Diodes

1

• Diodes

2
Function

• Electrical gate
• Current only flows one way
• Forward biased
• Current flows
• Reverse biased
• Blocks current

3
I-V characteristic
Forward Bias
Reverse Bias
0.7V Switch-on
Reverse saturation current
Depletion region narrowing
Breakdown
4
pn construction

• Semiconductor material
• n-type
• Excess electrons
• p-type
• Excess holes
• Join together
• Depletion region
• Redistribution of charge carriers
• Contact potential
• 0.7V

5
pn construction

• p-type material
• n-type material
• Holes diffuse into the n-type and swallow
  electrons
• Electrons diffuse into the p-type and fill
  holes
• Depletion region formed
• No free charge carriers
• 0.7V contact potential

6
Forward bias

• Depletion region narrows as applied voltage
  approaches 0.7V
• Applied voltage above 0.7V
• depletion region is removed
• charge carriers can flow

7
Reverse bias

• Depletion region extends
• Higher voltage
• Breakdown
• Current flow

8
Using breakdown

• During fabrication
• Breakdown voltage determined
• Specific breakdown voltage
• Structure designed to tolerate reverse current
• Provides stable voltage reference
• ZENER Diode
• Reverse bias connection

9
The Zener circuit Voltage regulator

• Always Vin gt Vout
• Diode breaks down
• Constant Vout voltage
• Set by the Zener
• eg 5.1V
• R limits maximum available current

10
Power dissipation

• Current always flows through the Zener
• Maintains breakdown voltage
• Power will be dissipated
• Heating effect
• Power rating consideration

11
LED

• Light Emitting Diode
• Gives out light as current flows 20mA
• After switch on 0.7V
• I-V characteristic
• Semiconductor material
• Different materials
• Different colours of light
• Protection resistor

12
Protection resistor

• VLED _at_ 1.2V
• Ityp _at_ 20mA
• Vin _at_ 5V

13
Photodiode

• pn junction diode
• Detects light
• photo
• Operation
• Reverse biased in dark
• current at 1nA
• Incident light on junction
• EHP generation
• Large current
• FAST reaction times

14
www.search for...

• Diodes
• Photodiodes
• pn junction