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111
Notes 22 April 2013Semiconductor Quantum Wells (QWs)
• A narrow gap semiconductor is sandwiched between layers of a wide band gap semiconductor
• Quantum confinement takes place when the well thickness is comparable to De Broglie wavelength of the particle
• Electron movement is confined in the quantum well growth direction
• Examples: GaAs/AlAs, InGaAs/AlInAs.
222
Application of QWs — Diode Laser
Disadvantages:
• Emission wavelength depends on material
• Very difficult to generate more than one color per laser
• Difficult to generate long wavelength, i.e., colors in the mid- to far- infrared region
n-AlGaAs
GaAs
p-AlGaAs
Electrode
+V
Conduction band
Valence bandBand gap
ElectrodeLight Light
• Operation involves both electrons and holes, so is called “bipolar”
• Junction E-B is forward biased, so electrons from the E (emitter) to the B (base)
• Junction B-C is reverse biased, so minority carrier electron concentration in B region at the B-C edge is close to zero.
• In the B region, there is large gradient of electron (minority carrier) concentration; the electron injected from E region will diffuse across the B region into the B-C space charge region
• An electric field due to the B-C reverse bias will sweep the electrons to the C (collector) region
• The B regions must be thinner than the minority carrier diffusion length in order to make as many electrons as possible to reach the C region.
Operation modes of BJT
9
Mode BE junction BC junction Currents
Active Forward Reverse ic = βiB
Cutoff Reverse Reverse iE= iB= iC=0
Saturation Forward Forward ic < βiB
Reverse active Reverse Forward ic = βRiB
Analysis:
Active mode: most useful bias mode when using a bipolar junction transistor as an amplifier
Cutoff mode: no electron injected to the base, all currents are zero. Used as “off” state in digital circuits or open switch
Saturation mode: used as “on” state in digital circuits or closed switch
Reverse active mode: emitter and collector regions switch roles. Seldom used.