Semiconductors, Variable Resistance, and PN Junctions

Posted on Nov 21, 2024 in Technology

Variable Resistance

Photoresistors (LDR) are photoresistive cells whose electrical resistance depends on the light they receive.

Thermistors (RTD) consist of a metal wire whose resistance varies with temperature. Rt = Ro (1 + αT).

Thermistors are resistors that vary with temperature but do not consist of metallic conductors; instead, they are semiconductors.

Varistors are components whose resistance varies according to the applied voltage (VDR). The relationship between voltage and current is not linear: I = k * Uα.

Semiconductors

Semiconductors are solid materials with conductivity intermediate between conductors and insulators. At low temperatures, they are insulators, while at high temperatures, they become good conductors. The characteristics of semiconductors can be interpreted with the band theory. Between the bands, there are gaps, forbidden energy bands called band gaps, which electrons can cross only if they gain enough energy.

  • In insulators, the lower energy band (valence band) is complete with electrons, but the upper band (conduction band) is empty and separated by a very wide band gap. In insulators, all conduction electrons are located in the valence band, which is filled, and they cannot move to produce a current.
  • Semiconductors have a band structure very similar to that of insulators, but the band gap is much smaller. Semiconductors are normally insulators, but increasing the temperature gives electrons enough energy to jump the band gap into the conduction band, leaving a hole in the valence band.

These holes can move through the crystal with the action of an external electric field, resulting in what is called intrinsic conductivity because it is due to the semiconductor’s own electrons, where two types of current carriers exist: electrons and holes.

Another way to excite conductivity is by adding impurities, which is called doping. Conductivity due to foreign atoms is called extrinsic conductivity. It can provide a free electron without creating a hole (n-type), where electrical conductivity is due to negatively charged conduction electrons. It can also create an electron acceptor, resulting in an excess of holes (p-type), where the semiconductor behaves as a carrier of positive charge.

PN Junction

In a PN junction, there is an excess of holes in the p-type material and electrons in the n-type material. Holes from the p-type region diffuse into the n-type region, and electrons from the n-type region diffuse into the p-type region.

  • Reverse bias: Conduction is only by minority carriers. This current is called leakage current.
  • Forward bias: Conduction is by majority carriers.

Semiconductor Diode: A semiconductor diode is an electronic component that allows current flow in one direction (forward bias) and blocks current flow in the opposite direction (reverse bias).

Zener Diode: A Zener diode is specially designed to operate in the reverse breakdown region. It works in its reverse zone at a particular Zener voltage. Other types of diodes include:

  • LED: Emits light when a certain current flows through it.
  • Schottky Diode: Formed by a metal-semiconductor junction (n-type), they are faster.
  • Photodiode: Exhibits reverse conduction when subjected to light.

Power Diode: Power diodes should have the following characteristics: the ability to conduct a high current with a small voltage drop in forward bias and withstand a high voltage with reduced leakage current in reverse bias.

The Relay

A relay is a specialized electrical switch that can control a high-power device with a much smaller power signal. It is a switch opened or closed by means of an electromagnet. When a button is used to trigger a relay, it may only remain active while the button is pressed. In some circuits, the relay continues to be on even when the button is released, which is known as latching or self-hold. This is achieved through an assembly with two additional drivers. When the button is pressed, current flows through the coil to activate the relay.

Types of Relays:

  • Apolar relays: Operate independently of the current direction.
  • Polarized relays: Depend on the direction of the exciting current.
  • Mercury switch relays: Use a mercury switch capsule.
  • Restraint relays.
  • Intermittent relays: The contacts are briefly opened or closed.
  • Resonance relays: Function at a certain frequency.
  • Protective relays.

Applications:

Elevators, TV remote control, garage door openers, etc.