The Devices: Diode Once Again

The Devices:Diode Once Again

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

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

SI SI SI SI SI SI SI SI SI SI SI SI N N N N N N SI SI SI SI SI SI n-type material • Donor Material w/an excess electron in the covalent bond w/Silicon • displays aNegative charge. • Majority Carriers are Electrons. I V

SI SI SI SI SI SI SI SI SI SI SI SI SI SI SI SI SI SI P P P P P P P P p-type material • Acceptor Material has a missing electron in the covalent bond w/Silicon, • displays a Positive charge. • Majority Carriers are Holes. I I V

Majority Current Carriers, Holesor Electrons. N Type Material: Donor Material with an excess electron in the covalent bond in Silicon & displays a Negativecharge. Majority Carriers are Electrons. P Type Material: Acceptor Material has a missing electron in the covalent bond in Silicon, & displays a Positive charge. Majority Carriers are Holes. N P Remember… • 2 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 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. 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. Depletion region

The pn Junction in Si Material anode cathode When 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.

The pn Junction in Si Material anode cathode 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).

- ++ Depletion Region + - +++ - + + + + + + + - - - + + - - - - - - - - + - - + + - - - - - - - - + + + + + + + + + - + - + + - + + - - - - - - + + + + + + - - - - - - + + + + - + + + + + + + + + + + + - - - - - - - - - - - ++++ Depletion Region I - Depletion Region I FORWARD BIASED REVERSED BIASED

Depletion Region • 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

Built-in Potential Where T is the thermal voltage ni is the intrinsic carrier concentration for pure Si (1.5 X 1010 cm-3 at 300K), so for

Models for Manual Analysis • Conducting diode replaced by voltage source VDon=0.7V • Good for first order approximation • Accurate • Strongly non-linear • Prevents fast DC bias calculations

+ V f V / D I = I (e D – 1) T D S – Typical Diode Parameters • Geometry, doping and material • constants lumped in Is Diffusion coefficient minority carrier concentration • Dn=25 cm2/sec • Dp=10cm2/sec • Wn=5 mm • Wp=0.7 mm • W2=0.15 mm • W1=0.03 mm • q=1.6*10-19 coulombs • pn0=0.3*105/cm3 • np0=0.6*104/cm3

0.1 ) A ( 0 D I –0.1 –25.0 –15.0 –5.0 0 5.0 V (V) D Secondary Effects: Breakdown • Cannot bear too large reverse biases • Drift field in depletion region will get extremely large • Minority carriers caught in this large field will get very energetic • Energetic carriers can knock atoms and create a new n-p pair • These carriers will get energetic, too, and so on: thus large currents! • Two types • Avalanche breakdown • Above mechanism • Zener breakdown • More complicated • Can damage diode

Diode SPICE Model • Required for circuit simulations • Must 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 assumptions • Additional series resistance accounts for body+contact • Nonlinear capacitance includes both CD and Cj

The Devices: Diode Once Again