Semiconductors Physics & Devices

What are the three classifications of materials based on conductivity?

Conductors, semiconductors, and insulators.

How many valence electrons do conductors usually have?

Three or fewer.

How many valence electrons do semiconductors have?

Four.

How many valence electrons do insulators have?

Five or more.

What are examples of conductors?

Silver, copper, gold, aluminum.

What are examples of insulators?

Glass, plastic, rubber, mica.

What are examples of semiconductors?

Silicon, germanium, and carbon.

What is valence?

The combining ability of an atom determined by outer electrons.

What is covalent bonding?

Sharing of valence electrons between atoms forming a crystal lattice.

What is ionization?

The process by which atoms gain or lose electrons, forming ions.

What is doping?

Adding impurities to a semiconductor to increase conductivity.

What are intrinsic semiconductors?

Pure semiconductors with very low impurity levels.

What are extrinsic semiconductors?

Doped semiconductors with increased conductivity.

What are the two types of extrinsic semiconductors?

N-type and P-type.

What impurity produces an N-type semiconductor?

Pentavalent impurity (5 valence electrons) like phosphorus or arsenic.

What impurity produces a P-type semiconductor?

Trivalent impurity (3 valence electrons) like boron or gallium.

What are free electrons?

Electrons that have gained enough energy to move freely through the lattice.

What is a hole?

A missing electron in the valence band that acts as a positive charge carrier.

What is a PN junction?

The boundary formed when P-type and N-type materials are joined together.

What is the depletion region?

An area near the PN junction depleted of free charge carriers.

What causes the depletion region to form?

Diffusion of electrons and holes creating fixed ions on each side.

What is barrier potential (VB)?

The potential difference across a PN junction that opposes further carrier diffusion; ≈0.7 V (Si), 0.3 V (Ge).

What does forward bias do to a diode?

Reduces the barrier potential and allows current flow.

What does reverse bias do to a diode?

Widens the depletion region and blocks current flow.

What is leakage current?

A small reverse current due to minority carriers, increasing with temperature.

What is breakdown voltage (VBR)?

The reverse voltage at which the diode conducts heavily and may enter avalanche.

What is avalanche breakdown?

A chain reaction where high-speed electrons knock other electrons free, causing heavy current.

What is the first diode approximation?

Forward-biased diode = short circuit, reverse-biased = open circuit.

What is the second diode approximation?

Forward-biased diode has 0.7 V drop (Si) or 0.3 V (Ge).

What is the third diode approximation?

Includes the bulk resistance (rB) of the diode.

How do you calculate a diode’s DC resistance?

RF = VF / IF.

Example: If a silicon diode has VF = 0.7 V and IF = 2 mA, find RF.

RF = 0.7 V / 0.002 A = 350 Ω.

How do you calculate bulk resistance of a diode?

rB = ΔV / ΔI.

Example: A diode’s voltage changes by 0.1 V when current changes by 100 mA. Find rB.

rB = 0.1 V / 0.1 A = 1 Ω.

What is an LED?

A Light-Emitting Diode that emits light when forward biased.

What determines LED color?

The semiconductor material and dopant used.

What is the typical forward voltage for LEDs?

Between 1.5 V and 2.5 V.

What is the typical forward voltage assumption for calculations?

2.0 V.

What is LED breakdown voltage range?

Between 3 V and 15 V (very low).

Why must LEDs be protected from reverse voltage?

They can be destroyed by small reverse voltages.

How do you protect an LED from reverse bias?

Connect a regular diode in parallel (reverse-parallel protection).

Example: Calculate LED current if RS = 470 Ω, VS = 5 V, VLED = 2 V.

I = (5 – 2)/470 = 6.38 mA.

Example: Determine RS for a 25 mA LED at 5 V supply, VLED = 2 V.

RS = (5 – 2)/0.025 = 120 Ω.

What is a Zener diode?

A diode designed to operate in reverse breakdown for voltage regulation.

What is zener voltage (VZ)?

The constant voltage maintained across the diode in breakdown.

What is zener current (IZ)?

The reverse current through the zener during regulation.

What is zener power dissipation formula?

PZ = VZ × IZ.

What is maximum zener current formula?

IZM = PZM / VZ.

Example: Calculate IZM for a 1 W, 10 V zener diode.

IZM = 1 W / 10 V = 0.1 A (100 mA).

Example: In a 6.2 V zener regulator with 12 V supply and 470 Ω resistor, find IZ.

IZ = (12 – 6.2)/470 = 12.34 mA.

What is a loaded zener regulator?

