Semiconductor Diodes (Part 2)

Fig1.11: Reverse-biased p-n junction

The number of minority carriers, however, that find themselves entering the depletion region will not change, resulting in minority-carrier flow vectors of the same magnitude indicated in Fig. 1.10 with no applied voltage. The current that exists under reverse-bias conditions is called the reverse saturation current and is represented by I_s.

The reverse saturation current is seldom more than a few microamperes except for high-power devices. In fact, in recent years its level is typically in the nanoampere range for silicon devices and in the low-microampere range for germanium. The term saturation comes from the fact that it reaches its maximum level quickly and does not change significantly with increase in the reverse-bias potential, as shown on the diode characteristics of Fig. 1.13 for V_D < 0 V.

The reverse-biased conditions are depicted in Fig. 1.12 for the diode symbol and p-n junction. Note, in particular, that the direction of Is is against the arrow of the symbol. Note also that the negative potential is connected to the p-type material and the positive potential to the n-type material—the difference in underlined letters for each region revealing a reverse-bias condition.

Fig 1.12: Reverse-bias conditions for a semiconductor diode

Forward Bias Condition (VD > 0 V) :

A forward-bias or “on” condition is established by applying the positive potential to the p-type material and the negative potential to the n-type material as shown in Fig. 1.14. For future reference, therefore:

“A semiconductor diode is forward-biased when the association p-type and positive and n-type and negative has been established.”

Fig 1.14: Forward-biased p-n junction.

The application of a forward-bias potential VD will “pressure” electrons in the n-type material and holes in the p-type material to recombine with the ions near the boundary and reduce the width of the depletion region as shown in Fig. 1.14. The resulting minority-carrier flow of electrons from the p-type material to the n-type material (and of holes from the n-type material to the p-type material) has not changed in magnitude (since the conduction level is controlled primarily by the limited number of impurities in the material), but the reduction in the width of the depletion region has resulted in a heavy majority flow across the junction.

An electron of the n-type material now “sees” a reduced barrier at the junction due to the reduced depletion region and a strong attraction for the positive potential applied to the p-type material. As the applied bias increases in magnitude the depletion region will continue to decrease in width until a flood of electrons can pass through the junction, re- sulting in an exponential rise in current as shown in the forward-bias region of the characteristics of Fig. 1.13.

Note that the vertical scale of Fig. 1.13 is measured in milliamperes (although some semiconductor diodes will have a vertical scale measured in amperes) and the horizontal scale in the forward-bias region has a maximum of 1 V. Typically, therefore, the voltage across a forward-biased diode will be less than 1 V. Note also, how quickly the current rises beyond the knee of the curve. It can be demonstrated through the use of solid-state physics that the general characteristics of a semiconductor diode can be defined by the following equation for the forward- and reverse-bias regions:

Where ,Is = reverse saturation current

k =11,600/ η with η = 1 for Ge and η = 2 for Si for relatively low levels of diode current (at or below the knee of the curve) and η = 1 for Ge and Si for higher levels of diode current (in the rapidly increasing section of the curve)

TK = TC + 273°

Fig 1.13: Silicon semiconductor diode characteristics.

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• Diode Characteristics (Part 2)
• Zener Diode Specification (Part 2)
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• QPSK Tutorial
• Differential phase shift keying ( DPSK ) Tutorial
• Low Pass Filters Circuit
• Applications of Filter Circuits
• Triangular and Square Wave Generator
• Diode Clipper (Part 2)
• QPSK Theory