Transcript i D

Chapter 2 Diodes and Diode Circuits
Outline
 Diode characteristics
 Load-line analysis
 The ideal-diode model
 Rectifier circuits
 Wave-shaping circuits
 Linear small-signal equivalent circuit
 Basic semiconductor concepts
 Physics of the junction diode
3.1 Diode characteristics
 Forward bias
 If the voltage vD across the diode is
positive, relatively large amounts of
current flow for small voltages.
 Reverse bias
 For moderately negative values of vD,
the current iD is very small. (reversebiased region)
 For sufficiently large reverse-bias
voltage vD, currents of large magnitude
flow. (reverse-breakdown region)
Figure 3.1 Semiconductor diode
3.1 Diode characteristics
 Small-signal diodes
 The voltage and current scales for the
forward-bias region are different than
for the reverse-bias region.
 The knee voltage
 The breakdown voltage
 Diodes that are intended to operate in
the breakdown region are called Zener
diodes.
Figure 3.2 volt-ampere characteristics for
a typical small-signal diode at 300K
 Zener diodes are used in applications
which a constant voltage in
breakdown is desirable.
Figure 3.3 Zener diode symbol.
3.2 Load-line analysis
 The volt-ampere characteristics is nonlinear, many of the techniques in
basic circuit theory do not apply.
 Much of the study of electronics is concerned with techniques for
analyzing circuits containing nonlinear elements.
 Graphical methods provide one approach to analyzing nonlinear circuits.
Example
Think about:
1.
For the left part, using KVL write an
equation for iD and vD.
2.
For the right part, the volt-ampere
characteristics of the diode is given.
3.
If we want to solve for the current iD
flowing through and the voltage vD
across the diode, what does that mean?
3.2 Load-line analysis
 The load line
Vss=RiD+vD
 The operating point is the intersection of the load line and
diode characteristic.
 The operating point represents the simultaneous solution
of equation (3.1) and the diode characteristics.
Exercise: P90 3.1 (a)(b)(c) choose any two of them
3.2 Load-line analysis
2
1
3
Figure 3.7 Diode characteristic for Exercise 3.1.
3.2 Load-line analysis
Think over
How do you solve for the current flowing through and the
voltage across the diode in a circuit if besides the diode, there
are more than one source and one resistor?
Preparation before using the load-line analysis:
Remove the diode, and try to find the Thevenin’s equivalent circuit for
the remaining circuit.
Example
R1
R2
iD
IS
3.2 Load-line analysis
Figure 3.27 Analysis of a circuit containing a singular nonlinear element can be
accomplished by load-line analysis of a simplified circuit.
3.3 The ideal diode model
The load-line analysis of diode circuits
provides insight and accurate results, yet it
is not suitable for the rapid analysis of
circuits containing more than one diodes.
The ideal diode model —— in the forward
direction, it is a perfect conductor with zero
voltage drop (short circuit) while in the
reverse direction, it acts as open circuit.
Figure 3.8 V-A characteristics for ideal diode
When analyzing a circuit containing ideal diodes, we may not know in
advance which diodes are on and which are off. Thus, we are forced to make
considered guesses. Then we analyze the circuit to find the currents in the
diodes assumed to be on and the voltages across the diodes assumed to be
off.
3.3 The ideal diode model
Example: Determine the diode states for the circuits shown below.
Assume ideal diodes.
D1 is on
D2 is off
D3 is off, D4 is off
Figure 3.10 Circuits for Exercise 3.4
3.4 Rectifier circuits
Half-wave rectifier circuits
 converts ac power into dc power
 forms the basis for electronic power supplies and battery-charging circuits
Figure 3.11 Half-wave rectifier with resistive load
3.4 Rectifier circuits
Half-wave rectifier circuits with smoothing capacitor
3.4 Rectifier circuits
Full-wave rectifier circuits
Full-wave rectifier circuits using
the diode bridge
3.8 Linear small-signal equivalent circuits

In this course, we mainly concern with electronic circuits (especially
amplifiers) in which dc supplier voltages are used to bias a nonlinear
device at an operating point and a small ac signal is injected into the
circuits.
 DC supply voltage —— bias the device at a suitable operating point
 AC signal —— what we want

