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Lecture 2 Basic Electronics

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					                                               6/25/2012




                Basic Electronics



  Semiconductor Diodes


                           Dr. Abid Karim
                      a ka rimpk@iqra.edu.pk




           Semiconductor Diode
• A Simplified Physical Structure:




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               pn-Junction Formation
•   On the moment of junction contact  carriers diffuse and quick
    recombination of electrons and holes occurs
•   Then forms a depletion region (space charge region) and a built-in
    potential  blocks a further diffusion.
•   However, a little amount of carrier diffusion still occurs because of
    natural probability (which is related to the barrier height)
•   The diffusion current I D (or leakage current)




                Energy Band Diagram




    At the i nstant of junction formation            At equi librium




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   Diode Operating Conditions
A diode has three operating conditions
   • No bias
   • Forward bias
   • Reverse bias

No Bias Conditions
 • No external voltage is
   applied: VD = 0 V
 • No current is flowing: ID = 0 A
 • Only a modest depletion
   region exists




   Diode Operating Conditions
Reverse Bias
External voltage is applied across the p-n
junction in the opposite polarity of the p-
and n-type materials.



                            • The reverse voltage causes the
                              depletion region to widen.
                            • The electrons in the n-type material
                              are attracted toward the positive
                              terminal of the voltage source.
                            • The holes in the p-type material are
                              attracted toward the negative
                              terminal of the voltage source.




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    Diode Operating Conditions
• This would result in
   – The broadening of the
     depletion region
   – Strengthening of the
     built-in electric field
   – The barrier potential
     would increase and
     becomes Vbi + VR
   – Reduction in diffusion
     current




   Diode Operating Conditions
Forward Bias
External voltage is applied across the p-n
junction in the same polarity as the p-
and n-type materials.


                               • The forward voltage causes
                                 the depletion region to narrow.
                               • The electrons and holes are
                                 pushed toward the p-n
                                 junction.
                               • The electrons and holes have
                                 sufficient energy to cross the
                                 p-n junction.




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  Diode Operating Conditions
Forward Bias Voltage
The point at which the diode changes from no-bias
condition to forward-bias condition occurs when the
electrons and holes are given sufficient energy to
cross the p-n junction. This energy comes from the
external voltage applied across the diode.

    The forward bias voltage required for a

       • gallium arsenide diode 1.2 V
       • silicon diode 0.7 V
       • germanium diode 0.3 V




  Forward-bias and Reverse-bias
          Connections




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       Diode VI Characteristics
                               Shockley Eq.



Note the regions
for no bias,
reverse bias, and
forward bias
conditions


Carefully note the
scale for each of
these conditions




       Diode VI Characteristics
• Detailed view of I-V curve




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               Breakdown Region
 • The third region of the diode curve is the Breakdown
   Region
 • The term is different from the physical breakdown or
   damage since it is a completely reversible process
   whereas the physical breakdown is not
 • This breakdown does not damage the diode
 • The absolute value of reverse voltage where the reverse
   current becomes infinity is called breakdown voltage
   (VBR)
 • There are two types of breakdown effects
    – Avalanche breakdown
    – Zener breakdown




  Zener vs. Avalanche Breakdown




• Zener breakdown is a result of the large electric field
  inside the depletion region that breaks electrons or
  holes off their covalent bonds.
• Avalanche breakdown is a result of electrons or holes
  colliding with the fixed ions inside the depletion region




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Zener vs. Avalanche Breakdown
• Zener effect is a high field effect and typically
  appears at around 5 V, while the avalanche
  effect appears voltage level of more than 6 V.
  However, both effects are somewhat mixed-up
• In general, Vz < 5V and Vav > 7V




               Zener Diode

A diode operated in reverse
bias at the Zener voltage (VZ)
is called Zener Diode

Common Zener voltages are
between 1.8 V and 200 V




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     Light Emitting Diode (LED)
• An LED emits photons when it is forward biased



• These can be in the infrared or visible spectrum
  depending upon the bandgap of the material
                       1.24
                            ( m)
                        Eg
• The forward bias voltage is usually in the range
  of 1.5 V to 3.5 V




          Temperature Effects
As temperature increases it adds energy to the diode
• It reduces the required forward bias voltage for
  forward-bias conduction
• It increases the amount of reverse current in the
  reverse-bias condition
• It increases maximum reverse bias avalanche
  voltage
Germanium diodes are more sensitive to temperature
 variations than silicon or gallium arsenide diodes




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        Temperature Effects




 Diode Approximations or Diode
       Equivalent Models
• There are three diode approximation or
  diode equivalent models:
  • First approximation or Ideal diode model
  • Second approximation or Practical diode
    model
  • Third approximation or Complete diode
    model