A voltage regulator where a load resistor is connected in parallel with the zener.

If RL = 200 Ω, VZ = 6.2 V, find IL.

IL = 6.2/200 = 31 mA.

What is a transistor?

A semiconductor device with three regions used for amplification or switching.

What are the three regions of a BJT?

Emitter, base, and collector.

What is the main function of the emitter?

To inject charge carriers into the base.

What is the function of the base?

To control the number of carriers flowing from emitter to collector.

What is the function of the collector?

To collect charge carriers and dissipate heat.

How is the emitter doped?

Heavily doped.

How is the base doped?

Lightly doped and very thin.

How is the collector doped?

Moderately doped and largest in size.

What are the two types of BJTs?

NPN and PNP transistors.

What are the majority carriers in an NPN transistor?

Electrons.

What are the majority carriers in a PNP transistor?

Holes.

What is the arrow direction on the emitter in an NPN transistor symbol?

Points outward.

What is the arrow direction on the emitter in a PNP transistor symbol?

Points inward.

How is an NPN transistor biased for operation?

Base-emitter junction forward biased, collector-base junction reverse biased.

What happens if both junctions are reverse biased?

Transistor is in cutoff region (OFF).

What happens if both junctions are forward biased?

Transistor is in saturation region (ON).

What happens when base-emitter is forward biased and collector-base is reverse biased?

Transistor operates in the active region (amplification).

What is the relationship between transistor currents?

IE = IB + IC.

Define αdc.

Ratio of collector current to emitter current in a common-base connection (αdc = IC / IE).

Define βdc.

Ratio of collector current to base current in a common-emitter connection (βdc = IC / IB).

How are αdc and βdc related?

βdc = αdc / (1 – αdc) and αdc = βdc / (βdc + 1).

Example: If αdc = 0.99, find βdc.

βdc = 0.99 / (1 – 0.99) = 99.

Example: If βdc = 150, find αdc.

αdc = 150 / (150 + 1) ≈ 0.993.

Example: Given IB = 20 µA and βdc = 100, find IC.

IC = βdc × IB = 100 × 20 µA = 2 mA.

Example: Given IC = 4.98 A and IB = 20 mA, find IE.

IE = IC + IB = 4.98 + 0.02 = 5.00 A.

Example: Given IE = 50 mA and IC = 49 mA, find IB.

IB = IE – IC = 50 – 49 = 1 mA.

What is the cutoff region in a transistor?

When IB = 0, and IC ≈ 0 (transistor is OFF).

What is the active region?

Where IC = βdc × IB; transistor acts as an amplifier.

What is the saturation region?

When increasing IB no longer increases IC (transistor fully ON).

What is a common-base connection?

Base is common to input and output; used for voltage amplification.

What is a common-emitter connection?

Emitter is common; provides both current and voltage gain.

What is a common-collector connection?

Collector is common; used for impedance matching (emitter follower).

How is base bias provided?

By applying a fixed voltage (VBB) through a base resistor RB.

Formula for base current IB in base bias circuit?

IB = (VBB – VBE) / RB.

Formula for collector current IC in base bias?

IC = βdc × IB.

Formula for collector-emitter voltage VCE?

VCE = VCC – IC × RC.

Example: VCC = 12V, RB = 470kΩ, β = 100, RC = 4.7kΩ, VBE = 0.7V. Find IB.

IB = (12 – 0.7)/470k = 0.024 mA (24 µA).

Example: Find IC and VCE.

IC = 24 µA × 100 = 2.4 mA; VCE = 12 – (2.4 mA × 4.7kΩ) = 0.72 V.

What is the advantage of voltage-divider bias?

It provides stable operating point despite transistor β variations.

Formula for VB in voltage divider bias?

VB = (R2 / (R1 + R2)) × VCC.

Formula for VE in voltage divider bias?

VE = VB – VBE.

Formula for IC in voltage divider bias?

IC ≈ VE / RE (assuming IE ≈ IC).

Example: VCC = 12V, R1 = 47kΩ, R2 = 10kΩ, RE = 1kΩ, VBE = 0.7V. Find VB, VE, IC.

VB = 12×(10/(47+10))=2.11V; VE=2.11–0.7=1.41V; IC≈1.41mA.

What is emitter bias?

A bias method using both +VCC and –VEE supplies for stability.

Formula for emitter current in emitter bias?

IE = (VEE – VBE) / (RE + (RB / (β + 1))).

What are the advantages of emitter bias?

Extremely stable Q-point and temperature independence.

What are transistor operating regions?

Cutoff, Active, Saturation, and Breakdown regions.