhole diffusion

electron diffusion

p n

hole driftelectron drift

ChargeDensity

Distancex+

-

ElectricalxField

x

PotentialV

W2-W1

(a) Current flow.

(b) Charge density.

(c) Electric field.

(d) Electrostaticpotential.

•Zero bias conditions•p more heavily doped than n (NA > NB)

•Electric field gives rise to potential difference in the junction, known as the built-in potential

EE314 Basic EE II

Forward Biashole diffusion

electron diffusion

p n

hole driftelectron drift

+ -

•Applied potential lowers the potential barrier, Idiffusion > I drift

•Mobile carriers drift through the dep. region into neutral regions•become excess minority carriers and diffuse towards terminals

EE314 Basic EE II

Reverse Biashole diffusion

electron diffusion

p n

hole driftelectron drift

- +

•Applied potential increases the potential barrier•Diffusion current is reduced•Diode works in the reverse bias with a very small drift current

EE314 Basic EE II

Diode Current

Ideal diode equation: