PN Semiconductor: Diode Characteristics and Applications | PDF

PN semiconductor After completing this section, you should be able to: ❏ Use a diode in common applications ❏ Recognize the electrical symbol for a diode and several diode package configurations ❏ Apply forward bias to a diode ◆ Define forward bias and state the required conditions ◆ D...

PN semiconductor After completing this section, you should be able to: ❏ Use a diode in common applications ❏ Recognize the electrical symbol for a diode and several diode package configurations ❏ Apply forward bias to a diode ◆ Define forward bias and state the required conditions ◆ Discuss the effect of forward bias on the depletion region ◆ Define barrier potential and its effects during forward bias ❏ Reverse-bias a diode ◆ Define reverse bias and state the required conditions ◆ Discuss reverse current and reverse breakdown The Semiconductor Industry What is a “clean room” inside the semiconductor industry? The Diode - a diode is made from a small piece of semiconductor material, usually silicon, in which half is doped as a p region and half is doped as an n region with a pn junction and depletion region in between. - The p region is called the anode and is connected to a conductive terminal. - The n region is called the cathode and is connected to a second conductive terminal. Forward Bias Condition ( VD > 0 V) To bias a diode, you apply a dc voltage across it. Forward bias is the condition that allows current through the pn junction. Reverse Bias Reverse-Bias Condition ( VD < 0 V) Is the condition that essentially prevents current through the diode. Reverse Current (Is) : The extremely small current that exists in reverse bias after the transition current dies out is caused by the minority carriers in the n and p regions that are produced by thermally generated electron-hole pairs. Or reverse saturation current General characteristics of a semiconductor diode can be defined by the following equation, referred to as Shockley’s equation, for the forward- and reverse-bias regions: Example : At a temperature of 27°C (common temperature for components in an enclosed operating system), determine the thermal voltage VT. Reverse Breakdown : Normally, the reverse current is so small that it can be neglected. However, if the external reverse-bias voltage is increased to a value called the breakdown voltage, the reverse current will drastically increase. Or breakdown potential (V_BV) The maximum reverse-bias potential that can be applied before entering the Zener region is called the peak inverse voltage (referred to simply as the PIV rating) or the peak reverse voltage (denoted by PRV rating). Comparison of Ge, Si, and GaAs commercial diodes. Note: Vk = knee voltage Silicon versus Germanium Silicon diodes have, in general, higher PIV and current rating and wider temperature ranges than germanium diodes. PIV ratings for silicon can be in the neighborhood of 1000 V, whereas the maximum value for germanium is closer to 400 V. Silicon can be used for applications in which the temperature may rise to about 200°C (400°F), whereas germanium has a much lower maximum rating (100°C). Temperature Effects The reverse saturation current (Is) will just about double in magnitude for every 10°C increase in temperature. DC or Static Resistance The application of a dc voltage to a circuit containing a semiconductor diode will result in an operating point on the characteristic curve that will not change with time. Example: Determine the dc resistance levels for the diode of Fig. 1.26 at (a) ID = 2 mA (b) ID = 20 mA (c) VD = -10 V AC or Dynamic Resistance The varying input will move the instantaneous operating point up and down a region of the characteristics and thus defines a specific change in current and voltage. The designation Q-point is derived from the word quiescent, which means “still or unvarying.” In general, therefore, the lower the Q-point of operation (smaller current or lower voltage) the higher the ac resistance. Average AC Resistance If the input signal is sufficiently large to produce a broad swing such as indicated in Fig. 1.30, the resistance associated with the device for this region is called the average ac resistance.