The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Electronics
Lecturer: Mr. M.G.D GOLOLO
B2 Lab 112
mphog@uj.ac.za
Department of Electrical and Electronic Engineering Science
University of Johannesburg
ETN2A Course 2022
Designed and Prepared by: Prof. Khmaies Ouahada
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Outline
1
The Diode
2
Diode models and analysis
3
Rectifiers
4
Zener diodes and voltage regulation
5
The bipolar junction transistor
6
BJT large signal model
7
The enhancement MOSFET
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Electrical Conduction in Semiconductor devices
show the Siliccon atomic structure: Electrons and Protons
Atomic table of molucules
Show the movement of Electrons and Protons
Introduce Holes and Electrons concepts
Doping and donor concepts
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
The pn Junction and the Semiconductor Diode
Now lets look at physical mechanisms from which the current
equations come.
Weve seen that holes and electrons move through a
semiconductor by two mechanisms drift and diffusion
In equilibrium, diffusion current (ID ) is balanced by drift
current (IS ). So, there is no net current flow. Drift current
comes from (thermal) generation of electron-hole pairs (EHP).
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Band Diagrams (1)
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Band Diagrams (2)
When the P-type material is contacted with the N-type
material, the Fermi levels mustbe at equilibrium.
Band bending: The conduction and valence bands bendto
align the Fermi levels.
Electrons diffuse from the N-side to the P-side and recombine
with holes at the boundary. Holes diffuse from the P-side to
the N-side and recombine with electrons at the boundary.
There is a region at the boundary of charged atoms called the
space-charge region (also called the depletion region b/c no
mobile carriers in this region).
An electric field is created which results in a voltage drop
across the region called the barrier voltage or built-in
potential.
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Current Equations
The forward bias current is closely approximated by.
i = Is (e v /nVT − 1)
where
VT = kT /q
where VT is the thermal voltage (≈ 25.8 mV at room temp T= 300K or 27C )
k = Boltzmans constant = 1.38x 10−23 joules/kelvin
T = absolute temperature
q = electron charge = 1.602 x 10−19 coulombs
n = constant dependent on structure, between 1 and 2 (we will assume n = 1)
Is = scaled current for saturation current that is set by diode size
Notice there is a strong dependence on temperature
We can approximate the diode equation for i >> Is
i∼
= −Is e v /nVT
In reverse bias (when v << 0 by at least VT ), then
i∼
= −Is
In breakdown, reverse current increases rapidly. . . a vertical line
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Real PN Junction Diode I-V Characteristic
Typical PN junction diode I-V characteristic is shown on the right. In forward
bias, the PN junction has a turn onvoltage based on the built-inpotential of the
PN junction. The turn on voltage is typically in the range of 0.5V to 0.8VIn
reverse bias, the PN junction conducts essentially no current until a critical
breakdown voltage is reached. The breakdown voltage can range from 1V to
100V. Breakdown mechanisms include avalanche and zener tunneling.
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Large-Signal Diode Models
Ideal Diode Model
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Large-Signal Diode Models
Offset Diode Model
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Small-Signal Diode Models (1)
Look at the simple diode circuit below. We can write two equations:
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Small-Signal Diode Models (2)
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Small-Signal Diode Models (3)
Some circuit applications bias the diode at a DC point (VD ) and superimpose a
small signal (vd (t))on top of it. Together, the signal is vD (t), consisting of both
DC and AC components
Graphically, can show that there is a translation of voltage to
current (id (t))
Can model the diode at this bias point as a resistor with
resistance as the inverse of the tangent of the i-v curve at that
point
iD (t) = IS e (VD +vd )/VT = IS e VD /VT e vd /VT
iD (t) = ID e vd /VT
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Small-Signal Diode Models (4)
And if vd (t) is sufficiently small then we can expand the
exponential and get an approximate expression called the
small-signal approximation (valid for vd < 10mV )
ID
vd
) = ID + id → id =
vd
iD (t) ∼
= ID (1 +
VT
VT
So, the diode small-signal resistance is. . .
rd =
Electronics
VT
ID
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Small-Signal Diode Models (5)
Perform the small signal analysis of the diode circuit biased with VDDby
eliminating the DC sources and replacing the diode with a small signal
resistance
The resulting voltage divider gives:
vd = vs
rd
R + rd
Separating out the DC or bias analysis and the small-signal analysis is a
technique we will use extensively
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Rectifier Circuits
One of the most important applications of diodes is in the design of rectifier
circuits. Used to convert an AC signal into a DC voltage used by most
electronics.
