Chapter 9 Semiconductors & Diodes

Jaesung Jang

Semiconductors PN Junction Rectifier Diodes DC Power Supply

Ref: Sedra/Smith, Microelectronic Circuits, 3rd ed., 1990, Chap. 3

1 SemiconductorSemiconductor MaterialsMaterials • Semiconductors conduct less than metal conductors but more than insulators. – Some common semiconductor materials are 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 manufactured today are made from silicon.

• Intrinsic semiconductors are semiconductors in their purest form. •Extrinsicsemiconductors are semiconductors with other atoms mixed in. – These other atoms are called impurity atoms. – The process of adding impurity atoms is called doping. 2 SemiconductorSemiconductor MaterialsMaterials (cont.)(cont.)

Figure below illustrates a bonding diagram of a silicon crystal. (Intrinsic Semiconductors)

Valence electrons: electrons in the outermost ring Si: 4 valence electrons

8 valence electrons are needed for stability. Figure on the right is stable.

3 SemiconductorSemiconductor MaterialsMaterials (cont.)(cont.)

Extrinsic Semiconductors

. Doping is a process to add impurity atoms to an intrinsic semiconductor.

. N-Type semiconductors are made by doping intrinsic semiconductor with a pentavalent (5) impurity.-> free electrons: majority current carriers

. Donors: Arsenic (As), antimony (Sb) or phosphorous (P)

Covalent bonds (공유결합)

4 SemiconductorSemiconductor MaterialsMaterials (cont.)(cont.)

Extrinsic Semiconductors

. P-Type semiconductors are made by doping intrinsic semiconductor with a trivalent (3) impurity. -> holes: majority current carriers

. Acceptors: Aluminum (Al), boron (B) or gallium (Ga)

. The diode is made by joining p- and 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. . Diodes have polarity.

Anode Lead

}Diodes

Cathode Lead 6 TheThe PNPN JunctionJunction && DiodeDiode (cont.)(cont.) . The important effect here is that when a free Electrically neutral electron leaves the n side and falls into a hole on the p side due to diffusion, two ions are created; a positive ion on the n side and a negative ion on the p side. Diffusion . Figure to the right shows a p-n junction with Electric field free electrons on the n side and holes on the p side.

. The free electrons are represented as dash (-) marks and the holes are represented as VO=0.7V for silicon small circles (○). Depletion region

7 TheThe PNPN JunctionJunction && DiodeDiode (cont.)(cont.) . The term bias is defined as a control voltage or current. . Forward bias: bias to allow for currents to flow easily through the diode. . n side: negative, p side: positive

. Reverse bias: bias to block currents through the diode. . n side: positive, p side: negative . In the reverse bias, an external voltage pulls majority current carriers away from the pn junction, widening the depletion zone. . When we increase the reverse bias voltage more and more, the pn diode will be filled with the depletion zone and a very small amount of electrical currents (reverse saturation currents) can flow.

currents 8 VoltVolt--AmpereAmpere CharacteristicCharacteristic CurveCurve ofof aa DiodeDiode • Figure below is a graph of diode current versus diode voltage for a silicon diode both forward- and reverse-bias voltages. • Forward-bias Region: About 0.6-0.8 V of forward bias, the diode current increases sharply. -> simple model for a diode has 0.6-V drop across it. • Reverse-bias Region: The current in the reverse direction is constant and is -15 order of 10 A. i ≈ - IS • Breakdown Region: A great deal of reverse current will flow when the magnitude

of the reverse voltage exceeds a threshold value of breakdown voltage.

nVT  nVT Volt-ampere i  I S e 1 I S e for i  I S -15 characteristic curve of a I S is a saturation constant and is of the order of 10 A, silicon diode. VT is the thermal voltage and about 25 mV at room temperatur e, and n is between 1 and 2 depending on the diodoe material and structure, which is normally 1.

+ _ 9 i CircuitCircuit ModelsModels forfor thethe SemiconductorSemiconductor DiodeDiode

.Three different approximations can be used when analyzing diode circuits. .The one used depends on the desired accuracy of your circuit calculations.

The first approximation treats a forward-biased diode like a closed switch with a voltage drop of zero volts, as shown below.

Ideal diode

10 CircuitCircuit ModelsModels forfor thethe SemiconductorSemiconductor DiodeDiode

The second approximation treats a forward-biased diode like an ideal diode in series with a battery, as shown below

0.6 V for Si Diode

11 CircuitCircuit ModelsModels forfor thethe SemiconductorSemiconductor DiodeDiode

. The third approximation of a diode includes the bulk resistance, rB. . The bulk resistance, rB is the resistance of the p and n materials. . The third approximation of a forward-biased diode is shown below.

12 CircuitCircuit ModelsModels forfor thethe SemiconductorSemiconductor DiodeDiode

First approximation

Second approximation Third approximation

13 CircuitCircuit ModelsModels forfor thethe SemiconductorSemiconductor DiodeDiode

Assume initially that the ideal diode does not conduct, and replace it with an open circuit.

R2 v1  VS  8V R1  R2

vD  v1  v2  3V  the diode is reverse biased.

Assume initially that the ideal diode does conduct, and replace it with a short circuit.

VS  v1 v1 v1  VB    v1  8.75V R1 R2 R3

v1  v2  VB  11V  the current is flowing in the reverse direction. 14 RectifierRectifier DiodesDiodes

. A circuit that converts the ac power- line voltage to the required dc value is called a dc power supply. .The most important components in power supplies are rectifier diodes (정류 다이오드). .Diodes are able to produce a dc output voltage because they are unidirectional devices allowing current to flow through them in only one direction.

.When the top of the transformer secondary voltage is positive, D1 is forward-biased, producing current flow in the load. . When the top of the secondary is negative, D1 is reverse-biased and acts like an open switch. This results in zero Half-wave rectifier current in the load, RL.

15 RectifierRectifier DiodesDiodes (cont.)(cont.)

Second approximation

Vac  30 V

Vspeak  1.414  30  42.42 V 42.42 - 0.7  41.72 V f = 60 Hz Vdc (average)  0.318 ( - average)  41.72  13.27 V

16 RectifierRectifier DiodesDiodes (cont.)(cont.)

. The circuit shown below is called a full-wave rectifier.

. When the top of the secondary is positive, D1 is forward-biased, causing current to flow in the load, RL. . When the top of the secondary is negative, D2 is forward-biased, causing current to flow in the load, RL. . The combined output voltage produced by D1 and D2 are shown in the next slide.

1   A sin 2ft

1   A sin 2ft

Full-wave rectifier

17 RectifierRectifier DiodesDiodes (cont.)(cont.)

Vac  15 V

Vspeak  1.414 15  21.21 V 21.21 - 0.7  20.51 V

Vdc (average)  2  0.318 ( - average)  20.51  13.04 V

18 RectifierRectifier DiodesDiodes (cont.)(cont.)

Filter

A half-wave rectifier with its output filtered by the capacitor, C.

t    V  V 1 e RLC  ripple out ( peak )     Output ripple voltage of the half-wave rectifier is illustrated. This ripple is going to be adjusted by a voltage regulator to produce a DC output. 19 SpecialSpecial DiodesDiodes

• Semiconductor diodes can be manufactured to emit different colors of light or regulate voltage.

• 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.

•A zener diode is a special diode that has been optimized for operation in the breakdown region and it is commonly used in voltage regulation.