Semiconductor Diode

DIODE : 


1)A diode is a one-way valve for electric current.



2)Diodes are a basic building block of all electronics and are used to control the direction of current flowing in circuits

Basic operation: 


Ideally it conducts current in only one direction

and acts like an open in the opposite direction

Operating Conditions:

• No Bias

• Forward Bias

• Reverse Bias

No Bias Condition:

No external voltage is applied: VD = 0V and no current is

flowing ID = 0A.

Forward Bias Condition:

The Forward bias voltage required for a :


• Silicon diode VT = 0.7V (proximately )

• Germanium diode VT = 0.3V (proximately )

Reverse Bias Condition:

Characteristics of an ideal diode:
Conduction Region

Look at the vertical line!
In the conduction region, ideally
            • the voltage across the diode is 0V,
            • the current is infinity ,
            • the forward resistance (R_F) is defined as R_F = V_F/I_F,
            • the diode acts like a short.

Equivalent Circuits:


Approximate model

Simplified Model

Ideal Model

Example:

Determine the Vf and If for the diode in Figure 1.2 for each of the diode model. Also find the voltage across the limiting resistor in each case. Asume rd=10 ohm and determined value of forward current.

Ideal diode   Practical Model   Complete Model

Vf=0V

If =10V/1k  

   =10mA

VRlimit=(10m)(1k)

         =10V

 Vf=0.7V

If=(10V – 0.7V)/1k

  =9.3/1k

  = 9.3mA

VRlimit=(9.3m)(1k)

         =9.3V

If=(10V – 0.7V)/1k+10 

  =9.3/1010

  = 9.21mA

Vf=0.7V+Ifrd

    =0.7 +(9.21m)(10)

    = 792mV

VRlimit=(If)(Rlimit)

         =9.21m(1k)

         =9.21V  

Sourch