ST AN1232 APPLICATION NOTE

AN1232

Application note

Ruggedness improvement

of RF DMOS devices

Introduction

RF amplifiers often experience impedance mismatch between output and load. Such an
impedance mismatch generates a reflected wave towards the RF power transistor, making a
much more stringent working environment for the transistor. Working conditions grow critical
when the load is disconnected from the output of the RF power transistor, since, in this case,
the reflected wave amplitude becomes comparable to the incident one.

RF transistors are able to withstand severe impedance mismatch conditions particularly
essential for applications such as plasma generators or nuclear magnetic resonators which
operate under rough conditions. DMOS devices used in such applications appeared to lack
the necessary ruggedness when operating under severe RF load mismatch conditions. This
weakness was believed to be intrinsic.

Based on the need to improve the ruggedness of RF power DMOS, an investigation was
carried out and a theoretical model simulating the failure mode mechanism was developed.
Finally, relevant corrective actions were undertaken.

July 2007 Rev 13 1/6

www.st.com

Proposed model AN1232

1 Proposed model

Figure 1. Npn Bipolar parasitic transistor

Under impedance mismatch conditions the RF power transistor is subjected to a reflected
wave with an amplitude that cannot be controlled and, in the worse case, with a voltage
value that can exceed the BV
clamp for this wave. Under such conditions the device is subjected to an electric current
whose amplitude is proportional to the power of the incident wave. Simultaneously, the
DMOS experiences a temperature rise proportional to the duration, voltage swing (BV
and amplitude of the above current. In the DMOS cross section, shown in Figure 1, the
presence of an npn bipolar parasitic transistor, in which the base and emitter are shorted by
means of the DMOS source metallization, are clearly noticeable.

of the device itself. Hence, the DMOS works as a voltage

dss

dss

)

Under static conditions the parasitic transistor is inhibited by the short circuit, but under
dynamic conditions, when the reverse breakdown current flows through the device, the short
circuit condition itself is modified. In fact, this current crosses the base-emitter junction
through the base resistance which is increased by the depletion layer due to the reverse
voltage. This results in a variation in V

be

We can assume that:

Equation 1

V

If I

(base distributed current) and Rbe (base distributed resistance) are large enough, the

dis

potential on the emitter side opposed to the short circuit is sufficient to turn on the bipolar
transistor, thus concentrating the current and destroying the device.

A simplified model was developed (see Figure 2) to simulate the mismatch condition on the
drain of the DMOS by means of the inductance L.

2/6

be

.

RbeI

•=

dis