Merely connecting up a piece of semiconducting material, either P or N type, to a source of current can be interesting, and a good subject for science experiments. But when the two types of material are brought together, the boundary between them, called the P-N junction, behaves in ways that make semiconductor materials truly useful in electronic components.
The Semiconductor Diode
Schematic symbol for a semiconductor diode.
above figure 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 above figure, electrons can move easily in the direction opposite the arrow, and holes can move easily in the direction in which the arrow points. But current cannot, under most conditions, flow the other way. Electrons normally do not move with the arrow, and holes normally do not move against the arrow.
If you connect a battery and a resistor in series with the diode, you’ll get a current to flow if the negative terminal of the battery is connected to the cathode and the positive terminal is connected to the anode, as shown in following figure A. No current will flow if the battery is reversed, as shown in following figure B. (The resistor is included in the circuit to prevent destruction of the diode by excessive current.)
It takes a specific, well-defined minimum applied voltage for conduction to occur through a semiconductor diode. This is called the forward breakover voltage. Depending on the type of material, the forward breakover voltage varies from about 0.3 V to 1 V. If the voltage across the junction is not at least as great as the forward breakover voltage, the diode will not conduct, even when it is connected as shown in following figure A.
Series connection of a battery, a resistor, a current meter, and a diode. At A, forward bias results in a flow of current. At B, reverse bias results in no current.
This effect, known as the forward break over effect or the P-N junction threshold effect, can be of use in circuits designed to limit the positive and/or negative peak voltages that signals can attain. The effect can also be used in a device called a threshold detector, in which a signal must be stronger than a certain amplitude in order to pass through.
How the Junction Works
When the N-type material is negative with respect to the P type, as in above figure A, electrons flow easily from N to P. The N-type semiconductor, which already has an excess of electrons, receives more; the P-type semiconductor, with a shortage of electrons, has some more taken away. The N-type material constantly feeds electrons to the P type in an attempt to create an electron balance, and the battery or power supply keeps robbing electrons from the P-type material. This condition is illustrated in following figure A, and is known as forward bias. Current can flow through the diode easily under these circumstances.
When the battery or dc power-supply polarity is switched so the N-type material is positive with respect to the P type, the situation is called reverse bias. Electrons in the N-type material are pulled toward the positive charge pole, away from the P-N junction. In the P-type material, holes are pulled toward the negative charge pole, also away from the P-N junction. The electrons are the majority carriers in the N-type material, and the holes are the majority carriers in the P-type material. The charge therefore becomes depleted in the vicinity of the P-N junction, and on both sides of it, as shown in following figure B. This zone, where majority carriers are deficient, is called the depletion region. A shortage of majority carriers in any semiconductor substance means that the substance cannot conduct well. Thus, the depletion region acts like an electrical insulator. This is why a semiconductor diode will not normally conduct when it is reverse-biased. A diode is, in effect, a one-way current gate—usually!
At A, forward bias of a P-N junction. At B, reverse bias of the same junction. Solid black dots represent electrons. White dots represent holes. Arrows indicate direction of charge-carrier movement.
Some P-N junctions can alternate between conduction (in forward bias) and nonconduction (in reverse bias) millions or billions of times per second. Other junctions are slower. The main limiting
factor is the capacitance at the P-N junction during conditions of reverse bias. As the junction capacitance of a diode increases, maximum frequency at which it can alternate between the conducting state and the nonconducting state decreases.
The junction capacitance of a diode depends on several factors, including the operating voltage, the type of semiconductor material, and the cross-sectional area of the P-N junction. If you examine following figure B, you might get the idea that the depletion region, sandwiched between two semiconducting sections, can play a role similar to that of the dielectric in a capacitor. This is true! In fact, a reverse-biased P-N junction actually is a capacitor. Some semiconductor components, called varactor diodes, are manufactured with this property specifically in mind.
The junction capacitance of a diode can be varied by changing the reverse-bias voltage, because this voltage affects the width of the depletion region. The greater the reverse voltage, the wider the depletion region gets, and the smaller the capacitance becomes.
Sometimes, a diode conducts when it is reverse-biased. The greater the reverse-bias voltage, the more like an electrical insulator a P-N junction gets—up to a point. But if the reverse bias rises past a specific critical value, the voltage overcomes the ability of the junction to prevent the flow of current, and the junction conducts as if it were forward-biased. This phenomenon is called the avalanche effect because conduction occurs in a sudden and massive way, something like a snow avalanche on a mountainside.
The avalanche effect does not damage a P-N junction (unless the voltage is extreme). It’s a temporary thing. When the voltage drops back below the critical value, the junction behaves normally again. Some components are designed to take advantage of the avalanche effect. In other cases, the avalanche effect limits the performance of a circuit. In a device designed for voltage regulation, called a Zener diode, you’ll hear about the avalanche voltage or Zener voltage specification. This can range from a couple of volts to well over 100 V. Zener diodes are often used in voltage-regulating circuits. For rectifier diodes in power supplies, you’ll hear or read about the peak inverse voltage (PIV) or peak reverse voltage (PRV) specification. It’s important that rectifier diodes have PIV ratings great enough so that the avalanche effect will not occur (or even come close to happening) during any part of the ac cycle.