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Simple Half-Wave Rectifier
Only lets through positive voltages and rejects negative voltages.
This example assumes an ideal diode.
What would the waveform look like if not an ideal diode?
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Full-Wave Rectifier
To utilize both halves of the input sinusoid use a center-tapped transformer . . .
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Bridge Rectifier
Looks like a Wheatstone bridge. Does not require a center-tapped transformer.
Requires 2 additional diodes and voltage drop is double.
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Zener diodes
Zener Diode : Works in the break down region when subjected to reverse bias.
Large variation in current. Voltage almost constant.
Used for voltage regulation. Upper limit of current depends on the power
dissipation rating of the device. It will not be constant voltage when current is
below IZK
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Zener diodes
Zener Diode : Works in the break down region when subjected to reverse bias.
Large variation in current. Voltage almost constant.
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Zener diodes in circuits (1)
V = VL =
IL =
RL Vi
R + RL
VL
RL
IR = IZ + I L
IR =
Electronics
Vi − VL
R
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Zener diodes in circuits (2)
When Zener is on Voltage across load is VZ This will remain till the current
through Zener reduces to knee.
VL
IL =
RL
IR = IZ + I L
IR =
Electronics
Vi − VL
R
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
The Bipolar Junction Transistor
The term Bipolar is because two type of charges (electrons and holes) are
involved in the flow of electricity.
The term Junction is because there are two pn junctions.
There are two configurations for this device.
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
NPN and PNP Transistors
NPN is more widely used
majority carriers are electrons so it operates more quickly
PNP is used for special applications.
we will concentrate on NPN
The terminals of the transistor are labeled as Base, Emitter, and Collector.
The emitter is always drawn with the arrow.
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Asymmetry of Transistor
Although in the NPN schematic, it looks like the collector and emitter can be
reversed, in reality the device is very inefficient in reverse connection and has
very little amplification (gain).
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Operation of NPN Transistor
In normal operation, the EB junction is forward biased and the BC junction is
reverse biased.
The base region is very thin so the ratio L1:L2 is typically about 150:1
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Behaviour(1)
Forward biasing of the EB junction causes a heavy flow of majority carriers
(electrons) from the n-type material into the base junction and also majority
carriers (holes) from the base region into the emitter region. We denote this
current Ie.
The transistor is made so that nearly all the current Ie consists of a flow of
carriers (electrons) from emitter to the base. This is achieved by making the
emitter much more heavily doped than the base.
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Behaviour(2)
The base region is very thin so that most of the electrons attracted to this region
pass straight through it (attracted by the collector which is positive relative to
the base) before there is much chance of recombination with the bases holes.
Because of this, the collector current is very nearly equal in value to Ie. Thus
with the current directions shown Ic=-αIe where α is close to unity (e.g.,
α=0.98).
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Behaviour(3)
The current that does not go through the collector forms the base current so
that we have Ib = −(1 − α)Ie.
From this
α
Ic
=
=β
Ib
1−α
Typically β=50 to 200
The parameter β is called the DC current gain and represents the current
amplification of the transistor.
