Behavior of a P-N junction

Simply having a semiconducting material, either P or N type, might be interesting, and a good object of science experiments. But when the two types of material are brought together, the P-N junction develops properties that make the semiconductor materials truly useful as electronic devices. Figure 1 shows the schematic symbol for a semiconductor diode, formed by joining a piece of P-type material to a piece of N-type material. The N-type semiconductor is represented by the short, straight line in the symbol, and is called the cathode.
The P-type semiconductor is represented by the arrow, and is called the anode. In the diode as shown in the figure, electrons flow in the direction opposite the arrow. (Physicists consider current to flow from positive to negative, and this is in the same direction as the arrow points.) But current cannot, under most conditions, flow the other way. Electrons normally do not flow in the direction that the arrow points. If you connect a battery and a resistor in series with the diode, you’ll get a current flow if the negative terminal of the battery is connected to the cathode and the positive

Figure 1: Schematic symbol for a semiconductor diode.

terminal is connected to the anode (Fig. 2A). No current will flow if the battery is reversed (Fig. 2B). The resistor is included in the circuit to prevent destruction of the diode by excessive current.
It takes a certain minimum voltage for conduction to occur. This is called the forward breaker voltage of the junction. Depending on the type of material, it varies from about 0.3 V to 1 V. If the voltage across the junction is not at least as great as the forward breaker value, the diode will not conduct. This effect can be of use in amplitude limiters, waveform clippers, and threshold detectors.

Fig. 2: Series connection of battery B, resistor R, milliammeter mA, and diode D. At A, forward bias results in current flow; at B, reverse bias results in no current.