The PN Diode as a Circuit Switch
Using the Ideal Diode Concept of a Switch to Analyse Simple Circuits
A diode consists of p-type and n-type semiconductor materials joined together. One may approximate an "ideal" diode to a switch and simplify analysis of simple circuits.
The concept of semiconductors is depicted in Figure 1. The addition of a small percentage of foreign atoms in a regular crystal lattice of silicon, or germanium, produces dramatic changes in their electrical properties. "Majority carriers" contributing to current flow are conceptually:
- Electrons in n-type semiconductors
- “Holes” in p-type semiconductors
When two different types of semiconductor materials are joined intimately, the movement of holes and electrons combine near the junction, resulting in a depletion layer at the juncture. A small barrier potential due to charged ions left behind is then created.
The circuit symbol used for a diode is a large black arrow, indicating conventional current flow in the direction from p-type to n-type materials. A bar represents the junction (Figure 2). If a battery is applied to the ends of the diode, the barrier potential provides resistance to any current flow.
Diode Characteristics
Applying a reverse bias potential to the diode (Figure 2), causes the depletion layer to widen, increasing resistance to flow. Majority carriers have more difficulty jumping across the junction and only a tiny current results. On the other hand, applying forward bias causes the depletion region to narrow and so lowers resistance to flow. In this case, majority carriers jump more easily across the junction and a large current ensues.
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.
The Ideal Diode Considered as a Switch
A useful tool to analyze circuits is to consider the diode's functionality approximated to that of a switch. Graphically, one may visualize switching as the region marked by a heavy line over 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 in Figure 3 is called a rectifier circuit as it smooths the sinusoidal input voltage by removing the negative voltage cycles. The circuit only has an input voltage, a diode, and a resistance across the output.
- During the time when 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 equivalent circuit of Figure 4a, which clearly indicates that Vout is equal to Vin. Similarly,
- During the time 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 of Figure 4b suggests output voltage must be zero.
These results are described qualitatively by the output trace showing clipping of all negative voltage phases.
Conclusions
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 idealized to that of a switch. This is a very useful idea that may be used in analyzing simple circuits.
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