Ch. 3: Semiconductor Electronics

When selecting a diode for flyback protection, what must you make sure of?

The current rating is high enough to handle the steady-state-current magnitude that occurs at the beginning of the flyback period.

What is a peak detector?

a circuit that outputs the peak/maximum voltage of a time varying input signal voltage. Consists Of: - A diode - A capacitor - Time varying Vin - Peak voltage Vout.

What is a CMOS device?

a complementary metal-oxide-semiconductor (CMOS) - an integrated circuit that contains complementary n- and p-channel MOSFETS

Draw a schematic of a zener diode voltage regulator circuit. Include the voltage current relationship plot.

- Exact same circuit as a rectifier circuit, except you replace the normal diode with a zener diode.

When selecting a diode for an application in circuit, what are the important specifications to consider?

1) Maximum forward current 2) Maximum reverse bias voltage where breakdown will occur.

What is a full-wave bridge rectifier? Draw one. Draw two diagrams of the input voltage versus time and the output voltage versus time for a circuit with a full-bridge rectifier.

Full bridge: rectifier that passes both positive and negative half of an AC signal A diode circuit that converts AC power into DC power. - Consists of 4 diodes Used in an device that require DC voltage to operate but is from an AC source. In this case, all diodes point uphill.

Draw a schematic and diagram of a n-channel enhancement-mode MOSFET.

G = gate S = source D = drain

What are the 3 leads on a npn BJT transistor? What function does each serve?

Collector (C): collects current into the device Base (B): controls the current flow (base of operations) Emitter (E): emits current from the device

What is a half-wave bridge rectifier? Draw a schematic. Draw two diagrams of the input voltage versus time and the output voltage versus time for a circuit with a half-bridge rectifier.

Half bridge: rectifier that passes only positive or negative half of an AC signal

What is a transistor made of? List all the components and label them on a diagram.

Semiconductors: both n-type and p-type 3 Electrical Contacts: 1. Source 2. Drain 3. Gate (insulated by an oxide layer)

What is the charge of a doped semiconductor?

Technically, a doped semiconductor is electrically neutral since there are still equal numbers of electrons and protons. However, the conductivity of the material is changed.

In a circuit, does the orientation of a diode matter?

Yes because a diode is a pn junction which can only pass current through it in one direction.

What is a common emitter transistor circuit? Draw a schematic.

You simply take a npn bipolar junction transistor, ground the emitter and add a resistor to the collector.

What is a fluid check (nonreturn) valve?

a device that allows fluid to flow in only one direction

What is a bipolar junction transistor (BJT)? Types? Primary functions?

a device that consists of 3 adjacent regions of doped silicon, each of which his connected to an external lead. Types: 1. npn transistor (most common) 2. pnp transistor Functions: 1. Switch - ON and OFF feature 2. Amplification

What is a rectifier? Types? Why do we care?

a device that effectively allows for an AC current to be converted into a DC current. Types: 1. Half-bridge 2. Full-bridge A diode can be used as a rectifier. Super important in any circuit that requires the use of DC power

What are light-emitting diodes? Draw a typical LED and its schematic. What is the typical voltage drop of a forward biased LED?

diodes that emit photons when forward biased. - Usually in plastic film that enhances the wavelength and may also help focus the light into a beam. Typical Voltage Drop Across an LED: 1.5-2.5 V

What are photodiodes? How do they work? Draw a circuit schematic.

special diodes designed to detect photons - Can be used to sense light in a circuit Basis for solar cells which converts sunlight into electrical energy Can think of them as inverse LEDs: rather than emitting light, they detect it. Imagery: photo diode solar cell - Image: the schematic of a photodiode on a solar panel.

What is doping (general)? In semiconductors? What is the purpose of doping a semiconductor? Why do we do it?

the act of injecting a foreign substance to improve performance - It's the secret weapon of semiconductors In a semiconductor its the act of inserting elements, either one with an extra electron or ones with a missing electron, in order to increase conductivity. Foreign elements = improved conductivity Why?: To increase and control the number of charge carriers

What is the depletion layer?

the layer between the n-type and p-type semiconductors in a transistor. - acts as a barrier between the n-type and p-type semiconductors in the transistor in the off state.

What is the basis for all digital devices?

the on-off action of the pn junction.

What is the contact potential? What is its value for silicon?

the voltage difference that develops due to the action that creates the depletion layer. Approximately: 0.6 - 0.7 V

What is a power transistor? Example?

transistors that are used in power applications that are specifically desired to pass large current and dissipate more heat. Ex. used to interface a microcontroller with a motor

How is the depletion layer created?

n-type electrons diffuse to holes in p-type, creating a barrier know as the the depletion layer

Why when designing a transistor switch do you want to ensure the transistor is fully saturated?