The shockley equation
i D  I S (e
vD
VT
 1)
Quiescent point
Q-point
IS: saturation current
VT: thermal voltage, about 26mV
Figure 3.31 Diode characteristic, illustrating the Q-point.
3.8 Linear small-signal equivalent circuits
1. DC resistance RD
RD=VDQ/IDQ
Q2
RD changes with the change of the current
flowing through the diode.
Q1
Think over: What is the relationship between the DC
resistances for forward bias and reverse bias?
2. Small signal resistance rd
rd 
dvD
diD
vD vDQ
The slope of the v-i characteristics evaluated
at the Q-point.
Exercise: Show rd=VT/IDQ
3.8 Linear small-signal equivalent circuits
Notation for currents and voltages in electronic circuits.
 vD and iD represent the total instantaneous voltage and current.
VDQ and IDQ represent the dc voltage and current at the Q point.
vd and id represent the (small) ac signals.
mA
9
7
Exercise:
5
Write the expression for IDQ, id(t) and iD(t).
20
Figure 3.32 Illustration of diode currents
ms
3.9 Basic semiconductor concepts
Materials used for the fabrication of electronic devices
Silicon (Si), Gemanium (Ge), Gallium arsenide (GaAs)
Intrinsic (pure) silicon － semiconductor
If voltage is applied to intrinsic
silicon, current flows. However,
the number of free electrons is
relatively small.
Figure 3.36 Intrinsic silicon crystal.
3.9 Basic semiconductor concepts
At room temperature, a small fraction of the electrons gain sufficient
thermal energy to break loose from their bonds.
Figure 3.37 Thermal energy can break a bond,
creating a vacancy and a free electron,
both of which can move freely through the crystal.
3.9 Basic semiconductor concepts
A hole can be regarded as a positive charge carrier that is free to move through the
crystal, whereas bound electrons can move only if a vacancy exists nearby.
In an intrinsic semiconductor, an equal
number of holes and free electrons are
available to move easily through the crystal.
When an electric field is applied to the
crystal, both types of carriers contribute to
the flow of current.
Generation — covalent bonds broken and free electrons and holes generated
Recombination — free electrons and holes combine to form a filled covalent bond
At a given temperature, an equilibrium exists at which the rate of recombination
equals the rate of generation of charge carriers.
The conductivity of an intrinsic semiconductor increases with temperature.
3.9 Basic semiconductor concepts
Extrinsic semiconductor can be obtained by adding small amount of suitable
impurities to the crystal greatly affects the relative concentration of holes
and electrons. (doping)

N-type semiconductor material
(phosphorus)
 donor: provide free electrons
 majority carrier – electrons
 minority carrier – holes
Figure 3.39 n-type silicon is created by adding valence five impurity atoms.
3.9 Basic semiconductor concepts

P-type semiconductor material
(Boron)
 acceptor: accept an extra electrons
 majority carrier – holes
 minority carrier – electrons
Figure 3.40 p-type silicon is created by adding valence three impurity atoms.

Diffusion: a concentration of charge carriers tends to spread with time
 Drift: The average motion of the charge carriers due to an applied electric field.
3.9 Basic semiconductor concepts
 The unbiased PN junction
 Diffusion occurs if a concentration
gradient exists.
 charges builds up on the p(n)-side
 An electric field is created from n  p
 A depletion region is formed at the
junction
 The primary effect of the electric field in
the depletion region is to repel the
further diffusion of majority carriers
across the junction.
 With no extra voltage applied, two equal,
but opposite, currents cross the junction,
so that zero net current flows.
3.9 Basic semiconductor concepts
 The PN junction with reverse bias
 The applied voltage helps the built-in barrier field in the depletion region.
 The majority carriers are pulled back from the junction
 The depletion region becomes wider.
 The minority carriers contribute to current under reverse bias.
 The reverse current is almost independent of the magnitude of the applied
voltage since the current is limited by the number of minority charges.
i D  I S (e
vD
VT
 1)
iD   I S
Typical Is for silicon is 10-14A.
3.9 Basic semiconductor concepts
 The PN junction with forward bias
 Forward bias acts in opposition to the built-in field in the depletion region.
 The depletion region becomes narrower.
 A large current flows across the junction.
 In practice, a barrier exists even with forward bias. If sufficient forward
bias were applied to reduce the barrier to zero, an excessively large current
would flow.
Summary
 Diodes are two-terminal devices that conduct current easily in one
direction, but not in the other.
 At room temperature, a small-signal silicon diode has a voltage of
approximately 0.6V when carrying current in the forward direction.
Under reverse bias, the current is very small, typically 1nA.
 Zener Diodes are intended to operate in the breakdown region.
 Circuits containing a nonlinear device can be analyzed using a graphical
technique called a load-line analysis.
 The ideal diode model is a short circuit for forward currents and an open
circuit for reverse voltages.
 Rectifier circuits are useful for converting ac into nearly constant dc.
Summary
 The analysis of nonlinear electronic circuits is often accomplished in
two steps: First, the dc operating point is determined, and a linear
small-signal equivalent circuit is found; second, the equivalent circuit is
analyzed.
 Dynamic (small-signal equivalent) resistance for a diode
 The Shockley equation relates the voltage and current in a PN junction.
 Carriers move through a semiconductor by diffusion when a
concentration gradient exists.
 When an electric field is applied, the carriers move by drift.
 Exercise
3.1, 3.2, 3.9(a)(b), 3.13, 3.15, 3.20, 3.53(optional), 3.63, 3.64, 3.68,
3.69, 3.75