Talking about Schottky Diodes and Barrier Height Adjustment for Specific Applications

This application note uses a true Schottky diode as the forward voltage drop of choice. This document describes low, medium, and high voltage level applications, as well as diodes with ideal dynamic behavior, fast reverse recovery PN diodes, true Schottky diodes, and application-specific barrier height tuning.

This application note uses a true Schottky diode as the forward voltage drop of choice. This document describes low, medium, and high voltage level applications, as well as diodes with ideal dynamic behavior, fast reverse recovery PN diodes, true Schottky diodes, and application-specific barrier height tuning.

According to the thermionic emission model, the pure Schottky barrier exhibits a forward voltage drop, which decreases linearly with the decrease of the barrier height; while the reverse current increases exponentially with the decrease of the barrier height. Therefore, there is a barrier height that can determine the sum of forward and reverse power dissipation for a particular application. However, discussions with Schottky diode users have shown that they are not looking for values ​​for forward and reverse power dissipation, but always for forward voltage drop. Reverse current values ​​are rarely required. It is essential to understand how Schottky diodes are used in order to objectively select the appropriate device.

low voltage applications

In high power applications with low circuit voltages and using Schottky diodes with blocking voltages below 25V, the diode forward power loss still dominates the power loss balance. The main application is Switch Mode Power Supplies (SMPS). It is argued here that a 4 mV reduction in forward voltage drop results in a reduction in forward power loss of about 1%. Therefore, the components created for this application have low barrier heights (less than 0.74 eV) and highly doped thin epitaxial drift layers. This results in devices with low forward voltage drop and high but still acceptable reverse current.

Medium and high voltage applications

On the other hand, reverse power losses in high-power applications using medium-voltage or high-voltage Schottky types (VRRM ranging from 45 V to 150 V) are comparable to forward power losses and may even be higher. Nevertheless, most users do not require low reverse current, but only low forward voltage drop.

Diodes with Ideal Dynamic Behavior

In addition to forward and reverser power losses, there is obviously a third quality that is difficult to quantify. However, as experience has shown, it has an effect on forward voltage drop.

I guess this quality is manifested by the dynamic characteristics and switching losses of real Schottky diodes. Due to their relatively short duration in ranges with expensive test equipment, furthermore, subtle differences in their dependencies cannot be discerned.

Fast reverse recovery PN diode

In contrast to ideal diodes, pn diodes with a minority carrier current component “remember” their previous conduction state after the forward current has dropped to zero. This is due to the fact that the injected minority carriers (holes in the n region) either decay exponentially with the adjusted minority carrier lifetime t or are swept away by the reverse current. The pn diode restores its reverse blocking capability with a delay after the current crosses zero. The minority carrier lifetime can be reduced by diffusing lifetime inhibitors (gold or platinum) into the n-region or exposing the diode chip to radiation.

true schottky diode

A true Schottky diode also injects minority carriers through its barrier, although it is orders of magnitude smaller. This phenomenon is called epitaxial layer modulation. Injection increases with barrier height, voltage type, forward current density, and junction temperature.

Due to the measurement difficulties of the above techniques, we simulated the turn-off behavior of a real Schottky diode. In Figure 1 below, the current and voltage waveforms are plotted against time for a Schottky diode of type 100 V with an active area of ​​0.323 cm2. Preset operating conditions are 50 A forward current, 300 A/µs commutation period, 25 V reverse bias voltage, and 25°C junction temperature. Three different materials with barrier heights of 0.74, 0.8 and 0.86 eV were considered. The turn-off energies are 0.86, 1.0, and 2.3 µW, respectively. The simulation model clearly shows that the remaining minority carriers from the conducting phase in the n-doped epilayer determine the initial conditions for the general solution of the differential equations of the LC circuit, which consists of a turn-off Inductor coil, junction capacitance, and a forced reverse voltage of 25 V Bias composition.

Due to the delay capability of a true Schottky diode to block reverse voltage after commutation – which increases with barrier height – the resonant circuit is resistant to excessive reverse currents (i.e. greater than the commutation turn-off slope times the as the square root of LC), excessive reverse voltage (ie more than twice the drive reverse voltage) and steep startup, excessive dv/dt (ie greater than the drive reverse voltage divided by the square root of LC). As the barrier height increases, the excess of dynamic parameters and switching losses becomes more pronounced.

Due to the delay ability of the actual Schottky diode to block reverse voltage after commutation – which increases with the height of the barrier – the resonant circuit responds to excessive reverse currents (i.e. greater than the commutation turn-off slope) Multiply by the square root of LC), excessive reverse voltage (ie greater than twice the drive reverse voltage) and steep startup, excessive dv/dt (ie greater than the drive reverse voltage divided by the square root of LC).

As the barrier height increases, the excess of dynamic parameters and switching losses becomes more pronounced. Due to the delay ability of the actual Schottky diode to block reverse voltage after commutation – which increases with the height of the barrier – the resonant circuit responds to excessive reverse currents (i.e. greater than the commutation turn-off slope) Multiply by the square root of LC), excessive reverse voltage (ie greater than twice the drive reverse voltage) and steep startup, excessive dv/dt (ie greater than the drive reverse voltage divided by the square root of LC). As the barrier height increases, the excess of dynamic parameters and switching losses becomes more pronounced.

Excessive reverse voltage (ie greater than twice the drive reverse voltage) and steep start-up, excessive dv/dt (ie greater than the drive reverse voltage divided by the square root of LC). As the barrier height increases, the excess of dynamic parameters and switching losses becomes more pronounced. Excessive reverse voltage (ie greater than twice the drive reverse voltage) and steep start-up, excessive dv/dt (ie greater than the drive reverse voltage divided by the square root of LC). As the barrier height increases, the excess of dynamic parameters and switching losses becomes more pronounced.

Talking about Schottky Diodes and Barrier Height Adjustment for Specific Applications

On the other hand, in order to increase the barrier height and type voltage, the increased modulation in the epitaxial layer reduces the resistivity and forward voltage drop of the epitaxial drift layer. As shown in Figure 2, this reduction may be more pronounced than the increase in voltage drop across the actual barrier. The numbers for our 100 V example at 232 A/cm2 and room temperature are: 1. A diode with a forward drop of 0.78 V has poor dynamic values ​​for a barrier of 0.86 eV, and; 2. 0.8 for a barrier of 0.74 eV The V forward voltage drop has a dynamic value. Therefore, true Schottky diodes with forward voltage drop above 80 V are not fast.

Talking about Schottky Diodes and Barrier Height Adjustment for Specific Applications

Application-specific adjustment of barrier light

In my opinion, deviations in dynamic behavior and corresponding switching losses of ideal diodes with junction capacitance are more detrimental to circuit designers than very high reverse currents. In fact, the reverse current of a diode with a barrier height of 0.74 eV is about 25 times higher than that of a diode with a barrier height of 0.86 eV. Beyond a certain limit, exponentially increasing reverse current (typically low barrier heights) becomes unacceptable. However, this depends on the respective application.

The Links:   G215HVN011 NL4864HL11-02A

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