Modeling Bidirectional Thyristors Using ATLAS

Introduction

Thyristors are semiconductor devices that exhibit multi-stable or bi-stable electrical characteristics, and can be switched between a high-impedance OFF state and a low-impedance ON state. Bidirectional thyristors are particularly useful for ac applications because they operate in the first and third quadrants of the I-V curve, depending on the anode and cathode polarities. It is useful to simulate and predict the important electrical characteristics of these devices, so they can be optimized for figures-of-merit such as breakover voltage, holding current, and switching speed.

In Part 1, we examine the principal voltage-current characteristics of two diac (diode ac switch) structures: the ac trigger diode and the p-n-p-n diode switch. In the next issue of the Simulation Standard, we will examine the transient electrical behavior of the p-n-p-n diode switch for large current pulses typically associated with ESD and other discharge phenomena. This characterization typically requires 3-D simulation of z-plane effects due to asymmetry of the electric fields in the device.

 

AC Trigger Diode

Figure 1 shows a typical ac trigger diode structure, constructed like a bipolar transistor with no base contact and equally doped emitter and collector regions. When either bias is applied to a contact, one junction is forward-biased and the other is reverse-biased. The current is limited initially by the reverse-biased junction leakage at lower voltages, and the device eventually breaks down at Vbr = BVCBO (1-)1/n , where is the common-base current gain, BVCBO is the avalanche breakdown voltage of the reverse-biased p-n junction, and n is an empirical constant [1]. As the current increases after breakdown, increases, causing a drop in contact voltage and negative-resistance region shown in Figure 2.



Figure 1. AC trigger diode structure
created in DevEdit.

 

Figure 2. Simulated I-V characteristic
of AC trigger diode structure from
ATLAS Curvetrace routine.



The data of Figure 2 was simulated in ATLAS from a device created analytically using the DevEdit device editor in 2-D mode. The solution was obtained in one sweep using the Curvetrace automatic curve tracing routine in ATLAS. The sweep ends when the controlling current contact reaches a current compliance limit of 20 ?A/?m.

 

Bidirectional p-n-p-n Diode Switch

Figure 3 shows a bidirectional p-n-p-n diode, or 5-layer diode structure, as created in DevEdit, based on an actual device (thus no direct geometric information is shown). If we consider a vertical axis of symmetry dividing the structure in to left and right halves, we can see two conventional Shockley diodes connected in anti-parallel. The structure can therefore be triggered into conduction by dV/dt, by exceeding the breakover voltage, or by increasing the temperature and saturation current until the common-base current gains sum to one. Also, due to the common-base current gains, the 5-layer thyristor will have carrier generation features, a larger range of negative resistance, and a smaller forward drop than the ac trigger diode. Figure 4 illustrates the simulated current-voltage characteristics of the p-n-p-n diode switch structure. Figure 5 shows the simulated impact ionization rate just before breakdown, and the current density at turn-on.

Figure 3. Bidirectional p-n-p-n diode switch
structure created in DevEdit.

 

Figure 4. Simulated I-V characteristic
of p-n-p-n diode from ATLAS.

 

Figure 5. Simulated impact ionization
rate at breakdown and total current
density after device turn-on.

 

Another important feature of the p-n-p-n diode is the emitter overlap, evident in Figure 3. This overlap creates a lateral current path, increasing the lateral voltage drop for a given current density, and helping to trigger the device at a lower current density. In Part 2 of this series, we will consider the operation of the p-n-p-n diode in greater detail, and also examine the introduction of emitter shunts, which break the monolithic emitter into curved areas, decreasing the associated electric fields and allowing greater control of critical voltages and currents [2].

 

Summary

The current-voltage characteristics of two classes of bidirectional thyristor structures is described theoretically and simulated using the ATLAS 2-D device simulator in automatic curve trace mode and in standard voltage/ current driven solution modes. Simulated results of steady-state I-V curves for the ac trigger diode and for the p-n-p-n diode are typical for these devices. Physical models used in the simulation solutions are standard drift-diffusion with Boltzmann carrier statistics, Shockley-Read-Hall recombination, concentration-dependent low-field mobility and parallel electric field-dependent high-field mobility, bandgap narrowing, and the Selberherr impact ionization models.

In Part 2 we analyze temporal behavior of the bidirectional p-n-p-n diode switch in three dimensions, based on more complex thermal and impact ionization models. This analysis will allow us to understand the mechanisms and geometries of the device that may be optimized using simulation.

 

References

  1. S.M. Sze, Physics of Semiconductor Devices, 2nd Edition, John Wiley & Sons, New York, 1981.
  2. A. Blicher, Thyristor Physics, Springer-Verlag, New York, 1976.

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