1 / 41

Microelectronics 2

Electrical Engineering 2. Lecture 10. Microelectronics 2. Dr. Peter Ewen. (Room G08, SMC; email - pjse). Metal plate (cathode pin). Fig. 47. (Discrete) Diode Fabrication. Al wire to anode pin. Gas of boron atoms. SiO 2. p-type. p-type. p-type. ~500 m. n-type . Aluminium.

niabi
Télécharger la présentation

Microelectronics 2

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Electrical Engineering 2 Lecture 10 Microelectronics 2 Dr. Peter Ewen (Room G08, SMC; email - pjse)

  2. Metal plate (cathode pin) Fig. 47 (Discrete) Diode Fabrication Al wire to anode pin Gas of boron atoms SiO2 p-type p-type p-type ~500 m n-type Aluminium Diode 3 Diode 2 Diode 1

  3. Before “contact” After “contact” - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + - - + + - + ΔV - + E = – - + - + - + - + - + Δx - + junction n-type p-type + - + - + - + - + - + - + - + - + - + - -ve +ve E Potential, V depletion region VB, Barrier Potential Distance

  4. - - - - - - - - - - + + + + + + + + + + - - - - - - - - - - + + EC EF EV EC EF EV + + + + + + + + junction Fig. 51 n-type p-type E Potential barrier, VB ΔE = qΔV Energy barrier, eVB Situation after “contact” depletion region

  5. Electron flows across a pn junction under zero bias –E + Fig. 52 p-type n-type F = -eE Electrons Drift current Diffusion current Electrons Electron energy eVB For zero applied bias the drift and diffusion flows balance – there is no net current across the junction. Eg depletion region

  6. Effect of bias on width of the depletion region 0 Fig. 55 -2 -14 Reverse bias (p-type -ve w.r.t. n-type) Volts -4 -12 -6 -10 -8 V VB - + - + - + - + - + - + - + - + + – n p Depletion region widens Potential VB+V VB Distance

  7. Effect of bias on width of the depletion region 0 Fig. 55 0.1 0.7 Forward bias (p-type +ve w.r.t. n-type) Volts 0.2 0.6 0.3 0.5 0.4 V VB + - + - + - + - + – n p - + - + Depletion region narrows Potential VB VB-V Distance

  8. A more accurate characteristic is obtained by plotting the diode equation: I / mA 5 4 3 2 1 Fig. 57 V0 Is 0.4 0.8 V / volts -0.5 -1 -1.5 -2 -2.5 -1 -2 -3 -4 -5 N.B. Is depends on temperature – for Si, Is doubles for every 6°C rise in temperature. I / nA I / mA

  9. LECTURE 10 PN JUNCTION DIODE  Static and dynamic resistance • Reverse breakdown • Avalanche breakdown • Zener breakdown  I-V characteristic of a real pn junction diode • Junction capacitance

  10. 3. Diode equationThe currents flowing through a Si diode are found to be -2.5 nA and 35 mA at voltages of -10 V and 0.85 V respectively. Determine the ideality factor, , for this diode.

  11. 3. Diode equation What is the value for Is? I / mA 50 40 30 20 10 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 V / volts 1 2 Is -10 -20 -30 -40 -50 Is = |I(-10V)| = 2.5nA I / mA

  12. Diode resistance Static resistance: Dynamic resistance: I / mA 5 4 3 2 1 V ΔI I ΔV Is V / volts 0.8 0.4 -1 -2 -3 -4 -5 For V >> 26 mV, I in mA and assuming T = 300 K and  = 1 I / mA

  13. Reverse Breakdown I / mA Fig. 58 VBD V / volts Diode equation prediction In a real device BREAKDOWN occurs: VBD – the BREAKDOWN VOLTAGE

  14. Reverse Breakdown • Breakdown occurs as a result of a high E field in the depletion region. Depletion region p n – – – – – + + + + + Drifting electrons E Drifting holes W W = width of depletion region V = applied bias If V is large or W is small, E will be large • Breakdown occurs only under reverse bias, so only drifting carriers are involved. These are accelerated by the field, i.e. they gain energy from it.

