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

DIODE AC EQUIVALENT

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.

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:

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 Where, During AC analysis the diode is equivalent to a resistor, rd

IDQ VDQ + - rd id DC equivalent AC equivalent

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

vi vi

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

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 = 0.465 mA id = 9.97 sin t (µA)

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

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