Lecture4 Diode Once Again.ppt

The Devices: Diode Once Again

Si Atomic StructureFirst Energy Level: 2 Second Energy Level: 8 Third Energy Level: 4 Electron Configuration:

Doping ProcessCovalent Bonding;Undoped Material Shares its 4 electrons w/other atoms and forms a pure crystal.Pentavalent Doping;Donor Material Impurities that have an excess of electrons. N type Material, called Electrons. - chargedTrivalent Doping;Acceptor Material Impurities that have missing electron, called Holes or P type Material. + charged.Doping: The process of adding impurities to the intrinsic material giving the material a Positive or Negative characteristic.

Donor Material w/an excess electron in the covalent bond w/Silicon displays a Negative charge.Majority Carriers are Electrons.n-type materialIV

Acceptor Material has a missing electron in the covalent bond w/Silicon, displays a Positive charge.Majority Carriers are Holes.p-type materialIVI

Remember2 Current Carriers: Majority & Minority Intrinsic impurities inherent in silicon result in current flow in the opposite direction to Majority flow. Becomes evident in heat, leakage and break down of the device. Minority Current carriers

The pn Junction in Si Material At the junction, electrons fill holes so that there are no free holes or electrons there. The actual junction becomes an insulating layer. This barrier must be overcome before current can flow through the pn junction. The pn junction is made from a single crystal with the impurities diffused into it. The n end has a surplus of negative electrons. The p end has a surplus of holes.Depletion region

The pn Junction in Si MaterialWhen a battery is connected as shown, the negative terminal pushes negative electrons towards the junction. The positive terminal pushes holes towards the junction. A high enough voltage will overcome the barrier and a current will flow through the pn junction.

There is a voltage across the diode of 0.7V for the silicon. The junction is said to be FORWARD BIASED. The p-type is the anode of the diode, the n-type the cathode, as shown by the diode symbol. The resistor limits the current to a safe level.anodecathode

When the battery is connected as shown, the positive terminal of the battery attracts negative electrons away from the barrier. The negative terminal attracts holes away from the barrier. The insulating barrier widens and no current flows.

The junction is REVERSED BIASED. If the reverse voltage is made high enough, then the junction will break down and electrons will flow from anode to cathode (under normal conditions, electrons flow from cathode to anode, when forward biased).The pn Junction in Si Materialanodecathode

REVERSED BIASEDFORWARDBIASED

Depletion RegionZero bias conditionsp more heavily doped than n (NA > NB)Electric field gives rise to potential difference in the junction, known as the built-in potential

Built-in PotentialWhere T is the thermal voltageni is the intrinsic carrier concentration for pure Si (1.5 X 1010 cm-3 at 300K), so for

Models for Manual AnalysisAccurateStrongly non-linearPrevents fast DC bias calculationsConducting diode replaced by voltage source VDon=0.7VGood for first order approximation

Typical Diode ParametersDn=25 cm2/secDp=10cm2/secWn=5 mmWp=0.7 mm W2=0.15 mmW1=0.03 mmGeometry, doping and material constants lumped in Is

Diffusion coefficientminority carrier concentrationq=1.6*10-19 coulombspn0=0.3*105/cm3np0=0.6*104/cm3

Secondary Effects: BreakdownCannot bear too large reverse biasesDrift field in depletion region will get extremely largeMinority carriers caught in this large field will get very energeticEnergetic carriers can knock atoms and create a new n-p pairThese carriers will get energetic, too, and so on: thus large currents!Two typesAvalanche breakdownAbove mechanismZener breakdownMore complicatedCan damage diode

Diode SPICE ModelRequired for circuit simulationsMust capture important characteristics but also remain efficient Extra parameter in the model: n (emission coefficient, 1 n 2)Fixes non-ideal behavior due to broken assumptionsAdditional series resistance accounts for body+contactNonlinear capacitance includes both CD and Cj

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