1. Review of Topics from Pre Mid term session
Kashif Shahzad
Lecture 9 & 10

2. A silicon atom (Si) Has 4 outer electrons
The outer electron shell needs 8 to be “full”
(standing wave pattern)
Silicon will try to lend or borrow 4

3. Silicon (group 4) bonds

4. A pure silicon crystal lattice

5. An Arsenic atom (As) Has 5 outer electrons
One surplus for fitting in to the lattice

6. Arsenic doping (group 5) – N type

7. A Gallium atom (Ga) Has 3 outer electrons
One short for fitting in to the lattice

8. Gallium doping (group 3) – P type
Holes are positive

9. A P-N Junction (N on left)

10. What causes the depletion?
Electrons move from left to right to fill the + holes
Where electrons and holes combine the area is “depleted” of current carriers

11. The junction transistor
Emitter (-)
Collector (+)
Electrons
Base
Electrons are negative 
(Original patent used point contact)

12. Introduction to BJT Amplifier
BJT (Review)

13. Bipolar Junction Transistors (BJTs)
The bipolar junction transistor is a semiconductor device constructed with three doped regions.
These regions essentially form two ‘back-to-back’ p-n junctions in the same block of semiconductor material (silicon).
The most common use of the BJT is in linear amplifier circuits (linear means that the output is proportional to input). It can also be used as a switch (in, for example, logic circuits). 
The description ‘bipolar’ arises because the device depends on the flow of both majority and minority carriers through the device. (bi means two). We shall look at field effect transistors later. They rely on just one carrier type (e.g. electrons) and can be regarded as ‘unipolar’ (uni means one).
Smith and Dorf pages , , (bias)

14. npn-BJT Structure n-type p-type
The ‘npn’ version of the BJT consists of two n regions separated by a p region (as the name suggests). A schematic of an npn transistor is shown.
n-type
p-type

15. BJT Structure
The three regions are known as the emitter, base and collector regions.
Electrical connections are made to each of these regions.

16. npn-BJT Structure E Emitter (n-type) Base (p-type) Collector (n-type)

17. npn BJT Symbol

18. npn BJT Symbol E B C

19. pnp BJT Symbol
In the symbol for a pnp BJT transistor the direction of the arrow on the emitter is reversed E B C

20. Still remember about BJT?
The emitter current (iE) is the sum of the collector current (iC) and the base current (iB)
iB << iE and iC OTHER PRAMETERS &
EQUATIONS?

21. BJT Basic structure and schematic symbol pnp type npn type approximate
equivalents
transistor
symbols
pnp type
npn type

22. Refresh.. Common-emitter current gain, β Common-base current gain, α
Range: 50 < β < 300
Common-base current gain, α
Range: always slightly less than 1
The current relationship between these 2 parameters are as follows:

23. Refresh.. BJT as amplifying device B-E junction is forward-biased
B-C junction is reverse-biased

24. BIASING OF BJT Remember…! for normal operation AND
emitter-base junction is always forward-biased
AND
collector-base junction is always reverse-biased

25. FORWARD BIASING E/B JUNCTION

26. REVERSE BIASING C/B JUNCTION

27. BIASING NPN TRANSISTOR

28. Common-Emitter Circuit
with an npn transistor
with a pnp transistor
with a pnp transistor biased with a positive voltage source

29. DC Analysis - Common-Emitter Circuit
Transistor current-voltage characteristics of the common-emitter circuit

30. DC Analysis - Common-Emitter Circuit
Common-emitter circuit with an npn transistor
Common-emitter dc equivalent circuit, with piecewise linear parameters

31. BJT as an Amplifier
Amplification of a small ac voltage by placing the ac signal source in the base circuit
Vin is superimposed on the DC bias voltage VBB by connecting them in series with base resistor RB:
Small changes in the base current circuit causes large changes in collector current circuit
Fig 4-20a & b (stacked)
END

32. BJTs – Testing

33. BJTs – Testing

34. Welcome review to FET

35. FET Plots and equations
The FET is a voltage controlled device.
The ‘control parameter’ is VGS –compare with IB in the BJT.

