CHAPTER 1 – INTRODUCTION

TO DIODES
TYPES OF DIODES
 CONTENT
1. SEMICONDUCTOR MATERIALS AND 3. DC MODEL AND ANALYSIS
PROPERTIES
 IDEAL MODEL
 ELEMENT & COMPOUND
 PIECEWISE LINEAR MODEL
SEMICONDUCTOR
 CONSTANT VOLTAGE DROP MODEL
 INTRINSIC & EXTRINSIC
SEMICONDUCTORS 4. AC MODEL
 P-TYPE AND N-TYPE 5. OTHER TYPES OF DIODES
SEMICONDUCTORS  SOLAR CELL
 DRIFT & DIFFUSION CURRENTS  PHOTODIODE
2. THE PN JUNCTION  LIGHT-EMITTED DIODE (LED)
 EQUILIBRIUM PN JUNCTION  SCHOTTKY BARRIER DIODE
 REVERSED-BIASED & FORWARD-  ZENER DIODE
BIASED PN JUNCTION
 IV RELATIONSHIP
ELEMENT & COMPOUND SEMICONDUCTORS

Compound semiconductor is
composed of elements from two
or more different groups of
periodic table.
 INTRINSTIC SEMICONDUCTORS
 Intrinsic semiconductor is a pure, single-crystal semiconductor with no impurities or lattice
defects.
 In an intrinsic semiconductor, the no. of holes equals to the no. of electrons. The
concentrations of electrons and holes are represented as ni, measured in cm-3.
 The valence electrons of semiconductor are shared among its atoms. This sharing of
electrons is known as covalent bonding.

Valence electrons
are electrons located
at the most outer
shell of an atom.
Silicon has 4 valence
electrons.
Valence electrons in Silicon Crystal lattice structure of Silicon
are shared in covalent bond
 ENERGY BAND DIAGRAM
 At T = 0K, all valence electrons occupy the valence band. Semiconductor behaves like an
insulator.
 When T increases, the valence electrons gain thermal energy. When the energy is
sufficient enough, the covalent bond can be broken. An electron-hole pair is generated.
 The valence electrons are now known as free electrons and exists in conduction band.
 The minimum energy needed by an electron to become a free electron from a valence
electron, is known as bandgap energy (Eg).

Energy band diagram of semiconductor Crystal lattice structure of Silicon

 EXTRINSTIC SEMICONDUCTORS
 Extrinsic semiconductor is a semiconductor having impurity in its crystal.
 An intrinsic semiconductor can be turned into extrinsic semiconductor when it is doped with
controlled amount of dopants (impurities).
 Doping semiconductor with donor atoms (Group V elements – P, As, Sb) creates n-type
semiconductor. Doping semiconductor with acceptor atoms (Group III elements – B, Al, Ga)
creates p-type semiconductor.
 Doping concentration for donor atoms (ND) and acceptor atoms (NA) is measured in cm-3.

Group V elements has 5 Group III elements has 3

valence electrons. When valence electrons. When
this impurity atom this impurity atom
displaced a Si atom, the displaced a Si atom, the
4 valence electrons 3 valence electrons
made covalence bonds made covalence bonds
with neighboring Si with neighboring Si
atoms, leaving some atoms, creating some
free electrons. Negative-charged electrons in n- holes.
type silicon and positive-charged
holes in p-type silicon
 DIFFUSION AND DRIFT CURRENTS
 Diffusion current is the current in semiconductor caused by variations in the dopant
concentration. Carriers flow from region of higher concentration to a region of lower
concentration.
 Drift current is the electric current, or movement of charged carriers, which is due to the
applied electric field. The direction of applied electric field will determine the direction
of carrier.
 Current in semiconductor material is normally measured as current density (current per
unit area of cross section, with unit in A/cm2]

Drift current in n-type and p-type Diffusion current in n-type and p-

semiconductor type semiconductor
 EXCESS CARRIERS (UNDER THERMAL NON-
EQUILIBRIUM)
 Carrier Generation = process whereby
electrons and holes are created. Conduction Band

 Carrier Recombination = process

whereby electrons and holes are
annihilated.
 When an external excitation is applied,
an electron-hole pair is generated.
These additional electrons and holes
are called excess electrons and excess
holes.
Valence Band
 These excess electrons and holes will
(1) Electron-Hole pair
not last forever. They will recombine generation
again to achieve equilibrium or (2a) Excess hole
(2b) Excess electron
steady-state value. This forms the basis (3) Electron-Hole pair
of solar cells and photodiodes. recombination
 EQUILIBRIUM PN JUNCTION
 Majority carrier in p-type region
is holes while majority carrier in
n-type region is electrons.
 At the boundary of pn junction,
some holes diffuse from p-
region to n-region, while some
electrons diffuse from n-region
to p-region. This creates a
space charge region
(depletion layer). An electric
field exists in the region due to
the static charges.
 The potential difference across
this region is called built-in
voltage, given by
Vbi = VT ln (NAND/ni2)

