ELECTRONIC
CIRCUITS
Solid State Fundamentals
■ Subtopics: 1) Semiconductor Diode, and 2) Diode Biasing and Models
■ At the end of the chapter, the learner should be able to:
– Explain the parameters of a semiconductor diode
– Analyze diode circuits using different equivalent models
Semiconductor Diode
Subtopic #1
Definition:
A diode is made from a small piece of
semiconductor material in which half is doped as a
p region and half is doped as an n region with a pn
junction and depletion region in between.
• The p region is called the anode and is
connected to a conductive terminal.
• The n region is called the cathode and is
connected to a second conductive terminal.
Definition.
Establishing predetermined voltages or currents at
various points of an electronic circuit for the
purpose of establishing proper operating conditions
in electronic components.
Definition.
Fixed DC voltage or current applied to a terminal of
an electronic component in order to establish
proper operating conditions for the component.
The condition
that allows
current through
the pn junction.
1. The negative side of VBIAS is connected to the n
region of the diode and the positive side is
connected to the p region.
2. The bias voltage, VBIAS, must be greater than the
barrier potential.
The condition
that essentially
prevents current
through the
diode.
Definition.
The extremely small current that exists in reverse bias
after the transition current dies out is caused by the
minority carriers in the n and p regions that are produced
by thermally generated electron-hole pairs.
Also called leakage current or reverse saturation
current.
• Normally, the reverse current is so small that it can be
neglected.
• However, if the external reverse-bias voltage is
increased to a value called the breakdown voltage
(peak reverse voltage or PIV, peak inverse voltage),
the reverse current will drastically increase.
• The high reverse-bias voltage imparts energy to the free
minority electrons.
• They collide with atoms with enough energy to knock
valence electrons out of orbit and into the conduction
band.
• The newly created conduction electrons are also high in
energy and repeat the process.
• If one electron knocks only two others out of their valence
orbit during its travel through the p region, the numbers
quickly multiply.
• As these high-energy electrons go through the depletion
region, they have enough energy to go through the n
region as conduction electrons, rather than combining with
holes.
• The multiplication of conduction electrons just discussed is
known as the avalanche effect, and reverse current can
increase dramatically if steps are not taken to limit the
current.
• When the reverse current is not limited, the resulting
heating will permanently damage the diode.
Diode Biasing and Models
Subtopic #2
• There is a slight increase in VF above 0.7 V as the
current increases due mainly to the voltage drop
across the dynamic resistance.
• Point A corresponds to a zero-bias condition.
• Point B corresponds to where the forward voltage is
less than the barrier potential of 0.7 V.
• Point C corresponds to where the forward voltage
approximately equals the barrier potential.
• Unlike a linear resistance, the resistance of the
forward-biased diode is not constant over the entire
curve.
• Because the resistance changes as you move along
the V-I curve, it is called dynamic or ac resistance.
• The dynamic resistance of a diode is designated 𝑟′𝑑 .
(Greatest below the knee)
• With 0 V across the diode, there is no reverse current.
• When a reverse-bias voltage is applied across a diode,
there is only an extremely small reverse current (IR)
through the pn junction.
• As you gradually increase the reverse-bias voltage,
there is a very small reverse current and the voltage
across the diode increases.
• When the applied bias voltage is increased to a value
where the reverse voltage across the diode (VR)
reaches the breakdown value (VBR), the reverse
current begins to increase rapidly.
• A typical rectifier diode has a breakdown voltage of
greater than 50 V.
• Some specialized diodes have a breakdown voltage
that is only 5 V.
𝐼𝑆1 = 𝐼𝑆
𝑘(𝑇
−𝑇
)
1
0
𝑒
where:
𝐼𝑆1 = saturation current at new temperature
𝐼𝑆 = saturation current at room temperature
𝑘 = 0.07 per degree Celsius
𝑇1 = new temperature
𝑇0 = room temperature
𝑉𝑇𝐻1 = 𝑉𝑇𝐻 + 𝑘(𝑇1 − 𝑇0 )
where:
𝑉𝑇𝐻1 = threshold voltage at new temperature
𝑉𝑇𝐻 = threshold voltage at room temperature
𝑘 = -2.5 mV/°C for Ge and -2 mV/°C for Si
𝑇1 = new temperature
𝑇0 = room temperature
1. The Ideal Diode Model
2. The Simplified (Practical) Diode Model
3. The Piecewise Linear (Complete) Diode Model
[1] A. Malvino and D. Bates, Electronic Principles, New York: McGraw-Hill Education,
2016.
[2] A. Agarwal and J. Lang, Foundations of Analog and Digital Electronic Circuits, San
Francisco: Elsevier Inc, 2005.
[3] M. E. Schultz, Grob's Basic Electronics, New York: McGraw-Hill Education, 2011.
[4] S. Gibilisco, Teach Yourself Electricity and Electronics, New York: McGraw-Hill
Companies, 2002.
[5] T. L. Floyd, Electronic Devices (Conventional Current Version), New Jersey: Prentice
Hall, 2012.
[6] R. Boylestad and L. Nashelsky, Electronic Devices and Circuit Theory, New Jersey:
Pearson Education, Inc., 2013
QUESTIONS?