1. Recall-Lecture 6 Diode AC equivalent circuit – small signal analysis
During AC analysis the diode is equivalent to a resistor, rd
IDQ
VDQ
+ - rd id
DC equivalent
AC equivalent

2. CALCULATE DC CURRENT, ID CALCULATE AC CURRENT, id
DC ANALYSIS
AC ANALYSIS
DIODE = MODEL 1 ,2 OR 3
CALCULATE rd
DIODE = RESISTOR, rd
CALCULATE DC CURRENT, ID
CALCULATE AC CURRENT, id

3. Other Types of Diodes Photodiode
Solar Cell
The term ‘photo’ means light. Hence, a photodiode converts optical energy into electrical energy. The photon energy breaks covalent bond inside the crystal and generate electron and hole pairs
Solar cell converts visible light into electrical energy. The working principle is the same as photodiode but it is more towards PROVIDING the power supply for external uses

4. Schottky Barrier Diode
Light Emitting Diode
An LED is opposite of photodiode this time, it converts electrical energy into light energy – Normally GaAs is used as the material for LED. During diffusion of carriers – some of them recombines and the recombination emits light waves.
A Schottky Barrier diode is a metal semiconductor junction diode. The metal side is the anode while the n-type is the cathode. But the turn on voltage for Schottky is normally smaller than normal pn junction diode

5. Breakdown Voltage
The breakdown voltage is a function of the doping concentrations in the n- and p-region of the pn junction.
Large doping concentrations result in smaller break-down voltage.
Reverse biased voltage – ET 
The electric field may become large enough for the covalent bond to break, causing electron-hole pairs to be created.
So, electrons from p-type are swept to n-region by the electric field and holes from the n-type are swept to the p-region
The movement will create reverse biased current known as the Zener Effect.

6. Zener Effect and Zener Diode
The applied reverse biased voltage cannot increase without limit since at some point breakdown occurs causing current to increase rapidly.
The voltage at that point is known as the breakdown voltage, VZ
Diodes are fabricated with a specifically design breakdown voltage and are designed to operate in the breakdown region are called Zener diodes. Circuit symbol of the Zener diode:
NOTE: When a Zener diode is reverse-biased, it acts at the breakdown region, when it is forward biased, it acts like a normal PN junction diode
Such a diode can be used as a constant-voltage reference in a circuit.
The large current that may exist at breakdown can cause heating effects and catastrophic failure of the diode due to the large power dissipated in the device.
Diodes can be operated in the breakdown region by limiting the current to a value within the capacities of the device.

7. Avalanche Effect
While these carriers crossing the space-charge region, they also gain enough kinetic energy.
Hence, during collision with other atoms, covalent bond is broken and more electron-holes pairs are created, and they contribute to the collision process as well. Refer to figure below
Electron with high kinetic energy e
atom h e
atom h e e
atom h

9. Basis For Comparison
PN Junction Diode
Zener Diode
Definition
It is a semiconductor diode which conducts only in one direction, i.e., in forward direction.
The diode which allows the current to flow in both the direction i.e., forward and reverse, such type of diode is known as the Zener diode.
Symbol
Reverse Current Effect
Damage the junction.
Do not damage the junction.
Doping Level
Low
High
Breakdown
Occurs in higher voltage.
Occur in lower voltage.

10. Zener Diode
10 k
Calculate the value of the current ID if VZ = 10V
ANSWER: ID = 0.2 mA

11. CHAPTER 2 Forward Biased, DC Analysis AC Analysis Reverse Biased
Model 1
V = 0
Model 2 V
Model 3
V and rf
Load Line  ID vs VD
At 300K VT = V
Forward Biased, DC Analysis
AC Analysis
Thermal equilibrium, depletion region
Reverse Biased
Must perform DC Analysis first to get DC diode current, ID
PN junction
Group 5
N-type
P-type
Group 3
Calculate
rd = VT / ID
Insulator
Conductor
Semiconductor
Extrinsic
Semiconductor: Group 4 eg. Silicon and Germanium
photodiode
Intrinsic
Other types of diode
Solar cells
Materials
Bandgap Energy
LED
CHAPTER 2
Zener Diode

12. Chapter 3 Diode Circuits

13. Voltage Regulator

14. A voltage regulator supplies constant voltage to a load.
Voltage Regulator - Zener Diode
A voltage regulator supplies constant voltage to a load.

15. The breakdown voltage of a Zener diode is nearly constant over a wide range of reverse-bias currents.
This make the Zener diode useful in a voltage regulator, or a constant-voltage reference circuit.
3. The remainder of VPS drops across Ri
2. The load resistor sees a constant voltage regardless of the current
1. The Zener diode holds the voltage constant regardless of the current

16. Example
A Zener diode is connected in a voltage regulator circuit. It is given that VPS = 20V, the Zener voltage, VZ = 10V, Ri = 222  and PZ(max) = 400 mW.
Determine the values of IL, IZ and II if RL = 380 .
Determine the value of RL that will establish PZ(max) = 400 mW in the diode.
ANSWER:
Part (a)
IL = mA
IZ = 18.7 mA
II = 45 mA
ANSWER:
Part (b)
PZ = IZ VZ
IZ = 40 mA
IL = = 5 mA
 RL = 2 k

17. For proper function the circuit must satisfied the following conditions.
The power dissipation in the Zener diode is less than the rated value
When the power supply is a minimum, VPS(min), there must be minimum current in the Zener diode IZ(min), hence the load current is a maximum, IL(max),
When the power supply is a maximum, VPS(max), the current in the diode is a maximum, IZ(max), hence the load current is a minimum, IL(min)
AND
Or, we can write

18. Considering designing this circuit by substituting IZ(min) = 0
Considering designing this circuit by substituting IZ(min) = 0.1 IZ(max), now the last Equation becomes:
Maximum power dispassion in the Zener diode is
EXAMPLE 1
Consider voltage regulator is used to power the cell phone at 2.5 V from the lithium ion battery, which voltage may vary between 3 and 3.6 V. The current in the phone will vary 0 (off) to 100 mA(when talking). Calculate the value of Ri and the Zener diode power dissipation

19. Solution:
The stabilized voltage VL = 2.5 V, so the Zener diode voltage must be VZ = 2.5 V. The maximum Zener diode current is
The maximum power dispassion in the Zener diode is
The value of the current limiting resistance is
IZ(min) = 0.1 IZ(max),
(3 – 2.5) (IZmax + 0) = (3.6 – 2.5) (0.1 IZmax mA)
0.5 IZmax = (1.1) (0.1 IZmax mA)
0.5 IZmax = Izmax
0.39 IZmax = 110
IZmax = mA

20. Example 2 Range of VPS : 10V– 14V RL = 20 – 100  VZ = 5.6V
Find value of Ri and calculate the maximum power rating of the diode

21. Solution:
The stabilized voltage VL = 2.5 V, so the Zener diode voltage must be VZ = 2.5 V. The maximum Zener diode current is
The maximum power dispassion in the Zener diode is
The value of the current limiting resistance is
IZ(min) = 0.1 IZ(max),
(10 – 5.6) (IZmax + 56 mA) = (14 – 5.6) (0.1 IZmax mA)
4.4 IZmax = (8.4) (0.1 IZmax mA)
4.4 IZmax = Izmax
3.56 IZmax =
IZmax = mA
PZmax = x = W
Ri = / = 13 