Electronic Circuits I. Instructor: Dr. Alaa Mahmoud

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1 Electronic Circuits I Instructor: Dr. Alaa Mahmoud alaa_y_emam@hotmail.com

2 Chapter 27 Diode and diode application

3 Outline: Semiconductor Materials The P-N Junction Diode Biasing P-N Junction Volt-Ampere Characteristic Curve Rectifier Circuit Special Diodes

4 Semiconductor (SC) Materials Semiconductors (SC) conduct less than metal conductors but more than insulators Common semiconductors: silicon (Si), germanium (Ge), and carbon (C) Silicon is the most widely used semiconductor material in the electronics industry Almost all diodes, transistors, and ICs are made from Si Semiconductors types Intrinsic semiconductors: Semiconductors in their purest form At room temperature the intrinsic sc acts like insulator Extrinsic semiconductors: Semiconductors with other atoms mixed in The other atoms are called impurity atoms The process of adding impurity atoms is called doping Doping increase the conductivity of SC Two types of extrinsic SC n-type p-type

5 Atomic structure: In its pure form, Si (and all SC) has 4 electrons in its outer shell (Valence electrons) Valence electron

6 Forming a crystal: Each Si atom shares its 4 valence electrons with other nearby atoms forming a solid crystalline structure This sharing is called covalent bonding Crystalline Si

the Si material At room temperature, or with thermal energy à electron-hole pairs are generated The hole acts like

7 Thermally generating electron-hole pairs: Energy band diagram for SC (a) at 0 o K and (b) when temperature increased (a) No free electrons at 0 0 K (b) Free electrons is generated At 0 o K, no free electrons float around the Si material At room temperature, or with thermal energy à electron-hole pairs are generated The hole acts like +ve charge because it attracts a free electron passing through the crystal Electrons-hole pairs is generated in Si crystal

8 Doping: the process of adding of impurity atoms in intrinsic semiconductor n-type SC Si is doped with a pentavalent impurity (5 electrons in its outer shell) e.g., Arsenic (As), antimony (Sb) or phosphorous (P) An electron has ve charge à n-type SC e à majority current carriers p à minority current carriers n-type semiconductor has +ve net charge n-type semiconductor called donor p-type SC Si is doped with a trivalent impurity (3 electrons in its outer shell) e.g., Aluminum (Al), boron (B) or gallium (Ga) A hole has +ve charge à p-type SC p à majority current carriers n à minority current carriers p-type semiconductor has -ve net charge p-type semiconductor called acceptor Donor energy level Acceptor energy level Conduction band Valence band

9 The P-N Junction Diode A diode: popular semiconductor device made by joining p- & n-type semiconductor materials The doped regions meet to form a p-n junction. Diodes are unidirectional devices that allow current to flow in one direction. The symbol for a diode is p n

Depletion zone: The figure shows a p-n junction : free electrons are represented as (-), on the n side holes are represented as ( ) on the p side When a free electron leaves the n

crystalline structure and not moving The area with +ve & -ve ions is called depletion region, because it is depleted of all charge carriers The movements of current carriers is

11 Depletion zone: The figure shows a p-n junction : free electrons are represented as (-), on the n side holes are represented as ( ) on the p side When a free electron leaves the n side into a hole on the p side, two ions are created; +ve on n side & ve on p side Electrons on n side diffuse across the junction to the p side The ions are fixed in the crystalline structure and not moving The area with +ve & -ve ions is called depletion region, because it is depleted of all charge carriers The movements of current carriers is called diffusion Diffusion creates a barrier potential V B For Si, V B = 0.7V, for Ge, V B = 0.3V V B will stop the diffusion of current carriers Depletion region Or Depletion layer Or Depletion zone

12 Biasing P-N Junction Bias voltage: is the DC voltage applied across the terminals of a P-N junction Forward bias Reverse-bias Positive voltage V is applied to p-region with respect to n-region If V >V B, free electron in the n side is repelled to p side to the +veterminal of battery The current I F flows easily through the diode I F decreases with temperature The voltage polarity is is reversed (+ve voltage applied to n-region) The external voltage pulls majority current carriers away from the pn junction This widens the depletion zone Ideally diode does not conduct any current I = zero A

