Electronic Principles, Diodes & Transistors (3b)

What is a diode and what are its key functions?

• A diode allows current to flow in one direction and blocks it in the opposite direction.

• Terminals:

  • Anode = positive

  • Cathode = negative

• Uses:

  • Early radio transmission

  • AC to DC conversion (power rectification) using rectifiers

  • Foundation for logic gates in computers

How is a diode constructed and what is its forward voltage behaviour?

• Construction: P-type semiconductor + P-N junction (depletion region) + N-type semiconductor

• Function: Conducts current from anode (P-side) to cathode (N-side), blocks reverse current

• Forward voltage drop:

  • Silicon diode: 0.7 V

  • Germanium diode: 0.3 V

• Diode acts as an insulator until this voltage is exceeded → then it is “turned on”

• Bias direction: Current flows in the allowed direction

• Recovery time: Time needed to overcome the forward voltage drop

What is diode bias, and what limits its reverse voltage?

• Bias: Direction of applied voltage (forward or reverse)

• Current flow: Anode (a, positive) → Cathode (k, negative) flows easily; reverse is blocked

• Forward voltage drop: Diode acts as an insulator until exceeded

• Reverse bias: Diode blocks current until Peak Inverse Voltage (PIV) is exceeded

• Breakdown: Occurs if reverse voltage exceeds PIV, allowing electrons to overwhelm the P-N junction

• Typical PIV values: 50V or higher

What is a Zener diode and how does it work?

• A diode that conducts in reverse when a precise PIV is reached

• Acts as a voltage-dependent switch or constant voltage diode

• Type of avalanche diode (reverse current triggered by voltage surge)

• Uses: Surge protection, voltage regulation

• Usually paired with a resistor to limit current and prevent exceeding maximum ratings

What is a Schottky diode and what are its key characteristics?

• Construction: Metal + metal-oxide film attached to an N-type semiconductor

• Features:

  • Very low forward voltage drop (0.15–0.45 V)

  • Fast recovery time

• Uses: Power rectifiers, radio-frequency circuits, and other specialised applications

What is an LED and how does it work?

• P-N junction diode made of direct band-gap materials

• Operation: Electrons combine with holes → photon emitted (light)

• Color: Determined by semiconductor material

• White light: Combination of coloured or phosphor coating

• Uses: Efficient lighting, infrared for remote controls

• Circuit connection:

  • Series: Efficient for powering multiple from one source

  • Parallel: Requires individual resistors for each

What is a photodiode and how does it work?

• Photodiode: A diode designed to detect light

• Unlike other diodes, its P-N junction is exposed to light

• Structure:

  • PIN: P-type, Intrinsic, N-type

  • NIP: N-type, Intrinsic, P-type

• Intrinsic semiconductor (I-type): Electrons excited via thermal energy or crystal defects

• Applications: Optical storage, telecommunications, photography, and other light-sensing electronics

What is a transistor and what are its main types?

An electronic component that amplifies signals or acts as a switch

• Function: Controls the amount and direction of current in a circuit

• Main types:

  • Bipolar Junction Transistors (BJTs)

  • Field Effect Transistors (FETs)

What is a Bipolar Junction Transistor (BJT) and how does it differ from diodes?

Essentially two diodes put end to end, but does not act as an insulator in both directions

• Leads:

  • Collector (C): analogous to diode anode

  • Emitter (E): analogous to diode cathode

  • Base (B): third lead that controls current flow

• Types: PNP and NPN, construction determines current direction and operation

What is the difference between PNP and NPN BJTs?

• PNP BJT:

  • Collector & emitter = positive regions (excess holes)

  • Depletion region = negative (excess electrons)

  • Base-emitter junction: reverse-biased

  • Base-collector junction: forward-biased

• NPN BJT:

  • Collector & emitter = negative regions (excess electrons)

  • Depletion region = positive (excess holes)

  • Base-emitter junction: forward-biased

  • Base-collector junction: reverse-biased

• Base voltage controls current flow through the transistor

How does a BJT amplifier work?

• Voltages applied to the collector and base control current flow.

• Electrons flow through the transistor by combining with holes, creating new holes behind them.

• Base voltage regulates this process, allowing amplification.

• In an NPN BJT:

  • Majority carriers = electrons (negative regions)

  • Minority carriers = holes (positive region)

• Current in the direction of majority carriers thins the depletion zone → faster electron transmission → amplified output

How do voltage levels affect a BJT’s operation?

Let VBV_BVB​ = base voltage, VCV_CVC​ = collector voltage, VEV_EVE​ = emitter voltage:

• VE < VB < VC: Transistor acts as an amplifier

• VE < VB > VC: Transistor acts as a conductor

• VC > VB < VE: Transistor acts as an open switch (cuts off current flow)

What is a Field Effect Transistor (FET) and how does it work?

• FET Terminals:

  • Gate (G): Controls current flow, analogous to BJT base

  • Drain (D): Collects current, analogous to BJT collector

  • Source (S): Supplies current, analogous to BJT emitter

  • Body (B): Substrate of the transistor

• Operation: Gate voltage opens/closes the channel between source and drain, controlling current flow

What are the types of FETs and their classifications?

• General types:

  • Enhancement mode

  • Depletion mode

• Further classification:

  • Bias type: N-channel or P-channel

  • Material or function-based

• MOSFET (Metal-Oxide Semiconductor FET): Commonly used in digital logic gates

What is a schematic diagram and what are its key points?

• Schematic diagram: Simplified graphical representation of a circuit, showing connections rather than physical appearance.

• Key points:

  • Wires = straight lines

  • Junctions = dots

  • Components labelled (R1, C1, L1, etc.)

  • Current direction and voltage polarity clearly marked

What is a series circuit and how is total resistance calculated?

• Series circuit: Components connected end to end

• Current: Same through all components

• Total resistance:

Rtotal=R1+R2+R3+…R_\text{total} = R_1 + R_2 + R_3 + \dotsRtotal​=R1​+R2​+R3​+…

What is a parallel circuit and how is total resistance calculated?

• Parallel circuit: Components connected across the same voltage source

• Voltage: Same across each branch

What is a mixed circuit?

A circuit that combines series and parallel connections of components