Peak Inverse Voltage

Pick inverse voltage(PIV)

The Peak inverse voltage (PIV) equals the peak value of the input voltage, and the diode must be capable of withstanding this amount of repetition reverse voltage. For the diode in figure, the maximum value of reverse voltage, designated as PIV, occurs at peak of each positive alternation of the input voltage when the diode is forward biased.
Or simply ~  Peak Inverse Voltage (PIV) or Peak Reverse Voltage (PRV) refer to the maximum voltage a diode or other device can withstand in the reverse-biased direction before breakdown. Also may be called Reverse Breakdown Voltage.

Importance of PIV

Typically this is the maximum reverse polarity voltage that can be applied to a diode before it starts to break down and conduct current.  Diodes always have a reverse leakage current, although usually very small for rectifier diodes (100nA - 500uA would be typical), so the reverse voltage specification is the voltage at which the diode conducts a defined amount of current in the reverse direction.

Why NAND and NOR gate is called universal gates

Best answers :~

•  NAND and NOR gates are called universal gates because they can be used in place of all basic gates that AND ,OR and NOT.
  They can be used to implement any circuitory. We can use only one type of universal gate to implement any circuit.
  
  Gates are used as switches.
  They use a set of output signals to produce the desired output signal.

• All other gates/functions can be implemented by NOR or NAND gates. so they are called universal gates. In fact, in chips, entire logic maybe built using only NAND (or NOR) gates. eg: inverter-- nand with inputs shorted. and ------ nand followed by a inverter(using nand). or--------- giving inverted inputs to nand gate. If you delve deep into realms of VLSI you maybe able to understand the reason .Implementing with NAND is easier when considering power and area of the chip.

• NAND and NOR gates are called universal gates because they can be used in place of all basic gates that AND ,OR and NOT. They are inexpensive to make and they can be used to make any circuit thoughcan use only one type of universal gate to make any circuit. 
  Gates are mainly switches which can be used to decide the output of any input.They can be used to get desired results by givin inputs.

Varactor Diode

Varactor Diode & its application

Varactor

A varactor diode uses a p-n junction in reverse bias and has a structure such that the capacitance of the diode varies with the reverse voltage. A voltage controlled capacitance is useful for tuning applications. The capacitance is controlled by the method of doping in the depletion layer. Typical values are from tens to hundreds of picofarads.

Application of Varactor Diode

Following are the varactor diode applications:
1.  It is used in variable resonant tank LC circuit. Here C part is varied using varactor diode.
2.   AFC(Automatic Frequency Control) where in varactor diode is used to set LO signal.
3.   Varactor is used as frequency modulator.
4.  It is used as frequency multiplier in microwave receiver LO. 5.  It is used as RF phase shifter

Difference between Conductor, Semiconductor and Insulator

                                                                                                    ~: gopalec201.blogspot.com :~

Formation of N - type semiconductor

Formation of N - type semiconductor

An N - type semiconductor is formed when a small amount of pentavalent impurity is added to a pure Germenium or Silicon crystal. The addition of pentavalent impurity produces a large no. of free electrons in the host crystal.
To explain the formation of N - type semiconductor, let us introduce a pentavalent impurity atom into the lattice of pure silicon crystal. The pentavalent atom has 5 valance electrons, but only 4 form covalent bonds with the neighbouring atoms. The 5th electron finds no place in the covalent bonding so becomes free. Since an impurity atom provides one free electron, an enormous increase occurs in the no. of free electrons. The impure semiconductor so obtained is then called as N - type semiconductor where N represents negative charge on an electron. Thus the majority carrier in N - type semiconductor are free electrons. Holes are also present in the N - type semiconductor. These are thermally generated and since they are relatively few, they are called minority carrier.
The pentavalent impurity atom are called donour because each donate a free electron to the host crystal.

Formation of P - type semiconductor

Formation of P-type semiconductor

A P - type semiconductor is formed when a small amount of trivalent impurity is added to pure Germenium or silicon atom crystal. The addition of trivalent impurity produces a large no. of holes tothe host crystals. To explain the formation of P - type semiconductor, let us introduce a trivalent impurity into the lattice of a pure silicon crystal. The trivalent atom has 3 valance electrons and form covalent bonds with neighbouring atoms. The 4th bond is incomplete . the trivalent atom then attracts an electron from an adjacent atom there by completing the 4th bond and forming a hole in the adjacent atom. Since a trivalent impurity atom provides 1 hole, an enormous increase occurs in the number of holes. The impure crystals so obtained is called P - type

semiconductor where P represents the positive charge on hole. Thus the majority carrier in a P - type semiconductor are holes. Free electrons are also present in the P - type semiconductor. These are thermally generated and since they relatively few, they are called minority carriers. The trivalent impurity atoms are called acceptors because each accepts an electron when the atom is introduced into the host crystal.