Page 344 - Electrical Engineering Dictionary

current amplitude is usually assumed to vary heterogeneous having dissimilar compo-
sinusoidally in time. nents in a system; in the context of comput-
ers, having different types or classes of ma-
Hertz, Heinrich Rudolf (1857-1894) chines in a multiprocessor or multicomputer
Born: Hamburg, Germany system.
Hertz is best known as the person who dis-
covered radio waves. Hertz had several pro-
heterojunction a junction between two
fessorships including posts at Berlin, Kiel,
crystals of different bulk composition, typ-
Karlsruhe, and Bonn. Hertz’s work was both
ically referring to semiconductor–semicon-
theoretical and experimental. He succeeded
ductor interfaces. The prototype example is
in proving important elements of Maxwell’s
AlAs/GaAs.
equations, and was able to demonstrate that
radio waves could be generated. He died
early from a degenerative bone disease. We heterojunction bipolar transistor (HBT)
honor his name by referring to the hertz as a proposedbyShockley(U.S.Patent2569347).
unit of frequency.
heterojunction field effect transistor
Hertzian dipole See Hertz dipole.
(HFET) a field effect transistor that uses
a heterojunction in the channel parallel to
Hertzian potential potentials used for the
the current flow direction to improve the
computation of electromagnetic field. By us-
carrier transport by separating the dopant
ing the Hertzian vector potentials of electric
from conduction region. Other names for
and magnetic type 5 e ,5 h , respectively, the
this device include MODFET, (MOdulation
fields are expressed as
Doped Field Effect Transistor and HEMT,
2
E = k 5 e +∇∇ · 5 e − jωµ∇× 5 h High Electron Mobility Transistor.
2
H = jωε∇× 5 e + k 5 h +∇∇ · 5 h
heterojunction laser See diode laser.
where is the dielectric permittivity, µ the
√
magnetic permeability, and k = ω µ, with
ω denoting the angular frequency. heterostructure refers to a composite
structure in which two dissimilar materials
heterodyne detection method of measur- are chemically joined at a common interface.
ing the frequency content of an optical sig- Examples are GaAs-AlGaAs, metal on semi-
nal in which that signal is combined with a conductor, oxide on semiconductor, etc.
known CW reference signal from a local os-
cillator by means of a square-law detector;
Heuristic specialproblem-specificknowl-
sometimes provides a more direct and sensi-
edge that can be used to help direct a search
tive representation of the frequency content
efficiently towards an appropriate, though
of a signal than homodyne detection.
possibly suboptimal, solution. See heuristic
search.
heterodyne receiver a receiver where the
low-level RF signal is mixed with a local
oscillator signal to produce an intermediate heuristic search a search that uses prob-
frequency (IF). The IF frequency is usually lem-specificknowledgeintheformofheuris-
between 10 to 100 MHz. It can be ampli- tics in an effort to reduce the size of the search
fied with a low noise amplifier. Heterodyne space or speed convergence towards a solu-
receivers have much better sensitivity and tion. Heuristic searches sacrifice the guaran-
noise characteristics than direct detection re- tee of an optimal solution for reduced search
ceivers. time.

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2000 by CRC Press LLC