1. Recall-Lecture 5 DC Analysis Representation of diode into three models
Ideal case – model 1 with V = 0
Piecewise linear model 2 with V has a constant value
Piecewise linear model 3 with V and forward resistance, rf

2. DIODE AC EQUIVALENT

3. Sinusoidal Analysis The total input voltage vI = dc VPS + ac vi
iD = IDQ + id
vD = VDQ + vd
IDQ and VDQ are the DC diode current
and voltage respectively.

4. The DC diode current IDQ in term of diode voltage VDQ
Total voltage
Total current
VDQ = DC voltage
vd = ac component
The DC diode current IDQ in term of diode voltage VDQ
If vd << VT , the equation can be expanded into linear series as:

5. During AC analysis the diode is equivalent to a resistor, rd
Therefore, the diode current-voltage relationship can be represented as
The relationship between the AC components of the diode voltage and diode current is
Or,
Where,
During AC analysis the diode is equivalent to a resistor, rd

6. IDQ
VDQ = V
+ - rd id
DC equivalent
AC equivalent

7. Example 1 DC Current DC Output voltage
Analyze the circuit (by determining VO & vo ).
Assume circuit and diode parameters of
VPS = 10 V, R = 5 kΩ, Vγ = 0.6 V & vi = 0.2 sin ωt
DC Current
DC Output voltage

8. vi vi

9. 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

10. EXAMPLE 1 VPS = 10V, R = 20k, V = 0.7V, and vi = 0.2 sin t (V).
Assume the circuit and diode parameters for the circuit below are
VPS = 10V, R = 20k, V = 0.7V, and vi = 0.2 sin t (V).
Determine the current, IDQ and the time varying current, id
ANSWERS
IDQ = mA
Id = 9.97 sin t (µA)

11. 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

12. 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

13. 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.

14. 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.

15. 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

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