where VT = thermal voltage In equilibrium, the net current is zero. The electron drift current and
electron diffusion current exactly balance out. Similarly, hole drift
= 0.026V at T = 300K
current and hole diffusion current also balance each other out.
 BIASED PN JUNCTION
 The pn junction is in forward-bias when +ve terminal of applied voltage is connected to p-
region while –ve terminal is connected to n-region. If the polarity is reversed, the pn junction
is in reverse-bias.
 In forward-biased pn junction, holes in p-region and electrons in n-region are pushed
towards the depletion layer. The width of the layer becomes narrower. When the applied
voltage is larger than cut-in voltage (V), minority carriers in the space-charge region will
diffuse into the respective region, thus creating a current in the pn junction.
 In reversed-biased pn junction, holes in p-region and electrons in n-region are attracted
towards the supply terminals. The width of the layer becomes wider and now function as an
insulation layer, preventing diffusion from taking place. Ideally, no current flow in the pn
junction.

The cut-in voltage (or

turn-on voltage) is the
minimum voltage
needed to turn on the
diode, i.e., overcome
the barrier and cause
current to flow.
 BREAKDOWN VOLTAGE IN REVERSE-BIASED
PN JUNCTION
 The maximum reverse bias voltage that can be
applied to a pn junction is limited by
breakdown.
 When the junction is reverse-biased, the electric
field in the space charge region increases. If
the electric field is large enough, covalent
bonds will be broken and electron-hole pairs
will be generated.
 Electrons are then swept into n-region while
holes are swept into p-region by the electric
field, generating large reverse-biased current.
The corresponding applied voltage is referred
to as breakdown voltage.
 There are two mechanisms that can cause
breakdown – avalanche multiplication
(avalanche breakdown) and tunneling of
carriers (Zener breakdown).
 APPLICATIONS OF PN JUNCTION
 A range of devices can be created using the  The first device to be explored is the pn
principles of pn junction. junction diode, which symbol is shown in
Figure below.
 ANALOGY OF PN JUNCTION DIODE
 A diode can be thought as a directional valve (check valve).
 In the forward direction, the diode (check valve) will exhibit a small resistance,
which will be a function of V.
 In the reverse direction, the diode resistance is very large and is treated as infinite
(i.e., diode is replaced by an open circuit.
 IV CHARACTERISTICS OF PN-JUNCTION DIODE

 In forward bias operation, diode will

not conduct significant current until
the bias reaches about 0.7V, which
is the diode internal barrier voltage.
After that point, forward current
increases rapidly for a very small
increase in voltage.
 In reverse bias operation, diode
blocks current except for an
extremely small leakage current. The
current blocking continues until
some breakdown voltage is
reached, resulting a sudden
increase in reverse current. IS = reverse-bias saturation current (in the range of 10-18 to 10-12).
(Actual value depends on doping concentrations and cross
sectional of pn junction)
n = ideality factor (in the range between 1 and 2)
vD = voltage across diode
VT = 26mV (thermal voltage at room temperature)
 EXAMPLE 1
Determine ID1 , ID2 , VD1 , VD2 for IS1 = IS2 = 10−13 A.

𝑰𝑫𝟏 = 𝑰𝑫𝟐 = 1 mA
vD

I D = I s (e VT
− 1)

𝟏𝟎−𝟑
 𝑽𝑫𝟏 = 𝑽𝑫𝟐 = 𝟎. 𝟎𝟐𝟔 𝒍𝒏 = 𝟎. 𝟓𝟗𝟗 𝑽
𝟏𝟎−𝟏𝟑
EXAMPLE 2

Determine ID1 , ID2 , VD1 , VD2 for IS1 = IS2 = 10−13 A.

𝐼𝑖
𝐼𝐷1 = 𝐼𝐷2 = = 0.5𝑚𝐴
2

0.5 × 10−3
𝑉𝐷1 = 𝑉𝐷2 = 0.026 𝑙𝑛 −13
= 0.581 𝑉
10
EXAMPLE 3
The reverse-saturation current of each diode is IS = 𝟔 𝒙 𝟏𝟎−𝟏4 𝐀. Determine the
input voltage 𝑽𝑰 to get an output voltage 𝑽𝑶 = 0.635 V.

