SEMICONDUCTORS SWITCH DIODE

The V-I characteristics of the silicon diode and germanium diode. The icon used to represent the diode is drawn in the upper left corner of the figure, together with the polarity markings used in describing the characteristics. The icon 'arrow' itself suggests an intrinsic polarity reflecting the inherent non-linearity of the diode characteristic.
As shown in the figure the diode characteristics have been divided into three ranges of operation for purposes of description. Diodes operate in the forward- and reverse-bias ranges. Forward bias is a range of 'easy' conduction, i.e., after a small threshold voltage level (>> 0.7 volts for silicon) is reached a small voltage change produces a large current change. In this case the diode is forward bias or in "ON" state. The 'breakdown' range on the left side of the figure happened when the reverse applied voltage exceeds the maximum limit that the diode can withstand. At this range the diode destroyed. On the other hand if the polarity of the voltage is reversed the current flows in the reverse direction and the diode operates in 'reverse' bias or in "OFF" state. The theoretical reverse bias current is very small.

Figure The diode V-I characteristics

In practice, while the diode conducts, a small voltage drop appears across its terminals. However, the voltage drop is about 0.7 V for silicon diodes and 0.3 V for germanium diodes, so it can be neglected in most electronic circuits because this voltage drop is small with respect to other circuit voltages. So, a perfect diode behaves like normally closed switch when it is forward bias (as soon as its anode voltage is slightly positive than cathode voltage) and open switch when it is in reverse biased (as soon as its cathode voltage is slightly positive than anode voltage). There are two important characteristics have to be taken into account in choosing diode. These two characteristics are:

• PEAK INVERSE VOLTAGE (PIV): Is the maximum voltage that a diode can withstand only so much voltage before it breaks down. So if the PIV is exceeded than the PIV rated for the diode, then the diode will conduct in both forward and reverse bias and the diode will be immediately destroyed.

• MAXIMUM AVERAGE CURRENT: Is the average current that the diode can carry.

It is convenient for simplicity in discussion and quite useful in making estimates of circuit behavior (rather good estimates if done with care and understanding) to linearize the diode characteristics. Instead of a very small reverse-bias current the idealized model approximates this current as zero. (The practical measure of the appropriateness of this approximation is whether the small reverse bias current causes negligible voltage drops in the circuit in which the diode is embedded. If so the value of the reverse-bias current really does not enter into calculations significantly and can be ignored.) Furthermore the zero current approximation is extended into forward-bias right up to the knee of the curve. Exactly what voltage to cite as the knee voltage is somewhat arguable, although usually the particular value used is not very important.

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