Chapter 3-1 Semiconductor Devices Diode

MEMS1082

Mechatronics Chapter 3-1 Semiconductor devices Diode

Department of Mechanical Engineering Semiconductor: Si

Department of Mechanical Engineering Semiconductor

Department of Mechanical Engineering N-type and P-type Semiconductors

There are two types of impurities: N-type - In N-type doping, phosphorus or arsenic is added to the silicon in small quantities. Phosphorus and arsenic each have five outer electrons, so they're out of place when they get into the silicon lattice. The fifth electron has nothing to bond to, so it's free to move around. It takes only a very small quantity of the impurity to create enough free electrons to allow an electric current to flow through the silicon. N-type silicon is a good conductor. Electrons have a negative charge, hence the name N-type. P-type - In P-type doping, boron or gallium is the dopant. Boron and gallium each have only three outer electrons. When mixed into the silicon lattice, they form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbor, moving the hole over a space. P-type silicon is a good conductor. Department of Mechanical Engineering N-type and P-type Semiconductors

Department of Mechanical Engineering Semiconductor device-diode

A diode is the simplest possible semiconductor device. A diode allows current to flow in one direction but not the other. You may have seen turnstiles at a stadium or a subway station that let people go through in only one direction. A diode is a one- way turnstile for electrons.

When you put N-type and P-type silicon together as shown in this diagram, you get a very interesting phenomenon that gives a diode its unique properties.

Department of Mechanical Engineering Diodes

Department of Mechanical Engineering Diode

Electron flow direction

Current direction

Department of Mechanical Engineering Diode depletion region

Department of Mechanical Engineering pn junction

 PN Junction

Department of Mechanical Engineering Diode depletion region

Department of Mechanical Engineering Diode forward and reverse bias

Department of Mechanical Engineering Shockley diode equation

Department of Mechanical Engineering Diode current and voltage

Department of Mechanical Engineering Diode Characteristic

Department of Mechanical Engineering Diode Characteristic at different scale

Department of Mechanical Engineering Diode measurement

 Meter with a “Diode check” function displays the forward voltage drop of 0.548 volts instead of a low resistance

Department of Mechanical Engineering Measurement of a diode

Measuring forward voltage of a diode without “diode check” meter function: (a) Schematic diagram. (b) Pictorial diagram

Department of Mechanical Engineering Load line of diode

 A circuit with a diode

Department of Mechanical Engineering Example

 For circuit, determine the current i

Department of Mechanical Engineering Example

 Circuit reduction to Thévenin equivalent circuit

Department of Mechanical Engineering Example

 Thévenin equivalent circuit

Department of Mechanical Engineering Example

 Draw load line to determine the diode voltage and current

Department of Mechanical Engineering Example

 Determine current i

Department of Mechanical Engineering Example

 Determine the current and voltage of the diode in the circuit. The diode characteristic is given in the right figure.

Department of Mechanical Engineering Example

Department of Mechanical Engineering Piecewise-linear approximation and small signal analysis

 Diode is nonlinear resistor

Department of Mechanical Engineering Piecewise-linear approximation and small signal analysis

 Diode piecewise-linear approximation

Department of Mechanical Engineering Piecewise-linear approximation and small signal analysis

 Small signal analysis

Department of Mechanical Engineering Piecewise-linear approximation and small signal analysis

 Small signal analysis

Department of Mechanical Engineering Piecewise-linear approximation and small signal analysis

 If we are only interested in the portion due to vs(t), we may set Es=0, and Ef =0, then

 Often, for practical purpose, we can assume Ef =0 in small signal equivalent circuit of a diode. For typical diodes, the

value of Rf is quite small, between 1Ω and 100Ω. Thus Rf can be neglected. Department of Mechanical Engineering Piecewise-linear approximation and small signal analysis

Department of Mechanical Engineering The ideal diodes

Department of Mechanical Engineering The piecewise- linear model of a diode, using an ideal diode

Ideal diode

Department of Mechanical Engineering Example

 Nonlinear resistors with a wide range of characteristics can be obtained, approximately, with circuit containing diodes, for example, a square-law device is two-terminal nonlinear resistor whose terminal voltage-current characteristic obey i = kv2 where k is normalization constant. The ideal characteristic is shown

