P-N Junction Diode

Topics: Diode, P-n junction, Zener diode Pages: 28 (10705 words) Published: October 19, 2014
CONTENTS
CONTENTS

+ 0 ) 2 6 - 4

Learning Objectives
➣ P-N Junction Diode
➣ Derivation of Junction
Resistance
➣ Junction Breakdown
➣ Junction Capacitance
➣ Equivalent Circuit of P-N
Junction
➣ Diode Fabrication

#

P-N JUNCTION
DIODE

➣ Grown Junction
➣ Alloy Junction
➣ Diffused Junction
➣ Epitaxial Junction
➣ Point Contact Junction
➣ The Ideal Diode
➣ The Real Diode
➣ Diode Circuits with D.C.
and A.C. Voltage Sources
➣ Diode Clipper and
Clamper Circuits
➣ Clippers
➣ Some Clipping Circuits
➣ Clampers
➣ Summary of Clamping
Circuits

Ç

CONTENTS
CONTENTS

Chemist, led the research for the molecular
diode (In the semiconductor industry,
called p-n junctions)

2060

Electrical Technology

52.1. P-N Junction Diode
(a) Construction
It is a two-terminal device consisting of a P-N junction
formed either in Ge or Si crystal. Its circuit symbol is shown in Fig. 52.1 (a). The P-and N-type regions are referred to
as anode and cathode respectively. In Fig. 52.1 (b), arrowhead indicates the conventional direction of current flow when forward-biased. It is the same direction in which hole
flow takes place.
Commercially available diodes usually have some
means to indicate which lead is P and which lead is N.
Standard notation consists of type numbers preceded by
‘IN’ such as IN 240 and IN 1250. Here, 240 and 1250
correspond to colour bands. Fig. 52.2 (a) shows typical
diodes having a variety of physical structures whereas Fig.
52.2 (b) illustrates terminal identifications. Also refer to the picture of two commercial diodes shown in Fig 52.1(c).

(a)

P

N

(b)
Cathode

Anode

a

(c)

k

a

k

a

k

Fig. 52.1

Fig. 52.2

The low-current diodes whose body is about 3 mm long can carry a forward current of about 100 mA, have saturation current of 5 µA at room temperature (25ºC) and can withstand a reverse voltage of 75 V without breaking down. The medium-current diodes can pass a forward current of about 500 mA and can withstand a reverse voltage of 250 V. The high-current diodes or power diodes can pass a forward current of many amperes and can survive several hundred volts of reverse voltage. (b) Diode Mounting

Low and medium-current diodes are usually mounted by soldering their leads to the connecting

P-N Junction Diode

2061

terminals. The heat generated by these
diodes (when operating) is small enough
to be carried away by air convection and
conduction along the connecting leads.
However, high-current stud-mounted
diodes generate large amounts of heat for
which air convection is totally inadequate.
For coolong, they need heat sinks made
of metals such as copper or aluminium
which are good conductors of heat. The
sink absorbs heat from the device and then
transfers it to the surrounding air by
convection and radiation since it has large
surface area.
(c) Working
A P-N junction diode is one-way
Fig. 52.3
device offering low resistance when
forward-biased [Fig. 52.3 (a)] and behaving almost as an insulator when reverse-biased [Fig. 52.3 (b)]. Hence, such diodes are mostly used as rectifiers i.e. for converting alternating current into direct current.

(d) V/I Characteristic
Fig. 52.4 shows the static voltage-current characteristics for a low-power P-N junction diode.

1. Forward Characteristic
When the diode is forward-biased and
the applied voltage is increased from zero,
hardly any current flows through the device
in the beginning. It is so because the external voltage is being opposed by the internal barrier voltage V B whose value is 0.7 V Forward biased diode.
for Si and 0.3 V for Ge. As soon as V B is
neutralized, current through the diode increases rapidly with increasing applied battery voltage. It is found that as little a voltage as 1.0 V produces a forward current of about 50 mA. A burnout is likely to occur if forward voltage is increased beyond a

certain safe limit.

2. Reverse Characteristic...
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