THE PN JUNCTION

Hundreds of different electronic devices have been produced since the invention of solid-state components. Solid-state devices are made by combining P-type and N-type materials. The device produced is determined by the number of layers of material used, the thickness of the layers of material, and the manner in which the layers are joined.

It is not within the scope of this text to cover even a small portion of these devices. The devices that are covered have been selected because of their frequent use in industry as opposed to communications or computers. These devices are presented in a straightforward, practical manner, and mathematical explanation is used only when necessary.

The PN junction is often called the diode. The diode is the simplest of all electronic devices. It is made by joining a piece of P-type material and a piece of N-type material (Figure 57 – 1). The schematic symbol for a diode is shown in Figure 57 – 2. The diode operates like an electric check valve in that it permits cur- rent to ﬂow through it in only one direction. If the diode is to conduct current, it must be forward biased. The diode is forward biased only when a positive voltage is connected to the anode and a negative voltage is connected to the cathode. If the diode is reverse biased, the negative voltage connected to the anode and the

positive voltage connected to the cathode, it will act like an open switch and no current will ﬂow through the device.

When working with solid-state circuits, it is important to realize that circuits are often explained assuming conventional current ﬂow as opposed to electron ﬂow. The conventional current ﬂow theory assumes that cur- rent ﬂows from positive to negative, while the electron ﬂow theory states that current ﬂows from negative to positive. Although it has been known for many years that current ﬂows from negative to positive, many electronic circuit explanations assume a positive to negative current ﬂow. There are several reasons for this assumption. One reason is that ground is generally negative and is considered to be 0 volts in an electronic circuit. Any voltage above, or greater, than ground is positive. Most people ﬁnd it is easier to think of some- thing ﬂowing downhill or from some point above to some point below. Another reason is that all of the arrows in an electronic schematic are pointed in the direction of conventional current ﬂow. The diode shown in Figure 57 – 2 is forward biased only when a positive voltage is applied to the anode and a negative voltage is applied to the cathode. If the conventional current ﬂow theory is used, current will ﬂow in the direction the arrow is pointing. If the electron theory of current ﬂow is used, current must ﬂow against the arrow.

A common example of the use of the conventional current ﬂow theory is the electrical system of an auto- mobile. Most automobiles use a negative ground system, which means that the negative terminal of the battery is grounded. The positive terminal of the battery is considered to be the “hot” terminal, and it is generally assumed that current flows from the “hot” terminal to
ground.

The diode can be tested with an ohmmeter (see Procedure 1 in the Appendix). When the leads of an ohmmeter are connected to a diode, the diode should show continuity in only one direction. For example, assume
that when the leads of an ohmmeter are connected to a diode, it shows continuity. If the leads are reversed, the
ohmmeter should indicate an open circuit. If the diode shows continuity in both directions, it is shorted. If the
ohmmeter indicates no continuity in either direction, the diode is open.

The diode can be used to perform many jobs, but it is most commonly used in industry to construct a rectiﬁer. A rectiﬁer is a device that changes, or con- verts, AC voltage into DC voltage. The simplest type of rectiﬁer is the half-wave rectiﬁer (Figure 57 – 3). The half-wave rectiﬁer can be constructed using only one diode. It gets its name from the fact that it will rectify only half of the AC waveform applied to it. When the voltage applied to the anode is positive, the diode is for- ward biased and current ﬂows through the diode, the load resistor, and back to the power supply. When the voltage applied to the anode is negative, the diode is reverse biased and no current will ﬂow. Since the diode permits current to ﬂow through the load resistor in only one direction, the current is direct current.

Diodes can be connected to produce full-wave rectiﬁcation, which means that both halves of the AC waveform are made to ﬂow in the same direction. One type of full-wave rectiﬁer is the bridge rectiﬁer (Figure 57 – 4). Notice that four diodes are required to construct the bridge rectiﬁer.

To understand the operation of the bridge rectiﬁer shown in Figure 57 – 4, assume that point X of the AC source is positive and point Y is negative. Current ﬂows to point A of the rectiﬁer. At point A, diode D4 is reverse biased and D1 is forward biased; therefore, the current ﬂows through diode D1 to point B of the rectiﬁer. At point B, diode D2 is reverse biased, so the cur- rent must ﬂow through the load resistor to ground. The current returns through ground to point D of the rectiﬁer. At point D, both diodes D3 and D4 are forward biased, but current will not ﬂow from positive to positive. Therefore, the current ﬂows through diode D3 to point C of the bridge, and then to point Y of the AC source, which is negative at this time. Since current ﬂowed through the load resistor during this half cycle, a voltage developed across the resistor.

Now assume that point Y of the AC source is positive and point X is negative. Current ﬂows from point Y to point C of the rectiﬁer. At point C, diode D3 is re- verse biased and diode D2 is forward biased. The cur- rent ﬂows through diode D2 to point B of the rectiﬁer. At point B, diode D1 is reverse biased, so the current must ﬂow through the load resistor to ground. The cur- rent ﬂows from ground to point D of the bridge. At point D, both diodes D3 and D4 are forward biased. Since current will not ﬂow from positive to positive, the current ﬂows through diode D4 to point A of the bridge and then to point X, which is now negative. Current ﬂowed through the load resistor during this half cycle, so a voltage developed across the load resistor. Notice that the current ﬂowed in the same direction through the resistor during both half cycles. Bridge rectiﬁers in single cases are shown in Figure 57 – 5.

In industry three-phase power is used more often than single-phase power. Six diodes can be connected to form a three-phase bridge rectiﬁer that will change three-phase AC voltage into DC voltage (Figure 57 – 6).

When the diode is to be connected in a circuit, there must be some means of identifying the anode and the cathode. Diodes are made in different case styles, as shown in Figure 57 – 7, so there are different methods of identifying the leads. Large stud mounted diodes of- ten have the diode symbol printed on the case to show proper lead identiﬁcation. Small plastic case diodes often have a line or band around one end of the case (Figure 57 – 8). This line or band represents the line in front of the arrow on the schematic symbol of the diode. An ohmmeter can always be used to determine the proper lead identiﬁcation if the polarity of the ohmmeter leads is known. The positive lead of the ohm- meter must be connected to the anode to make the diode forward biased.