Department of Mechanical Engineering Example

 This device may be used in modulator, e.g., to attain a voice signal to high-frequency carrier wave, as is done in amplitude modulation (AM) radio transmission. Design a square-law device to approximate the ideal characteristics for 0 ≤ v ≤ 5V with a normalization constant k=0.001

Department of Mechanical Engineering Example

 A circuit using ideal diodes D1 and D2 and voltage sources E1 and E2

 Use V=5V; E1 < E2

 Initially 0≤v≤ E1,the diodes are reverse biased and open, the curve will have

slope 1/R3

 For E1 ≤v≤ E2,D1 closes, and D2 open, the input resistance will be R3llR1

 For E2 ≤v≤ 5V,D1 and D2 close, the input resistance will be R3llR1llR2

 Suppose E1 =2.0V and E2=3.5V

2 I1 = kE1 = 4mA 2 I2 = kE2 =12.25mA

2 I = kV = 25mA Department of Mechanical Engineering Example

 Noting the slope of each portion, we obtain E E − E R = 1 = 500Ω R R = 2 1 =182Ω R = 286Ω 3 1 2 − 1 I1 I2 I1

V − E2 R1 R2 R3 = =118Ω R2 = 333Ω I − I2

 Replacing the actual diode with their piecewise-linear approximation using

R f =10Ω, E f = 0.5V

R1 = 276Ω R2 = 323Ω R3 = 500Ω

E1 =1.5V and E2=3.0V Department of Mechanical Engineering Ideal transformer

Department of Mechanical Engineering Rectifiers

 Half-Wave Rectifier  The transformer isolates the load from the source

Department of Mechanical Engineering Rectifiers

 Half-Wave Rectifier

The average dc value of vL vL = Vs sinωt 0 ≤ ωt ≤ π π = π ≤ ω ≤ π 1 vL 0 t 2 VL = Vs sinωt d(ωt) 2π ∫0 V = s π

Department of Mechanical Engineering Rectifiers

 Representing the Half-Wave Rectifier voltage by Fourier series

vL = VL + a1 sinωt + a2 sin 2ωt +...... + b1 cosωt + b2 cos 2ωt +...... The Fourier coefficients can be determined as 2 T 2 T an = vL (t)sin nωt dt; bn = vL (t)cos nωt dt T ∫0 T ∫0 For the Half-Wave Rectified voltage

T π 2 1 Vs a1 = vL (t)sinωt dt = Vs sinωt sinωt d(ωt) = T ∫0 π ∫0 2 2 T 1 π an = vL (t)sin nωt dt = Vs sinωt sin nωt d(ωt) = 0 T ∫0 π ∫0

Department of Mechanical Engineering Rectifiers

2V 2V b = 0; b = − s , b = 0; b = − s ; b = 0 1 2 3π 3 4 15π 5

Thus the Fourier series for the Half-Wave Rectified signal

V V 2V 2V v (t) = s + s sinωt − s cos 2ωt − s cos 4ωt +..... L π 2 3π 15π

Department of Mechanical Engineering Rectifiers

 Filtering the Half-Wave Rectifier

Capacitor has lower impedance to higher frequencies

Department of Mechanical Engineering Rectifiers

 Filtering the Half-Wave Rectifier

Larger C can be used to increase the time constant RC

Department of Mechanical Engineering Rectifiers

 Effects of actual diodes

Department of Mechanical Engineering Rectifiers

 Effects of actual diodes

Department of Mechanical Engineering The Full-Wave Rectifiers

 The full-wave rectifier

Department of Mechanical Engineering The Full-Wave Rectifiers

 The full-wave rectifier

The average dc value of vL 1 π VL = Vs sinωt d(ωt) π ∫0 2V = s π

Thus the Fourier series for the Full-Wave Rectified signal 2V 4V 4V v (t) = s − s cos 2ωt − s cos 4ωt +..... L π 3π 15π

Department of Mechanical Engineering The Full-Wave Rectifiers

 Effect of actual diodes

Department of Mechanical Engineering The Full-Wave Bridge Rectifier

 A bridge rectifier makes use of four diodes in a bridge arrangement to achieve full-wave rectification. This is a widely used configuration, both with individual diodes wired as shown and with single component bridges where the diode bridge is wired internally.

