Physics of Semiconductors Common Q

Photoelectric Effect

When light shining on a metal causes electrons to be emitted

 

Einstein's Prediction

Light comes in tiny packets of energy, and the energy of each packet is related to the frequency of the light

Photoelectric Effect

When light shining on a metal causes electrons to be emitted

Einstein’s Prediction

Light comes in tiny packets of energy, and the energy of each packet is related to the frequency of the light

DETAILED EXPLANATION

• Photons transfer their energy to electrons in the metal

• One photon gives all its energy to one electron

• Electrons emitted directly from the surface have the most energy

• Max energy of emitted electrons depends on the light's freq & metal's work function

  • Ek = hf - W

• Stopping potential (the voltage needed to stop electrons) is measured

• Electron energy depends on light freq, not intensity, and that stopping potential increases linearly w/ freq

• Validated light is quantised and the energy of a photon is given by E=hf

SIMPLIFIED Explanation

• Light Shines on a metal

  • Causes electrons to be emitted

• Stopping Voltage applied to stop the electrons

  • Stopping Voltage related to kinetic energy of electrons

• Varying light freq

  • Allows us to observe the relationship w/ light freq & electron kinetic energy

• Results match Einstein’s Prediction

  • Electron energy depends on light freq

1. When atoms are far apart, their electrons occupy distinct energy levels

2. As atoms get closer together, their wave functions begin to overlap

3. The electron energy levels cannot be the same in the bonded atoms. Thus the discrete energy levels of individual atoms split into a large number of closely spaced energy levels

4. These closely spaced energy levels form an energy band

   1. e.g. In Si, the 3s & 3p bands merge into a single band, which then splits into the valence and conduction band

Work function for a material:

The min energy needed to remove an electron from the Fermi level at the surface of the material into a vacuum

Unit:

 eV or J (electron-volts or joules)

0K; Sharp Step:

@ Absolute 0, Distribution is a perfect square w/ sharp transition between filled & empty states

Increasing Temp:

• Rounded Curve

• As temp increases, step becomes a rounded curve  as electrons gain enough thermal energy to occupy higher states

Fermi Level (EF):

• Fixed

• Fermi level = in middle of band gap

Low Temp: Sharp Step

High Temp: Rounded Curve

Intrinsic (undoped) semiconductor: Fermi level near middle of band gap

n-type doping

• Extra Donor Atoms contribute free electrons to conduction band

  • More free electrons in conduction band

  • Fermi level higher than Ei, closer to conduction band

 

p-type doping

• Extra Acceptor Atoms contribute holes to valence band

  • More holes in valence band

  • Fermi level lower than Ei, closer to valence band

Reverse-bias drift current in a p-n junction:

• Caused by the electric field in the depletion region.

• This electric field sweeps thermally generated minority carriers (electrons in p-region) across the junction

 

Why reverse-bias drift current is limited to small value

• Depends on rate of thermal generation of electron-hole pairs

• Generation rate is quite low in semiconductors

• Number of minority carriers available to drift across the junction; therefore is limited

• Contact Potential: formed @ junction of p-n diode & electric field forms in depletion region

  • Contact potential causes a drift current where holes drift from + to - & free electrons drift from - to +

  • In reverse bias, the depletion region widens, but the num of thermally generated minority carriers remain small

• Reverse saturation current, sometimes denoted as I0 or Is, is caused by this small thermal drift current

  • The current is limited to small value of the reverse saturation current

i)

• Put the given I0, q, V, k, T into the formula

ii)

• Same as i) but -0.5V

Values not the same but same method

Emitter Efficiency: measure of how well an emitter injects the desired carriers into base of BJT

Pnp: emitter injects mostly holes into the base

Npn: emitter injects mostly electrons into the base

 

What determines its value: Should be close to 1 as possible

Influenced by:

Doping concentrations: Heavily doped emitter & Lightly doped base enhance efficiency

Recombination: Minimizing recombination at emitter-base junction improves efficiency

Material Properties: Diffusion coefficients affect carrier injections

Cutoff:

• BJT is off

• Both junctions reverse biased

• No current flow

Saturation

• BJT fully on

• Both junctions forward biased

• Max current flow

Forward Active

• BJT amplifying

• EB junction forward-biased

• CB junction reverse biased

• Current Amplification

When VDS = 0 and VGS is increased from 0V to just above VT (threshold voltage)

• A positive charge accumulates on the gate

  • Positive gate charge attracts Negative charge to the underlying p-type substrate

• This attraction forms a depletion region in substrate

• As VGS increases:

  • Negative charge @ surface of substrate increases to where p-type substrate becomes intrinsic

• When VGS reaches VT:

  • Inversion occurs (n-type channel forms between source & drain regions)

MOSFETS:

• Primarily Controlled by Voltage

• Easier to manage for amplification

• Generally offer higher current gain

 

BJT:

• Controlled by Current

• Slightly better high-frequency performance