Bipolar Junction Transistors (BJTs)

Introduction

•The invention of the transistor was the beginning of a technological revolution that is still continuing.

•All of the complex electronic devices and systems today are an outgrowth of early developments in semiconductor transistors.

•Two basic types of transistors are the bipolar junction transistor (BJT), and the field-effect transistor (FET).

•The BJT is used in two broad areas

•as a linear amplifier to boost or amplify an electrical signal,

•as an electronic switch.

•The term bipolar refers to the use of both holes and electrons as current carriers in the transistor structure.

BJT Structure

The BJT has three regions called the emitter, base, and collector. Between the regions are two pn junctions as indicated. B (base) C (collector) n p n Base-Collecto r junction Base-Emitter junction B 3 C p n p E (emitter) npn E pnp

The base is a thin lightly doped region compared to the

heavily doped emitter and

moderately doped collector regions.

BJT Structure

BJT Operation

In normal operation, the base-emitter (BE) is forward- biased and the base-collector (BC) is reverse-biased.

npn – + – + BC reverse- biased BE forward-biased

For the npn type shown, the collector is more positive than the base, which is more positive than the emitter.

6

+

C n

–

E n

B p

–

+

BJT Operation

In normal operation, the base-emitter (BE) is forward- biased and the base-collector (BC) is reverse-biased.

For the pnp type, the voltages are reversed to maintain the forward-reverse bias. – + + – BC reverse- biased + – BE forward-biased – + pnp 7

C p

E p

B n

BJT Operation (npn)

Direction of electron flow:

8

I_E = I_C + I_B

B

C

E

BJT Operation (npn)

•The heavily doped n-type emitter region has a very high density of conduction-band (free) electrons.

•These free electrons easily diffuse through the forward- biased BE junction into the lightly doped and very thin p-type base region.

•The lightly doped p-type base has a low density of holes, which are the majority carriers.

•A small percentage of the total number of free electrons injected into the base region recombine with holes and move as valence electrons through the base region, forming small base electron current.

•When the electrons that have recombined with holes as valence electrons leave the crystalline structure of the base, they become free electrons in the metallic base lead and produce the external base current.

BJT Operation (npn)

10

•Most of the free electrons that have entered the base do not recombine with holes because the base is very thin (no enough holes).

•As the free electrons move toward the reverse-biased BC junction, they are swept across into the collector region by the attraction of the positive collector supply voltage.

•The free electrons move through the collector region, into the external circuit forming collector current, and then return into the emitter region along with the base current (I_E = I_C + I_B).

•The emitter current is slightly greater than the collector current because of the small base current that splits off from the total current injected into the base region from the emitter (I_E = I_C +

BJT Currents

11

I_{E I}

E

I

B

I

C IC

I B n p n p n p – ₊

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

The emitter current is the sum of the collector current and the small base current. That is, I_E = I_C + I_B.

+ – – + I C I B + – +

IE IE

DC Bias Circuits (Common Emitter)

DC Beta (β_DC) and DC Alpha (α_DC)

13

The dc current gain of a transistor is the ratio of the dc collector current (I_C) to the dc base current (I_B) and is designated dc beta (β_DC).

Typical values of β_DC range from less than 20 to 200 or higher.

The ratio of the dc collector current (I_C) to the dc emitter current (I_E) is the dc

alpha (α_DC).

The alpha is a less-used parameter than beta in transistor circuits.

Typically, values of α

DC range from 0.95 to 0.99 or greater, but αDC is

always less than 1. The reason is that I

C is always slightly less than IE by the amount of IB

(I

DC Beta (β_DC) and DC Alpha (α_DC)

14

Example:

If I_E = 100 mA and I_B = 1 mA, then

I_C = I_E – I_B = 100 – 1 = 99 mA and

Current & Voltage Analysis

15

I_B: dc base current

I_E: dc emitter current

I_C: dc collector current

KCL: I_E = I_C + I_B

V_BE: dc voltage at base with respect to emitter

V_CB: dc voltage at collector with respect to base

V_CE: dc voltage at collector with respect

to emitter KVL: VCE = VCB + VBE

V_BB forward-biases the BE junction, and V_CC reverse-biases the BC junction. When BE is forward-biased, it is like a forward-biased diode and has a

Current & Voltage Analysis

16

V_BE = 0.7 V

V_CE = V_CC – I_C R_C

V_CB = V_CE – V_BE

V_RB = V_BB – V_BE

V_RB = I_B R_B

I_B = (V_BB – V_BE ) / R_B

V_RC = I_C R_C

I_B R_B = V_BB – V_BE

V_CE = V_CC – V_RC

> 0

Operation of BJTs

BJT will operates in one of following four regions:

1. Cutoff region (digital circuit/switching)

•

When I

=0, BEJ is RB and BCJ is RB

2. Linear / Active region (Amplifier)

•

BEJ is FB and BCJ is RB

3. Saturation region (digital circuit/switching)

•

BEJ is FB and BCJ is also FB

4. Breakdown region (never recommended)

Linear / Active Region

Cut off Region

Saturation Region

The collector characteristic curve shows the relationship of the three transistor currents.

BJT Characteristics

22

I

C Breakdown

The curve shown is for a fixed base current. The first region is

CE

the saturation region. As V

is

B

C

A

0 0.7 V V

CE(max) V_CE Saturation region Active region region

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.

