As frequency increases, the action of small-signal rectifier diodes begins to deteriorate. They are no longer able to switch off fast enough to produce a well- defined half-wave signal. The solution to this problem is the Schottky diode.
Problems with Diodes at High Frequency
Before describing this special-purpose diode, let us look at the problem that arises with ordinary small-signal diodes.
Charge Storage in Diodes at High Frequency
Figure 1(a) shows a small-signal diode, and Fig. 1(b) illustrates its energy bands. As you can see, conduction-band electrons have diffused across the junction and traveled into the p region before recombining (path A).
Similarly, holes have crossed the junction and traveled into the n region before recombination occurs (path B). The greater the lifetime, the farther the charges can travel before recombination occurs.
For instance, if the lifetime equals 1 s, free electrons and holes exist for an average of 1 s before recombination takes place. This allows the free electrons to penetrate deeply into the p region, where they remain temporarily stored at the higher energy band.
Similarly, the holes penetrate deeply into the n region, where they are temporarily stored in the lower energy band.
The greater the forward current, the larger the number of charges that have crossed the junction. The greater the lifetime, the deeper the penetration of these charges and the longer the charges remain in the high and low energy bands. The temporary storage of free electrons in the upper energy band and holes in the lower energy band is referred to as charge storage.
Charge Storage Produces Reverse Current
When you try to switch a diode from on to off, charge storage creates a problem. Why? Because if you suddenly reverse-bias a diode, the stored charges will flow in the reverse direction for a while. The greater the lifetime, the longer these charges can contribute to reverse current.
For example, suppose a forward-biased diode is suddenly reverse biased, as shown in Fig. 2(a). Then a large reverse current can exist for a while because of the flow of stored charges in Fig. 2(b). Until the stored charges either cross the junction or recombine, the reverse current will continue.
Reverse Recovery Time
The time it takes to turn off a forward-biased diode is called the reverse recovery time trr. The conditions for measuring trr vary from one manufacturer to the next. As a guide, trr is the time it takes for the reverse current to drop to 10 percent of the forward current.
For instance, the 1N4148 has a trr of 4 ns. If this diode has a forward current of 10 mA and it is suddenly reverse biased, it will take approximately 4 ns for the reverse current to decrease to 1 mA.
Reverse recovery time is so short in small-signal diodes that you don’t even notice its effect at frequencies below 10 MHz or so. It’s only when you get well above 10 MHz that you have to take trr into account.
Poor Rectification at High Frequencies
What effect does reverse recovery time have on rectification? Take a look at the half-wave rectifier shown in Fig. 3(a). At low frequencies, the output is a half-wave rectified signal. As the frequency increases well into megahertz, however, the output signal begins to deviate from the half-wave shape, as shown in Fig. 3(b).
Some reverse conduction (called tails) is noticeable near the beginning of the reverse half-cycle. The problem is that the reverse recovery time has become a significant part of the period, allowing conduction during the early part of the negative half-cycle.
For instance, if trr 54 ns and the period is 50 ns, the early part of the reverse half-cycle will have tails similar to those shown in Fig. 3(b). As the frequency continues to increase, the rectifier becomes useless.
Eliminating Charge Storage
The solution to the problem of tails is a special-purpose device called a Schottky diode. This kind of diode uses a metal such as gold, silver, or platinum on one side of the junction and doped silicon (typically n-type) on the other side. Because of the metal on one side of the junction, the Schottky diode has no depletion layer.
The lack of a depletion layer means that there are no stored charges at the junction. When a Schottky diode is unbiased, free electrons on the n side are in smaller orbits than are the free electrons on the metal side. This difference in orbit size is called the Schottky barrier, approximately 0.25 V.
When the diode is forward biased, free electrons on the n side can gain enough energy to travel in larger orbits. Because of this, free electrons can cross the junction and enter the metal, producing a large forward current. Since the metal has no holes, there is no charge storage and no reverse recovery time.
The Schottky diode is sometimes called a hot-carrier diode. This name came about as follows. Forward bias increases the energy of the electrons on the n side to a higher level than that of the electrons on the metal side of the junction.
This increase in energy inspired the name hot carrier for the n-side electrons. As soon as these high-energy electrons cross the junction, they fall into the metal, which has a lower-energy conduction band.
Due to lack of charge storage, the Schottky diode can switch off faster than an ordinary diode can. In fact, a Schottky diode can easily rectify frequencies above 300 MHz.
When it is used in a circuit like Fig. 4(a), the Schottky diode produces a perfect half-wave signal like Fig. 5-34b even at frequencies above 300 MHz.
Figure 4(a) shows the schematic symbol of a Schottky diode. Notice the cathode side. The lines look like a rectangular S, which stands for Schottky. This is how you can remember the schematic symbol.
Applications of Schottky Diodes
The most important application of Schottky diodes is in digital computers. The speed of computers depends on how fast their diodes and transistors can turn on and off. This is where the Schottky diode comes in. Because it has no charge storage, (explained further) the Schottky diode has become the backbone of low-power Schottky TTLs, a group of widely used digital devices.
Since a Schottky diode has a barrier potential of only 0.25 V, you may occasionally see it used in low-voltage bridge rectifiers because you subtract only 0.25 V instead of the usual 0.7 V for each diode when using the second approximation. In a low-voltage supply, this lower diode voltage drop is an advantage.