Electronic Circuits and Applications

1
Electronic Circuits and Applications
2
Chapter Three Physical Electronics Of
Transistors

• Contents
• Active devices and Control Elements
• Bipolar Transistors as Control Elements
• Field-Effect Transistors as Control Elements

3
3.0 Active devices and Control Elements (1)

• 1. What is an active device?????
• An active device is a device capable of
  controlling the flow of electrical energy from a
  source to a load. If an active device is to be
  useful for increasing the power level of signal,
  then the power required at the control inputs
  must be much less than the power delivered to the
  load, the balance coming from the dc power
  sources. The combination of the active device and
  its associated power source then functions as an
  amplifier, and is said to have power gain. The
  op-amp is an active device.
• 2. What is a control element?????
• A control element is a network element that
  can be used for the control of power flow. A
  control element normally requires at least three
  electrical terminals, with the v-i characteristic
  at the output terminal pair being dependent on
  the voltage or current at the other terminals.
  The transform is a control element.

4
3.0 Active devices and Control Elements (2)

• 3. Transistor and classification
• The transistor is made from semiconductor.
  It is the most widely used active device in
  modern electronic circuits. Transistors can be
  divided into two general categories
• Bipolar transistor (BJT) ??????
• Field-effect transistor (FET) ??????

5
3.1 Bipolar Transistors as Control Elements (1)

• 3.1.1 The Physical Basis of Transistor Operation

Injection??, Diffusion??, and Collection??
A typical bipolar transistor structure is shown
as
It consists of a p-type central region, called
the base??, which is sandwiched between two
n-type regions, called the emitter??? and the
collector???. This arrangement is known as an NPN
transistor. It is also possible to construct a
complementary form, the PNP transistor, by using
p-type material for the emitter and collector
regions and n-type material for the base. In
either case, the bipolar transistor consists of
two p-n junctions that share a common region, the
base, between them.
6
3.1 Bipolar Transistors as Control Elements (2)

• To understand how the bipolar transistor operates
  as a control element, we can draw on our
  understanding of the p-n junction diode developed
  in Section 7.3. In the diode a forward bias
  produces a significant current resulting from a
  net flow of holes and electrons from the regions
  where they are majority carriers to the region
  where they are minority carriers. This injection
  process is reviewed schematically in following
  figure. 

The total terminal current I is given by the sum
of the two current components arising from the
hole flow and the electron flow.
7
3.1 Bipolar Transistors as Control Elements (3)
Under reverse bias, the p-n junction diode is
characterized by a small saturation current,
arising from the collection of minority carriers
from their respective regions by the electric
field in the space-charge layer. The magnitude of
this reverse saturation current depends on the
available concentration of minority carriers, and
is small in the diode.
The bipolar transistor works as a control element
by combining injection at one of its p-n
junctions with collection at the other p-n
junction. 
8
3.1 Bipolar Transistors as Control Elements (4)

• In the normal gain or the active gain region???
  of operation, the emitter-base junction is
  maintained in forward bias, and the
  collector-base junction is held in reverse bias.
  By doping the emitter region much more heavily
  than the base, most of the injection of minority
  carriers is made to occur into the base side of
  the junction. Thus, under forward bias
  conditions, there is a large buildup of minority
  carriers ( electrons in an NPN transistor, holes
  in a PNP transistor) on the base side of the
  emitter-base junction. While this buildup of
  electron concentration is taking place at the
  emitter end of the base region, the
  reverse-biased collector-base junction keeps the
  concentration of minority electrons very low at
  the collector end of the base region. This
  combination of a forward bias at the emitter-base
  junction and a reverse bias at the collector-base
  junction thus establishes a large concentration
  gradient of minority carriers across the base
  region. Normal thermal motions, therefore,
  produce a diffusive flow of minority electrons
  through the base region, from the emitter end,
  where they are in excess, to the collector end,
  where they are swept across the collector-base
  junction into the collector region. 

9
3.1 Bipolar Transistors as Control Elements (5)
10
3.1 Bipolar Transistors as Control Elements (6)

• Above figure illustrates this flow of electrons
  from the emitter to the base by injection??,
  across the base by diffusion??, and into the
  collector by collection??. Nearly all the
  electrons entering the base region from the
  emitter reach the collector. A small fraction,
  however, recombine?? with holes to form complete
  covalent bonds. Because of this recombination
  process, and because of the injection of holes
  from the base to the emitter, some holes must be
  supplied to the base via the base terminal. These
  holes cannot be supplied from the collector
  because they are the minority carrier there, and
  are few in numbers.
• In summary, the collector-base junction behaves
  as a reverse-biased diode whose saturation
  current is controlled by the injection of
  electrons at the emitter-base junction. The
  collector current is independent of the
  collector-base voltage, provided that a reverse
  bias is established at the collector-base
  junction. Thus, the basic property of a control
  element, in this case the dependence of the
  output (collector) current on an input variable
  (emitter current or emitter-base voltage) has
  been demonstrated.

