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The P-N Diode as a SwitchUsing the Ideal Diode Concept of a Switch to Analyse Simple Circuits
Jun 21, 2010 © Harry P. Schlanger
A diode consists of dissimilar semiconductor materials joined together. One may approximate an 'ideal' diode to a switch and simplify analysis of simple circuits.
The diode has p-type and n-type semiconductor materials joined together. This concept was presented
by this author in a previous article,
The P-N Diode Junction and is continued here.
Controlling the Depletion Region
Applying a reverse bias potential to the diode, causes the depletion layer to widen, increasing resistance
to flow. On the other hand, applying a forward bias causes the depletion region to narrow and so lowers
resistance to flow.
Bias Voltage Characteristics
A graph of current flow versus applied voltage is
non-linear
. The reverse-bias current is very small, in
the order of micro amps, but the forward-bias current is much larger, in the order of milliamps. With
forward bias voltage progressively applied, the diode does not initially conduct as it needs to overcome
the energy of the barrier potential, up to the value corresponding to the switch-on voltage. At this point,
the current in the diode runs away and the switching effect is created.
Ideal Diode Considered as a Switch
A useful tool to analyse circuits is to consider the diode's functionality when approximated to that of a
switch.
Graphically, one may visualise switching as the region marked by the positive
y-axis and negative x-axis. For example, reverse bias causes zero current (negative x-axis) and is
therefore open circuit – i.e., the diode is "OFF". Forward biasing causes a large current at zero voltage
(positive y-axis) and therefore corresponds to a closed circuit – i.e., the diode is "ON".
A practical diode model improves over this ideal model by taking account of the small switch-on voltage
source of 0.7V for silicon or 0.3V for germanium.
Analysing a Rectifier Circuit
The simple circuit shown below is called a rectifier circuit as it smooths the sinusoidal input
voltage by removing the negative voltage cycles.
The circuit only has input voltage, Vin, a diode, and a resistance across the output, Vo.
During the cycle when the input voltage is positive, i.e. Vin > 0, the diode is forward biased,
therefore voltage across this "ideal"
diode is zero, the diode is turned "ON". This observation is represented by the graphic below. The
equivalent circuit (a) clearly indicates that Vo is equal to Vin.
Similarly, during the cycle when Vin < 0, the diode is reverse biased, therefore current flowing through the
diode must be zero – the diode is turned "OFF". The equivalent circuit (b) suggests output
voltage must be zero.
These results are described in (c) by the output trace, Vo, showing clipping of all negative voltage
phases.
In conclusion, a diode consists of two different semiconductor materials brought together in intimate
contact. The junction becomes depleted of majority carriers and a small barrier potential forms. Applying
external voltage either reduces the width of the barrier if the voltage is forward-biased, or increases
the width of the barrier when reverse-biased. This behaviour has the effect of switching from low to
high resistance, respectively, and can be idealised to that of a switch. This is a very useful idea
that may be used in analysing simple circuits.
References:
- Bail Doug, et al. Heinemann Physics 12. Units 3 and 4. Port Melbourne, Melbourne Australia. 2nd Edition. 2004.
The copyright of the article The P-N Diode as a Switch: Using the Ideal Diode Concept of a Switch to Analyse Simple Circuits is owned by Harry P. Schlanger. Permission to republish in print or online must be granted by the author in writing.
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