EENG 3510 Chapter 3 Diodes

Chapter 3 Homework 3.2 (c & d), 3.3 , 3.9, 3.19, 3.23

3.1.1 Current-Voltage Characteristic diode circuit symbol i–v characteristic equivalent circuit in the reverse direction equivalent circuit in the forward direction

3.1.1 Current-Voltage Characteristic an external circuit to limit the forward current the reverse voltage

3.1.2 A Simple Application: The Rectifier

3.1.3 Another Application: Diode Logic Gates (In a positive-logic system) AND gate (in a positive-logic system) OR gate Y = A + B + C Y = ABC

Example 3.2 Find values of I and V

Example 3.2a Find values of I and V 1. Both diodes are conducting. Voltage at B is zero. ID2 = 10 V -0 V / 10 k = 1 mA I + ID2 = (0 – (-10) V ) / 5 k = 2 mA I + 1 = 2mA I = 1 mA 5. V = 0

Example 3.2b Find values of I and V Assume both diodes are conducting. VB = 0 ID2 = (10 V – 0 V) / 5k = 2 mA I + 2 = (0 – (-10)) V / 10 k I = - 1 A This is not correct. Assume D1 is off and D2 is on. ID2 = (10 V – (-10 V) )/ 15k = 1.33 mA V = VB = -10 V + (10 k X 1.33 mA) V = -10 V + 13.3 V = 3.3 V

Exercise 3.4a - Find: I & V I = 5 V / 2.5 K = 2 mA V V = 0, Why ?

Exercise 3.4b - Find: I & V I = 0 A, Why? V V = 5 V, Why ?

Exercise 3.4c - Find: I & V V I = 0 A, Why? V = 5 V, Why ?

Exercise 3.4d - Find: I & V V I = 5 V / 2.5 K = 2 mA V = 0, Why ?

Exercise 3.4e - Find: I & V I = 3 V / 1 K = 3 mA V = 3 V, Why ?

Exercise 3.4f - Find: I & V I = 4 V / 1 K = 4mA V = 1 V, Why ?

3.2 Terminal Characteristics of Junction Diodes

3.2 Terminal Characteristics of Junction Diodes

3.2.1 The Forward-Bias Region Is: Saturation current, in the order of 10-15A, doubles in value for every 5°C rise in temperature n = 1, 2: material and physical structure V = forward voltage

3.2.1 The Forward-Bias Region (cont.) Silicon diodes conduct when the forward voltage = 0.7 volts Germanium diodes conduct when the forward voltage = 0.3volts

Example Given: A forward biased diode, forward voltage drop is 0.7 V at 2 mA, n = 1 at 0.6 V Find : the current i2

3.2.2 The Reverse-Bias Region If |v|>> |VT|(25mV) i≅-Is (Saturation current)

3.2.3 The Breakdown Region If the power dissipated in the diode is limited to a “safe” level, the breakdown is normally not destructive VZK: Z →Zener, K →Knee

3. 3 Modeling the Diode Forward Characteristics 3. 3 3.3 Modeling the Diode Forward Characteristics 3.3.1 The Exponential Model Graphical Analysis

3. 3 Modeling the Diode Forward Characteristics 3. 3 3.3 Modeling the Diode Forward Characteristics 3.3.3 Iterative Analysis Using theExponential Model

3. 3 Modeling the Diode Forward Characteristics 3. 3 3.3 Modeling the Diode Forward Characteristics 3.3.5 The Piecewise-Linear Model

3. 3 Modeling the Diode Forward Characteristics 3. 3 3.3 Modeling the Diode Forward Characteristics 3.3.5 The Piecewise-Linear Model (cont.) Piecewise-linear model of the diode forward characteristic and its equivalent circuit representation

3. 3 Modeling the Diode Forward Characteristics 3. 3 3.3 Modeling the Diode Forward Characteristics 3.3.6 The Constant-Voltage-Drop Model Development of the constant-voltage-drop model of the diode forward characteristics. A vertical straight line (B) is used to approximate the fast-rising exponential. Observe that this simple model predicts VD to within 0.1 V over the current range of 0.1 mA to 10 mA.

3. 3 Modeling the Diode Forward Characteristics 3. 3 3.3 Modeling the Diode Forward Characteristics 3.3.6 The Constant-Voltage-Drop Model The constant-voltage-drop model of the diode forward characteristics and its equivalent-circuit representation.

3. 3 Modeling the Diode Forward Characteristics 3. 3 3.3 Modeling the Diode Forward Characteristics 3.3.9 Use of the Diode Forward Drop in Voltage Regulation A voltage regulator is a circuit whose purpose is to provide a constant dc voltage between its output terminals The output voltage is required to remain as constant as possible in spite of Changes in the load current drawn from the regulator output terminal Changes in the dc power-supply voltage that feeds the regulator circuit

3. 3 Modeling the Diode Forward Characteristics 3. 3 3.3 Modeling the Diode Forward Characteristics 3.3.9 Use of the Diode Forward Drop in Voltage Regulation

3. 4 Operation in the Reverse Breakdown Region –Zener Diodes 3. 4 3.4 Operation in the Reverse Breakdown Region –Zener Diodes 3.4.1 Specifying and Modeling the Zener Diode Circuit symbol for a zener diode.

