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 the positive side of the voltage source to the negative. This convention holds true for all semiconductors symbol possessing “arrowheads” the arrow points in the permitted direction of conventional flow, and against the permitted direction of electron flow. Diode behavior is analogous to the behavior of a hydraulic device called a check valve. A check valve arrows fluid flow through it in only one direction.

Lik e check valves, diodes 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. Let’s take a closer look at the simple battery-diode-lamp circuit shown earlier, this time investigating voltage drops across the various components.

A forward-biased diode conducts current and drops small voltage across it, leaving most of the battery voltage dropped across the lamp. If the battery’s polarity is reversed, the diode become reversed-biased, and drops all of the battery’s voltage leaving none for the lamp. If we consider the diode to be a self actuating switch (closed in the forward-bias mode and open in the reversed-bias mode), this behavior makes sense. The most substantial difference is that the diode drops a lot more voltage when conducting than the average mechanical switch (0.7 volts verses tens of mille volts).

This 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 is applied across the semi conductor diode, a thin depletion region exists  around  the region of the P-N junction, preventing current flow, fig. below  (a) the  depletion region is almost devoid  of the available charge carriers, and acts  as an  insulator.