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1. CHAPTER – TWO
Semiconductor diode and applications
 Semiconductors
 Biasing & Operation of Diode
 Applications

2. Fundamentals of semiconductors
 Atoms having 4 valence electrons are called semiconductors like silicon & germanium.
 At absolute 0 T, all electrons stay bonded so behaving as insulator.
 At high T, many electrons gaining sufficient energy will break the bond and freely move to able conduct.
 Minimum energy to dislodge no of electrons is a fundamental property of conductivity called band-gap
energy ( Eg).
 i.e. high for Insulator, small for conductors & moderate for semiconductors

3.  Pure intrinsic semiconductor has equal electrons & holes.
 Addition of impurity (dopant) is possible to get extrinsic N & P type semiconductor.
 N-type is created by doping pentavalent donor atom such as phosphorus.
 Electrons are majority carriers while holes are minority carriers.
 P-type is created by doping trivalent acceptor atoms such as boron.
 Holes are majority carriers while electrons are minority carries.

4. PN junction diode & Operational characteristics
 P&N type are joined to form two-terminal component called diode that conduct current in only one
direction.

5.  Initially, free electrons begin to migrate across the junction to fill up holes.
 Such back & forth continues until a built-in depletion barrier prevent any more crossing.
 Eventually, equal & opposite charged ions will create an electric field with +ve N-side & -ve P-side.
 Normally, potential difference of 0.6 - 0.7v for Si & 0.3 - 0.35 v for Ge will exist.
 Providing external voltage supplied between those two ends can give extra energy needed to overcome
the barrier.
 It is known as biasing which determine the diode character.

7.  Forward biased : Both majority carriers are forced to move creating a current flow, ID.
 ID becomes large quickly as VD increases beyond cut-in value of 0.3v (ge) & 0.6v(si).

8.  Reverse Biased : Very small drift current (Is) furtherly increase when VR increases.


9.  Small increase beyond a certain value called reverse breakdown voltage, VBR, results a very large
value of Is can finally cause total failure or breakdown.
 So diode act like short to conducting & open to non-conducting character.

10.  Thus, created current is:
VD= voltage across diode
Is= reverse saturation current,
VT= thermal voltage (26mv)
Ƞ= ideality factor (1 for ge & 2 for si)

11. Diode Applications
i. Rectification
 Important part of power supply to convert bi-directional (AC) into unidirectional (DC) signal.
 Half-wave rectifier only utilize half cycle of input.
 During T/2 (0-t1), Vi(t) > 0, diode is ON and ideally V0=Vi ,
 During (t1-t2), Vi(t) < 0, diode is OFF & ID=V0=0

12.  The output is average DC value , Vdc = 0.318 Vm

13.  Full wave rectifier utilizes both cycle of input.
Center-Tap full-wave Rectifier:
 During +ve cycle, D1 on and D2 off so +Vo is available.
 During -ve cycle, D1 off and D2 on so +Vo is available.

14.  Note the +Vo due to current through load resistance has same direction during both cycles.

15. Advantages:
 Double efficiency than half wave rectifier.
 Higher output power
 No saturation because of opposite DC currents flow.
Disadvantages:
 Requires two diodes & special transformer.

16.  Full-wave bridge Rectifier: for +ve half cycle, D1 & D2=On, D3 & D4 remain OFF.

17.  For -ve half cycle, D1 & D2=Off, D3 & D4 remain On.

19.  For further use of more constant DC form, the pulsating output has to be smoothed by connecting
resistor & capacitor or inductor to filter out residual ripples.

20. Advantages of bridge rectifier :
 No special transformer.
 Higher utilization effectively used in both cycles.
 Smaller size
 Less ripple factor & double ripple frequency.
Disadvantages of bridge rectifier:
 Requires 4 diodes.
 Voltage drop across 2 diodes will reduce efficiency.

21. 2. Clipping and Clamping
 Wave shaping that either clip (limit) or clamp (shift) signal.
 Entire or some input portion above or below a given DC reference ( – or +) can be clipped.
 Series & parallel Clipper (limiter) circuits.

26.  Clamping can shift signals through adding or subtracting some offset level.
 Positive clamper shifts upward.

27.  Negative Clamper shifts downward.

29. Types of diodes
 Light Emitting Diode (LED): produce visible bandwidth emitted when current passes.
 It convert electrical signal into light emission of variety colors.
 used as lamp, indicators, displays (digital voltmeters).
 Photodiodes: produce current when exposed to photon energy falling over it.
 It work in helpful reverse bias conditions to convert light into electrical signal.
 used for many sensors detection, solar cells, instruments.
 Laser diode: coupling LED & photodiode known as opto-isolator which exhibit high sensitivity to
detect & convert light.
 used in CD drives, DVD players, optic devices, photo-detectors, optical fiber communications, digital
transmission.

31.  Zener Diodes : reversely operated to work in breakdown conditions & regulate a stable rating.
 used for power regulation, electronic stabilization of current and voltage.
 Schottky-barrier diode (SBD): has working voltage to perform at lower ranges(0.15 - 0.4).
 used in rectifier, digital fast switching.
 Varactor diode: provide voltage capacitance changed with preferred function.
 Used as variable capacitors in auto tuning of radio receivers.