13 Leakage current I R For ideal diode, the current in the reverse bias direction is zero For real (practical) diode, in a reverse bias direction, diode will conduct a small amount of current called leakage current Leakage current is due to the action of the minority carriers in each sides (electrons in p-side & holes in the n-side) Leakage current affected by increasing temperature i-v curve for an ideal diode Breakdown Voltage V BR V BR is the voltage at which there is a sudden increase in the leakage current (I R ) V BR occurs when the diode is reverse-biased By increasing reverse bias voltage à I R flows until breakdown or avalanche is reached Avalanche effect produced by thermally produced free electrons Diodes should not be operated in this voltage range i-v curve for a real diode

Volt-Ampere Characteristic Curve Beyond 0.

14 Volt-Ampere Characteristic Curve Beyond 0.6 V of forward bias, the diode current I F increases sharply In the reverse-bias condition, only a small current I R flows until breakdown or avalanche is reached Leakage current I R Barrier potential V B 0.6V DC Resistance R F = V F / I F

15 Rectifier Circuit Function: Convert alternating current/voltage (ac) to direct current/voltage (dc) Power supply: a circuit that converts the ac power-line voltage to dc value Rectifier diode: is the most important components in power supplies, which convert ac line voltage to dc voltage Why diodes are able to produce a dc output voltage? because diode is unidirectional device that allows current to flow through it in one direction only Normally rectifier circuits consist of transformer T that is connected to a diode D and a load R L T Input (ac) Output (dc)

transformer secondary voltage is ve à D 1 is reverse-biased and acts like an open switch à zero current flows in the

16 Half-Wave Rectifier The circuit shown is called a half-wave rectifier It consists of one T & one D When the top of the transformer secondary voltage is +ve à D 1 is forward-biased à current flows in the load R L When the top of the transformer secondary voltage is ve à D 1 is reverse-biased and acts like an open switch à zero current flows in the load R L The output voltage is a series of +ve pulses Frequency of the output voltage is the same as the input voltage

Full-Wave Rectifier The circuit shown is called a fullwave rectifier It consists of one T & two D When the top of the transformer secondary voltage is +ve à D 1 is forward-biased à current flow in

17 Full-Wave Rectifier The circuit shown is called a fullwave rectifier It consists of one T & two D When the top of the transformer secondary voltage is +ve à D 1 is forward-biased à current flow in the load R L When the top of the transformer secondary voltage is ve à D 2 is forward-biased à current flow in the load R L The combined output voltage produced by D 1 and D 2 is +ve pulses Frequency of the output voltage is the twice the input voltage

Full-Wave Bridge Rectifier The circuit shown is called a full-wave bridge rectifier When the top of the transformer secondary voltage is +ve à diodes D 1 & D 2 are forward-biased à

18 Full-Wave Bridge Rectifier The circuit shown is called a full-wave bridge rectifier When the top of the transformer secondary voltage is +ve à diodes D 1 & D 2 are forward-biased à current flow in R L When the top of the transformer secondary voltage is ve àd 3 & D 4 are forward-biased à current flow in R L Output voltage has twice the frequency of the input voltage T

Special Diodes A light-emitting diode (LED): is a

forward-biased The color of light emitted by an

doping LED symbol is Zener diode: is a special

diode symbol is Zener diode is reverse-biased

19 Special Diodes A light-emitting diode (LED): is a diode that emits a certain color light when forward-biased The color of light emitted by an LED is determined by the type of material used in doping LED symbol is Zener diode: is a special diode that operates in the breakdown region Zener diode symbol is Zener diode is reverse-biased with +ve terminal of battery connected to the diode cathode - +

Lesson 08. Name and affiliation of the author: Professor L B D R P Wijesundera Department of Physics, University of Kelaniya.

Lesson 08. Name and affiliation of the author: Professor L B D R P Wijesundera Department of Physics, University of Kelaniya. Lesson 08 Title of the Experiment: Identification of active components in electronic circuits and characteristics of a Diode, Zener diode and LED (Activity number of the GCE Advanced Level practical Guide