Lecture (07) Optical Diodes Bipolar Junction...

By:

Dr. Ahmed ElShafee

Dr. Ahmed ElShafee, ACU : Fall 2019, Electronic Circuits I1

Lecture (07)Optical DiodesBipolar Junction

Transistor

Zener No load and Full load

• The zener diode maintains a nearly constant voltage across RLas long as the zener current is greater than IZK and less than IZM.

Dr. Ahmed ElShafee, ACU : Fall 2019, Electronic Circuits2

• In no load, RL = inf, all current go through zener diode

• With when RL is connected, current is divided between load and zener diode, while load voltage still constant because of zener regulating action.


Example 01

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Optical diodes

The Light‐Emitting Diode (LED)

• The difference in energy between the electrons and the holes corresponds to the energy of visible light.

• When recombination takes place, the recombining electrons release energy in the form of photons. Dr. Ahmed ElShafee, ACU : Fall 2019, Electronic Circuits6

• emitted light tends to be monochromatic (one color) that depends on the band gap (and other factors) (Various impurities are added during the doping process to establish the wavelength of the emitted light).

• A large exposed surface area on one layer of the semiconductive material permits the photons to be emitted as visible light


LED Semiconductor Materials

• IR : gallium arsenide (GaAs)

• Visible (orange pal green) : gallium arsenide phosphide (GaAsP)

• Visible (red, green, yellow) : gallium aluminum arsenide phosphide (GaAlAsP).

• Blue : silicon carbide (SiC)

• Ultra blue : gallium nitride (GaN)


LED Biasing

• the maximum VF for LEDs is between 1.2 V and 3.2 V, depending on the material.

• Reverse breakdown for an LED is much less than for a silicon rectifier diode (3 V to 10 V is typical).


• The amount of power output translated into light is directly proportional to the forward current, and temperature


Light Emission

• An LED emits light over a specified range of wavelengths as indicated by the spectral output curves


• radiation patterns for small LEDs

• LEDs are directional light sources (unlike filament or fluorescent bulbs).

• The radiation pattern is generally perpendicular to the emitting surface;


• bright LEDs are becoming popular for lighting because of their superior efficiency and long life


Optical diodes applications

• Standard LEDs are used for indicator lamps and readout displays on a wide variety of instruments,

• A common type of display device using LEDs is the seven‐segment display.


• common application of an infrared LED is in remote control units for TV, DVD, gate openers, etc.

• The IR LED sends out a beam of invisible light that is sensed by the receiver in your TV, for example.


• For each button on the remote control unit, there is a unique code.

• When a specific button is pressed, a coded electrical signal is generated that goes to the LED, which converts the electrical signal to a coded infrared light signal.

• The TV receiver recognizes the code and takes appropriate action, such as changing the channel or increasing the volume


High‐Intensity LEDs

• LEDs that produce much greater light outputs than standard LEDs are found in many applications including traffic lights, automotive lighting, indoor and outdoor advertising and informational signs, and home lighting.


• LEDs in an array are usually connected either in a series‐parallel or a parallel arrangement.

• A series connection is not practical because if one LED fails open, then all the LEDs are disabled.

• For a parallel connection, each LED requires a limiting resistor.

• To reduce the number of limiting resistors, a series‐parallel connection can be used,


Example 05

• An array of green leds (20 mA, 2.5 Volt). Design a 12 volt to minimize the number of limiting resistors of 60 leds


Volt of 3 leds in series @ 20 mA

V = 3 x 2.5 = 7.5 Volts

Shunt resistor volt = 12 – 7.5 = 4.5 Volts

Rlimit = 4.5/20 mA = 225 ohm


LED Displays

• LEDs are widely used in large and small signs and message boards for both indoor and outdoor uses, including large‐screen television.

• Full‐color screens use a tiny grouping of high‐intensity red, green, and blue LEDs to form a pixel

• Red, green, and blue (RGB) are primary colors and when mixed together in varying amounts, can be used to produce any color in the visible spectrum


• The light emission from each of the three diodes can

• be varied independently by varying the amount of forward current.

• Yellow is added to the three primary colors (RGBY) in some TV screen applications.


•


The Organic LED (OLED)

• An OLED is a device that consists of two or three layers of materials composed of organic molecules or polymers that emit light with the application of voltage.

• The color of the light depends on the type of organic molecule in the emissive layer.


The Photodiode

• The photodiode is a device that operates in reverse bias, as shown

• Iʎ where is the reverse light current.

• The photodiode has a small transparent window that allows light to strike the pn junction.


• A photodiode differs from a rectifier diode in that when its pnjunction is exposed to light, the reverse current increases with the light intensity.

• When there is no incident light, the reverse current Iʎ , is almost negligible and is called the dark current

• An increase in the amount of light

intensity, expressed as irradiance

(mW/cm2), produces an increase in

the reverse current


•


•


By:

Dr. Ahmed ElShafee


Lecture (07.02)Bipolar Junction

Transistor I

Agenda

• BJT structure

• BJT operation


BJT structure

• The BJT is constructed with three doped semiconductor regions

• One type consists of two n regions separated by a p region (npn), and the other type consists of two p regions separated by an n region (pnp).

• The term bipolar refers to the use of both holes and electrons as current carriers in the transistor structure


• In order for a BJT to operate properly as an amplifier, the two pn junctions must be correctly biased with external dc voltages.

• In this section, we mainly use the npn transistor for illustration.

• The operation of the pnp is the same as for the npn except that the roles of the electrons and holes, the bias voltage polarities, and the current directions are all reversed.



in

out

Control signal

•


Biasing

base‐emitter (BE) junction is forward‐biased

base‐collector (BC) junction is reverse‐biased

This condition is called forward‐reverse bias


BJT Operation

• CE battery +ve terminal attract electrons from the collector N‐region.

• Collector N‐region compensate electrons depletion region, which become more and more wider.


• BE battery +ve terminal start attracting the small amounts of electrons (minority carrier ) from base p‐type region, then starts attracting electrons from depletion region, which become thinner and thinner, then electrons start to flow from Emitter N‐region


• Now base p‐type now has a new electrons being transferred from CE battey –ve terminal through emitter n‐type.

• Electrons stream divided between

– Directly to BE battery +ve terminal

– CE battery +ve terminal through emitter n‐type Dr. Ahmed ElShafee, ACU : Fall 2019, Electronic Circuits I38

• The more +ve potential applied to base from BE battery +ve terminal, a more electrons being transferred from CE battery –ve terminal, through emitter N‐region, a more electrons in turn flow to CE battery +ve terminal.



• VBB forward‐biases the base‐emitter junction,

• VCC reverse‐biases the base‐collector junction.

• In practice both VCC, and VBB derived from single battery with necessarily voltage divider on base terminal


• DC Beta (βDC) and DC Alpha (αDC)

• DC gain called (βDC) is the ratio between IC/IB, which called hFEin data sheet, range from less than 20 to 200 or higher.

• (αDC) is the ratio between IC/IE, in range from 0.95 to 0.99 or greater


Example

•


βDC

Example

•


βDC

Transistor DC Model


Thanks,..

See you next week (ISA),…


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Lecture (07) Optical Diodes Bipolar Junction...

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