Varactor Diode Capacitance Equation

Negative Resistance and Saturation

• A device possesses negative resistance characteristic to increase its current with a decrease in voltage.
• A device reaches saturation with an additional increase in Emitter current in the active region.

Unijunction Transistor (UJT)

• Applications of UJT include thyristors, oscillators, and pulse generators.

Varactor Diode

• A Varactor diode is a type of diode that operates as a voltage-dependent capacitor.
• It operates under reverse bias.
• The capacitance of a Varactor diode is expressed as C = εA / (d - Δd), where ε is the permittivity of the semiconductor, A is the area of the p-n junction, d is the width of the depletion region, and Δd is the change in the width of the depletion region.
• The capacitance of a Varactor diode decreases with an increase in the p-type region.
• As the reverse bias voltage increases, the depletion region of a Varactor diode widens.
• Advantages of using Varactor diodes include high capacitance and low losses.
• Varactor diodes are operated under reverse bias to take advantage of the voltage-dependent capacitance.
• The depletion region of a Varactor diode should be kept small to increase the storage capacity of charge.

UJT Circuit

• The total Inter-base resistance RBB in the circuit is given by the formula RBB = R1 + R2.
• The voltage at point A when a voltage VBB is applied between the two base terminals B1 and B2 is given by the equation VA = (R2 / (R1 + R2)) * VBB.
• The intrinsic stand-off ratio is denoted by η and has a value range of 6 to 12.

Diode Characteristics

• When no Emitter voltage is supplied to the circuit, the diode is said to be in cut-off mode.
• The barrier potential value for a diode made of silicon semiconductor is approximately 0.7 V.
• The Emitter potential that puts the diode in forward biased condition is known as the knee voltage.

Photodiode

• The main function of a photodiode is to convert light into an electrical signal.
• A photodiode consists of a PN junction diode with a transparent window to allow light to illuminate the PN junction.
• Photodiodes are operated under reverse bias.
• When light illuminates the PN junction of a photodiode, electron-hole pairs are produced.
• Electron-hole pairs are produced in a photodiode when photons interact with the semiconductor material.
• When electron-hole pairs enter the depletion region of a photodiode, they are separated by the electric field, resulting in a flow of current.

Schottky Diode

• Another name for a Schottky diode is a hot carrier diode.
• The forward voltage drop of a Schottky diode is lower compared to a P-N junction diode.
• A Schottky diode is formed when aluminum or platinum metal is joined with an N-type semiconductor.
• The depletion layer formed between a metal and n-type semiconductor in a Schottky diode is known as the Schottky barrier.
• Schottky diodes are widely used in high-speed switching power circuits due to their fast switching times and low forward voltage drop.