I
𝟎.𝟔𝟑𝟓
ID3 = (𝟔 × 𝟏𝟎−𝟏𝟒 ) exp = 2.426 mA
𝟎.𝟎𝟐𝟔
𝟎. 𝟔𝟑𝟓
𝑰𝑹 = = 𝟎. 𝟔𝟑𝟓 𝒎𝑨
𝟏𝒌
𝐼𝐷1 = 𝐼𝐷2 = 2.426 + 0.635 = 3.061 𝑚𝐴

𝟑. 𝟎𝟔𝟏 × 𝟏𝟎−𝟑
𝑽𝑫𝟏 = 𝑽𝑫𝟐 = 𝟎. 𝟎𝟐𝟔 𝒍𝒏 = 𝟎. 𝟔𝟒𝟏 𝑽
𝟔 × 𝟏𝟎−𝟏𝟒
𝑉𝐼 = 2(0.641) + 0.635 = 1.917 𝑉
EXAMPLE 4
(a) The diode cut − in voltage is 0.7 V . Determine the diode current 𝑰𝑫 and
diode voltage 𝑽𝑫 . (b) What would happen if R1 is increased to 50 kΩ.

10 kΩ
EXAMPLE 5
The diode cut − in voltage is 0.7 V . Determine the diode current 𝑰𝑫 and diode
voltage 𝑽𝑫 .

I
EXAMPLE 6
The diode cut − in voltage is 0.7 V . The diode is on for supply voltage, Vps
between 5 and 10 V. The min doped current is 2 mA. The max dissipated power
allowed in the diode is 1 mW. Determine the required resistance values, R1 and R 2 .

I
EXAMPLE 7
The diode cut − in voltage is 0.7 V . Determine I and 𝑽𝑶 .

I
 DC ANALYSIS & AC ANALYSIS

DC ANALYSIS AC ANALYSIS (SMALL-SIGNAL ANALYSIS)

 The analysis determines the behavior or  The analysis determines the small-signal
response of a circuit with only DC supply response of a circuit with only AC supply
(voltage or current) and no AC supply. (voltage or current) and no DC supply.
 The results of this analysis is generally  In AC analysis, non-linear components (diodes
referred to as bias operating points or and transistors) have to be linearized at the DC
quiescent point (Q-point). operating point.
 In DC analysis,  In AC analysis,
 All AC voltage sources are shorted-  All DC voltages sources are shorted-
circuited circuited
 All AC current sources are opened-  All DC current sources are opened-
circuited
circuited
 All large capacitors are opened-circuited
 All large capacitors are short-circuited

The results from DC and AC analysis need to be summed

together to produce total instantaneous value.
 DIODE DC ANALYSIS – IDEAL MODEL
 An ideal diode will “conduct” or
“turned on” when the voltage across
diode is greater than zero (forward
bias). Current then flows through the
diode. Under forward bias, the ideal
diode is modelled as a “closed-circuit
or “short-circuit”.
 An ideal diode will “turned off” when
the voltage across diode is less than
zero (reverse bias). No current flow.
Under reverse bias, the ideal diode is
modelled as an “opened-circuit.
 DIODE DC ANALYSIS – PWL MODEL
 In piecewise linear (PWL) model, the current-
voltage characteristics of a real diode is
approximated using two linear segments.
 The diode will “conduct” or “turned on”
when the voltage across diode is greater
than cut-in voltage (V). Current then flows
through the diode.
DIODE DC ANALYSIS – CVD MODEL
 Constant voltage drop (CVD) model is similar
to PWL model, except that the forward
diode resistance is considered to be 0.
Hence, a vertical slope at cut-in voltage (V).
 The diode will “conduct” or “turned on”
when the voltage across diode is greater
than cut-in voltage (V). Current then flows
through the diode.
DIODE DC ANALYSIS – SUMMARY

(CVD
model)
 DC ANALYSIS METHODOLOGY
 Identify the state of diodes (ON or OFF). If unsure, make assumptions.
 Replace diode with appropriate model (ideal, PWL or CVD).
 Solve I and V using KCL, KVL and other circuit techniques.
 Check your assumptions. Make sure that there is no contradiction in DC
operating points.

If real diode model is used,

Example of contradictions: - When VD and ID are both unknowns, use
- Diode is ON but current flows in the iterative analysis or graphical analysis to
opposite direction (i.e., negative determine its operating point.
current), indicating diode supposed - When one value is known, use KCL and
to be OFF. KVL techniques to solve the other
- Diode is OFF but voltage across diode unknown.
is positive (forward biased), indicating
diode supposed to be ON.
- When more than one diodes are ON,
a short-circuit occurs.
 DC ANALYSIS (REAL DIODE) – GRAPHICAL
SOLUTION
 Load line = a linear relationship between ID
and VD for a given voltage supply and
resistance, R.
 Load line equation can be obtained by
deriving the KVL equation from the circuit.
VDD = IDR + VD
 The load line must be plotted on the same
graph as the IV characteristics of the
diode.
 When VD = 0, ID = VDD/R → y-intercept (VDQ, IDQ)
 When ID = 0, VD = VDD → x-intercept
 The intersection of load line and diode IV
curve is the operating point (Quiescent
point) of the circuit.
 SMALL-SIGNAL ANALYSIS
 Small-signal analysis is
performed after dc
analysis is carried out to
determine its operating Q-
point.
 At Q-point, the diode’s
small-signal resistance, rd
can be determined.
 SMALL-SIGNAL ANALYSIS – AC MODEL
 In small-signal analysis, replaced
the nonlinear diode with
linearized small-signal resistance,
rd in the ac equivalent circuit.
 The circuit can then be solved
using KCL and KVL techniques.
 Equivalent Circuits

