GaAs FETs as Control Devices
APN2015
Gallium arsenide MESFETs are being used in RF control
device applications as switches and attenuators. They are
very easily adapted to monolithic circuit form, dissipate
essentially no power and can easily be designed into
broadband circuits.
The RF signal flows from source to drain, while the RF
isolated gate is the voltage control. The high impedance
鈥渙ff state鈥?is attained by applying a DC voltage on the gate
more negative than the 鈥減inch-off鈥?voltage (V
P
). In this
condition the source-drain channel is 鈥減inched off.鈥?The
capacitance is typically 0.25 pF per mm of gate
periphery (see Figure 1A). The 鈥渙n鈥?state occurs when
zero DC bias is applied to the gate (see Figure 1B). The
channel from source to drain is 鈥渙pen鈥?and represents a
2.5鈥?.5
鈩?/div>
resistance per mm of gate periphery.
A configuration of FETs used in series and shunt normally
produce the optimum switch or attenuator performance in
monolithic circuits (see Figure 2). The only DC current that
flows is the leakage of the gate-source and gate-drain
reverse biased junctions (when negative voltage is applied
to gate). Typical current drain is < 25 碌A(chǔ) @ -5 V. This
leakage is a function of the wafer fabrication process, the
device periphery, and magnitude of applied voltage. For
isolation of gate voltage control, 2.5鈥? k鈩?resistor is
incorporated monolithically in the gate. Since no further
external bias circuitry is required, the switch is inherently
broadband.
The power handling of the switches is primarily limited by
the current handling capability, which is related to the IDSS
of the FET. The IDSS is a function of the gate periphery
which then determines the source-drain capacitance.
The input series FET in switches have a nominal 1 dB
compression of 1 W/mm.
Digital attenuators use FETs configured as 鈥淭鈥?or 鈥淧I鈥?pads
to achieve a given attenuation value (Figure 3). For the low
loss state V
2
is set to 0 V and V
1
is set to -5 V. For the
attenuation state V
2
is set to -5 V and V
1
is set to 0 V.
Typically digital attenuators are composed of multiple bit
values (e.g. 2, 4, 8, 16 dB AD220-25).
-5 V
5 k鈩?/div>
Gate
Source
Drain
N
SI
Figure 1A. MESFET Control Device in
High Impedance State (鈥淥ff鈥?State)
0V
5 k鈩?/div>
Gate
Source
Drain
N
SI
Figure 1B. MESFET Control Device in
Low Impedance State (鈥淥n鈥?State)
Voltage variable attenuators (VVAs) use the channel
resistance of the FET as the actual resistance of the circuit
components. The resistance is a nonlinear relationship
with control voltage as shown in Figure 4 (FET with a
鈥減inch-off鈥?voltage of 1.5 V). Figure 5 shows a TEE VVA
that uses two series FETs and one shunt FET.
Skyworks Solutions, Inc.
[781] 376-3000
鈥?/div>
Fax
[781] 376-3100
鈥?/div>
Email
sales@skyworksinc.com
鈥?/div>
www.skyworksinc.com
Specifications subject to change without notice. 9/03A
1
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