Department of Mechanical Engineering Bridge Rectifiers

Various types of Bridge Rectifiers Note that some have a hole through their centre for attaching to a heat sink

Department of Mechanical Engineering The Full-Wave Bridge Rectifier

 Bridge Rectifier

Department of Mechanical Engineering The Full-Wave Bridge Rectifier

 Bridge Rectifier with RC Filter and LC filter

Department of Mechanical Engineering The Voltage Limiter

 Limiter using ideal diodes and batteries

Department of Mechanical Engineering The Voltage Limiter

 Limiter using ideal diodes and batteries

Department of Mechanical Engineering The Voltage Limiter

 Limiter using ideal diode and batteries

Department of Mechanical Engineering The Voltage Limiter

 Limiter using ideal diode and batteries Load voltage is limited for source voltage

RL + Rs RL + Rs − V2 < vs (t) < V1 RL RL

Department of Mechanical Engineering The Voltage Limiter

 Limiter using ideal diode and batteries

Department of Mechanical Engineering Example

 For a limiter shown below, assume identical piecewise-

linear diodes with Rf=100Ω, Ef=0.5V, V1=V2=10V, RL=100Ω, Rs=100Ω, and vs(t)=50sinωt V, sketch vL(t)

Department of Mechanical Engineering Zener Diodes

 A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener, who discovered this electrical property.

Department of Mechanical Engineering Zener Diodes

 Device characteristic  Piecewise-linear of Zener diode characteristic

Department of Mechanical Engineering Zener Diodes

 Piecewise-linear model

Department of Mechanical Engineering Zener Diode Regulator

 In this circuit, a typical voltage reference or regulator, an input voltage,

UIN, is regulated down to a stable output voltage UOUT. The intrinsic voltage drop of diode D is stable over a

wide current range and holds UOUT relatively constant even though the input voltage may fluctuate over a fairly wide range. Because of the low impedance of the diode when operated

like this, Resistor R is used to limit IDiode = (UIN - UOUT) / R current through the circuit.

Department of Mechanical Engineering Zener Diode Regulator

 R must be small enough that the current through D keeps D in reverse breakdown. The value of this current is given in the data sheet for D. For example, the common BZX79C5V6 device, a 5.6 V 0.5 W Zener diode, has a recommended reverse current of 5 mA. If insufficient current exists through D, then

UOUT will be unregulated, and less than the nominal breakdown voltage. When calculating R, allowance must be made for any current through the external load, not shown in this diagram, connected across

UOUT.

 R must be large enough that the current through D does not destroy the device. If the current through D is ID, its breakdown voltage VB and its maximum power dissipation P , then I V < P . MAX D B MAX Department of Mechanical Engineering Zener Diode regulator

− Vs,max Vz Pmax Vz Imax = = + Rs + Rmin Vz RL

− Vs,min Vz Vz Imin = = Rs + Rmax RL

Department of Mechanical Engineering Example

 A source voltage varies between 120V and 75V. The source resistance is zero, and the load resistance is 1kΩ. It is desired to maintain the load voltage at 60V. Determine the value of a regulator resistor R that will accomplish this and the required power rating of the zener.

1. A zener having a zener voltage of 60V is selected 2. The maximum value of regulator resistance V 60 V −V = z = = s,min z Imin 60mA Rmax = = 250Ω RL 1000 Imin

3. The power rating is determined when Vs=Vs,max. And zener draw the maximum current − Pmax Vs,max Vz Vz Imax = = − = 0.18A Vz R RL

Pmax =10.8W Department of Mechanical Engineering Light Emitting Diode

Department of Mechanical Engineering Light Emitting Diode

An LED will begin to emit light when the on-voltage is exceeded. Typical on voltages are 2–3 volts

Department of Mechanical Engineering Connect Light Emitting Diode in Series

Connecting LEDs in series If you wish to have several LEDs on at the same time it may be possible to connect them in series. This prolongs battery life by lighting several LEDs with the same current as just one LED. All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V for each LED (4V for blue and white) plus at least another 2V for the resistor. To work out a value for the resistor you must add up all the LED voltages and use this for VL.

Example calculations: A red, a yellow and a green LED in series need a supply voltage of at least 3 × 2V + 2V = 8V, so a 9V battery would be ideal. VL = 2V + 2V + 2V = 6V (the three LED voltages added up). If the supply voltage VS is 9V and the current I must be 15mA = 0.015A, Resistor R = (VS - VL) / I = (9 - 6) / 0.015 = 3 / 0.015 = 200, so choose R = 220 (the nearest standard value which is greater).

Department of Mechanical Engineering