1. The Devices: Diode Once Again

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

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

4. SI SI SI SI SI SI SI SI SI SI SI SI SI SI SI SI SI SI N N N N N N  Donor Material w/an excess electron in the covalent bond w/Silicon  displays a Negative charge.  Majority CarriersElectrons.  Majority Carriers are Electrons. n-type material I V

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

6. Remember… HolesElectrons Majority Current Carriers, Holes or Electrons. N Type Material Negative N Type Material: Donor Material with an excess electron in the covalent bond in Silicon & displays a Negative charge. Majority CarriersElectrons. Majority Carriers are Electrons. N P Type Material Positive P Type Material: Acceptor Material has a missing electron in the covalent bond in Silicon, & displays a Positive charge. Majority CarriersHoles Majority Carriers are Holes. P 2 Current Carriers MajorityMinority 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

7. 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

8. The pn Junction in Si Material 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. anode cathode

9. 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 Material anode cathode

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

11. Depletion Region Zero bias conditions p more heavily doped than n (N A > N B ) Electric field gives rise to potential difference in the junction, known as the built-in potential

12. Built-in Potential Where  T is the thermal voltage n i is the intrinsic carrier concentration for pure Si (1.5 X 10 10 cm -3 at 300K), so for

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

14. Typical Diode Parameters V D I D =I S (e V D /  T – 1) + – D n =25 cm 2 /sec D p =10cm 2 /sec W n =5  m W p =0.7  m W 2 =0.15  m W 1 =0.03  m Geometry, doping and material constants lumped in I s Diffusion coefficient minority carrier concentration q=1.6*10 -19 coulombs p n0 =0.3*10 5 /cm 3 n p0 =0.6*10 4 /cm 3

15. Secondary Effects: Breakdown l 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! l Two types »Avalanche breakdown –Above mechanism »Zener breakdown –More complicated l Can damage diode

16. Diode SPICE Model l 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 l Additional series resistance accounts for body+contact l Nonlinear capacitance includes both C D and C j