Tunnel Diode - Quantum Device Lab

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npn-Junction Transistor - consists of a thin p-doped region called the base electrode connected to an n-doped emitter and collector electrode, see figure - an npn transistor acts as an amplifier for small signals applied between the emitter and base electrode that are amplified into a large base-collector signals - the energy band structure of the npn transistor at zero bias is shown - the current in an npn transistor is carried by electrons - a pnp transistor would work in an analog way for holes being the predominant charge carrier phys4.19 Page 3 Transistor Bias - for transistor operation the emitter base junction is weakly forward biased and the base collector junction is strongly reverse biased - the current from the heavily doped emitter into the base is carried by electrons - electrons diffuse across the thin (~ 1 μm) weakly hole doped base electrode into the baseemitter junction and are accelerated by the large reverse bias into the collector - the input signal power is then amplified at constant current by the large base collector reverse bias voltage to a larger output power - a limitation of the npn transistor amplifier is its low input impedance (or low input resistance), also its power consumption and integration density are not the best - for some applications an amplifier with higher input impedance, such as a field effect transistor, is advantageous phys4.19 Page 4
Field Effect Transistor (FET) - an FET consists of a n-type channel connecting source and drain and contacted by a p-type gate; it is widely used as an alternative to npn junction transistors - electrons move from source to drain along an n-type channel - the pn junction is reverse biased to create a depletion region at the interface, the carrier density and the source-drain current depend sensitively on the magnitude of the reverse bias - in reverse bias little current flows into the pn junction giving it a high input impedance Metal Oxide Semiconductor Field Effect Transistor (MOSFET) - semiconductor gate replaced by a metal film separated from the channel by a thin oxide layer - MOSFETs have high input impedance (up to 10 15 Ω) due to capacitively coupled gate and are also compatible with high integration density phys4.19 Page 5 Superconductivity - usual electrical conductors, even the very best ones, have finite resistance determined by temperature and impurities in the material - at very low temperatures some metals, alloys and some special chemical compounds can transport current without resistance, an effect called superconductivity - Kammerlingh Onnes discovered that resistance of mercury (Hg) decreased like that of other metals down to T c ~ 4.15 K but then lost all of its resistance to immeasurable levels below that critical temperature T c - the resistivity is actually zero as tested in persistent current measurements - usual critical temperatures for metallic superconductors are in the range 0.1 - 10 K - it is interesting to note that good usual conductors such as copper (Cu) and silver (Ag) do not become superconducting phys4.19 Page 6
Page 1: Tunnel Diode - in a heavily doped p
Page 5 and 6: Meissner Effect - superconductors a

Tunnel Diode - Quantum Device Lab

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