Electronic DevicesEighth Edition

Floyd

Chapter 11

Phototransistors

- Photodiode with amplifier

- B/E exposed to radiation.

- Collector current is a linear function of irradiance. (assuming a constant beta).

- Linearity is over a much narrower range than a photodiode or photoconductor.

- IC versus VCE is plotted based on steps of irradiance.

- Responstivity of a phototransistor (RE) is for a specified black body radiation (usually at a colour temp. of 2870 K)

Phototransistor

- Responsivity is the ratio of electrical output to the applied radiation. It is a figure of merit.

- Dark current is the main factor in limiting detection sensitivity.

- Dark current is a function of a.) ambient temperature (20

deg. C causes a decade increase in dark current) and 2.) VCE.

- Rise time is poor due to a.) the combined capacitance of the B/E and C/E junctions and b.) the lifetimes of the

carriers in the depletion region of the junction.

VCC

Vout

VCC C

E

Q1Q2

n-type

p-typen

EmitterBase

Collector

Window

Optocouplers

Optotisolator.

=>Electrically Insulated (insulation resistance ohms).

=>Capacitance < 1 pf.

=>Insulation voltage strength of several KV.

Eliminates ground effects from 2 systems with different ground potentials.

1210

Optocouplers

•Specifications:

1.) Operating speed.

a.) BW – typically refers to NRZ ( )

b.) Baud rate.

2.) Propagation delay.

3.) CTR (forward current transfer ratio)

4.) Isolation: ability to separate desired and stray input signals. Differential modes (desired) versus Common

mode (undesired).

BITB tf 1=

100×=

F

O

I

ICTR

Optocouplers

5.) Insulation.

6.) Limiting parameters: Pmax, current, voltage

ratings,same as typical semiconductor data.

7.) Aging characteristics: Ex. CTR deterioration due to

LED source of optocoupler.

Signal Sources:

a.) D.C.

b.) pulse

c.) A.C.

U1

A C

K E

U2

Q1

Selected Key TermsSelected Key TermsSelected Key Terms

4-layer diode

Thyristor

SCR

The type of 2-terminal thyristor that conducts

current when the anode-to-cathode voltage reaches

a specified “breakover” value.

A class of four-layer (pnpn) semiconductor

devices.

Silicon-controlled rectifier; a type of three

terminal thyristor that conducts current when

triggered by a voltage at the single gate terminal

and remains on until anode current falls below a

specified value.

Selected Key TermsSelected Key TermsSelected Key Terms

LASCR

Diac

Triac

Light-activated silicon-controlled rectifier; a four

layer semiconductor device (thyristor) that

conducts current in one direction when activated

by a sufficient amount of light and continues to

conduct until the current falls below a specified

value.

A two-terminal four-layer semiconductor device

(thyristor) that can conduct current in either

direction when properly activated.

A three-terminal thyristor that can conduct current

in either direction when properly activated.

Selected Key TermsSelected Key TermsSelected Key Terms

SCS

UJT

PUT

Silicon-controlled switch; a type of four-terminal

thyristor that has two gate terminals that are used

to trigger the device on and off.

Unijunction transistor; a three terminal single pn

junction device that exhibits a negative resistance

characteristic.

Programmable unijunction transistor; a type of

three terminal thyristor (physically more like an

SCR than a unijunction) that is triggered into

conduction when the voltage at the anode exceeds

the voltage at the gate.

Excercise

Basic 4-layer device: Exercise 1

part 2

A certain 4-layer diode is biased in the forward-blocking region with an anode-to-cathode voltage of 20V. Under this bias condition, the anode-current is 1uA. Determine the resistance of the diode in the forward-blocking region.

Basic 4-layer device: Exercise 1

part 2

If the anode current is 2uA and VAK=20V, what is the 4-layer diode’s resistance in the forward-blocking region?

Basic 4-layer device: Exercise 2

part 1Determine the value of anode current in the diagram when the device is on, VBR(F)=100V. Assume VBE=0.7 and VCE(sat)=0.1V for the internal transistor structure.

Basic 4-layer device: Exercise 2

(contd…)

What is the resistance in the forward-conduction region of the 4-layer diode in the figure?