  15. Avalanche Breakdown Si Si Si Si Si Si Si Fig. 59(a) Si Si Si Si Si VREV EEnergy of drifting carriers Strong E field in depletion region

  16. Avalanche Breakdown Si Si Si Si Si Si Si Fig. 59(a) Si Action Replay Si Si Si Si Strong E field in depletion region

  17. Zener Breakdown • Electrons torn directly from • bonds by the high E field Si Si Si Si Si Si Si Fig. 59(b) Si Si Si Si Si Strong E field in depletion region

  18. Both effects are perfectly reversible provided power dissipation in the device is limited. • VBD depends on temperature. I / mA VBD V / volts operating point

  19. Temperature Dependence of VBD AVALANCHE BREAKDOWN: |VBD| INCREASES as temperature increases because: More thermal vibrations (phonons) around at higher temperatures Carriers make more collisions So there is less time between collisions for them to get energy from the E field So the E field and hence VBD must increase to compensate for this.

  20. ZENER BREAKDOWN: |VBD| DECREASES as temperature increases because To rip an electron directly out of a bond requires an energy equal to the energy gap, Eg Eg decreases as temperature increases

  21. ELECTRON ENERGY Temperature increases, structure expands Energy levels of the Isolated atom Equilibrium spacing INTERATOMIC SPACING Fig 19: Variation of the energy bands with interatomic spacing for silicon (and also germanium and carbon).

  22. The range of voltages over which Zener and avalanche breakdown occur: Fig. 60 Avalanche breakdown Zener breakdown 200 12 11 10 9 8 7 6 5 4 3 2 1 0 VBD (volts) N.B. In this voltage range both mechanisms occur simultaneously, so the different temperature effects cancel and |VBD| can be independent of temperature. Zener Diodes: The breakdown effect can be used as the basis of a voltage reference – diodes designed for this application are termed “Zener Diodes” Fig. 61

  23. Fig. 62 I / mA Characteristic for real device ______ 100 50 Characteristic predicted by the diode equation - - - • Characteristic becomes • more linear at large forward voltages due to resistance of p and n regions (R1 and R2) Is -5 -10 0.4 0.8 V / volts -50 -100 I / nA I / mA

  24. Depletion region Fig. 63 p n contact contact junction R1 R2 Rs Rs is the resistance of the depletion region – this is given by the diode equation. R1 & R2represent the resistances of the actual semiconductor material in the p and n regions outside the depletion region: typically R1 & R2are ~5Ω. Static resistance: I / mA 50 30 10 V Rs small I Rs large V / volts 0.8 0.4

  25. 4. Diode bulk resistance In a diode for which  =1, bias voltages of 0.18 V and 0.29 V produce forward currents of 1 mA and 10 mA respectively. What is the bulk resistance of this device if it can be assumed that this resistance has negligible effect on the current for bias voltages below 0.2 V?

  26. 4. Diode bulk resistance The bulk resistance is R1 + R2. “…this resistance has negligible effect on the current for bias voltages below 0.2V ”  diode equation is accurately obeyed for V < 0.2V For I = 1mA, V = 0.18V: Since 0.18V >> 26mV we can approximate the diode equation by

  27. For I = 10mA, V=0.29V: Re-arranging the diode equation above to obtain V on the left-hand side we can calculate what the voltage across the diode should be for 10mA current: But the voltage across the terminals is 0.29V!

  28. 10 mA represents the real diode R1 0.29V ‘resistanceless’ diode – represents the depletion region R2 The “missing” voltage (0.29V - 0.24V) must be dropped across the bulk resistance, RB (= R1+R2), of the diode (i.e. the resistance of the contacts and the semiconductor material outside the depletion region).

