What is a diode

•  It is the first active electronic device  
• Diode is a two terminal active semi-conductor 
  
• Conducts only in forward biased condition 
• Mostly used for switching and rectification 
• The first semiconductor diodes, called cat's whisker diodes, developed around 1906

• Forward bias :-  +ve to Anode (A) and -ve to Cathode (K).

• Reverse bias :-   +ve to Cathode (K) and -ve to Anode (A).

• Forward voltage drop :- 0.7V.

• PIV :- Peak Inverse Voltage - ( Max. Voltage can be apply in reverse bias ).

• DC resistance : Vd / Id

• AC resistance : ⌂Vd / ⌂Id

Used for

•      Rectification
•      Switching 

Inventions related

• 1904 - Vac cum diode  - JA Fleming

• 1906 - Cat's whisker diode 
• 1938 - Shotkey diode ( hot carrier diode ) - Walter H Shotkey 
• 1958 - Tunnel diode -Leo Esaki
• 1963 - Gunn diode - Gunn

Check a diode

• Reverse bias   : High resistance ( more than 2000K typ ).
• Forward bias   : Low resistance ( 500R - 800R typ ).