AN3022 Datasheet, PDF (1/6 Pages) M/A-COM Technology Solutions, Inc. – Establishing the Minimum Reverse Bias for a PIN Diode in a High-Power Switch

AN3022
Establishing the Minimum Reverse Bias for a PIN Diode in a
High-Power Switch
Rev. V2
Abstract - An important circuit design parameter in a
high-power p-i-n diode application is the selection of an
appropriate applied dc reverse bias voltage. Until now,
this important circuit parameter has been chosen either
conservatively, using the magnitude of the peak RF volt-
age, or by empirical trials to determine a possible lower
value. This paper explores the reverse bias requirement
for a p-i-n diode operating in a high power microwave
environment. It demonstrates that the minimum reverse
bias voltage is equivalent to the p-i-n diode’s self-
generated dc voltage under similar RF conditions. A
concise expression for this self-generated voltage is de-
veloped and experimentally verified and will assist the
design engineer in more accurately selecting an appro-
priate minimum value for the applied reverse bias voltage
setting.
1. Introduction
A fundamental property of a p-i-n diode is its ability to
control large radio frequency (RF) and microwave signals
with much lower values of dc current and voltage. While
there are design rules for selecting a minimum level of
forward current based on allowable ohmic loss and dis-
tortion requirements [1], [2], there are no existing design
rules on which to base the selection of the minimum level
of applied dc reverse bias voltage.
The instantaneous voltage across the p-i-n diode (both
RF and dc) must never exceed its avalanche breakdown
voltage (V_BR in Fig. 1(a)), where high reverse current
densities may cause p-i-n diode failure. Safe operation
will result if the instantaneous voltage never forces the p-
i-n diode into forward conduction or into avalanche
breakdown (Fig. 1(b)). However, this requires that the
applied dc voltage be at least equal to the peak RF volt-
age and that the breakdown voltage be at least twice the
peak RF voltage (V_RF). In many applications, high ap-
plied reverse bias voltages are often not available or are
too expensive to implement. Frequently p-i-n diodes are
operated in the so-called conditionally safe region, where
an instantaneous excursion of voltage into the forward
conducting region may be tolerated (Fig. 1c)). If a dc
reverse bias voltage in this region is chosen, circuit per-
formance parameters such as loss, distortion, and reli-
ability must not be compromised.
It is conditionally safe region where most high-power
(greater than 1 kW) p-i-n diode switches are designed to
operate. The applied dc reverse bias voltage must be
large enough to prevent excessive conduction during the
positive portion of the RF signal. If excessive conduction
does occur, the p-i-n diode loss will increase and the
diode will be subject to failure.
In absence of any theory or analytic design guidelines,
the design engineer may choose a dc bias voltage equal
to the peak RF voltage, resulting in extremely conserva-
tive and costly designs; more frequently, however the
design is based on empirically matching a p-i-n diode to
an available voltage. This paper presents a guide for the
p-i-n diode circuit designer, similar to the forward bias
case, for selecting a minimum applied reverse bias volt-
age based on diode and circuit operating parameters.
The investigation of the relationship of the reverse bias
requirement was prompted by experimental observations
of p-i-n diode distortion under zero applied bias open
circuit conditions, using a test circuit of the type shown in
Figure 2. The 10⁹ Ω resistor was inserted in the
voltmeter line to increase the effective voltmeter internal
resistance from its nominal 10⁷ Ω value to better
approximate an open circuit across the diode. The
voltage read was then approximately 1% of the diode
voltage.
A self-generated reverse bias dc voltage was developed
across the p-i-n diode that allowed the diode to operate
in its high-impedance state with good stability. The
magnitude of the self-generated dc voltage was
influenced primarily by the peak RF voltage level, the
frequency, and the thickness of the I-region. Upon
application of an equivalent externally applied dc bias,
the distortion generated was identical to the self-
generated dc voltage. However, when the applied dc
voltage was lower than the self-generated voltage,
unstable performance would occur, manifested by large
increases in distortion signals and by heating of the p-i-n
diode, which often led to device failure. Published
experimental results by other investigators [3] show
similar device and circuit parameters that affect the
degree of forward conduction in the p-i-n diode.
An analysis of the p-i-n diode leading to a concise
expression for the safe minimum operating dc reverse
bias voltage is presented. The expression indicated how
the p-i-n diode I-region and circuit parameters such as
frequency, duty factor, and peak RF voltage affect the
magnitude of the minimum reverse bias voltage. The
derived expression is verified using experimental
measurements of the developed open-circuit zero bias
1
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