  29. Fig. 62 I / mA Characteristic for real device ______ 100 50 Characteristic predicted by the diode equation - - - • Characteristic becomes • more linear at large forward voltages due to resistance of p and n regions (R1 and R2). -5 -10 Is 0.4 0.8 V / volts 2. Slope due to surface leakage. -50 -100 I / nA

  30. Fig. 64 Some current manages to leak around the corners of the pn junction at the surface. Reverse bias: -ve SiO2 p-type n-type Aluminium +ve

  31. Fig. 62 I / mA Characteristic for real device ______ 100 50 Characteristic predicted by the diode equation - - - • Characteristic becomes • more linear at large forward voltages due to resistance of p and n regions (R1 and R2). VBD -5 -10 Is 0.4 0.8 V / volts 2. Slope due to surface leakage. -50 -100 3. Breakdown occurs at VBD I / nA

  32. Diode Capacitance depletion region parallel plate capacitor Fig. 65(a) d W - + - - + + - - + + - - + + - - + + - - + + p n junction A = plate area; ε = permittivity of medium between the plates CJ is the “junction capacitance” K = a constant; VB = barrier potential V = applied bias (-ve for reverse bias) n = ½ for an abrupt junction

  33. Typically: 1pF < CJ < 1000pF The varactor or varicap diode is a diode designed specifically for use as a voltage-controlled capacitor: or Circuit symbol: Used in e.g. tuning circuits: {C = C'CJ/(C'+CJ)} C' Fig. 65(b): Tuning circuit incorporating varactor diode + - L V

  34. 5. Diode capacitance The capacitance of a Si pn diode depends on the reverse voltage, V, as shown in the table below. Determine K and n in the expression for CJ and calculate the value of V for CJ = 4pF.

  35. 5. Diode capacitance For Si, VB ≈ 0.7V, hence we can write down two equations for the two unknowns, n and K: Note ‘+’ signs here: V is the applied bias and for reverse bias, V is -ve. Dividing (2) by (1):

  36. To find K, put n = 0.39 in (1) or (2), e.g. Re-arranging for V:

  37. SUMMARY • STATIC & DYNAMIC RESISTANCE I / mA I / mA I / mA I / mA I / mA I / mA I / mA I / mA 5 5 5 5 5 5 5 5 V V V V 4 4 4 4 4 4 4 4 D I I I I 3 3 3 3 3 3 3 3 2 2 2 2 2 2 2 2 I D V V 1 1 V V 1 1 1 1 1 1 I I I I I I I I s s s s s s s s V / volts V / volts V / volts V / volts 0.8 0.8 V / volts V / volts 0.8 0.8 0.8 0.8 V / volts V / volts 0.4 0.4 0.4 0.4 0.8 0.8 0.4 0.4 0.4 0.4 - - 1 1 - - 1 1 - - 1 1 - - 1 1 - - 2 2 - - 2 2 - - 2 2 - - 2 2 - - 3 3 - - 3 3 - - 3 3 - - 3 3 - - 4 4 - - 4 4 - - 4 4 - - 4 4 - - 5 5 - - 5 5 - - 5 5 - - 5 5 I / mA I / mA I / mA I / mA I / mA I / mA I / mA I / mA

  38. REVERSE BREAKDOWN MECHANISMS • AVALANCHE BREAKDOWN – carriers drifting across depletion region collide with electrons in the bonds, knocking them out and creating more carriers. ZENER BREAKDOWN – electrons are pulled directly out of the bonds. Both mechanisms are reversible provided excessive device heating is avoided. • VBD depends on temperature – For avalanche breakdown: |VBD| as T For Zener breakdown: |VBD| as T

  39.  I-V CHARACTERISTIC OF A REAL DIODE The I-V characteristic for a real pn junction diode differs from that predicted by the diode equation in several respects:  It is more linear at high forward currents because of series resistance. • The reverse current is not constant but depends on the reverse voltage because of leakage around edges of the junction. • Breakdown occurs at VBD. Fig. 62

  40. DIODE CAPACITANCE  The diode has a capacitance CJ associated with it due to the space charge in the depletion region. CJ depends on the applied reverse bias, V, and the way the junction is made (through n ) •  Varactor diodes are specifically designed to act as voltage-controlled capacitors.

More Related