Transient Voltage Suppressors

Transient Voltage Suppressors are based on Avalanche and Zener diodes optimized for carrying high currents, tailored for specific breakdown voltages and often in arrangements of several diodes allowing multiple signal lines to be protected by the same protection device. Diodes are formed in a semiconductor, usually silicon, at a junction between n and p doped regions. TVS devices provide protection with a combination of forward bias and reverse bias breakdown conduction. Basic diode properties will be discussed below.

Figure 1 Diagrams of an abrupt np junction

Diode properties can be understood qualitatively by considering an abrupt np junction as shown in Figure 1 . Consider two pieces of silicon, an n+ sample heavily doped with donor impurities and a p sample less heavily doped with acceptor impurities. We will conceptually join them together at x=0 as in Figure 1 a. The n+ region will start with an electron density equal to the number of donor impurities. The p region will have a hole density equal to the acceptor impurity concentration. This situation will not last. At the boundary the electrons and the holes will recombine. The result will be a region around the junction that is depleted of mobile carriers, both electrons and holes. This is known as the depletion region and is shown as the region of zero carrier concentration in Figure 1 b. With the carrier concentration depleted in the region of the junction there are regions of net charge as in Figure 1 c. To the left of zero there is a net positive charge from electron donors that have given up electrons To the right of zero there is a net negative charge from electron acceptors that have accepted an extra electron. The width of the depletion region on each side of the junction depends on the doping concentration of donor and acceptors on each side of the junction. One constraint is that the total charge across the junction needs to be zero. This dictates that the depletion width on the heavily doped side of the junction is narrower than on the lightly doped side. The net charge density in the region of the junction creates an electric field which peaks at the metallurgical junction, Figure 1 d. Integration of the electric field across the junction creates the junctions built in voltage as shown in Figure 1 e. Note that the resultant electric field and potential is such that electrons are forced toward the n region and holes are pushed toward the p region.

Figure 2 Diode I-V curves for three Zener diodes with differing reverse bias breakdown voltages. The voltage is applied to the n region relative to the p region.

Figure 2 shows sample current versus voltage curves for 3 diodes with different reverse bias breakdown voltages. For negative voltages applied to the n region relative to the p region, large current begins to flow at voltages below -0.6V. For positive voltage negligible current flows up to a junction breakdown voltage that is doping level dependent. The basic diode properties can be understood by continuing to consider the potentials and electric fields in the depletion region.

Figure 3 depicts a diode under forward bias conditions. Forward basing involves placing a negative potential on the n region relative to the p region. The resulting potential is illustrated in Figure 3 e where the applied voltage reduces the built in diode potential. The reduction in the potential drop across the junction reduces the electric field, Figure 3 d as well as the depletion widths as seen in Figure 3 b and c. The reduction in the built in diode voltage and the narrowing of the depletion region begins to allow current to flow across the diode. A detail of the forward bias current versus voltage behavior of three Zener diodes with different reverse bias breakdown voltages is shown in Figure 4 .

Figure 3 Diode properties under forward bias conditions. Black lines are without external bias and red lines are with an external bias in the forward bias condition.

Figure 4 Forward Bias Current - Voltage characteristics of 3 different Zener diodes of different reverse bias breakdown voltages.

Reverse bias conditions are illustrated in Figure 5 . In reverse bias a positive voltage is applied on the n region relative to the p region. This increases the potential across the junction above the built in bias, as shown in Figure 5 e and increases the electric field as shown in Figure 5 d. The result is a widening of the depletion region and the potential and electric field restrict the flow of current even more than with no external bias applied. This accounts for the wide range of voltage over which very little current flows across the diode.

Figure 5 Diode properties under reverse bias conditions. Black lines are without external bias and red lines are with and external reverse bias voltage.

At high enough reverse bias voltages current does begin to flow, as can be seen in Figure 2 . If the junction doping levels are not extremely high, current begins to flow by avalanche breakdown. The applied voltage creates an electric field in the depletion region. This field will accelerate the few carriers that are thermally injected into the depletion region. At high fields, carrier collisions can create additional electron hole pairs. Where there had been a single carrier there are now three. If this process repeats itself, avalanche occurs resulting in high currents. Electric fields are higher for more heavily doped materials due to the narrower depletion width. More heavily doped junctions therefore experience avalanche breakdown at lower voltages. Avalanche breakdown is characterized by a very rapid increase in current over a very narrow voltage range, as is seen in Figure 6.

Figure 6 I-V curves for diodes with different breakdown voltages. The breakdown voltage is defined as the current for 5mA at room temperature.

At very high doping levels a second breakdown mechanism occurs. At high doping levels the electric field can become so great that electrons are essentially torn away from individual atoms and become free carriers. This process is known as Zener Breakdown and is usually described as quantum mechanical tunneling of electrons in the valance band into the conduction band. Zener breakdown has a more gradual increase in current versus voltage than avalanche breakdown does as shown for the 5.1V diode in Figure 6 .

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