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

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

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

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

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.

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.

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

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.

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

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 = 0.026 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

Chapter 3 Diode Circuits

Voltage Regulator

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

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

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 = 26.3 mA IZ = 18.7 mA II = 45 mA ANSWER: Part (b) PZ = IZ VZ IZ = 40 mA IL = 45 -40 = 5 mA  RL = 2 k

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

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

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 + 100 mA) 0.5 IZmax = (1.1) (0.1 IZmax + 100 mA) 0.5 IZmax = 0.11 Izmax + 110 0.39 IZmax = 110 IZmax = 282.05 mA

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

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 + 280 mA) 4.4 IZmax + 246.4 = (8.4) (0.1 IZmax + 280 mA) 4.4 IZmax + 246.4 = 0.84 Izmax + 2352 3.56 IZmax = 2105.6 IZmax = 591.46 mA PZmax = 5.6 x 591.46 = 3.312 W Ri = 8.4 / 647.46 = 13 