1

Chapter 9Semiconductors & Diodes

Jaesung Jang

SemiconductorsPN Junction

Rectifier DiodesDC Power Supply

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

2

Semiconductor MaterialsSemiconductor Materials• 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.

• Extrinsic semiconductors are semiconductors with other atoms mixed in.

– These other atoms are called impurity atoms.– The process of adding impurity atoms is called doping.

3

Semiconductor Materials (cont.)Semiconductor Materials (cont.)Figure below illustrates a bonding diagram of a silicon crystal.

(Intrinsic Semiconductors)

Valence electrons: electrons in the outermost ringSi: 4 valence electrons

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

4

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)

Extrinsic Semiconductors

Covalent bonds (공유결합)

Semiconductor Materials (cont.)Semiconductor Materials (cont.)

5Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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)

Extrinsic Semiconductors

Semiconductor Materials (cont.)Semiconductor Materials (cont.)

6

The PN Junction & DiodeThe PN Junction & Diode The diode is made by joining p- and n-type semiconductor materials. The doped regions meet to form a p-njunction. Diodes are unidirectional devices that allow current to flow in one direction. Diodes have polarity.

}Diodes

Cathode Lead

Anode Lead

}Diodes

Cathode LeadCathode Lead

Anode LeadAnode Lead

7

The important effect here is that when a free 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.

Figure to the right shows a p-n junction with 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 small circles (○).

VO=0.7V for siliconDepletion region

Electrically neutral

Electric field

Diffusion

The PN Junction & Diode (cont.)The PN Junction & Diode (cont.)

8

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.

The PN Junction & Diode (cont.)The PN Junction & Diode (cont.)

currents

9

VoltVolt--Ampere Characteristic Curve of a Ampere Characteristic Curve of a 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 order of 10-15A. 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.

Volt-ampere characteristic curve of a silicon diode.

i+ _

1.normally iswhich structure, and material diodoe on the depending 2 and 1between is and e,temperatur

roomat mV 25about and voltage thermal theis A,10 oforder theof is andconstant saturation a is

for 115-

nVI

IieIeIi

T

S

SnV

SnV

STT

10

Circuit Models for the Semiconductor DiodeCircuit Models for the Semiconductor Diode

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

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

Ideal diode

11

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

Circuit Models for the Semiconductor DiodeCircuit Models for the Semiconductor Diode

0.6 V for Si Diode

12

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.

Circuit Models for the Semiconductor DiodeCircuit Models for the Semiconductor Diode

13

First approximation

Second approximation Third approximation

Circuit Models for the Semiconductor DiodeCircuit Models for the Semiconductor Diode

14

Circuit Models for the Semiconductor DiodeCircuit Models for the Semiconductor Diode

biased. reverse is diode theV3

V8

circuit.open an it with replace andconduct,not does diode ideal that theinitially Assume

21

21

21

vvv

VRR

Rv

D

S

direction. reverse in the flowing iscurrent theV11

V75.8

circuit.short ait with replace andconduct, does diode ideal that theinitially Assume

21

13

1

2

1

1

1

B

BS

Vvv

vR

VvRv

RvV

15

Rectifier DiodesRectifier Diodes 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 (정류다이오드).Diodes are able to produce a dc output voltage because they are unidirectionaldevices 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 current in the load, RL.

Half-wave rectifier

16

Second approximation

V 13.2741.72average) -( 318.0 V 41.72 0.7 - 42.42

V 42.4230414.1V 30

average)(

dc

peaks

ac

V

VV

f = 60 Hz

Rectifier Diodes (cont.)Rectifier Diodes (cont.)

17

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.

ftA 2sin1

ftA 2sin1

Full-wave rectifier

Rectifier Diodes (cont.)Rectifier Diodes (cont.)

18

V 13.0420.51average) -( 318.02

V 20.51 0.7 - 21.21

V 21.2115414.1V 15

average)(

dc

peaks

ac

V

VV

Rectifier Diodes (cont.)Rectifier Diodes (cont.)

19

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

Filter

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.

CR

t

peakoutrippleLeVV 1)(

Rectifier Diodes (cont.)Rectifier Diodes (cont.)

20

Special DiodesSpecial Diodes• 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.