11
3.1 Bipolar Transistors as Control Elements (7)
3.1.2 Circuit Symbols and Terminal Variables for
Bipolar Transistors
1. NPN bipolar transistor
2. PNP bipolar transistor
The voltage subscripts denote the terminals
between which the voltage is measured, with the
first subscript indicating the positive reference
terminal and the second the negative reference
terminal.
The only difference between NPN and PNP device
symbols is the direction of the arrow on the
emitter lead, which indicates the actual
direction of forward current in the emitter-base
junction. The reference directions for the
terminal currents are all defined entering the
device, irrespective of the direction of positive
current flow. Thus at least one of the terminals
current must be algebraically negative.
12
3.1 Bipolar Transistors as Control Elements (8)
3.1.3 Regions of Transistor Operation
Since the transistor has three terminals, there
are six possible ways to connect it in a given
circuit.
Input Output   Connection Input Output Connection 
B-E C-E Common-emitter C-E B-E No using
B-C E-C Common-collector E-C B-C No using
E-B C-B Common-base C-B E-B No using
There are four regions of transistor operation.
1. Active-Gain region??? E-B junction is
forward biased C-B junction is reverse biased
2.   Saturation region??? E-B junction is
forward biased C-B junction is forward biased
3.   Cutoff region??? E-B junction is reverse
biased C-B junction is reverse biased
4. Reverse region??? E-B junction is reverse
biased C-B junction is forward biased
13
3.1 Bipolar Transistors as Control Elements (9)
3.1.4 v-i Characteristics of BJT

1. The input characteristic of BJT

The input characteristic of a transistor is
similar to that of a diode, which can be
expressed as (IBSlt0).
14
3.1 Bipolar Transistors as Control Elements (10)

1. The output characteristic of BJT

In active region, there are the relationships of
currents
Where, ?F is the forward short-circuit
common-base current gain. It is a positive number
whose magnitude is slightly less than 1. ?F is
the forward short-circuit common-emitter current
gain. It is a positive number whose magnitude can
be quite large.
15
3.1 Bipolar Transistors as Control Elements (11)
3.1.5 Maximum Voltage and Current Limits
1. Maximum power dissipation PD, max (
???????PCM)
As was the case for diodes, the temperature rise
of a transistor produced by internal power
dissipation ultimately limits the allowed
voltages and currents. The maximum permissible
power dissipation for a given transistor will
depend on its physical size, its method of
construction and mounting, and the maximum
expected ambient operating temperature.
Evaluation of these thermal parameters will
produce a maximum power dissipation rating to the
device under the given set of operating
conditions. This rating serves to limit the
maximum internal temperature of the structure to
a safe level, usually from 150C to 200C in
silicon units. Because the ?F is closed to 1, in
the active-gain region, nearly all the power
dissipation occurs at the collector-base
junction. The power dissipation of a transistor
is
The operating region of a transistor whose power
dissipation is less than the PD, max is called
safe operating region?????.
16
3.1 Bipolar Transistors as Control Elements (12)
2. C-E sustaining voltage (CE??)VCEO (
???????U(BR)CEO)
When the base is opened, if the C-E voltage is
increased to magnitude that the avalanche
multiplication occurs, the value of C-E voltage
is called the sustaining voltage between
collector and emitter.
3.  Maximum collector current IC, max (
???????ICM)  
The maximum value of collector current that a
transistor can carry is limited by the current
carrying capacity of the fine wire used to
connect the active regions of the device to its
terminal leads. If excessive current occurs, this
wire will melt, causing an open circuit at one or
more of the device terminals.
17
3.1 Bipolar Transistors as Control Elements (13)

• 3.1.6 Transistor Operation in the Reverse Region
• The bipolar transistor is a somewhat
  symmetrical device, since a region of one type of
  semiconductor is placed between two regions of an
  oppositely doped semiconductor. Thus it is
  perfectly possible to interchange the roles of
  collector and emitter and operate the transistor
  in reverse with the Emitter-Base junction
  reverse-biased and the Collector-Base junction
  forward-biased. Normally, however, the emitter
  and collector are fabricated very differently,
  with the result that superior amplifying
  characteristics are obtained in the normal
  connection, and very poor amplifying
  characteristics are obtained in reverse region.
  Nevertheless, transistor characteristics do
  continue into the third quadrant (the reverse
  region), and a current gain  ?R can be defined
  there analogous to ?F in the active-gain region.
  In our course we generally ignore reverse-region
  operation, as it is encountered relatively rarely
  in signal-processing circuits.