Because the power dissipated by the transistor is minimized. and If the transistor is not fully saturated, it gets hot faster, and can fail.

Compare and contrast BJTs and FETs.

Both: - Both have 3 terminals - Both control current through 2 terminals by applying a voltage to the third terminal BJTs: - A current amplifier - Nomenclature: 3 Terminals: base, collector, emitter - NOT scalable FETs: - A transconductance amplifier (output current is controlled by an input voltage) - Nomenclature: 3 Terminals: gate, drain, source - Scalable nature

1) What is a conductor? What is an insulator? What is a semiconductor? 2) Give example of each. 3) Draw an image of each and emphasize the differences.

C: material with "free" electrons - Easy for large current to flow through - Ex. metals I: material with electrons "tightly" wound - Do not usually sustain large electric currents - Ex. rubber, plastic S: material with electrons "medium" bonded - Properties somewhere between insulators and conductors - Ex. silicon, germanium

How does a full-bridge rectifier work? Draw a diagram of its function.

2 diodes for positive AC wave from voltage source. Opposite 2 diodes for negative AC wave. - Parallel diodes are always forward biased or reversed biased together. Result: The current remains in the same direction through the output circuit

What is a voltage limiter/clipper? What does it consist of? Draw a schematic of a voltage limiter circuit.

A device that limits the output voltage to values between V_L and V_H. Consists of 2 diodes and voltage sources in parallel with the input voltage source and the load over which the output voltage is measured.

What is a hole in a semiconductor? Example?

A gap in the valence band where an electron should be. Ex. by adding an element like boron with only 3 valence electrons to the semiconductor silicon, there is a hole where the fourth electron of boron should be.

What is a flyback / kickback / snubber diode? Draw a diagram of the circuit.

Diode circuit that provides protection against switch arcing and voltage/current spikes that can result when an inductive load is switched off Switch: transistor or relay instead Inductive load: inductor, motor coil, solenoid, etc.

How do you remember which is positive and which is negative: anode vs. cathode?

Don't P.A.N.I.C.! Positive Anode Negative Is Cathode

Why are LEDs used frequently in lighting applications?

Due to their high efficiency in converting electrical energy into light.

How does a FET work? Analogy?

Electrons flow into or "down the drain" and are "sourced" or flow out of the source, and the flow/ current is "gated" or controlled by the gate. Analogy: A kitchen sink 1) The electrons flow out of the source, the water line, and down the drain and is the flow/current is controlled by the gate/sink handles. n-channel enhancement-mode MOSFET: 1) A capacitor is applied across the p-type substrate. 2) When a voltage is applied across the capacitor, electrons form two regions between the n-type source and drain. The top part has extra electrons that are attracted to the capacitor, and the region right below that has no holes due to electrons filling them. 3) Effectively, this creates a channel and a new depletion region that allows electrons to flow between the source and drain.

How is AC power converted to DC power? What are the steps in the process? Draw a diagram of AC-to-DC converter with a full bridge rectifier.

1. AC power is stepped down with a transformer. - Why: to get desired DC peak voltage 2. Rectifier inverts negative halves of AC waves. Retains entire AC signal. 3. Filter and voltage regulator smooth out the AC current. - Why: to get cleaner more stable DC signal

What are the common emitter characteristics of a transistor?

1. Cutoff region: no collector current flows 2. Active region: collector current is proportional to base current 3. Saturation region: collector current is strictly controlled by the collector circuit, assuming sufficient base current.

When designing a transistor switch, what must you do? How do you do this?

1. Ensure the transistor is fully saturated when on. How: V_CE must be at min = 0.2 V for a BJT V_BE is forward biased = 0.7 V

What are the advantages of a FET over a BJT?

1. FETs are scalable; BJTs are NOT. - This means millions of FETS can be fitted to a single wafer. - FETs take up less space. 2. More efficient. Operate at lower input currents. - BJTs waste a small amount of its main current when it's switched on. - FETS do NOT. - FETS consume less power and generate less heat. 3. FETS are quieter. - FETS only operate with one type of charge carrier.

If a diode's leads have been trimmed, how to tell the anode from the cathode? How to remember?

1. LED polarity-testing - unnecessary 2. Smaller component on inside of LED = positive side 3. Flat side on lip at base of plastic casing = negative side Connection: - On a typical outlet, the smaller slit is the positive side and the larger slit is the neutral/negative side.

What are some applications of diode circuits (5)?

