More applications of pn junction
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More Applications of PN Junction. ES230 Jack Ou. PN Junction Based Devices. Energy Band Diagram Zener Diode Light to current conversion Solar Cell Photodiode Current to light conversion LED. Energy Band Diagram. Conduct current. Electrons tend to fill up the low energy bands first.

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Pn junction based devices
PN Junction Based Devices

  • Energy Band Diagram

  • Zener Diode

  • Light to current conversion

    • Solar Cell

    • Photodiode

  • Current to light conversion

    • LED

Energy band diagram
Energy Band Diagram

Conduct current

Electrons tend to fill up the low energy bands first.

Note: A totally filled band can not conduct current, just as the water

In a totally filled bottle does not slosh about.


When light is absorbed by a semiconductor sample and electron-hole

pairs are created, the number of electrons and holes increase

in proportion to the light intensity.

By putting two electrodes on the semiconductor and applying a voltage

between the electrodes, we can measure the change in resistance

and detect changes in light intensity.

Example cds cell
Example: CdS Cell

A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

Energy band diagram of a semiconductor under an applied voltage
Energy Band Diagram of a Semiconductor Under an Applied Voltage

Electrons roll downhill like stones.

Holes float up like bubles!

More applications of pn junction

Energy band Voltage

Diagram of

A pn junction

Pn junction under reverse bias
PN Junction under Reverse Bias Voltage

Reverse: Connect

the + terminal to the

n side.

Depletion region widens.

Therefore, stronger E.

Minority carrier to cross

the PN junction easily

through drift.

Current is composed

mostly of drift current contributed

by minority carriers.

np to the left and pn to the right.

Current from n side to p side,

the current is negative.


Energy band diagram of a reverse biased pn junction
Energy Band Diagram of a Reverse Biased PN Junction Voltage




Stronger E field in the

depletion region


c02f31 Voltage

IS=Reverse Saturation=leakage current

Small since np and pn are small.

c02f32 Voltage

The diode current is proportional to area.

Complete i v characteristic
Complete I-V Characteristic Voltage

If the current is limited to a reasonable value

by the external circuit so that the heat dissipation

is not excessive, the PN junction can operate

in the breakdown region safely.

Tunneling zener breakdown
Tunneling(Zener) Breakdown Voltage

N side

P side

Only a small distance separates the large

number electrons on the P-side valence band and

the empty states in the N-side conduction band.

Tunneling of electrons can occur

Appear for reverse voltage from 3-8V.

Avalanche breakdown
Avalanche Breakdown Voltage

(P side)


Application zener diode
Application: Zener Diode Voltage

A Zener diode is a PN junction diode designed to operate in the breakdwon

Region with a breakdown voltage that is tightly controlled by a manufacturer.

Solar cell
Solar Cell Voltage

A solar cell is structurally identical to a PN junction. The only difference is that it is transparent on one side to allow light reaching the PN junction.

Solar cell1
Solar Cell Voltage

When light shines on the PN junction, the minority carriers that are generate by light close to the junction can diffuse to the junction and be swept across the junction by the built-in E field and cause a current to flow out of the P terminal through the external short circuit and back into the N-terminal. This is current is called short circuit current , ISC. Note the direction of ISC.

More applications of pn junction

The total diode (solar cell ) current is the sum of the current generated

by the voltage and that generated by light.

Generated by light

PN junction

Photodiode current generated

A reverse-biased PN junction is called a photodiode. Photodiode

are used for light-sensing applications.

Example of a photodiode circuit
Example of a Photodiode Circuit current generated

  • Light is applied to the pn junction

  • Electrons are dislodged from covalent

  • bonds.

  • Electron-hole pair is created.

  • Electron is attracted to the positive terminal

  • of the battery.

  • Current flows through the diode is proportional

  • to light intensity.

  • Application: Digital camera.

Light emitting diode
Light Emitting Diode current generated

When the injected minority carriers recombine with the majority carriers, photons are emitted.

Direct and indirect semiconductor
Direct and Indirect Semiconductor current generated





k, the wave vector, represents the direction and the wavelength of the electron.

To be an efficient LED, the electrons and the holes need to have

matching k, just so that they can recombine easily.

Optoelectronic devices
Optoelectronic Devices current generated