18
3.2 Field-Effect Transistors as Control Elements
(1)
In Chapter 7 we found that the current and
voltage in an n-type semiconductor bar were
linearly related through the conductance of the
bar
Since the conductance G in the two-terminal bar
is a constant, there is no possibility of
utilizing the bar as a control element. However,
through a more complex structure, it becomes
possible to vary the conductance between two
terminals in response to a third-terminal control
voltage, and this forms the basic mechanism of
operation for the group of semiconductor control
elements known as field-effect transistors
(FETs). Examination of above equation shows that
if the conductance of a bar is to be controlled,
there are two possible ways in which it may be
accomplished. Either the carrier concentration n
or the geometry (A/L) must be made responsive to
voltage. The electronic charge q is a fundamental
constant not subject to control, and the mobility
?e is also an independent constant in the
first-order analysis of FETs
19
3.2 Field-Effect Transistors as Control Elements
(2)
3.2.1 Junction Field-Effect Transistors ????????
A schematic diagram of an N-type channel junction
FET (N??????????) is shown as
A JFET (junction field-effect transistor) is a
unipolar, voltage-controlled transistor that uses
an induced electrical field to control current.
The current through the JFET is controlled by its
gate voltage, vGS. The more negative the voltage,
the smaller the current. A JFET consists of a
length of n-type or p-type doped semiconductor
material called a channel. The ends of the
channel are called the source and the drain. In
an n-channel JFET, the gate is p-type material
surrounding the channel. In a p-channel device it
is n-type material. There are two types of JFET
N-channel JFET and P-channel JFET
It operates on the basis of varying the
cross-sectional area of a semiconductor bar. The
basic bar is shown as n-type, and this portion of
the device is called the channel. The terminals
at the ends of the bar are labeled source and
drain. Along one side of the bar is a p-type
region, which forms a p-n junction along the
length of the bar. The p-region is termed the
gate and is connected to the third or control
terminal of the structure. This general form of
FET is known as the junction field-effect
transistor JFET.
20
3.2 Field-Effect Transistors as Control Elements
(3)
3.2.2 Metal-Oxide-Semiconductor Field-Effect
Transistors
???????? ???(??MOS?)
In this structure a metal gate electrode is
spaced from the conducting channel by a thin
insulator. Two regions, more heavily doped than
the channel and denoted n, serve as the source
and drain contracts. Typically, the insulator is
a layer of silicon oxide approximately 0.1 micron
(??) thick, grown directly on the n-type silicon
surface. This structure is called the
insulator-gate field-effect transistor (IGFET),
or the metal-oxide-semiconductor (MOS)
field-effect transistor. The p-type substrate
(??) is required to give the assembly sufficient
physical size and strength for handling and does
not play a direct role in the electrical
performance.
21
3.2 Field-Effect Transistors as Control Elements
(4)

• The MOSFET substrate is usually connected to its
  most negatively biased part, usually the source
  lead. An N-channel MOSFET has an inward-pointing
  substrate arrow. A P-channel MOSFET has an
  outward-pointing arrow.
• N-channel and P-channel MOSFET are identical
  except their voltage polarities are reversed.
• There are two main categories of MOSFET
• Depletion MOSFET ???MOSFET
• Enhancement MOSFET ???MOSFET

22
3.2 Field-Effect Transistors as Control Elements
(5)

• 1. Enhancement MOSFET ???MOS????
• An enhancement MOSFET has no physical channel
  between the drain and the source, unlike the
  depletion MOSFET. Instead, the substrate extends
  all the way to the silicon dioxide (SiO2) layer.
• An enhancement MOSFET works only with positive
  gate- source voltages. A positive gate-source
  voltage above a minimum threshold value (VT)
  creates an inversion layer of charges in the
  substrate region adjacent to the SiO2 layer. The
  conductivity of this induced channel is enhanced
  by increasing the gate-source voltage on the
  positive side.
• Enhancement MOSFETs are extensively used in
  digital circuits and large-scale integration
  (LSI) applications.