Indeed the use of the transistor as an amplifier is one of its main applications
Another major application is using the transistor as a switch
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Physical Structure Symbols
NPN
PNP - similar, but:
N- and P-type regions interchanged
Arrow on symbol reversed
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Operating Modes
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
BJT - Operation in Active Mode (1)
voltage polarities for NPN
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
BJT - Operation in Active Mode (2)
IEn , IEp both proportional to e VBE /VT
IC ≈ IEn
⇒ IC ≈ IS e VBE /VT
IB ≈ IEp << IEn
⇒ IC = βIB
IS = SATURATION CURRENT (type
where
10−15
β
to
large
10−12
A)
VT = THERMAL VOLTAGE = kT/e ≈ 25 mV at 25o C
β = COMMON-EMITTER CURRENT GAIN (type 50 to 250)
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Active Mode Circuit Model
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
BJT Operating Curves
INPUT-OUTPUT IC vs VBE (for IS = 10−13 A)
ACTIVE REGION:
IC ≈ 0 for VBE <≈ 0.5V
IC rises very steeply for VBE >≈ 0.5V
VBE ≈ 0.7V over most of useful IC range
IB vs VBE similar, but current reduced by factor β
CUT-OFF REGION:
IC ≈ 0
Also IB , IE ≈ 0
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Summary of BJT Characteristics
Also IE = IB + IC (always)
THIS TABLE IS IMPORTANT - GET TO KNOW IT !
For PNP table:
Reverse order of suffices on all voltages in table
i.e. VCB → VBC etc
Reverse arrows on currents in circuit
i.e. arrows on IB , IC point out of PNP device, while arrow on IE points in.
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Using Small-Signal Models
Steps for using small-signal models.
Determine the DC operating point of the BJT
in particular, the collector current
Calculate small-signal model parameters: gm , rπ , re
Eliminate DC sources
replace voltage sources with shorts and current sources with
open circuits
Replace BJT with equivalent small-signal models
Choose most convenient one depending on surrounding
circuitry
Analyze
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Graphical Analysis (1)
Can be useful to understand the operation of BJT circuits
First, establish DC conditions by finding IB (or VBE )
Second, figure out the DC operating point for IC
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Graphical Analysis (2)
Apply a small signal input voltage and see ib
See how ib translates into VCE
Can get a feel for whether the BJT will stay in active region of operation
What happens if RC is larger or smaller?
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Classification of field-effect Transistor
In three-phase circuits the 3 voltages sources are 120o apart.
Polyphase generation and transmission of electricity is more advantageous and
economical.
three-phase instantaneous power is constant over time.
single-phase line losses are 50% greater than three-phase losses
(for the same load power, voltage, pf), i.e.,
PSingle = 3/2PThree .
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Overview of Enhancement Mode MOSFET
Most widely used field effect transistor (enhancement type).
Lets look at its structure and physical operation.
3 terminal device (gate, source, drain).
Additional body (or bulk) terminal (generally at DC and not
usedfor signals).
Two types:
nMOSand pMOS.
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
nMOS Transistor
Four terminal device: gate, source, drain (and body)
No connection between the gate and drain/source (separated
by oxide)
Voltage on gate controls current flow between source and drain
Gate-oxide-body stack looks like a capacitor
Gate and body are conductors
SiO2 (oxide) is a good insulator
Called Metal-Oxide-Semiconductor (MOS) capacitor
Gate no longer made out of metal, but poly
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Basic nMOS Operation (1)
Body is commonly tied to ground (0V)
When the gate is at a low voltage (VG = 0):
P-type body is at low voltage
Source-body and drain-body diodes are OFF (reverse bias)
Depletion region between n+ and p bulk
No current can flow, transistor is OFF
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Basic nMOS Operation (2)
When the gate is at a high voltage
Positive charge on gate of MOS capacitor
Negative charge attracted to oxide in the body (under the
gate)
Inverts channel under the gate to n-type
Now current can flow through this n-type channel between
source and drain
Transistor is ON
Electronics
Lecturer: Mr. M.G.D GOLOLO
The Diode
Diode models
Rectifiers
Zener diodes
The BJT
BJT signal
MOSFET
Biasing MOSFET
In three-phase circuits the 3 voltages sources are 120o apart.
Polyphase generation and transmission of electricity is more advantageous and
economical.
three-phase instantaneous power is constant over time.
single-phase line losses are 50% greater than three-phase losses
(for the same load power, voltage, pf), i.e.,
PSingle = 3/2PThree .
Electronics
Lecturer: Mr. M.G.D GOLOLO