36. Loose equivalences
FET
Drain
Source
Gate
BJT
Collector
Emitter
Base

37. FET ( Field Effect Transistor)
Few important advantages of FET over conventional Transistors
Unipolar device i. e. operation depends on only one type of charge carriers (h or e)
Voltage controlled Device (gate voltage controls drain current)
Very high input impedance ( )
Source and drain are interchangeable in most Low-frequency applications
Low Voltage Low Current Operation is possible (Low-power consumption)
Less Noisy as Compared to BJT
No minority carrier storage (Turn off is faster)
Self limiting device
Very small in size, occupies very small space in ICs
Low voltage low current operation is possible in MOSFETS
Zero temperature drift of out put is possiblek

38. Types of Field Effect Transistors (The Classification)
JFET
MOSFET (IGFET)
n-Channel JFET
p-Channel JFET
FET
Enhancement MOSFET
Depletion MOSFET
n-Channel EMOSFET
n-Channel DMOSFET
p-Channel DMOSFET
p-Channel EMOSFET

39. The Junction Field Effect Transistor (JFET)
Figure: n-Channel JFET.

40. SYMBOLS Gate Drain Source Gate Drain Source Gate Drain Source
n-channel JFET
Offset-gate symbol
n-channel JFET
p-channel JFET

41. Figure: n-Channel JFET and Biasing Circuit.
Biasing the JFET
Figure: n-Channel JFET and Biasing Circuit.

42. (Note: The two gate regions of each FET are connected to each other.)
Operation of JFET at Various Gate Bias Potentials
Figure: The nonconductive depletion region becomes broader with increased reverse bias.
(Note: The two gate regions of each FET are connected to each other.)

43. Operation of a JFET Drain - N Gate P P + + - DC Voltage Source - N +

44. Figure: n-Channel FET for vGS = 0.
Simple Operation and Break down of n-Channel JFET
Figure: n-Channel FET for vGS = 0.

45. Biasing Circuits used for JFET
Fixed bias circuit
Self bias circuit
Potential Divider bias circuit

46. How to use it
Using a MOSFET

47. Thyristors review

48. Limitation of power semiconductor devices
Majority carrier devices, like Schottky diode, MOSFET exhibit very fast switching responses, controlled essentially by the charging of the device capacitances.
However, forward voltage drops of these devices increases quickly with increasing breakdown voltage.
Minority carrier devices, like BJT, IGBT can exhibit high breakdown voltages with relatively low forward voltage drop.
But they can have longer switching times due to stored minority charges.
Energy is lost during switching transitions, due to a variety of mechanisms.
The resulting average power loss, or switching loss, is equal to this energy loss multiplied by the switching frequency.
So need of a mechanism to have a compensation between these issues.

49. THYRISTOR
Thyristor, a three terminal, four layers solid state semiconductor device, each layer consisting of alternately N-type or P-type material, i.e; P-N-P-N, that can handle high currents and high voltages, with better switching speed and improved breakdown voltage .
Name ‘thyristor’, is derived by a combination of the capital letters from THYRatron and transISTOR.
Thyristor has characteristics similar to a thyratron tube which is a type of gas filled tube used as a high energy electrical switch and controlled rectifier.
But from the construction view point, a thyristor (pnpn device) belongs to transistor (pnp or npn device) family.
This means that thyristor is a solid state device like a transistor and has characteristics similar to that of a thyratron tube.

50. THYRISTORS
Thyristor (famous as Silicon Control Rectifier-SCR) can handle high currents and high voltages.
Typical rating are 1.5kA & 10kV which responds to 15MW power handling capacity.
This power can be controlled by a gate current of about 1A only.
Thyristor a three terminal (Anode, Cathode and Gate), three junctions and four layers solid-state semiconductor device, with silicon doped alternate material with P-N-P-N structure.
Thyristor act as bistable switches.
It conducts when gate receives a current pulse, and continue to conduct as long as forward biased (till device voltage is not reversed).
They stay ON once they are triggered, and will go OFF only if current is too low or when triggered off.