23
3.2 Field-Effect Transistors as Control Elements
(6)

• 2. Depletion MOSFET ???MOS????
• Like a JFET, a depletion MOSFET consists of
  a length of P-type (for a P-channel MOSFET) or
  N-type (for an N-channel MOSFET) semiconductor
  material called the channel, which is formed on a
  substrate of the opposite type. This substrate
  reduces the channel between the drain and the
  source. Free electrons from the source to the
  drain have to pass through this narrow channel.
  The two ends of the channel are called the source
  and the drain.
• The metal gate of the MOSFET is insulated
  from the channel by a thin layer of silicon
  dioxide (SiO2), so negligible gate current flows
  during operation. The more negative the
  gate-source voltage is in an n-channel MOSFET,
  the more the gate depletes the channel of its
  conduction electrons, thus reducing the drain
  current. At VP, the channel is totally depleted
  and the drain-to-source current is cut off.
  Alternatively, the more positive the voltage, the
  bigger the channel size, thus increasing the
  current flow. A P-channel MOSFET is identical to
  an N-channel MOSFET, except its voltage
  polarities are opposite.
• Depletion MOSFET are used in automatic-gain
  control (AGC) circuits.

24
3.2 Field-Effect Transistors as Control Elements
(7)

• 3.2.3 Circuit Symbols and Terminal Variables of
  Field-Effect
• N-Channel JFET N????????It is always operated
  with the gate-source p-n junction reverse biased.
  The arrowhead terminal is the gate. The more
  negative the gate-source voltage, the narrower
  the channel becomes, due to the effect of
  depletion layers. When the gate-source voltage
  reaches a value of VGS(off), the depletion layers
  touch, cutting off the drain current.

25
3.2 Field-Effect Transistors as Control Elements
(8)

• P-Channel JFET P????????
• Its operation is identical to that of an
  n-channel JFET, except that the gate-source must
  be positively biased. The arrowhead terminal is
  the gate.
• The more positive the gate-source voltage, the
  narrower the channel becomes, due to the effect
  of depletion layers. When the gate-source voltage
  reaches a value of VGS(off), the depletion layers
  touch, cutting off the drain current.

26
3.2 Field-Effect Transistors as Control Elements
(9)
3. Depletion P-MOSFET P?????MOS??????
4. Depletion N-MOSFET N?????MOS??????
5. Enhanced P-MOSFET  P?????MOS??????
6. Enhanced N-MOSFET  N?????MOS??????
27
3.2 Field-Effect Transistors as Control Elements
(10)
3.2.4 The v-i Characteristics of Field-Effect
Transistor
1. N-channel JFET
Operating region VPltvGSlt0, vDSgt0
Transfer Characteristic
Output Characteristic
For vDSltvGS-VP and vGS-VPgt0 Region I
For vDSgtvGS-VP and vGS-VPgt0 Region II
Variable resistance
Saturation
28
3.2 Field-Effect Transistors as Control Elements
(11)
2. P-channel JFET
Operating region VPgtvGSgt0, vDSlt0
Transfer Characteristic
Output Characteristic
For vDSgtvGS-VP and vGS-VPlt0 Region I
For vDSltvGS-VP and vGS-VPlt0 Region II
Variable resistance
Saturation
29
3.2 Field-Effect Transistors as Control Elements
(12)
3. n-channel depletion MOSFET
Operating region VPltvGSgt0, vDSgt0
Transfer Characteristic
Output Characteristic
For vDSltvGS-VP and vGS-VPgt0 Region I
For vDSgtvGS-VP and vGS-VPgt0 Region II
Variable resistance
Saturation
30
3.2 Field-Effect Transistors as Control Elements
(13)
3. n-channel depletion MOSFET
Operating region VPgtvGS vDSlt0
Transfer Characteristic
Output Characteristic
For vDSltvGS-VP and vGSltVP Region II
For vDSgtvGS-VP and vGSltVP Region I
Variable resistance
Saturation
31
3.2 Field-Effect Transistors as Control Elements
(14)
5. n-channel enhancement MOSFET
Operating region VTltvGS, vDSgt0
Output Characteristic
Transfer Characteristic
For vDSltvGS-VT and vGS-VTgt0 Region I
For vDSgtvGS-VT and vGS-VTgt0 Region II
Variable resistance
Saturation
32
3.2 Field-Effect Transistors as Control Elements
(15)
6. p-channel enhancement MOSFET
Operating region VT gtvGS, vDSlt0
Transfer Characteristic
Output Characteristic
For vDSgtvGS-VT and vGS-VTlt0 Region I
For vDSltvGS-VT and vGS-VTlt0 Region II
Variable resistance
Saturation
33
3.2 Field-Effect Transistors as Control Elements
(16)
3.2.5 Maximum Limits of Voltage and Current
1. PD, maxthe maximum allowable dissipated
power.
vDS ? iDltPD, max
2. VDS, max the reverse breakdown voltage
vDS ltVDS, max
3. ID, max the limited drain current
iDltID, max
34
Chapter Three Physical Electronics Of Transistors
End of Chapter Three