1. Rectifier: AC-DC conversion 2. Flyback: current surge protection 3. Clamping: diode clamp - limit voltage to a sensitive circuit 4. Peak detector: output max voltage from time varying input voltage 5. Voltage limiter/Voltage clipper: the output voltage is limited or clipped between a maximum output voltage of V_H and a minimum output voltage of V_L

How do transistors work?

Analogous to a mechanical switch. - 2 States: off (0) and on (1) Contain both n-type and p-type semiconductors and 3 electrical contacts: source, drain, and gate. 1. Electrons diffuse from n-type to p-type in the transistor which creates a depletion layer. Effectively in the off state. 2. The depletion layer acts a barrier, and prevents the flow of electric current through the transistor. 3. The transistor is switched to the on state by applying a small voltage to the gate that allows the electrons to overcome the repulsion from the depletion layer and creates a conducting channel.

Why is arcing bad? Why are current spikes bad?

Arcing: Because it can damage the switch contacts. Current Spikes: - Can damage sensitive components in the circuit - Create fast changing magnetic fields that can generate electromagnetic interference (EMI)

Why are zener diodes excellent tools for use as voltage regulators?

B/c they can maintain a stable DC voltage value in the presence of a variable supply voltage and a variable load resistance

3.4: Bipolar Junction Transistors

COMPLETE

3.5: Field Effect Transistors

COMPLETE

What is the diode equation? Define each variable in the equation.

I_D = current through junction I_0 = reverse saturation current q = 1.6 X 10^-19 (charge of 1 electron) k = 1.381 X 10^(-23) (Boltzmann's constant) V_D = forward bias voltage across the junction T = absolute temperature in Kelvin

How do you model an ideal diode that is forward biased? Reversed biased? How do you model a real diode that is forward biased? Reverse biased?

Ideal: Forward biased = short circuit Reverse biased = open circuit Real: Forward biased = replace with 0.7V voltage drop Reverse biased = open circuit

What does the current-voltage curve for an ideal semiconductor diode look like? For a real diode? Draw a diagram with the curves.

Ideal: Real: exponential

How do you determine how much base current and input voltage are required to saturate a transistor? In other words, how do you guarantee a transistor is in saturation?

In Saturation: V_BE = 0.7 V V_CE = 0.2 V Therefore, you use those two values and other information provided from the problem in conjunction with the BJT equations to solve for V_in or I_b.

What impacts the intensity of light of an LED? How many amperes of current is required to dimly light an LED?

It is directly related to the amount of current flowing through the device. Only a few milliamperes of current will dimly light an LED

What is a breakdown voltage?

It is the voltage at which a reversed biased diode will fail. The voltage that when applied to a diode is large enough to cause a large reverse current to flow.

On a diode, what is the longer lead? The shorter one?

Longer lead = anode (positive lead) Shorter lead = cathode (negative lead)

What is the procedure to analyze a diode circuit assuming ideal diodes? For real diodes?

METHOD OF LOCI: Location - Spring Creek Trail 1. Assume current directions over all elements. - Starting Point: An arrow hovering above the mile marker on the trail. 2. Replace all reverse biased diodes with open circuits. Replace all forward biased diodes with short circuits. - Bridge @ Quarter Mile: imagine bridge going forward as normal, raised drawbridge coming back "going in reverse" 3. Compute voltage drop and currents in the circuit loops using KVL and KCL. Second Bridge: Giant KVL and KCL signs 4. If you get a negative current, assumption was wrong. Switch current direction and reanalysis. Underpass: - Positive = positivitron (Optimus Prime giving me a thumbs up and letting me pass) - Negative = negatron (Megatron blocking my path and forcing me to reverse directions and start over) 5. Continue repeating steps 3 and 4 until there are no inconsistencies. Real: The only difference for the real diode analysis is instead changing forward biased diodes to short circuits, you simply replace with a small voltage source (0.7 V)

What is the current-voltage relationship for an ideal diode (concept)? What does this imply?

Model Implies: forward biased = diode fully on, no voltage drop across reverse biased = diode fully off, no reverse saturation current Thus, Ideal Diode has: - Zero resistance when forward biased - Infinite resistance when reverse biased How to Remember: Analogy to non-return valve

What's the difference between n-type and p-type doping? What do their names mean?

Names: named after what types of charges can move through them. n-type: negative electrons can move p-type: positive holes can move

Are the collector and emitter terminals on a npn BJT transistor interchangeable? Why or why not?

No because the emitter is more heavily doped than the collector.

When does an inductor generate a voltage?

Only when the current changes: Remember: V = L dI/dt

How does a voltage limiter/voltage clipper circuit work?