51. Thyristor Schematic Representation

52. Characteristics of Thyristors

53. Thyristor Operating modes
Thyristors have three modes :
Forward blocking mode: Anode is positive w.r.t cathode, but the anode voltage is less than the break over voltage (VBO) .
only leakage current flows, so thyristor is not conducting .
Forward conducting mode: When anode voltage becomes greater than VBO, thyristor switches from forward blocking to forward conduction state, a large forward current flows.
If the IG=IG1, thyristor can be turned ON even when anode voltage is less than VBO.
The current must be more than the latching current (IL).
If the current reduced less than the holding current (IH), thyristor switches back to forward blocking state.
Reverse blocking mode: When cathode is more positive than anode , small reverse leakage current flows.
However if cathode voltage is increased to reverse breakdown voltage , Avalanche breakdown occurs and large current flows.

54. Thyristor turn-ON methods
Thyristor turning ON is also known as Triggering.
With anode positive with respect to cathode, a thyristor can be turned ON by any one of the following techniques :
Forward voltage triggering         
Gate triggering
dv/dt triggering
Temperature triggering
Light triggering

55. Forward Voltage Triggering
When breakover voltage (VBO) across a thyristor is exceeded than the rated maximum voltage of the device, thyristor turns ON.
At the breakover voltage the value of the thyristor anode current is called the latching current (IL) .
Breakover voltage triggering is not normally used as a triggering method, and most circuit designs attempt to avoid its occurrence.
When a thyristor is triggered by exceeding VBO, the fall time of the forward voltage is quite low (about 1/20th of the time taken when the thyristor is gate-triggered).
However, a thyristor switches faster with VBO turn-ON than with gate turn-ON, so permitted di/dt for breakover voltage turn-on is lower.

56. Gate Triggering
Turning ON of thyristors by gate triggering is simple and efficient method of firing the forward biased SCRs.
In Gate Triggering, thyristor with forward breakover voltage (VBO), higher than the normal working voltage is chosen.
This means that thyristor will remain in forward blocking state with normal working voltage across anode and cathode with gate open.
Whenever thyristor’s turn-ON is required, a positive gate voltage b/w gate and cathode is applied.
With gate current established, charges are injected into the inner p layer and voltage at which forward breakover occurs is reduced.
Forward voltage at which device switches to on-state depends upon the magnitude of gate current.
Higher the gate current, lower is the forward breakover voltage .
When positive gate current is applied, gate P layer is flooded with electrons from cathode, as cathode N layer is heavily doped as compared to gate P layer.
As the thyristor is forward biased, some of these electrons reach junction J2.
As a result, width of depletion layer around junction J2 is reduced.
This causes junction J2 to breakdown at an applied voltage lower than forward breakover voltage VB0.
If magnitude of gate current is increased, more electrons will reach junction J2, thus thyristor will get turned ON at a much lower forward applied voltage.

57. dv/dt triggering
With forward voltage across anode & cathode of a thyristor, two outer junctions (A & C) are forward biased but the inner junction (J2) is reverse biased.
The reversed biased junction J2 behaves like a capacitor because of the space-charge present there.
As p-n junction has capacitance, so larger the junction area the larger the capacitance.
If a voltage ramp is applied across the anode-to-cathode, a current will flow in the device to charge the device capacitance according to the relation:
If the charging current becomes large enough, density of moving current carriers in the device induces switch-on.
This method of triggering is not desirable because high charging current (Ic) may damage the thyristor.

58. Temperature Triggering
During forward blocking, most of the applied voltage appears across reverse biased junction J2.
This voltage across junction J2 associated with leakage current may raise the temperature of this junction.
With increase in temperature, leakage current through junction J2 further increases.
This cumulative process may turn on the SCR at some high temperature.
High temperature triggering may cause Thermal runaway and is generally avoided.

59. Light Triggering
In this method light particles (photons) are made to strike the reverse biased junction, which causes an increase in the number of electron hole pairs and triggering of the thyristor.
For light-triggered SCRs, a slot (niche) is made in the inner p-layer.
When it is irradiated, free charge carriers are generated just like when gate signal is applied b/w gate and cathode.
Pulse light of appropriate wavelength is guided by optical fibers for irradiation.
If the intensity of this light thrown on the recess exceeds a certain value, forward-biased SCR is turned on. Such a thyristor is known as light-activated SCR (LASCR).
Light-triggered thyristors is mostly used in high-voltage direct current (HVDC) transmission systems.

