When a voltage is applied to a diode, electrons flow from the N-type side through the depletion zone and into the P-type side if the diode is forward biased. This causes current to flow. If the voltage is reversed, the depletion zone widens and no current flows, making the diode act as an open switch. Diodes can be used as rectifiers to convert AC to DC or as switches that allow current in one direction but not the other depending on bias polarity.

2. Junction Diode
N Type P Type
extra electrons holes (missing electrons)
depletion zone

3. What happens when a voltage source
is applied to the 2 ends of the
diode?

4. • Electrons are pushed from the negative
supply into the N side of the diode.
• Holes are pushed (electrons are drained)
from the P side of the diode into the
positive side of the supply.

5. What happens when a voltage source
is applied to the 2 ends of the
diode?

6. As more and more charges build up,
easier for charges to move across the
depletion zone
Current flows as electrons flow from N type
through holes of P type out to positive source

7. Forward Biased
• To cross depletion zone, must apply
enough voltage (pressure or push) to get
electrons past negatively charged region
of depletion zone.
• Forward bias voltage about 0.7 V for
silicon diodes and 0.3 V for germanium
diodes.

8. What happens if the voltage
polarity is reversed?

9. What happens if the voltage
polarity is reversed?
depletion zone
widens

10. Reversed Bias
• When diodes are reverse biased, depletion
zone widens
• Harder for charges to cross depletion zone
• High enough voltage will force charges
across depletion zone
– Breakdown voltage = voltage level that causes
diodes to conduct when reverse biased
– Heat from large reverse current destroys diode

13. Diodes act as switches
• When forward biased, diode acts as a
short (very low resistance).
• When reverse biased, diode acts as an
open circuit (huge resistance).

14. Testing Diodes
• Multimeter with + lead to anode and
– lead to cathode should show low
resistance.
• Multimeter with + lead to cathode and
– lead to anode should show high
resistance.
anode cathode
(end current flows into) (end current flows out of)

17. Types of diodes
Rectifier diodes
– convert AC to DC, handles higher power
• 1N4001 to 1N4007 IF = 1A, PIV = 50V to 100V
Signal diodes or switching diodes
– faster response at high frequencies
• 1N4148 IF = 200mA, PIV = 100V
Zener diode
– holds voltage level steady
Schottky diode
– fast recovery time, low voltage, low noise