BJT Characteristics

Consider point A on the characteristic curve:

•Assume that V_BB is set to produce a certain value of I_B and V_CC is zero.

•For this condition, both the BE junction and the BC junction are forward- biased, because the base is at approximately 0.7 V while the emitter and the collector are at 0 V.

•I_B is through the BE junction, because of the low impedance path to ground and, therefore, I_C is zero.

•When both junctions are forward-biased, the transistor is in the saturation region of its operation.

Consider the portion of the curve between points A and B:

•Saturation is the state of a BJT in which I_C has reached a maximum and is independent of I_B (I_C ≠ β_DC I_B). I_B is constant, but I_C is increasing.

•When V_CC is increased, V_CE increases as I_C increases.

•I_C increases as V_CC is increased because V_CE remains less than 0.7 V due to the forward-biased BC junction.

V_CE = V_CB + V_BE = negative + 0.7 < 0.7 and I_C = (V_CC – V_CE) / R_C

BJT Characteristics

Consider the portion of the curve between points B and C:

•Ideally, when V_CE exceeds 0.7 V, the BC junction becomes reverse-biased and the transistor goes into the active, or linear, region of its operation.

•Once the BC junction is reverse-biased, I_C levels off and remains

essentially constant for a given value of I_B as V_CE continues to increase.

•Actually, I_C increases very slightly as V_CE increases due to widening of the BC depletion region.

•This results in fewer holes for recombination in the base region which effectively causes a slight increase in β_DC.

•For this portion of the characteristic curve, the value of I_C is determined only by the relationship I_C = β_DC I_B.

Consider the portion of the curve to the right of point C:

•When V_CE reaches a sufficiently high voltage, the reverse-biased BC junction goes into breakdown; and I_C increases rapidly.

•A transistor should never be operated in this breakdown region.

BJT Characteristics

It can be read from the curves. The value of β_DC is nearly the same wherever it is read.

BJT Characteristics

•A family of collector characteristic curves is produced when

I_C versus V_CE is plotted for several values of I_B.

•When I_B = 0, the transistor is in the cutoff region although there is a very small collector leakage current.

•Cutoff is the non-conducting state of a transistor.

•The amount of collector leakage current for I_B = 0 is exaggerated on the graph for illustration.

Cutoff

In a BJT, cutoff is the condition in which there is no base current (I_B = 0), which results in only an extremely small leakage current (I_CEO ≈ 0) in the collector circuit (due mainly to thermally produced carriers). For practical work, this current is assumed to be zero.

In cutoff, neither the BE junction, nor the BC junction are forward- biased.

The subscript CEO represents

collector-to-emitter with the base open.

Saturation

In a BJT, saturation is the condition in which there is maximum I_C. The saturation current is determined by the external circuit (V_CC and R_C in this case) because the CE voltage is minimum (V_CE ≈ 0.2 V). V_CE = V_CC –I_C R_C

In saturation, an increase of I_B has no effect on the collector circuit

and _{C DC B}

the relation I = β I is no longer valid.

Saturation

31

•When the BE junction becomes forward-biased and I_B is increased, I_C also increases (I_C = β_DC I_B) and V_CE

decreases as a result of more drop across R

C, VCE = VCC –

I_C R_C

•When V_CE reaches its saturation value, V_CE(sat), the BC junction becomes forward-biased and I_C can increase no further, even with a continued increase in I_B.

DC Load Line

32

the transistor. It is drawn by connecting the

saturation

The DC load line represents the circuit that is external to

and cutoff points.

The transistor characteristic curves are shown superimposed on the load line.

The DC Operating Point

For a transistor circuit to amplify it must be properly biased with dc voltages.

The dc operating point between saturation and cutoff is called the Q-point.

The goal is to set the Q-point such that that it does not go into saturation or cutoff when an a ac signal is applied.

DC Load Line

35

What is the saturation current for the circuit? Assume V_CE = 0.2 V in

saturation.

+

+

VCC

– 15

V V BB 3 V – R C R B

β_DC = 200 220 kΩ 3.3 kΩ C(sat) I = V CC

R_C 3.3 kΩ − 0.2 V

= 15 V − 0.2 V = 4.48 mA

B

Is the transistor saturated? I ₌ 3.0 V − 0.7 V _{= 10.45 μA}

220 kΩ

I_C = β I_B = 200 (10.45 μA) = 2.09 mA

(repeated)

BJT As An Amplifier

A BJT 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.

BJT As Switch

A BJT can be used as a switching device in digital circuits 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).

I_C

V

CE

I_B = 0 Cutoff

0 V

CE(sat) VCC

I

C(sat)

Saturation

In cutoff, the transistor

looks like an open switch.

In saturation, the transistor

looks like a closed switch.

R B 0 V R C I C =

0 +V CC R C C E +V CC

I_B = 0 _–

+ R

B

A Sample of Common Transistor Packages

Voltage-Divider Bias

• Voltage-divider bias is the most widely used type of bias circuit. Up to this point a separate dc source, V_BB, was used to bias the base-emitter junction because it could be varied independently of V_CC and it helped to illustrate transistor operation.

• A more practical bias method is to use V_CC as the single bias source. To simplify the schematic, the battery symbol is omitted and replaced by a line termination circle with a voltage indicator (V_CC) as shown.

Voltage-Divider Bias

Emitter Bias

Collector-Feedback Bias