60. Thyristor Gate Control Methods
An easy method to switch ON a SCR into conduction is to apply a proper positive signal to the gate.
This signal should be applied when the thyristor is forward biased and should be removed after the device has been switched ON.
Thyristor turn ON time should be in range of 1-4 micro seconds, while turn-OFF time must be between 8-50 micro seconds.
Thyristor gate signal can be of three varieties.
D.C Gate signal
A.c Gate Signal
Pulse

61. Thyristor Family Members
SCR: Silicon Controlled Rectifier
DIAC: Diode on Alternating Current
TRIAC : Triode for Alternating Current
SCS: Silicon Control Switch
SUS: Silicon Unilateral Switch
SBS: Silicon Bidirectional Switch
SIS: Silicon Induction Switch
LASCS: Light Activated Silicon Control Switch
LASCR: Light Activated Silicon Control Rectifier
SITh : Static Induction Thyristor
RCT: Reverse Conducting Thyristor
GTO : Gate Turn-Off thyristor
MCT: MOSFET Controlled Thyristor
ETOs: Emitter Turn ON thyristor

62. Silicon-Controlled Rectifier (SCR)
SCR is a synonym of thyristor
SCR is a four-layer pnpn device.
Has 3 terminals: anode, cathode, and gate.
In off state, it has a very high resistance.
In on state, there is a small on (forward) resistance.
Applications: motor controls, time-delay circuits, heater controls, phase controls, etc.

63. Summary: Thyristors The thyristor family:
double injection yields lowest forward voltage drop in high voltage devices.
More difficult to parallel than MOSFETs and IGBTs
The SCR:
highest voltage and current ratings, low cost, passive turn-off transition
The GTO:
intermediate ratings (less than SCR, somewhat more than IGBT). Slower than IGBT.
Slower than MCT.
Difficult to drive.
The MCT:
So far, ratings lower than IGBT.
Slower than IGBT.
Easy to drive.
Still emerging devices?

64. Stepper Motors
Relax
Its not over yet

65. Stepper Motors
A stepper motor is a “pulse-driven” motor that changes the angular position of the rotor in “steps”
Define
β = the step angle (per input pulse)
Resolution = the number of steps/revolution
θ = total angle traveled by the rotor
= β X # of steps
n = the shaft speed = (β X fp) / 360°
fp = # of pulses/second

66. Variable-Reluctance Stepper Motor
Toothed Rotor and Toothed Stator
Principle of Operation:
Reluctance of the magnetic circuit formed by the rotor and stator teeth varies with the angular position of the rotor
Here, energize coils A and A’ (Phase A)
Rotor “steps” to align rotor teeth 1 and 4 with stator teeth 1 and 5

67. Variable-Reluctance Stepper Motor
Energize coils B and B’ (Phase B)
Rotor steps “forward”
Rotor teeth 3 and 6 align with Stator teeth 1 and 5
Let Ns = # of teeth on the stator
Nr = # of teeth on the rotor
β = Step Angle in space degrees

68. Variable-Reluctance Stepper Motor
Energize Phase C
Rotor steps forward another 15°

69. Variable-Reluctance Stepper Motor
Energize Phase D
Rotor steps forward another 15°

70. Variable-Reluctance Stepper Motor
Repeat the sequence
Energize Phase A
Rotor steps forward again

71. Variable-Reluctance Stepper Motor
Switching Circuit for the stepper motor
Close switches in order 1, 2, 3, and 4 to turn the rotor “clockwise”
Close switches in reverse order - 4, 3, 2, and 1 to change rotation to the opposite (counter-clockwise) direction

72. Variable-Reluctance Stepper Motor Typical Driver Circuit
F = “filter” block
C = Up/Down Counter
D = Decoder
S = Electronic Switch (transistor)

73. Typical Switching Circuit
Decoder provides logic output to turn QA On/Off
Amplifier A “conditions” the logic pulses
When QA turns On, it conducts current in the motor phase A winding
When QA turns Off, D and RS conduct current to “discharge” the phase winding

74. Stepper Motion Move clockwise or counter clockwise
Move in full or half steps
Half Steps
Time
Full Steps

75. IC Package
Stepper motors are driven by a stepper motor driver chip.

76. Pin Connections
Outputs
Logic Control

77. Input Truth Table
< 0.8V
> 2.0 V

78. Block Diagram
Supply
Outputs
Control
Reset