**Semiconductor diode**

**The semiconductor diode is formed by simply bringing n- and p-type materials together (constructed from the same base—Ge or Si), as shown in Fig. 1. At the instant the two materials are “joined” the electrons and holes in the region of the junction will combine, resulting in a lack of carriers in the region near the junction.**

**This region of uncovered positive and negative ions is called the depletion region due to the depletion of carriers in this region.**

**Since the diode is a two-terminal device, the application of a voltage across its terminals leaves three possibilities: no bias ( = 0 V), forward bias ( > 0 V), and reverse bias ( < 0 V). Each is a condition that will result in a response that the user must clearly understand if the device is to be applied effectively.**

**No Applied Bias** **( = 0 V)**

**Under no-bias (no applied voltage) conditions, any minority carriers (holes) in the n-type material that find themselves within the depletion region will pass directly into the p-type material. The closer the minority carrier is to the junction, the greater the attraction for the layer of negative ions and the less the opposition of the positive ions ****in the depletion region of the n-type material. For the purposes of future discussions we shall assume that all the minority carriers of the n-type material that find themselves in the depletion region due to their random motion will pass directly into the p-type material. Similar discussion can be applied to the minority carriers (electrons) ****of the p-type material. This carrier flow has been indicated in Fig. 1 for the minority carriers of each material.**

**The majority carriers (electrons) of the n-type material must overcome the attractive forces of the layer of positive ions in the n-type material and the shield of negative ions in the p-type material to migrate into the area beyond the depletion region of the p-type material. However, the number of majority carriers is so large in the n-type material that there will invariably be a small number of majority carriers with sufficient kinetic energy to pass through the depletion region into the p-type material. Again, the same type of discussion can be applied to the majority carriers (holes) of the p-type material. The resulting flow due to the majority carriers is also shown in Fig. 1.**

**A close examination of Fig. 1 will reveal that the relative magnitudes of the flow vectors are such that the net flow in either direction is zero. This cancellation of vectors has been indicated by crossed lines. The length of the vector representing hole flow has been drawn longer than that for electron flow to demonstrate that the magnitude of each need not be the same for cancellation and that the doping levels for each material may result in an unequal carrier flow of holes and electrons. In summary, therefore:**

**In the absence of an applied bias voltage, the net flow of charge in any one direction for a semiconductor diode is zero.**

**The symbol for a diode is repeated in Fig. 2 with the associated n- and p-type regions. Note that the arrow is associated with the p-type component and the bar with the n-type region. As indicated, for = 0 V, the current in any direction is 0 mA.**

**Reverse-Bias Condition ( < 0 V)**

**If an external potential of V volts is applied across the p-n junction such that the positive terminal is connected to the n-type material and the negative terminal is connected to the p-type material as shown in Fig. 3, the number of uncovered positive ions in the depletion region of the n-type material will increase due to the large number of “free” electrons drawn to the positive potential of the applied voltage. For similar reasons, the number of uncovered negative ions will increase in the p-type material. The net effect, therefore, is a widening of the depletion region. This widening of the depletion region will establish too great a barrier for the majority carriers to overcome, effectively reducing the majority carrier flow to zero as shown in Fig. 3.**

**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 with no applied voltage.**

**The current that exists under reverse-bias conditions is called the reverse saturation current and is represented by .**

**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. 6 for < 0 V. The reverse-biased conditions are depicted in Fig. 4 for the diode symbol and p-n junction. Note, in particular, that the direction of 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.**

**Forward-Bias Condition ( > 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. 5. 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.**

**The application of a forward-bias potential 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. 5. 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, resulting in an exponential rise in current as shown in the forward-bias region of the characteristics of Fig. 6. Note that the vertical scale of Fig. 6 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 = 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)**

** = + 273°**

**A plot of Eq. (1) is provided in Fig. 6. If we expand Eq. (1) into the following form, the contributing component for each region of Fig. 6 can easily be described:**

**For positive values of the first term of the equation above will grow very quickly and overpower the effect of the second term. The result is that for positive values of , will be positive and grow as the function y = appearing in Fig. 7. At = 0 V, Eq. (1) becomes = ( – 1 ) = (1 – 1) = o mA as appearing in Fig. 6. For negative values of the first term will quickly drop off below
, resulting in = - , which is simply the horizontal line of Fig. 6. The break in the characteristics at = 0 V is simply due to the dramatic change in scale from mA to μA.**

**Note in Fig. 6 that the commercially available unit has characteristics that are shifted to the right by a few tenths of a volt. This is due to the internal “body” resistance and external “contact” resistance of a diode. Each contributes to an additional voltage at the same current level as determined by Ohm’s law (V = IR). In time, as production methods improve, this difference will decrease and the actual characteristics approach those of Eq. (1).**

**It is important to note the change in scale for the vertical and horizontal axes. For positive values of the scale is in milliamperes and the current scale below the axis is in microamperes (or possibly nanoamperes). For the scale for positive values is in tenths of volts and for negative values the scale is in tens of volts.**

**Initially, Eq. (1) does appear somewhat complex and may develop an unwarranted fear that it will be applied for all the diode applications to follow. Fortunately, however, a number of approximations will be made in a later section that will negate the need to apply Eq. (1) and provide a solution with a minimum of mathematical difficulty.**

**Before leaving the subject of the forward-bias state the conditions for conduction (the “on” state) are repeated in Fig. 8 with the required biasing polarities and the resulting direction of majority-carrier flow. Note in particular how the direction of conduction matches the arrow in the symbol (as revealed for the ideal diode).**

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**Keywords – Semiconductor diode, Semiconductor diodes, pn junction diode, pn junction diodes, pn diode, forward bias, forward bias and reverse bias, semiconductor diode, forward bias diode, semiconductor, semiconductors, reverse bias, reverse bias diode.**

**Reference :- Robert L. Boylestad and Louis Nashelsky, Electronic device and circuit theory, Seventh edition, Prentice Hall**