111 Notes 22 April 2013 Semiconductor 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 band Band gap Electrode 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 ModeBE junctionBC junctionCurrents ActiveForwardReversei c = βi B CutoffReverse i E = i B = i C =0 SaturationForward i c < βi B Reverse activeReverseForwardi c = β R i B 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.