OMU-409MECHATRONICS

SEMICONDUCTOR ELECTRONICS - II

Asst. Prof. Özgür ÜNVER December 20th, 2019

BIPOLAR JUNCTION TRANSISTOR (BJT)

The bipolar junction transistor was the remarkable invention that led to the electronic age, integrated circuits, and ultimately the entire digital world,

Semiconductor diode consists of adjacent regions of p-type and n-type silicon, each connected to a lead,

A bipolar junction transistor (BJT), in contrast, consists of three adjacent regions of doped silicon, each of which is connected to an external lead,

There are two types of BJTs; npn (most common) and pnp transistors,

Collector and emitter are not interchangeable!

Negative output

Positive output

Videos• http://www.youtube.com/watch?v=Te5YYVZiOKs&feature=fvwrel

• http://www.youtube.com/watch?v=GDbzCal0NCQ

• http://www.youtube.com/watch?v=-JWVdgD0dek

• Video 3.3

http://www.youtube.com/watch?v=Te5YYVZiOKs&feature=fvwrelhttp://www.youtube.com/watch?v=GDbzCal0NCQhttp://www.youtube.com/watch?v=-JWVdgD0dekMovies/3.3 transistors.wmv

NPN BIPOLAR JUNCTION TRANSISTORVCE is the voltage between the collector and emitter, and VBE is the voltagebetween the base and emitter.

For the transistor to be on;• The base-to-emitter junction must be forward biased (VBE =0.7V, so VB =VE+ 0.7V)• When this is the case, a large collector current can flow (IC > 0) with a small base

current ( IB

PNP & NPN SENSORS

BJTBJT may function as a current amplifier

Collector current is proportional to the base current with an amplification factor known as the beta for the transistor.

Manufacturers often use the symbol hFE instead of beta.

For typical BJTs, beta is on the order of 100, but it can vary significantly among transistors.

Beta is also temperature and voltage dependent

it can be used to amplify current or to simply switch current on and off.

We focus on switching design (not amplifying) in our mechatronic applications.

COMMON EMITTER TRANSISTOR CIRCUIT

Cut-off region: where no collector current flows,

As the base current is gradually increased, the base-to-emitter diode of the transistor begins to conduct when VBE is about 0.6V.

At this point IC begins to flow and is roughly proportional to IB ( IC= Beta x IB).

Active region: where collector current is proportional to base current,

Saturation region: where collector current is strictly controlled by the collector circuit assuming sufficient base current.

COMMON EMITTER TRANSISTOR CIRCUITWhen designing a transistor switch, we need to guarantee that the transistor is fully saturated when it is on.

If the transistor is not fully saturated, it gets hot faster and can fail.

In full saturation;

https://www.youtube.com/watch?v=MSYHjMMgQd4https://www.youtube.com/watch?v=_Gvx8RPK-lA

https://www.youtube.com/watch?v=MSYHjMMgQd4https://www.youtube.com/watch?v=_Gvx8RPK-lA

COMMON EMITTER TRANSISTOR CIRCUIT

How to determine how much base current and input voltage are required to saturate a transistor?

Example

Example

BIPOLAR TRANSISTOR SWITCHOFF• When Vin is less than 0.7 V, the BE

junction of the transistor is not forward biased ( VBE < 0.7 V), and the transistordoes not conduct ( IC =IE=0).

ON• When the BE junction is forward biased (

VBE =0.7 V), the transistor conducts.

• Current passes through the CE circuit, and Vout is close to ground potential (0.2 Vfor a saturated BJT).

• We assume that there is enough base current to saturate the transistor.

Can serve as a semiconductor switch to turn on or off an LED, electric motor, solenoid, electric light, or some other load.

EXAMPLE – LED SWITCH

DESIGNING A TRANSISTOR SWITCH

VC > VB > VE

To be ON VBE must be 0.7 V.

The collector current IC is independent of base current IB when the transistor is saturated, as long as there is enough base current to ensure saturation.

The minimum base current required IB_min ≈ IC / β.

IB_ideal = 10 x IB_min (beta may vary among components, with temperature, and with voltage; and the load resistance may change as current flows through it)

Calculate the maximum values of IC and IB to ensure that they fall within the manufacturer’s specifications.

BIPOLAR TRANSISTOR PACKAGES

FLYBACK DIODEA flyback diode is a diode used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed.

DARLINGTON TRANSISTOR• Darlington pair usually comes in a single package.

• The advantage of this combination is that the current gain is the product of the two individual transistor gains and can exceed 10,000.

• They may often be found in power circuits for mechatronic systems.

http://www.youtube.com/watch?v=L1Ue2j14mss&feature=related

http://www.youtube.com/watch?v=lr3qqsFKKNw

http://www.youtube.com/watch?v=L1Ue2j14mss&feature=relatedhttp://www.youtube.com/watch?v=lr3qqsFKKNw

PHOTOTRANSISTOR AND OPTOISOLATOR• A special class of transistor is the phototransistor, whose junction between the

base and emitter acts as a photodiode.

