Diode Info

What is this symbol for

Varactor

What is the potential uses for a varactor

A variable capacitor

What is the normal operating bias for a varicap

Reverse bias

What is a varicap’s normal operation

As a variable capacitor

What happens to the depletion region when a varicap diode is in reverse bias

It expands

What does the depletion region in a varicap act as

capacitor dielectric

How do the P and N regions act in a varicap diode

As capacitor plates

Are the P and N religions conductive in varicap diodes

Yes

How does increasing the reverse bias voltage in a varicap diode increase the capacitance

As this widens the depletion region and the Separation between the P and N increases, this increases the plate separation

What happens when reverse voltage in a varicap diode decreases

The depletion region narrows so the capacitance increases

What is the equation for dialectic constant

Material permittivity/permittivity of free space

Why is a varicap diode highly doped

To increase the capacitance

What is the capacitance of a varactor controlled by

reverse bias voltage, diode size, doping method, alignment of charges

Do varactors have a high or low capacitance

low

What are the applications of varactor diodes

tuning circuits, high, very high and ultra high frequency signals

What is the drawbacks of varicap

If an AC signal is used there will be harmonic distortion

How can harmonic distortion be avoided in varacaps

by having 2 connected next to each other

What is the normal biasing operation for a light emitting diode

Forward bias

What is this symbol for

LED

What is the forward voltage drop of an LED

1.2 to 3.2

when do LED's experiance reverse breakdown

between 3 and 10 volts which is quicker than normal diodes

What intially happens to the electrons when an LED is in forward bias

Electrons cross the PN junction from the n type material and recombine with the holes in the P-type material

What happens in a forward bias LED when the elections recombine with the holes

The electrons release energy in the form of photons

Why in a forward bias LED do the electrons release photons when they recombine with the holes

As there is a difference in energy that corresponds to the energy of monochromatic visible light

What is the power output translated into light directly proportial to in LED 's

The forward current

What is light output dependent on (LEDs)?

Temperature

What happens in an LED when the temperature increases

The current decreases and so does the light output

What is a reverse bias LED's behaviour the same as

A normal diode

What is this symbol for

Photodiode

What is the normal biasing operation of a photodiode

Reverse bias

How do photodiodes allow light in

Through a very small window which lets the light hit the PN junction

What happens in a photodiode when the light hits the PN junction

It starts conducting as it creates charge carriers in the depletion region by increasing their energy and making them jump to the conduction band

What increases the reverse current in a photo diode

light intensity

What happens to a photo diode in reverse bias when there is no light

No current flow, practically there may be a small amount

What happens to an LED in reverse bias when there is light

A current will flow

What is the small amount of revese current called in a reverse-bias LED with no light incident

Dark current

What is the all the current though a photodiode

Reverse current

What are the two factors that impact reverse current in a photodiode

Reverse voltage and light intensity

What are the applications of photodiodes

Automatic lights

What is this symbol for

Schottky Diode

What is the structure of a Schottky diode

An N-type region bound to a metal

Examples of metals used in a Schottky diode

Gold/Silver/Platinum

What does the structure of a Schottky diode mean for the depletion region, and why

No depletion region as there is only 1 type of charge carrier

What is the charge carrier in a Schottky diode

Electrons

What is the typical voltage drop of a Schottky diode

0.3

What are the uses of a Schottky diode

High current ratings used for fast switching and high frequency applications

Why is there no reverse leakage current in Schottky diodes

As there is no minority charge carriers

What happens to a Schottky diode in reverse bias

No current, it has a higher reverse leakage current

What happens in terms of conductance for a Schottky diode in forward bias

It conducts immediately

What happens in a Schottky diode in forward bias

A number of electrons are injected into the metal region and will lose some energy and join the free electrons in the metal

What is the symbol for

Pin diode

What is the structure of a pin diode

It has P and N regions, however they have an intrinsic semiconductor region between them

What is an intrinsic semiconductor

A semiconductor that hasn’t been doped

What does a PIN diode act as in reverse bias

A constant capacitor - meaning the plate separation is the same

In reverse bias what does the intrinsic region of a PIN capacitor act as

Dielectric

In reverse bias what does the P and N regions of a PIN capacitor act as

Capacitor plates

How does a PIN diode act in forward bias

As a current controlled variable resistor

What happens to the intrinsic region of a PIN diode in forward bias

Charge carriers flood it and enable it to conduct

What happens to the forward resistance of the intrinsic region as current increases

It decreases

What are the applications of PIN diodes

Attenuator applications - volume control, DC controlled microwave switches

What is an attenuator

An electronic device or circuit that reduces the amplitude of a signal without distorting its waveform

What is this symbol for

Tunnel diode

What are doping levels in a tunnel diode and what does this increase

High, increasing both majority and minority charge carriers

What happens to a tunnel diode in reverse bias

There is a high reverse current due to the amount of minority charges and there is no breakdown

What are tunnel diodes made of

Geranium or gallium arsenide

What effect does the heavy doping have on the depletion region of a tunnel diode

The depletion region is very narrow

What special characteristics do tunnel diodes have

Negative resistance

What is negative resistance

When voltage increases, current decreases

What are the potential applications of a tunnel diode

Oscillator circuits and microwave amplifier applications

What does having a high amount of doping in a tunnel diode allow the electrons to do

They can tunnel though the PN juction in any biasing

What happens to a tunnel diode in forward bias between A and B

The diode will start conducting after a very small amount of voltage and the current increases

What happens to a tunnel diode in forward bias between B and C

It shows negative resistance, and a potential barrier will start to form again

What happens to a tunnel diode in forward bias after point C

It begins to act as a normal diode again

What is this the symbol for

Zener Diode

What does a Zener diode behave like in forward bias

A standard diode

How does a Zener diode act in reverse bias before the breakdown point

Like a standard diode, all the voltage is dropped over it

What happens to the Zener diode at the breakdown point

Instead of breaking down it sustains that voltage and maintains that output

What is Zener voltage

The voltage at which the diode begins to conducts a significant amount of current and maintains a constant voltage

What is the ideal approximation for a Zener diode in forward bias

It will conduct as soon as it has a positive voltage

What is the ideal approximation for a Zener diode in reverse bias

It will have a constant Zener voltage drop

What is V_z (diodes)

Zener voltage

What is the practical approximation for a Zener diode in reverse bias

There is a small increase in Zener voltage as the input voltage increases

Why does the Zener voltage increase as the input voltage increases

Due to the impedance (dynamic resistance)

What is Z_z

Zener impedance

What is the equation for Zener impedance

Change in Zener voltage/change in current

What is I_zk

The minimum amount of current needed to keep the diode in the Zener region

What is I_zm

The maximum about of Zener current that can be sustained before the diode damages

What is the equation for I_zm

Pd/Vz

What is P_D (diodes)

The max power the diode can take

To use a Zener diode to regulate, what do you want the Zener voltage to be

Slightly below the bottom of the ripple