The limiter circuit will limit the output voltage between the values of V_H and V_L. - If V_in < V_H or V_in > V_L, both diodes are reverse biased, and V_out = V_in. - If V_in > V_H, the diode connected to is forward biased and V_out = V_H. If , V_in < V_L the diode connected to is forward biased and V_out = V_L.

How do you design a BJT transistor switch?

To be ON, the base-to-emitter voltage (VBE)VBE must be 0.7 V. The collector current I_C is independent of base current I_B when the transistor is saturated, as long as there is enough base current to ensure saturation. The minimum base current required can be estimated by first determining the collector current ICIC and then applying IBmin≈IC/β.IBmin≈IC/β. For a given input voltage, the input resistance must be chosen so that the base current exceeds this value by a conservative margin (e.g., 2-5 times larger). The reasons for this are that β may vary among components, with temperature, and with voltage; and the load resistance may change as current flows through it. It is also important to calculate the maximum values of ICIC and IBIB to ensure that they fall within the manufacturer's specifications, and add or change series resistors if the currents are too large.

Why is it important to include a resistor in series with a diode in a circuit? What is the typical value of this resistor placed in series with a digital circuit with a 5 volt source?

To limit current, and prevent excessive forward current that can destroy the diode. Typical: 330 ohm resistor

What are the different types of diodes?

Types: - small-signal diode - power diode - light-emitting diode - 2-digit, 7-segment LED display

What happens when you apply a voltage across a semiconductor?

Valence electrons jump to the conductance band and then move freely in the electric field to produce current. Note: the current produced is smaller than that of a conductor

What is a forward biased junction (conceptual/molecular level)? What happens at this type of junction? Draw a diagram.

a pn junction that is connected to a voltage in the following way: - Positive side of source goes to anode (p-side) - Negative side of source goes to cathode (n-side) How to Remember: positive to positive, negative to negative: correct way to connect a battery and thus current will flow through Voltage overcomes the contact potential, diminishes the depletion layer, and current flows through.

What is a reversed biased junction (conceptual/molecular level)? What happens at this junction? Draw a diagram.

a pn junction that is connected to a voltage in the following way: - Positive side of source goes to cathode (n-side) - Negative side of source goes to anode (p-side) Inhibits diffusion of electrons and current - A reverse saturation current does flow but it is super small

What is a diode? Analogy?

a pn junction that only passes current in one direction. Analogous to a fluid check (nonreturn) valve

What is a PN junction? What makes up the PN junction? Draw a diagram.

a region in a semiconductor where a p-type region is created next to a n-type region. Parts: - Anode: p-type side of the diode - Cathode: n-type side of the diode - Depletion layer: layer between n-type and p-type sides

What is a pull up resistor?

a resistor that is connected to a voltage source, often 5 V, that pulls up the value of the voltage value of an output voltage. For example, a pull up resistor can make the value of a pin on a microcontroller high when a switch is open and then low when the switch is closed.

What is a pull down resistor?

a resistor that is connected to ground and inserted in a circuit to drop the voltage to zero.

What is a zener diode? Characteristics? Other names? Draw a schematic and voltage current relationship for a zener diode. How to Remember?

a special class of diodes that exploits the phenomenon known as diode breakdown where a large reverse current flows in a diode when a certain voltage (the breakdown voltage) is exceeded. Characteristics: - Steep breakdown curves - Well-defined breakdown voltage values - Therefore, can maintain a nearly constant voltage value at many varying currents Also called: avalanche or voltage regulator diodes How to Remember: zener sounds like Zeus - Zeus's weapon is the lightning bolt (breakdown voltage) - Image: Zeus hitting a snowcapped mountain and causing an avalanche.

What is an IGBT?

an insulated-gate bipolar transistor - It essentially combines the best aspect of the BJT and the the MOSFET into a transistor.

What are dopants? What types exist?

elements added to a semiconductor to increase its conductivity n-type: example - silicon doped with phosphorous - n = negative: b/c negative charges can move due to the extra electron from phosphorous - Also called the donor dopant p-type: example - silicon doped with boron - p = positive: b/c the positive hole can move that is a result of boron having one less electron than silicon - Also called the acceptor dopant

What is a FET? What does it consist of? What types/families of FETS exist?

field-effect transistor that serves a similar function to a BJT. Consists Of: - 3 doped semiconductor regions - A gate - A drain - A source Types/Families: 1) Enhanced-mode metal-oxide-semiconductor-FETs (MOSFETs) 2) Depletion-Mode MOSFETs 3) Junction field-effect transistors (JFETs) - Note: Each of the 3 above FETs come in both n an p types

What is a diode clamp? What purpose does it serve?

it is a circuit that limits or "clamps" the voltage going into a sensitive circuit component.