• An optoisolator is composed of an LED and a phototransistor separated by asmall gap.

• The light emitted by the LED causes current to flow in the phototransistor circuit

• With no common ground, the optoisolator creates a state of electrical isolation between the input and output circuits by transmitting the signal optically rather than through an electrical connection

An opto isolator is an electronic device that prevents high voltages from components in one side of a circuit from damaging or interfering with components on the other side of the same circuit.

ANGULAR POSITION OF A ROBOTIC SCANNERQuestion: The head of the scanner is rotated through 360 by a DC motor. Your problem here is to track the angular position of the scan head. How could you do this if you want an on-board computer to use the sensed values?

Answer: The solution requires a sensor that provides a digital output.

• An LED-phototransistor pair, also known as a photo-interrupter, is at the heart of the design.

• The pair, which is readily available in a single package, produces a beam of light that can be broken or interrupted.

• A slotted disk must be designed to attach to the shaft of the motor driving the scan head and to pass through the gap in the photo-interrupter pair.

• Each slot in the disk provides a digital pulse as it interrupts the light beam during rotation.

PHOTO INTERRUPTER

Assume you have only 8 slots but you need 1 degree of accuracy, as a mechanical engineer what would you do?

Hint: Mechanical solution

http://www.youtube.com/watch?v=cn83jR2mchwhttp://www.youtube.com/watch?v=pAZGtymRMpg&feature=relatedhttp://www.youtube.com/watch?v=-xPl9xZVmx0http://www.youtube.com/watch?v=fvjWcG5A_UU&feature=related

http://www.youtube.com/watch?v=cn83jR2mchwhttp://www.youtube.com/watch?v=pAZGtymRMpg&feature=relatedhttp://www.youtube.com/watch?v=-xPl9xZVmx0http://www.youtube.com/watch?v=fvjWcG5A_UU&feature=related

FIELD-EFFECT TRANSISTORS (FET)• Both the BJT and FET are three-terminal devices

• Both BJTs and FETs operate by controlling current between two terminals using a voltage applied to a third terminal

• BJT is a current amplifier (assuming no saturation)

• In contrast, in FET, output current is controlled by an input voltage (transconductance amplifier).

• In contrast to the BJT base, the FET gate draws no direct current (DC)

• There are three families of FETs: • metal-oxide-semiconductor FETs (MOSFETs)→ 2 different types,

• junction field-effect transistors (JFETs).

http://www.youtube.com/watch?v=QO5FgM7MLGghttp://www.youtube.com/watch?v=v7J_snw0Eng

http://www.youtube.com/watch?v=QO5FgM7MLGghttp://www.youtube.com/watch?v=v7J_snw0Eng

MOSFET• MOSFET has a p-type substrate and an n-type source and drain that form pn

junctions with the substrate

• There is a thin silicon dioxide layer insulating the gate from the substrate.

MOSFETWhen closed, the MOSFET mimics a very large resistor (~ 100 MΩ -10e12 Ω), and no current flows between the drain and source. The MOSFET is said to be in cutoff.

As Vgs is gradually increased beyond a gate-to-source threshold voltage Vt, the n-channel begins to form.

Vt depends on the particular MOSFET considered but a typical value is about 2 V.

MOSFET• With a positive Vgs larger than Vt , as Vds is increased from 0, we enter the

active region, also called the ohmic region, of the MOSFET.

• In this region, as Vgs is further increased, the conduction channel grows correspondingly, and the MOSFET appears to function like a variable resistor whose resistance is controlled by Vgs.

• when Vgs -Vt reaches Vdd , there is no longer an electric field at the drain end of the MOSFET. Therefore, the width of the n-channel shrinks to a minimum value close to the drain resulting in what is called pinch-off.

• This pinch-off limits a further increase in drain current, and the MOSFET is said to be in saturation.

• The drain-to-source resistance, called R on , is minimal (usually less than 5 Ω) as it enters the saturation region

where conduction begins ( Ids > 0), is about 3.5 V. Also note that the drain-to-sourcevoltage Vds doesn’t drop to zero when the MOSFET is fully on. This is due to thedrain-to-source resistance Ron of the device, which creates a small voltage drop ( Vds Ids Ron ).

APPLICATION AREAS OF MOSFETS• MOSFETs can be used to make excellent high-current voltage-controlled switches.

• Some MOSFETs are designed as analog switches, where signals can be gated

• Control signal Vg should be larger than the largest value of the input signal Vin plusthreshold voltage Vt , the drain to source channel will conduct with a low resistance,and the output signal will almost track the input ( Vout ≈Vin ).

• The controlling parameter for the MOSFET is gate voltage Vg .

• Recall that with the BJT, the controlling parameter is base current IB .