Diodes

1. Diodes Construction operation Construction operation

2. Function Electrical �gate� Current only flows one way Forward biased Current flows Reverse biased Blocks current Note current flows in the way the arrow is pointingNote current flows in the way the arrow is pointing

3. I-V characteristic 4 regions of the diode I/V characteristic See excel model Note reverse saturation current exaggerated for display4 regions of the diode I/V characteristic See excel model Note reverse saturation current exaggerated for display

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 Doped substrate rather than two materials brought togetherDoped substrate rather than two materials brought together

5. 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 pn construction No free charge carriers in th depletion region so no current flows Thermal excitation of semiconductor lattice causes EHP generation within the depletion region that gives rise to the the reverse saturation currentNo free charge carriers in th depletion region so no current flows Thermal excitation of semiconductor lattice causes EHP generation within the depletion region that gives rise to the the reverse saturation current

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 Forward bias removes the depletion region so there are free charge carriers so current can flow when a pd is appliedForward bias removes the depletion region so there are free charge carriers so current can flow when a pd is applied

7. Reverse bias Depletion region extends Higher voltage Breakdown Current flow Depletion region extends Structural breakdownDepletion region extends Structural breakdown

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 Always reverse biasedAlways reverse biased

9. The Zener circuit: Voltage regulator Always Vin > Vout Diode �breaks down� Constant Vout voltage Set by the Zener eg 5.1V R limits maximum available current Vout across the Zener is constantVout across the Zener is constant

10. Power dissipation Current always flows through the Zener Maintains breakdown voltage Power will be dissipated Heating effect Power rating consideration Zener always has current flowing through itZener always has current flowing through it

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 Forward biased Gives out light Low current wrt bulb Used as indicators e.g. TV on standbyForward biased Gives out light Low current wrt bulb Used as indicators e.g. TV on standby

12. Protection resistor VLED @ 1.2V Ityp @ 20mA Vin @ 5V Some LEDs don�t need protection resistor�check data book. Must have Ityp for them to give out lightSome LEDs don�t need protection resistor�check data book. Must have Ityp for them to give out light

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 LED in reverse Set in circuit at slightly reverse biased pd. In light the depletion region is removed �conduction VERY fast response times Used in optical commsLED in reverse Set in circuit at slightly reverse biased pd. In light the depletion region is removed �conduction VERY fast response times Used in optical comms

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