Unit-1 Semiconductor Diodes PDF
Unit-1 Semiconductor Diodes PDF
Unit-1 Semiconductor Diodes PDF
By
RAJARAO MANDA
Asst. Professor SS,
Department of electronics,
UPES
REFERENCE TEXT BOOKS:
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Extrinsic or
Intrinsic or pure
impure
semiconductors
semiconductors
INTRINSIC SEMICONDUCTORS
which is made of the semiconductor material in its extremely
pure form. Ex: Silicon and Germanium
𝑑𝑛
𝑎𝑛𝑑 𝐽𝑛 = 𝑛𝜇𝑛 qE + 𝑞𝐷𝑛 → (5)
𝑑𝑥
Consider a doped semiconductor with non-uniform hole
concentration (p) and there is no external voltage applied across
the diode.
𝑑𝑉
𝐸=− 𝑤ℎ𝑒𝑟𝑒 𝑉 = 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
𝑑𝑥
𝑉𝑇
−𝑑𝑉 = dp
𝑝
𝑥2 𝑥2
𝑑𝑝
− න 𝑑𝑉 = න
𝑝
𝑥1 𝑥1
−(𝑉2 − 𝑉1 ) = 𝑉𝑇 (ln 𝑝2 − ln 𝑝1 )
𝑝1 = 𝑝2 𝑒 𝑉21/𝑉𝑇
𝑝𝑝 𝑁𝐴 𝑁𝐷
𝑉0 = 𝑉𝑇 ln = 𝑉𝑇 ln
𝑝𝑛 𝑛𝑖 2
Diode symbol
𝐼𝐷 = 𝐼𝑠 (𝑒 𝑉𝐷 Τη𝑉𝑇 − 1)
𝑉𝐷
DC 𝑜𝑟 𝑠𝑡𝑎𝑡𝑖𝑐 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑅𝑑𝑐 =
𝐼𝐷
𝑑𝑉𝐷 η𝑉𝑇
𝐴𝐶 𝑜𝑟 𝑑𝑦𝑛𝑎𝑚𝑖𝑐 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑟𝑎𝑐 = 𝑟𝑑 = =
𝑑𝐼𝐷 𝐼𝐷
LOAD LINE ANALYSIS
The applied load will normally have an important impact
on the point or region of operation of a device.
If the analysis is performed in a graphical manner, a line
can be drawn on the characteristics of the device that
represents the applied load.
The intersection of the load line with the characteristics
will determine the point of operation of the system.
Such an analysis is called load-line analysis. The
intersecting point is known as operating or quiescent (Q)
point.
Diode configurations in the circuit analysis
Example 1: Determine ID and V0 for the given circuit shown in fig.
E=
Example 3:
.
For the period 0 → T/2 (+ ve half cycle)
During negative half cycle (T/2 → T )
PERFORMANCE PARAMETERS
𝐼𝑑𝑐 2 𝑅𝐿
η=
𝐼𝑟𝑚𝑠 2 (𝑅𝐹 +𝑅𝐿 )
𝐼𝑚 𝐼𝑚
For half wave rectifier: 𝐼𝑑𝑐 = and 𝐼𝑟𝑚𝑠 =
𝜋 2
0.406𝑅𝐿
η= ≈ 0.406
(𝑅𝐹 +𝑅𝐿 )
2𝐼𝑚 𝐼𝑚
For full wave rectifier: 𝐼𝑑𝑐 = and 𝐼𝑟𝑚𝑠 =
𝜋 2
0.812𝑅𝐿
η= ≈ 0.812
(𝑅𝐹 +𝑅𝐿 )
RIPPLE FACTOR
Ripple factor is a measure of effectiveness of a rectifier
circuit and defined as a ratio of RMS value of ac
component to the dc component in the rectifier output.
𝐼𝑚 𝐼𝑚
For half wave rectifier: 𝐼𝑑𝑐 = and 𝐼𝑟𝑚𝑠 = ,
𝜋 2
ripple factor = 1.21
2𝐼𝑚 𝐼𝑚
For full wave rectifier: 𝐼𝑑𝑐 = and 𝐼𝑟𝑚𝑠 = ,
𝜋 2
ripple factor = 0.48
CLIPPER (VOLTAGE LIMITERS)
Clippers are the circuit that employ diodes to remove a
portion of an input signal without distorting the remaining
part of the applied waveform.
Depending on the orientation of the diode, the positive or
negative region of the input signal is “clipped” off.
Clipper Circuit - 1:
• If vi < VR, diode is reversed biased and
does not conduct. Therefore, vo = vi
• if vi > VR, diode is forward biased and
thus, vo= VR.
By KVL,
Vi = IR + V0 and V0 = VD+VR
Vi = IR + VD + VR
VD = Vi – IR – VR
By KVL,
Vi = IR + V0 and V0 = – VD+VR
Vi = IR – VD + VR
VD = – Vi + IR + VR
Vi = IR+VD1 + VR
VD1 = Vi – IR – VR > 0 ; D1 -- FB
Vi >VR
Vi – IR – VR < 0 D1 --- RB
Vi < VR
Or Vi = IR – VD2 – VR
VD2 = – Vi + IR – VR > 0 D2 ---FB
Vi < – VR
Vi – IR – VR < 0 D2 ---- RB
Vi > – VR
CLAMPERS
A clamper adds a dc level to an ac voltage
The network must have a capacitor, a diode, and a resistive
element
For sinusoidal input
Voltage multiplier (double)
ZENER DIODES
A major application for Zener diodes is as a type of
voltage regulator for providing stable reference voltages
for use in power supplies, voltmeters, and other
instruments.
Zener diodes are designed to operate in reverse
breakdown.
If a diode is sufficiently reverse biased the diode will
allow the conduction of a wide range of currents in
reverse direction and the diode voltage approximately
constant at some value. This is known as breakdown
region.
Two types of reverse breakdown mechanisms in a Zener
diode: (1) avalanche and (2) Zener.
AVALANCHE BREAKDOWN
Avalanche breakdown occurs when a high reverse bias voltage
is applied to a diode and large electric field (or greater junction
potential) is created across the depletion region.
The effect is dependent on the doping levels in the region of the
depletion layer.
Minority carriers in the depletion region associated with small
leakage currents acquires energy and accelerated by the field so
that they collide with crystal ion and imports sufficient energy
to disrupt a covalent bond.
A new hole-electron pair is created in addition to the original
carrier. These carriers also may pick up sufficient energy from
the applied field, collide with crystal ion and create another
electron hole pair.
Thus each new carrier may in turn produce additional carrier
through collision and the action of disrupting bonds. This
cumulative process is referred as avalanche multiplication. It
results in large reverse current and the diode is said to be in the
region of avalanche breakdown.
ZENER BREAKDOWN
Breakdown occurs with heavily doped junction regions (i.e.
highly doped regions are better conductors).
Because of the high doping levels, the depletion layer is very
thin and, as a consequence, a strong field (106 V/cm, or greater)
exists across it.
Near the Zener breakdown voltage (VZ), the field is intense
enough to pull electrons from their covalent bonds and create
current. This is known as Zener break down.
The Zener break down occurs at a field of approximately 2 ×
107 𝑉/𝑚
Zener diodes with breakdown voltages of less than
approximately 6 V operate predominately in Zener breakdown.
Those with breakdown voltages greater than approximately 6 V
operate predominately in avalanche breakdown. Both types,
however, are called Zener diodes.
Zener diode symbol