More Applications of PN Junction By D. P. Narsale

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. Note: A totally filled band can not conduct current, just as the water In a totally filled bottle does not slosh about.

Photoconductor 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 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 Electrons roll downhill like stones. Holes float up like bubles!

Energy band Diagram of A pn junction

PN Junction under Reverse Bias 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. E

Energy Band Diagram of a Reverse Biased PN Junction np pn np Stronger E field in the depletion region pn

c02f31 IS=Reverse Saturation=leakage current Small since np and pn are small.

c02f32 The diode current is proportional to area.

Complete I-V Characteristic 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 P side N 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 (P side) (N-side)

Application: Zener Diode 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.

Forward Biased Diode

Graphical Illustration

Solar Cell 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 Cell 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.

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 A reverse-biased PN junction is called a photodiode. Photodiode are used for light-sensing applications.

Example of a Photodiode Circuit 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 When the injected minority carriers recombine with the majority carriers, photons are emitted.

Direct and Indirect Semiconductor 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