When ac signal is small, the dc operation can be decoupled from

the ac operation.
First perform dc analysis using the dc equivalent circuit (a).
Then perform the ac analysis using the ac equivalent circuit (b).
Example 8
OTHER TYPES OF DIODES
 OTHER DIODE: PHOTOVOLTAIC CELL
 A solar cell is a pn junction device with no direct
applied voltage across the junction.
 The pn junction has the ability to convert solar energy
(photons) into electrical energy (current).
 When light hits the space-charge region of the pn
junction, electron-hole pairs are generated. They are
then quickly swept out of the region by the electric
field, thus creating a photocurrent.
 OTHER DIODE: PHOTODIODE
 Photodiodes is similar to solar cells except that the pn
junction is operated with reverse-bias voltage.
 When light hits the space-charge region of the pn
junction, electron-hole pairs are generated. They are
then quickly swept out of the region by the electric
field, thus creating a photocurrent.
 OTHER DIODE: LIGHT-EMITTING DIODE (LED)
 LEDs are made from compound semiconductors. They
convert current to light.
 When the pn junction is forward-biased, electrons and
holes flow across the space-charge region and
become excess minority carriers.
 The electron and holes can recombine and a photon
or light wave can be emitted.
OTHER DIODE: LIGHT-EMITTING DIODE (LED) 2
 OTHER DIODE: SCHOTTKY BARRIER DIODE
 Unlike pn junction, Schottky diode is composed of
metal made in contact with n-type semiconductor.
 The current-voltage characteristics of Schottky diode
is very similar to pn junction diode, but with two major
differences:
 Current is resulted from the flow of majority carriers over
the potential barrier.
 The reverse-saturation current IS for a Schottky diode is
larger than that of a pn junction
 OTHER DIODE: ZENER DIODE
 For a pn junction, the applied reverse-bias voltage cannot be increased without limit. At
some point, breakdown will occur and the current will increase rapidly. The voltage at this
point is called breakdown voltage.
 A Zener diode can be designed to have a specific breakdown voltage, |VZ|.
 Zener diodes are normally operated with reverse-bias voltage.
 Example 9
Given VZ = 5.6V
rZ = 0
Find a value for R such that the
current through the diode is
limited to 3mA

VPS − VZ
I=
R
VPS − VZ 10V − 5.6V
R= = = 1.47k
I 3mA
PZ = I ZV Z= 3mA  5.6V = 1.68mW
 Example 10

Given V (pn) = 0.7V

V (SB) = 0.3V
rf = 0 for both diodes
Calculate ID in each diode.

VPS − V
I=
R
4V − 0.7V
I= = 0.825mA for the p - n junction diode
4k
4V − 0.3V
I= = 0.925mA for the Schottky diode
4k
Example 11

First, determine if the diode is on or

off. Is the open circuit voltage for
the diode greater or less than V?
The voltage at the node connected to the p side of the diode is

2k 5V/(4k) = 2.5V

The voltage at the node connected to n side of the diode is

2k 5V/(5k) = 2V

The open circuit voltage is equal to the voltage at the p side minus
the voltage at the n side of the diode:

Voc = 2.5V – 2V = 0.5V.

To turn on the diode, Voc must be ≥ V.

 SUMMARY
 A pn junction diode is “turned on” or “conducting” when a forward
bias is applied to the diode. If a reverse bias is applied, the diode is
“turned off” or “non-conducting”.
 The current that flows through the pn junction is due to the
movement of minority carriers.
 DC analysis on diode circuits can be simplified by modelling the non-
linear diode using diode equivalent circuits. Three models were
discussed; ideal, piecewise linear (PWL) and constant voltage drop
(CVD).
 Graphical techniques can be applied to determine the operating
point of the nonlinear diode, when both VD and ID are unknown.
 If a circuit has both dc and ac supplies, then ac analysis also need
to be performed. In ac analysis, the non-linear diode is replaced with
a small-signal resistance, rd.
 COURSE OUTCOME

CO1- Understand the characteristics of

diode, and its DC and AC models and
behavior in relation to circuit analysis.