The Devices: Diode Once Again Si Atomic Structure Electron Configuration: First Energy Level: 2 Second Energy Level: 8 Third Energy Level: 4 Doping Process Doping: The process of adding impurities to the intrinsic material giving the material a Positive or Negative characteristic. N SI P SI SI SI SI SI N SI SI SI SI SI SI SI SI SI SI SI SI P SI SI SI SI SI SI SI SI SI SI Covalent Bonding; Pentavalent Doping; Trivalent Doping; Undoped Material Shares its 4 electrons w/other atoms and forms a pure crystal. Donor Material Acceptor Material Impurities that have an excess of electrons. N type Material, called Electrons. - charged Impurities that have missing electron, called Holes or P type Material. + charged. n-type material N I Donor Material w/an excess electron in the covalent bond w/Silicon displays a Negative charge. Majority Carriers are Electrons. SI N SI SI SI N SI SI N SI SI SI SI SI SI SI N SI SI SI N SI SI V p-type material P I Acceptor Material has a missing electron in the covalent bond w/Silicon, displays a Positive charge. Majority Carriers are Holes. SI SI P SI SI SI P SI SI SI SI SI SI SI P SI SI SI P SI SI SI I P P V P Remember… Majority Current Carriers, Holes or Electrons. N Type Material: N P Type Material: P 2 Current Carriers: Donor Material with an excess electron in the covalent bond in Silicon & displays a Negative charge. Majority Carriers are Electrons. Acceptor Material has a missing electron in the covalent bond in Silicon, & displays a Positive charge. Majority Carriers are Holes. 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. Depletion region 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 in Si Material cathode anode 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 cathode anode 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). FORWARD BIASED + - - REVERSED BIASED - I + + + + - - - - + + + + - - - - + + + + - - - - - + - - - - + - - - + + + + + + + + + - + - - - + + - - + + - - + + - Depletion Region + + - - + + - - + + - - - - - - + + + + - - - - + + + + - - - - + + + + Depletion Region Depletion Region - I + + + + + + + + + Depletion Region •Zero bias conditions hole diffusion electron diffusion p •p more heavily doped than n (NA > NB) •Electric field gives rise to potential difference in the junction, known as the built-in potential (a) Current flow. n hole drift electron drift Charge Density + x Distance - Electrical Field (b) Charge density. x (c) Electric field. V Potential -W 1 W2 x (d) Electrostatic potential. Built-in Potential N A ND 0 T ln 2 n i Where T is the thermal voltage kT T 26mV (at 300 K ) q ni is the intrinsic carrier concentration for 15 1 pure Si (1.5 X 1010 cm-3 at 300K), so forN A 10 3 , cm 10151016 0 26 ln mV 638mV 10 2 1.5 *10 1 N B 1016 3 , cm Models for Manual Analysis + ID = IS(eV D/T – 1) VD ID + + VD – (a) Ideal diode model •Accurate •Strongly non-linear •Prevents fast DC bias calculations – VDon – (b) First-order diode model •Conducting diode replaced by voltage source VDon=0.7V •Good for first order approximation Typical Diode Parameters Geometry, doping and material constants lumped in Is Diffusion coefficient minority carrier concentration + VD ID = IS(eV D/T – 1) – •Dn=25 cm2/sec •q=1.6*10-19 coulombs •Dp=10cm2/sec •pn0=0.3*105/cm3 •Wn=5 mm •np0=0.6*104/cm3 •Wp=0.7 mm •W2=0.15 mm •W1=0.03 mm I S qAD ( WnpWn 02 W pn Wp 01 ) D p typical value I S 10 17 A / mm 2 D n 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! 0.1 Two types » Avalanche breakdown ID (A) – Above mechanism » Zener breakdown – More complicated 0 –0.1 –25.0 –15.0 –5.0 VD (V) 0 5.0 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 I D I S (eVD /nT 1) RS + VD - ID CD