A diode, in lay terms, is a semiconductor that has two electrodes and that passes electric current in one direction only. An ideal diode offers no impedance to current flow in .one direction and an infinite impedance in the other. A typical solid-state diode has a very low forward resistance (resulting in a 0.5 to 1.5V drop) and a reverse current of a few milliamperes when blocking several hundred volts, as shown in Fig. 1.

What causes current flow?

A typical semiconductor diode has what are called N-type and P-type materials. These materials are created by altering their crystal structure; in other words, some atoms within their crystal are replaced with atoms from another element.

An N-type material is created by adding atoms from an element that has more electrons in its outer shell than the crystal. This provides more free electrons and since they have a negative charge, the material is called N-type ("N" for negative).

The same procedure is used to create P-type material, except that atoms from an element having fewer electrons in its outer shell are added to the crystal. This results in empty holes in the crystalline structure and these represent positive charges; hence, the name P-type ("P" for positive).

When voltage is applied to a diode, the holes in the P-type material are filled with the free electrons from the N-type material; thus, the current flow from negative potential to positive potential. (Note that in electron theory, current flows from negative to positive, whereas conventional current flow is positive to negative.)

The above is a very simplified explanation; however, it provides the justification for uni-directional current flow.

Which end is which?

When you apply the proper polarity to a diode, it's called forward bias and results in forward current. When you apply the opposite polarity, it's called reverse bias and this results in a reverse current that is very close to zero.

As shown in Fig. 2, the symbol for a diode has two parts: a straight line representing the cathode and a triangle representing the anode. As we discussed earlier, electrons flow from the cathode to the anode.

Manufacturers denote anode and cathode locations in varying manners. One method has the diode symbol on the surface of the diode; another uses a band around the diode indicating the cathode; a third method has the cathode end physically larger than the anode end; and a fourth method has the cathode end beveled.

If you're unsure about the polarity, you can use an ohmmeter as verification. An ohmmeter's polarity is usually denoted by the markings on its face (+ or -) or by color coding (red for positive; black for negative). However, the ohmmeter's battery actually determines the external polarity.

To find the polarity (forward and reverse bias) of a diode, you place the diode in one direction between the known polarities of the ohmmeter and take a resistance reading; then, reverse the diode's direction and take another reading. A low resistance reading indicates forward bias while a high resistance reading indicates reverse bias. Since the ohmmeter's polarity is known, the diode end connected to the negative lead during forward bias must be the cathode; and the end connected to the positive lead must be the anode.

Testing diodes

While diodes are very reliable, they certainly aren't indestructible. What can damage them? Anything from high voltages to improper connections to overheating. As such, you may have to verify the condition of a diode during troubleshooting of inoperative solid-state equipment.

Open and shorts are the two most common problem areas. Here's how to interpret ohmmeter readings to determine a diode's condition.

Open diode. When taking two sets of resistance readings, one with test leads connected positive-to-anode, negative-to-cathode, and one with negative-to-anode, positive-to-cathode, high resistance readings are obtained in both cases.

Shorted diode. When taking two sets of resistance readings, one with test leads connected positive-to-anode, negative-to-cathode, and one with negative-to-anode, positive-to-cathode, low resistance readings are obtained in both cases.

Good diode. When taking two sets of resistance readings, one with test leads connected positive-to-anode, negative-to-cathode, and one with negative-to-anode, positive-to-cathode, the first connection provides a low resistance reading and the second connection provides a high resistance reading.

If a diode is open or shorted, it must be replaced.

Diode capacity and derating

The amount of current at which a diode can operate without damaging itself is limited by two factors: the size of its heat sink, which helps dissipate heat, and its P-type material, N-type material (PN) junction temperature rise.

Most diodes are rated to operate at 25[degrees]C, or at room temperature. This is a reflection of the diode's PN junction temperature rise. If the ambient temperature increases, the diode can't dissipate as much heat and must then be derated. This means that the diode's maximum operating current must be reduced. Manufacturers offer derating tables to help you find these limits.

Solid-state equipment manufacturers usually install heat sinks to permit internal diodes to operate at or close to maximum operating current. In retrofit or trouble-shooting activities, you may want to install a heatsink to overcome frequent diode damage. Again, this information is available from solid-state equipment manufacturers for your use.

Typical diode application

Diodes are usually seen in rectifiers, gates, modulators, and detectors. Let's talk about rectifiers here. While AC power is generally the most available, many electronic devices require DC power for operation. As such, AC power must be converted to DC. Circuits that provide this conversion are called rectifiers and these circuits use semiconductor diodes.

The simplest form of a rectifier circuit is the half-wave rectifier, as shown in Fig. 3. The circuit includes a load resistor, [R.sub.L], along with a diode across an AC source. A rectifier circuit affects the AC voltage by cutting it in half or, as is commonly said, rectifying it. Because the signal now travels in only one direction, it's called pulsating DC. This type of power supply circuit is only one in which diodes are used.