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Diode First Approximation




Diode First Approximation
=>   VF   =   0V
=>   IF   =   VBIAS /RLIMIT
=>   IR   =   0A
=>   VR   =   VBIAS




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Diode Second Approximation




Diode Second Approximation

=>   VF        =   0.7 V       (S ilicon)

=>   VF        =   0.3 V       (Germanium)

=>   VBIAS     =   VF          +            VRLIMIT

=>   VRLIMIT   =   IF RLIMIT

=>   IF        =   VRLIMIT / RLIMIT

=>   IR        =   0A

=>   VR        =   VBIAS




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     Diode Third Approximation




     Diode Third Approximation
=>    VD   =   0.7 V    +         IF rd     = VF   + IF rd
=>    IF   =   (VBIAS + VF) / (RLIMIT + rd)

=>    IR   ≠   0A

=>    VR   +   VRLIMIT =          VBIAS

=>    VR   ~   VBIAS

=>    IR   =   VRLIMIT / RLIMIT




                                          ‘
                                          d




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                           Example
Determine the forward voltage and forward current for the
diode in the diagram for each of the diode models. Also find
the voltage across the limiting resistor in each case. Assume
r ’ d = 10 Ω at the determined value of forward current.

Ideal Model:
VF = 0 V, IF = 10 mA, VRLIMIT = 10 V

Practical Model:
VF = 0.7 V, IF = 9.3 mA, VRLIMIT = 9.3 V

Practical Model:
VF = 0.792 V, IF = 9.21 mA,
VRLIMIT = 9.21 V




                           Example
Determine the reverse voltage and reverse current
for the diode as shown, for each of the diode
models. Also find the voltage across the limiting
resistor in each case. Assume IR = 1 μA.




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              Resistance Levels
Semiconductors react differently to DC and AC
currents.
There are three types of resistance:

      • DC (static) resistance
      • AC (dynamic) resistance
      • Average AC resistance




        DC (Static) Resistance
• For a specific applied
  DC voltage VD, the
  diode has a specific
  current ID, and a
  specific resistance RD
                VD
         RD
                ID
• Adiode can be replaced
  with a fixed resistance
  along the diode curve
• DC resistance will
  decrease with
  increasing V and I.




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       AC (Dynamic) Resistance
    Suppose a AC signal is applied with DC to a diode



                                                    The derivative of a
                                                    function at a point is
                                                    equal to the slope of
                                                    the tangent line
                                                    drawn at that point.




    Dynamic or ac resistance is defined as
                            dVD        26 mV
                       rd
                            dI D          ID




       AC (Dynamic) Resistance
•    All the resistance levels determined thus far have been defined
     by the p-n junction and do not include the resistance of the
     semiconductor material itself (called body or bulk resistance)
     and the resistance introduced by the connection between the
     semiconductor material and the external metallic conductor
     (called contact resistance)
•    These additional resistance levels can be included in Eq. by
     adding resistance denoted by rB as
                                      26 mV
                            rd                 rB
                                        ID
•    rB ranges from a typical 0.1 for high power devices to 2 for
     low power, general purpose diodes. In some cases rB can be
     ignored
•    In reverse bias region
                                 rd




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       Average AC Resistance

             ΔVd
       rav          pt.to pt.
             ΔI d

    AC resistance can be
    calculated using the
    current and voltage
    values for two points
    on the diode
    characteristic curve




                    Diode Testing
Diode Checker
•    Many digital multimeters
     have a diode checking
     function
•    The diode should be tested
     out of circuit
     A normal diode exhibits its
     forward voltage:

• Gallium arsenide 1.43 V
• Silicon diode 0.7 V
• Germanium diode 0.3 V




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              Diode Testing
Ohmmeter
• An ohmmeter set on a low Ohms scale can be
  used to test a diode. The diode should be
  tested out of circuit.




              Diode Testing
Curve Tracer
A curve tracer displays the characteristic curve of
a diode in the test circuit. This curve can be
compared to the specifications of the diode from a
data sheet.




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Diode
Specification
Sheet




       Diode Specification Sheet
 Data about a diode is presented uniformly for many
 different diodes. This makes cross-matching of diodes for
 replacement or design easier.
  1. Forward Voltage (VF) at a specified current and temperature
  2. Maximum forward current (IF) at a specified temperature
  3. Reverse saturation current (IR) at a specified voltage and
     temperature
  4. Reverse voltage rating, PIV or PRV or VBR, at a specified
     temperature
  5. Maximum power dissipation at a specified temperature
  6. Capacitance levels
  7. Reverse recovery time, trr
  8. Operating temperature range




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Diode Symbol and Packaging




The anode is abbreviated and the cathode is abbreviated K




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Description: Lecture 2 Basic electronics