PN Junction Diode Semiconductor Elements 1 Diode Cases 2 1
Basic Diode Feature The essential feature of a diode is at e magnitude of e current greatly depends on e polarity of applied voltage. Basicaly e diode conducts current in only one direction. t is applied in rectifiers for converting an alternating current into a dirrect current. 3 PN Junction Diode Structure An ideal diode can only conduct current in one direction. Diode specific electrical properties depends on it s structure. 4 2
Junction Diode Schematic Symbol A diode is a nonlinear semiconductor device wi two electrodes: an anode and a caode. An arrow points from e anode to e caode to remind at e current will only flow easily in is direction. 5 Diode's Mode of Operation A diode's mode of operation depends on e various conditions possible at e pn junction. They are caused by e polarity of e applied voltage. 6 3
Unibiased Diode Equal number of plus and minus charges Electrically neutral as a whole 7 PN Junction Formation Majority carriers free electrons and holes, diffuse (spread) across e junction because difference in concentration in bo side of e crystal. Unlike e free carriers, e ions will never move. They remains fixed due to eir covalent bonds in e crystal structure. 8 4
Depletion Layer almost completely depleted of mobile charge carriers When an electron leaves e n side, it will leave behind an uncompensated positive ion. A positive space charge is created to e right of e junction in e n-region. Similarly a negative space charge will be created to e left of e junction in e p-region. 9 Barrier Potential & Electric Field Diffusion current D D Majority carriers Drift current E E Minority carriers The uncompensated positively- and negatively-charged ions at are wiin e depletion layer generate an electric field E o e barrier potential U o. At room temperature (25 o C) e barrier potential has for Si diodes a voltage of approximately 0.7V and for Ge diodes a voltage of about 0.3V. 10 5
Forward Bias The barrier potential will reduce to U o - U s and e electric field to E < E o. A forward current is always composed of majority carriers whose energies are sufficient to overcome e barrier potential. Current will easily flow in forward-biased diode. 11 everse Bias The barrier potential will increase to U o + U s and e electric field to E > E o. The diffusion of majority carriers across e junction has been greatly diminished. A very small reverse current, composed of ermally produced minority carriers still cross e junction. This makes e reverse current independent from e direction of e voltage polarity and of e barrier potential. 12 6
This image cannot currently be displayed. VA Characteristic A diode is a nonlinear device. t conducts in only one direction. An ideal diode functions like a switch open and closed. n forward-biased diode, e current increases very rapidly wi U. The reverse current is very small. 13 deal Diode Equation U = T ϕ s ( e 1) s = SJ s s - Saturation current J s - Current density S - Junction area ϕ T - Thermal potential kt ϕ T = q ϕ T = T (K) 11600 ϕ T = 0.0258 V for T = 25 o C U = ϕ ln + 1 T S 14 7
deal Diode Equation = s e U ϕ T = s 15 Saturation Current = f ( W, T, S) s A Si diode has a much smaller s an a Ge diode. There are fewer minority carriers in Si diodes an in Ge diodes. s doubles wi every 10 o C increase. Because e reverse current is caused by ermally created minority carriers, it will also be highly sensitive to temperature changes. 16 8
VA Characteristics of Si and Ge Diode 17 eal Diode Forward egion ecombination current, base ohmic resistance r B, r B = f () are considered. U PN U B r B = S e U r B mϕt ( ) U = U PN + r B p r E d r B n F U PN.r B Омичен участък _ Е + U B U + _ U F 18 9
eal Diode everse egion Generation current, leakage current and breakdown are considered. = + + + S G ут B G U U U, V ут S Si ~ G G >> S G Ge ~ S Breakdown region S >> G, na Si diode = S + ут ( G >> S ), Ge diode = G + ут ( S >> G ) 19 Forward Voltage and Temperature du U TKU F = = const dt T TKU F - 2 mv/ o C 20 10
Diode atings Max junction temperature T jmax < T i, where n=p=n i Max power P jmax P = U Power dissipated in diode Tj Ta U = Tj Ta P = Power conducted into e ambient surroundings P max T i = max T a 1 P max = Ta + 1 T j max 21 Max Power & Max Current 1 P max = Ta + 1 T j max U F F max = P max T = j max T a Tj max P jmax, W Fmax, A T a, o C 25 o C T jmax 22 11
Current-Limiting esistor E U F = < F max The current-limiting resistor has e function to keep e diode current smaller an e maximum rating. 23 Heat emoval indicates efficiency in removing heat from e transistor in units o K/W. = + jc ca >> ca jc The less ermal resistance e higher power rating. Thermal energy can be easily reduced rough conduction and radiation from e device's case. = + + jc ch ha Heat sink 24 12
everse Breakdown At a reverse breakdown voltage U B, e current will rapidly increase wi only small changes in e voltage. Thermal breakdown Electrical breakdown Avalanche breakdown U B Zener breakdown 25 Avalanche Breakdown M = B 1 = U 1 U B n Minority carriers can be accelerated by e field and generate anoer two electron- hole pairs on collisions wi crystal atoms. The process may continue, causing e increase of e current. U B >7V Avalanche breakdown occurs in wide depletion layers wi a reverse bias higher an 7V. 26 13
Zener Breakdown When an electric field of e barrier potential is large enough to break e covalent bonds additional free electrons and holes are created. Such effect is called Zener effect or Zener breakdown. This requires high electric fields on e order of 300 000 V/cm. Zener breakdown occurs in very narrow depletion layers wi a reverse bias less an 5V. 27 Thermal Breakdown This breakdown occurs if U Tj Ta > U,V U B1 U B2 U,V Avalanche Thermal Surface r < 0 Zener T 2 > T 1 T 1 T 2, ma, ma 28 14
Load Line and Operating Point E E Q U Q = Q Q E E 2 E 1 F E = U + E 2 > E 1 E 1 E 2 1 F 2 > 1 = 1 U + E U F U F E 1 Е 2 E 29 Equivalent Circuits 30 15
Diode Testing 31 Examples + 10 k ma =? U вх = 10 V 100 V _ Si фиг. 1 + 1k K 1 K 2 V=? 10V _ Si ШД V фиг. 1 32 16
Work wi ac Signal Dynamic resistance U F u(t) r = U E + _ U F E-U m U E+U m r = du d ϕt = + s 33 Graphical calculation of r B F r r = U U U F F Values of dynamic parameters are valid in operation point only. W hen e position of operating point changes e value of e dynamic parameter also changes. The value of forward dynamic resistor is very small (less an 100 ohms), but in reverse direction is very large (around Mohms) 34 17
Examples 10 k 10V U вх U вх (t) Si U (t) изх t 1V + _ -10V фиг. 1 U изх t 35 18