DIODES AND RECTIFIERS

A diode is an electrical device which allows the free flow of current in only direction and blocks the flow of current in the opposite direction. The most common kind of diode in modern circuit design is the semiconductor diode although other technologies exist.

Semiconductor diodes are symbolized in schematic diagrams below.

When a diode is placed in a simple battery lamp circuit, the diode will either allow or prevent the flow of current through the lamp, depending on the polarity of the applied voltage.

 

When the polarity of the battery is such that electrons are allowed to flow through the diode, the diode is said to be forward biased. Conversely, when the battery is backward and the diode blocks the current, the diode is said to be to be reverse biased. A diode may be thought of as like a switch “closed” when forward biased and “open” when reversed biased.

Oddly enough, the direction of the diode symbols arrowhead points against the direction of electron flow. This is because the diode symbol was invented by engineers who predominantly use conventional flow notation in their schematics, showing current as a flow of charge from positive (+) side of the voltage source to the negative (-). This convention holds true for all semiconductor symbols possessing arrow head. The arrow points in the permitted direction of electron.

Diode behavior is analogous to the behavior of a hydraulic device called a check valve. A check valve allows fluid to flow through it in only one direction.

Like valve, diode are essentially pressure operated (voltage operated) devices. The essential difference between forward bias and reverse bias is the polarity of the voltage dropped across the diode. A diagram is shown below investigating the voltage drops across the various components.

 

Diode circuit measurements: (a)               forward biased. (b) Reversed biased

A forward biased diode conducts current and drops a small voltage across it, leaving most of the battery voltage dropped across the lamp. If the battery’s polarity is reversed, the diode becomes reversed biased and drops almost all f the battery’s voltage leaving non for the lamp. The most substantial difference is that the diode drops a lot of more voltage when conducting than the average mechanical switch (0.7 volts versus tens of millivolts)

The forward bias voltage drop exhibited by the diode is due to the action of the depletion region formed by the P-N junction under the influence of an applied voltage. If no voltage applied is across a semiconductor diode, a thin depletion region exists around the region of the P-N junction, preventing current flow.

 

If a reversed biasing voltage is applied across the P-N junction, this depletion region expands, further resisting any current through it.

 

Conversely, if a forward biasing voltage is applied across the P-N junction, the depletion region collapses becoming thinner. The diode becomes less resistive to current through it. In order for sustained current to go through the diode; though the depletion region must be fully collapsed by the applied voltage. This takes a certain minimum voltage to accomplish called the forward voltage as illustrated in the figure below.