The article covers the construction, I-V characteristics, and detailed specifications of tunnel diodes, emphasizing their unique conduction behavior due to heavy doping and thin depletion regions.
Adding a controlled amount of impurities can change the conduction capabilities of a semiconductor. The conduction capabilities of a tunnel diode are changed in a unique way by adding a higher concentration of impurities than a standard diode.
Tunnel Diode Construction
A tunnel diode is a two-element semiconductor device with construction similar to that of a conventional silicon diode. This device has an anode connected to the P-type material and a cathode connected to the N-type material. The two materials are joined at a common point or junction. The tunnel diode is, however, quite different beyond this point. The P-type and N- type materials, for example, are heavily doped. This means that the material has a rather high concentration of impurities. Typical doping levels may be one hundred to several thousand times that of a conventional silicon diode.
Heavily doping the semiconductor material of a tunnel diode causes the N-type material to have a large number of free electrons and the P-type material to have a large number of holes. The high current carrier content of the crystal material causes the width of the depletion region to be very thin. In general, depletion zone width is only 1/100 of that of a regular diode. This permits current carriers to “tunnel” through the barrier rather than move across it when bias voltage is reduced.
Normally, electrons must have sufficient external energy to cross the surface barrier of a junction, as described in P–N junction diodes. Due to the increased number of current carriers and the thin barrier that exists in tunnel diodes, electrons tend to pass through the barrier with an extremely small amount of energy. In many cases, ambient temperature may be sufficient to cause some conduction. This can occur in some cases when very little bias voltage has been applied. Current carriers tend to move across the barrier as if it did not exist. Figure 1 shows an example of electrons tunneling through the barrier of a P–N junction with a small amount of external voltage applied.
Figure 1. Tunneling electrons. The thin depletion zone of a tunnel diode enables current carriers to pass through the barrier with a small amount of bias voltage.
Tunnel Diode I–V Characteristics
The I–V characteristic of a tunnel diode is quite different from that of other diodes. The tunnel diode has three distinct characteristics that make it electrically different. Each characteristic depends on a value of bias voltage applied to the device. Within a certain range of bias voltage, a tunnel diode is conductive in both directions. See the I–V characteristics curve of Figure 2 (right and left of the peak voltage point) and the area on each side of the valley voltage.
In the second range of bias voltage, the tunnel diode has a negative resistance characteristic. This is located between the forward voltage peak (VP) and the valley voltage (VV) point. Negative resistance refers to an area where an increase in voltage causes a decrease in current.
The third characteristic of bias voltage deals with an area beyond the valley voltage point. Increasing the forward voltage beyond the VV point of a tunnel diode causes it to respond as a conventional silicon diode. This part of the curve shows that an increase in forward voltage causes a corresponding increase in forward current.
Figure 2. I–V characteristics curve of a tunnel diode.
The operation of a tunnel diode can be changed according to the value and polarity of the applied bias voltage. The three bias conditions discussed occur within a few millivolts. The general area of operation is somewhere between 0 and 600 mV. The exact voltage for each bias condition is dependent on the material used in the construction of the diode. Germanium or gallium arsenide can be used in the construction of most tunnel diodes. The doping level of the P-type and N-type materials has a great deal to do with other characteristics. The peak current (IP), for example, can vary from a few microamperes to 100 A. The peak voltage (VP), however, is limited to a maximum value in the range of 600 mV. For this reason, a tunnel diode can be easily damaged. A multimeter with a 1.5-V cell energizing the ohmmeter could damage a tunnel diode by simply measuring its resistance. When using the tunnel diode, one must use extreme care in connecting it into a circuit and altering the circuit so that it will produce a desired operating condition.
Tunnel Diode Symbols
Three common symbols for the tunnel diode are shown in Figure 3. Two of these symbols resemble the conventional silicon diode symbol with a slight modification. The third symbol, which is widely used, consists of a line and a half-circle. Note the parts of the symbol that are used to identify the anode and cathode. For the standard diode symbol, the anode and cathode remain unchanged. For the line-half-circle symbol, the anode is represented as a line, and the half-circle denotes the cathode. These symbols may all be drawn within a circle or with the circle omitted.
Figure 3. Tunnel diode symbols and crystal structure.
Tunnel Diode Specifications
Tunnel diodes are usually packaged in a special housing. Figure 4 shows a representative package. Note the dimensions of this device. It is extremely small in comparison with a conventional silicon diode. As a rule, the enclosure is usually metal. The anode is insulated from the metal housing, and the cathode is attached to it. Since tunnel diodes are used primarily in high-frequency signal applications, metal enclosures are purposely used to isolate the internal diode structure from stray electromagnetic fields.
Figure 4. Tunnel diode package.
The data from a tunnel diode specification sheet is shown in Figure 5. The tunnel diode represented in the sheet is a low-power device. Note that the data is listed for three different conditions of operation: minimum, typical, and maximum. This particular diode is used for oscillators, high-frequency amplification, and high-speed switching applications. The specification sheet generally lists a number of characteristics that influence the high-frequency response of the tunnel diode, such as:
- Capacitance of the Junction (C),
- Terminal Lead Inductance (LS),
- Negative Resistance (−R), and
- Lead Resistance (RS).
Figure 5. Tunnel Diode Datasheet Specifications
Tunnel Diode Review Questions
1. Heavily doping the P-type and N-type materials of a tunnel diode causes the depletion zone to be very _____.
2. The _____ effect is due to the heavy doping of P-type and N-type materials and a thin depletion zone.
3. _____ refers to an area of a tunnel diode where an increase in voltage causes a decrease in current.
4. An increase in forward voltage beyond the _____ point causes a tunnel diode to respond as a regular diode.
Answers
1. thin
2. tunneling
3. Negative resistance
4. valley voltage or VV
Key Takeaways
Tunnel diodes are distinct semiconductor devices characterized by their heavily doped P-N junctions, which result in a thin depletion region and enable quantum tunneling of charge carriers. This unique property leads to their unusual I-V characteristics, including negative resistance and high-frequency behavior. The detailed specifications, such as peak voltage and current, highlight the importance of careful handling to avoid damage. While tunnel diodes are compact and efficient for specific high-speed and high-frequency applications, their operation requires precise control of bias voltage and circuit integration.