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Analogue and digital electronics
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2
Section 1: PN Junction
1.1 Basic Semiconductor Concept
1.1.1
I. Boron
II. Aluminum
They all have 3 valence electrons (Laube).
1.1.2
I. Phosphorus
II. Antimony
They have 5 valence electrons (Laube).
1.2 PN junction (diode) basic
1.2.1
Depletion zone of P-N junction is formed by intimate contact between p-type and
n-type semiconductors such that crystal structure remains continuous at the boundary.
The junction has a transition of a very short thickness, 1micro, from p-type material to n-
type material. The p-type which forms the anode has acceptor atoms while n-type which
forms the cathode, has donor atoms. Majority carriers in p-type are holes while minority
charge carriers are electrons. Majority charge carriers through diffusion process, move

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from high concentration region to a region of low concentration hence forming
concentration gradient. For n-type majority carriers are electrons while minority carriers
are holes. Positive and negative ions are immobile charge carriers. At the junction, holes
in p-type recombine with electrons in the n-type leaving only acceptor and donor
immobile ions in the depletion layer. When enough electrons have accumulated the p-
region, more electrons diffusing from n-region are repelled. A similar case for holes from
p-region diffusing to the n-region are repelled. The action leads to a state of equilibrium
with fixed immobile negative and positive ions at near the junction with no charge
carriers. Barrier voltage across the depletion layer barricades the movement of charge
carriers across the depletion layer (Nave).
1.2.2
The junction is forward biased by applying a positive voltage at the p-type and negative
voltage at the n-type, mobility majority charge carriers results to more flow of current as
a result of reduced depletion layer (Nave).
When reverse voltage is applied, a constant small reverse current flow due to mobility
of minority charge carriers as a result of increased depletion layer.
1.2.3
The ebullient circuits and corresponding Voltage verses Current curves of a diode when
considering it as 1) ideal diode, 2) practical diode with voltage drop, 3) practical diode
with voltage drop and resistance

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1) Ideal diode
2) Practical diode with voltage drop

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3) Practical diode with voltage drop and resistor
1.3 PN Junction (Diode) Applications
1.3.1
If the diode has a forward voltage drop 𝑉𝐷 = 0.6𝑉 and 𝑅𝐷 = 10Ω, use the practical
diode DC model to calculate current flowing in the circuit in Fig 1.3.1. (3 Marks) 5 V
100Fig 1.3.1
I D = ( 5−V D ) V
RΩ 100
I D = (5−0.6 ) V
100 Ω
I D =0.044 A

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1.3.2
It is a biased combination diode clipping circuit.
The clipped waveform
1.3.3
This is a full wave bridge rectifier. It consists of four diodes and during each cycle, only two
diodes conduct.
Full wave rectifier