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23. Thermionic emission occurs when a metal is
heated to a high temperature, the free electrons in
the metal gain kinetic energy sufficient to escape
through the surface of the metal.
24. The electron emission from the cathode depends
on the temperature of the filament and the work
function of the material of the cathode.
25. The thermionic diode is a two electrode (cathode
and plate) device based on thermionic emission.
26. A diode allows unidirectional flow of electrons,
i.e. only when the plate is positive with respect to
cathode. Hence it is also called a valve.
) is defined as
27. The plate resistance of diode (R
p
the ratio of change in plate potential to the
corresponding change in plate current. i.e.
V
p
.
R
p
I
p
28. The triode valve consists of three electrodes
e.g. cathode, plate and grid enclosed in an evacuated
glass bulb.
29. Grid influences the space charge and controls
the flow of plate current.
30. When the grid is given a negative potential with
respect to cathode, it repels the electrons escaping
from the cathode and increases the effect of space
charge, at sufficiently negative grid potential the
plate current falls to zero. This value of grid potential
is known as cut off grid bias.
31. If the grid is given a positive potential with
respect to cathode, it attracts the electrons and
decreases the effect of space charge, then increasing
the plate current. In this case a current flows into
the circuit, thus grid modifies the function of valve.
32. Grid is always kept at small negative potential
with respect to cathode.
33. The two important characteristic of the triode
valves are (i) plate or anode characteristics and
(ii) mutual characteristic curves.
34. The amplification factor of a triode valve is defined
as the ratio of a small change in plate potential to
a change in grid potential in the opposite direction,
so that the plate current remains constant.
35. The mutual conductance or trans conductance
(gm) of triode valve is defined as the ratio of small

change in plate current to the corresponding change
in grid potential.
36. The characteristic of a valve in presence of load
are called dynamic characteristics of the valve.
37. The load line is a graphical representation of
the distribution of the plate supply voltage across
the load and across the internal plate resistance of
the valve.
38. Triode can be used as an amplifier, oscillator,
modulator and demodulator.
39. The amplifying action of the triode is based on
the fact that a small change in grid voltage causes
a large change in plate current.
40. An oscillator is an electronic device which
generates A.C voltage from D.C. power. It is basically
a positive feed back amplifier with infinite voltage
gain.
41. Semiconductors have conductivity in between
that of metals and insulators.
42. Pure germanium and silicon crystals in their
natural state are called intrinsic semiconductors.
43. Suitable impurities are added to intrinsic
semiconductors in order to increase their conductivity
is known as doping.
44. Doped semiconductors are called extrinsic
semiconductors.
45. In n-type semiconductor, the number of density
of electrons is approximately equal to that of donor
atoms and very large as compared to number density
n >> n
holes. Thus n
e d h
46. The number density of holes in p-type semi
conductor is approximately equal to that of acceptor
atoms and is very large as compared to number
n >> n
density of electrons. Thus n
h a e
47. Junction diode is a semiconducting device
equivalent to diode valve. It is formed by placing
a p-type crystal in contact with n type crystal and
subjecting to high pressure so that it becomes a
single piece.
48. The potential difference developed across the
junction is called potential barrier.
49. When a battery is connected to the diode with
p-section connected to positive pole and n-section
to the negative pole, the junction diode is said to
be forward biased.

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