Exam 2 Semiconductor Devices and Circuits

BJT (bipolar junction transistor)

a transistor constructed with three doped semiconductor regions of a BJT

Emitter

the most heavily doped of the three semiconductor regions of a BJT

Base

one of the three semiconductor regions of a BJT. The base is thin and lightly doped compared to the other regions

Collector

the largest of the three semiconductor regions of a BJT

Beta

the ratio of dc collector current to the dc base current in a BJT; current gain from base to collector

Saturation

the state of a BJT in which the collector current has reached a maximum and is independent of the base current

Cutoff

the nonconducting state of a transistor

Phototransistor

a transistor in which base current is produced when light strikes the photosensitive semiconductor base region

The region on the characteristic curve in which the current changes only slightly with an increase in V_CE is called the

a. saturation region

b. cutoff region

c. breakdown region

d. active region

d. active region

β_DC is defined as the ratio of

a. collector current to base current

b. collector current to emitter current

c. emitter current to base current

d. emitter current to collector current

a. collector current to base current

When a BJT is in saturation, the

a. collector current does not change with an increase in base current

b. base current cannot increase

c. collector to emitter voltage is maximum

d. all of the above

a. collector current does not change with an increase in base current

When a BJT is cutoff, the

a. voltage from collector to emitter is near zero

b. collector current is near zero

c. base-emitter junction is forward-biased

d. none of the above

none of the above

The lower end of the dc load line touches the x-axis at

a. saturation

b. cutoff

c. breakdown

d. 0.7 V

b. cutoff

For the circuit shown, the base current is

a. 1.0 μA

b. 1.16 μA

c. 10 μA

d. 11.6 μA

c. 10 μA

For the circuit shown, the saturation current is

a. 200 μA

b. 2.0 mA

c. 3.79 mA

d. 14.8 mA

c. 3.79 mA

For the phototransistor circuit, assume there is sufficient light to saturate Q₁. In this condition,

a. Q₂ is also saturated

b. the diode is conducting

c. the relay is energized

d. none of the above

d. none of the above

An optocoupler is a single package containing

a. two transistors

b. an LED and a phototransistor

c. a phototransistor and a relay

d. an LED and a relay

b. an LED and a phototransistor

The transistor package that is a TO-3 case is

c.

BJT regions

three regions called the emitter, base and collector

BJT base

thin lightly doped region

BJT emitter

Heavily doped

BJT collector

moderately doped

npn bjt diagram

pnp bjt diagram

Normal BJT Operation

the base-emitter is forward-biased and the base-collector is reverse biased

npn BJT operation

the collector is more positive than the base, which is more positive than the emitter

pnp BJT operation

The voltages are reversed to maintain the forward-reverse bias

BJT current direction

The direction of conventional current is in the direction of the arrow on the emitter terminal

The emitter current is the ___ of the collector current and

sum

The emitter current equation

I_E = I_C + I_B

(emitter current is the sum of the collector current and small base current)

BJT Characteristics curve for fixed base current

The collector characteristic curves show the relationship of the three transistor currents

The collector characteristic curves illustrate the relationship of the three transistor currents. By setting up other values of base current, ____

a family of collector curves is developed

β_DC is

the ratio of collector current to base current. it can be read from the curves. the value of β_DC is nearly the same wherever it is read

β_DC is the ratio of collector current to base current:

β_DC = I_C / I_B

What is the β_DC for the transistor shown?

Choose a base current near the center of the range - in this case I_B3 which is 30 μA

Read the corresponding collector current - in this case, 5.0 mA. Calculate the ratio:

β_DC = I_C / I_B = 5.0 mA / 30 μA = 167

BJT Characteristics for a fixed base current

• The first region is the saturation region

• As V_CE is increased, I_C increases until B

• Then, it flattens in the region between points B and C, which is the active region

• After C, is the breakdown region

In a BJT, cutoff

• is the condition in which there is no base current, which results in only an extremely small leakage current (I_CEO) in the collector circuit. For practical work, this current is assumed to be zero.

• In this condition, neither the base-emitter junction, nor the base-collector junction are forward-biased

I_CEO

An extremely small leakage current in the BJT collector circuit in cutoff. For practical work, this current is assumed to be zero

In a BJT ___, neither the base-emitter junction, nor the base-collector junction are forward biased

cutoff

In a BJT, saturation is the condition in which

there is maximum collector current

In a BJT, saturation is determined by

the external circuit (V_CC and R_C ) because the collector-emitter voltage is minimum (around 0.2 V)

Why is the saturation current determined by the external circuit (V_CC and R_C in this case) ?

