### Categorized |Diodes, Power Devices

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Posted on 04 December 2019

# Static Behavior of Fast Diodes

Modern fast switching devices require fast diodes as freewheeling diodes in the power circuit. In the predominant applications which use inductive loads, the freewheeling diode is commutated from conductive to blocking state with every turn-on operation of the switch. Here, storage charges are to be depleted gently in order to avoid induced voltage spikes and high-frequency oscillations. For this reason, these diodes are also referred to as soft-recovery diodes. They are also instrumental to switch performance. When designing these devices, a compromise between conflicting requirements has to be found. Two main types of fast diodes exist: the Schottky diode, and pin- diodes in epitaxial or diffused design.

Fast diodes essentially tend to display the same static behaviour as rectifier diodes.

Figure 1. Reverse and forward diode voltage

### On-state behaviour

The maximum forward voltage VF indicates that, at a specified current, the forward voltage drop across the diode must not exceed the specified limit value. This specification is made for room temperature and a higher temperature, typically the maximum recommended operating temperature. In forward direction, the current must overcome the diffusion voltage of the pn-junction and the resistance of the adjacent n- region. The voltage drop is given by

$V_F = V_{diff} + V_{ohm}$

The diffusion voltage at the pn-junction depends on the amount of doping of both sides of the pn-junction and it is typically in the range of 0.6...0.8 V. The ohmic share depends on the base width wB (proportionate to the blocking voltage) and the charge carrier density. For fast diodes with a blocking voltage of 600 V and above, the ohmic part dominates. The charge carrier lifetime of free- wheeling diodes has to be kept so short that the on-state voltage will depend exponentially on the base width wB and the charge carrier lifetime τ.

$V_{ohm} = \frac{3 \cdot \pi k T}{8q} \cdot e^{\frac{w_B}{2 \sqrt{D_A \cdot \tau}}}$

with the ambipolar diffusion constant $D_A = 2 \frac{\mu_n \mu_p}{\mu_n + \mu_p} \frac{kT}{q}$

where

k : Boltzmann constant; 1.38066_10-23 I/K

q: electronic charge; 1.60218_10-19

T: absolute Temperature [K]

Here, μn and μp represent electron and hole mobility, provided the n- region is flooded with free electrons and holes. Due to this exponential correlation, it is important to select the smallest possible wB .

The diffusion voltage has a negative temperature coefficient, while the ohmic voltage part has a positive temperature coefficient. Depending on which part is dominant, there will be an intersection of the on-state characteristic "Hot" and "Cold" at different current levels, typically within the rated current range or up to 3-4 times the rated current.

### Blocking behaviour

The reverse voltage VR indicates that, at a specified value, the reverse current must not exceed the limit for IR . Specifications in the databooks are made for an operating temperature of 25°C. In case of lower temperatures, the blocking capability will decrease, for example by about 1.5 V/K for a 1200 V diode. For components which are operated at temperatures below room temperature, this has to be taken into account in the circuit layout. At higher temperatures, the blocking voltage will increase accordingly. At the same time, the reverse current will also rise, doubling roughly every 10 K. For this reason, a reverse current is also specified for a high temperature (125°C or 150°C). For gold-diffused components, the reverse current increase may be very strong, possibly causing problems due to thermal instability in systems operated at high temperatures.

Figure 2. Example of the reverse current of a 1700 V CAL diode, parameter Tj

The base width wB not only affects the on-state voltage, but also has a crucial impact on the blocking voltage. Two cases can be distinguished between (Figure 3): if wB has been dimensioned such that the space charge zone cannot protrude into the n+ region (triangular field shape), this is called Non-Punch-Through (NPT) dimensioning in line with the terminology used for IGBTs [12]. If wB has been dimensioned such that the space charge region protrudes into the n+ region, the field shape will be trapezoidal and the diode is called a Punch-Through (PT) diode. This, however, is not actual punch-through, where the space charge region would reach the area of the other doping type. This designation has nonetheless become widely accepted.

Figure 3. Diode dimensioning for triangular (a) and trapezoidal (b) field shape for 0 ≤ w ≤ wB

For an ideal NPT diode, wB is selected such that at maximum reverse voltage, the end of the triangular field is located at this point.

For the charge carrier lifetimes presently in use, the difference in forward voltage for PT dimensioning and NPT dimensioning is approximately 0.8 V. For this reason, PT dimensioning is to be given preference if possible.

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