3. 4 Operation in the Reverse Breakdown Region –Zener Diodes 3. 4 3.4 Operation in the Reverse Breakdown Region –Zener Diodes 3.4.4 A Final Remark In recent years, zener diodes are replaced in voltage-regulator design by specially designed integrated circuits (ICs) that perform the voltage regulation function much more effectively and with greater flexibility than zener diodes.

3.5 Rectifier Circuits 120(N2/N1) V Coils wound around an iron core Remove pulsation Remove ripple

3.5.1 The Half-Wave Rectifier Transfer characteristic of the rectifier circuit Equivalent circuit of the half-wave rectifier with the diode replaced with its battery-plus-resistance model. Input and output waveforms, assuming that rD ! R.

3.5.1 The Half-Wave Rectifier (cont.) Two important parameters: 1) Current-handling capability: the largest current the diode is expected to conduct 2) Peak inverse voltage (PIV): the diode must be able to withstand without break

3.5.2 The Full-Wave Rectifier Full-wave rectifier utilizing a transformer with a center-tapped secondary winding transfer characteristic assuming a constant-voltage-drop model for the diodes; PIV = 2Vs-VD input and output waveforms

3.5.3 The Bridge Rectifier Most Popular Rectifier Circuit Configuration The bridge rectifier input and output waveforms

3.5.4 The Rectifier with a Filter Capacitor The Peak Rectifier A simple circuit used to illustrate the effect of a filter capacitor. Note that the circuit provides a dc voltage equal to the peak of the input sine wave. The circuit is therefore known as a peak rectifier or a peak detector. Input and output waveforms assuming an ideal diode.

3.5.4 The Rectifier with a Filter Capacitor The Peak Rectifier

3.5.4 The Rectifier with a Filter Capacitor The Peak Rectifier Waveforms in the full-wave peak rectifier

3.7.1 Basic Semiconductor Concepts Simplified physical structure of the junction diode. (Actual geometries are given in Appendix A.)

3.7.1 Basic Semiconductor Concepts (cont.) Two-dimensional representation of the silicon crystal. The circles represent the inner core of silicon atoms, with +4 indicating its positive charge of +4q, which is neutralized by the charge of the four valence electrons. Observe how the covalent bonds are formed by sharing of the valence electrons. At 0 K, all bonds are intact and no free electrons are available for current conduction.

3.7.1 Basic Semiconductor Concepts (cont.) At room temperature, some of the covalent bonds are broken by thermal ionization. Each broken bond gives rise to a free electron and a hole, both of which become available for current conduction.

3.7.1 Basic Semiconductor Concepts (cont.) The concentration of free electrons n, and the concentration of holes p

3.7.1 Basic Semiconductor Concepts (cont.) Diffusion: electrons (holes) will diffuse from the region of high concentration to the region of low concentration Drift:when an electric field is applied across a piece of silicon, free electrons and holes are accelerated by the electric field E. The positively charged holeswill drift in the direction of E, while the negatively charged electronswill drift in a direction oppositeto that of electric field.

3.7.1 Basic Semiconductor Concepts (cont.) Doping of a silicon crystal to turn it into n type or p type is achieved by introducing a small number of impurity atoms. Ex: phosphorus A silicon crystal doped by a pentavalent element. Each dopant atom donates a free electron and is thus called a donor. The doped semiconductor becomes n type.

3.7.1 Basic Semiconductor Concepts (cont.) Doping of a silicon crystal to turn it into n type or p type is achieved by introducing a small number of impurity atoms. Ex: boron A silicon crystal doped with a trivalent impurity. Each dopant atom gives rise to a hole, and the semiconductor becomes p type.

3.7.2 The pn Junction Under Open-Circuit Conditions Equilibium: Is= ID, Maintained by the barrier voltage V0 (a) The pn junction with no applied voltage (open-circuited terminals). (b) The potential distribution along an axis perpendicular to the junction.

3.7.2 The pn Junction Under Open-Circuit Conditions Barrier Voltage:

3.7.2 The pn Junction Under Open-Circuit Conditions Width of the Depletion Region

3.7.3 The pn Junction Under Reverse-Bias Conditions Anode Cathode Depletion width: increases Barrier voltage v0: increase I = IS-ID The pn junction excited by a constant-current source I in the reverse direction. To avoid breakdown, I is kept smaller than IS. Note that the depletion layer widens and the barrier voltage increases by VR volts, which appears between the terminals as a reverse voltage.

3.7.3 The pn Junction Under Reverse-Bias Conditions Anode Cathode Depletion width: decrease Barrier voltage v0: decrease I = ID -IS The pn junction excited by a constant-current source supplying a current I in the forward direction. The depletion layer narrows and the barrier voltage decreases by V volts, which appears as an external voltage in the forward direction.