The document discusses semiconductor diodes, their biasing, operation, and various applications. It covers the fundamentals of semiconductors, the characteristics of pn junction diodes, and different types of diodes including rectifiers and specialty diodes like LEDs and Zener diodes. Additionally, it highlights the advantages and disadvantages of different rectifier types and the basic principles of clipping and clamping in signal processing.

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.