Because the collector-emitter voltage is minimum (around 0.2 V)

In saturation, an increase of base current ___

has not effect on the collector circuit and the relation I_{C =} β_DC I_B is no longer valid

DC Load Line

Represents the circuit that is external to the transistor. It is drawn by connecting the saturation and cutoff points. The transistor characteristic curves are shown superimposed on the load line. The region between the saturation and cutoff points is called the active region

The transistor characteristic curves are

shown superimposed on the load line

The region between saturation and cutoff points is

called the active region

What is the saturation current and cutoff voltage for the circuit? Assume V_CE = 0.2 V in saturation.

Solution:

I_SAT = (V_CC - 0.2 V) / R_C

I_SAT = (15 V - 0.2V) / 3.3kohm = 4.48 mA

V_CO = V_CC = 15 V

Follow-up: Is the transistor saturated?

I_B = (3.0 V - 0.7 V) / 220kohm = 10.45 μA

I_C = βI_B = 200 (10.45 μA) = 2.09 mA

Since I_C < I_SAT, it is not saturated

Is the transistor saturated?

I_B = (3.0 V - 0.7 V) / 220kohm = 10.45 μA

I_C = βI_B = 200 (10.45 μA) = 2.09 mA

Since I_C < I_SAT, it is not saturated

Data Sheets

Give manufacturer’s specifications for maximum operating conditions, thermal, and electrical characteristics. For example, an electrical characteristic is β_DC, which is given as h_FE

The 2N3904 shows

a range of β‘s on the data sheet from 100 to 300 for I_C and the electrical characteristic β_DC, which is given as h_FE

DC Quantities subscripts

Use uppercase roman subscripts. the second letter in the subscript indicates the reference point (Ex. V_CE)

Is V_CE an AC or DC quantity?

DC

What does the second letter the subscript of a DC quantity indicate?

the reference point

AC Quantities and time barying signals use __ subscripts

lower case, italic (Ex: V_ce)

What kind of subscript is V_ce?

AC Quantity

Internal transistor resistances

indicated as lower case quantities with a prime and an appropriate subscript. Ex: r^’_e

What resistance does the subscript r^’_e show?

Internal transistor resistances

External resistances

indicated as capital R with either a captial or lower case subscript depending on if it is a DC or ac resistance. (Ex: R_C and R_c)

What type of resistance does R_C and R_c show?

External resistance

BJT Amplifiers

amplifies AC signals by converting some of the DC power from the power supplies to AC signal power. An ac signal at the input is superimposed in the dc bias by capacitive coupling. The output ac signal is inverted and rides on a dc level of V_CE

A BJT can be used as a ___ in logic circuits

switching device to turn on or off current to a load

As a switch, the transistor is normally in ____

either cutoff (load is OFF) or saturation (load is ON)

As a switch, a BJT transistor in cutoff

looks like an open switch and load is OFF

As a switch, a BJT transistor in saturation

looks like and closed switch and load is ON

Phototransistor

produces base current when light strikes the exposed photosensitive base region, which is the active area. Has high gain and are more sensitive to light than photodiodes

In a typical phototransistor circuit, the base lead is

left open

Characteristic curves for a phototransistor

based on light flux (mW/cm²) to the base rather than base current in an ordinary transistor

The output from the phototransistor can

be used to activate or deactivate a relay

Is either transistor ON for the circuit when there is no incident light?

With no incident light, Q₁ will be biased OFF. Q₂ will be forward-biased through R and is ON. Collector current in Q₂ causes the relay to be energized

Phototransistor Diagram

Optocouplers

A single package containing an LED and a phototransistor. They transfer a signal from one circuit to another while providing a high degree of isolation

Optocoupler diagram

What do optical couplers do?

transfer a signal from one circuit to another while providing a high degree of isolation

CTR

The current transfer ratio, which is a measure of efficiency and a key specification for optocouplers. It is the ratio of output current to input current and values are typically from 50-110% for standard optocouplers

Optocouplers are used when

data or signals needs to be transferred from a control circuit to a power circuit without making electrical connections between circuits. (Ex: The logic circuits in a traffic light controller need to be isolated from the lights themselves)

An application for optocouplers is

as a transducer to detect a light path such as a hole in a rotating disk

Optocouplers are useful for

isolating patients from the monitoring instruments

TO-92 transistor

SOT-23 transistor

TO-18 transistor

TO-3 transistor

TO-220AB Transistor

TO-225AA

BJT Basic Test

• Use the Diode Test function of your DMM.

• To test the base-emitter junction of a BJT, connect the positive lead to the base and the negative lead to the emitter.

• You should see a voltage close to 0.7 V for an npn transistor.

• Reversing the leads will show OL for “overload”

• The procedure is repeated to test the base-collector junction