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Designing with Diodes: Why Choose AlGaAs?

For decades, solid state control components such as PIN diodes have been used in RF and microwave control devices, such as switches and attenuators. PIN diodes act as charge-controlled variable RF resistors, producing low insertion loss, large isolation, excellent power handling and excellent linearity, and in many cases better than any field effect transistor can produce. The range of impedance that a PIN diode can present can be as large as 5 or 6 decades, the extremes of which approximate an open and a short circuit.

PIN diodes can be placed either in series or in shunt with transmission lines, such as microstrip, coplanar waveguide and more. The resistance and capacitance of the PIN diode determine the insertion loss and isolation, respectively, for a series connection, or the converse for a shunt connection.

(Image source: MACOM Technology Solutions)

The PIN diode is a 3-layer device, composed of:

  • the anode, an acceptor-doped (p-type) P layer
  • an undoped (intrinsic) I layer
  • the cathode, a donor-doped (n-type) N layer

When this structure is approximated as a right-cylindrical section, we can see that the area of the junction and the thickness of the I layer determine the capacitance (C) of the PIN diode when it is not conducting and the series resistance (R) of the diode when it is biased into conduction, according to the elementary equations:

The permittivity of the I layer (e) and its resistivity (r) are determined by the type of material comprising the diode. The thickness, also known as the length (l) of the I layer, determines or affects several performance parameters, including, among other parameters, the capacitance of the diode, the resistance of the diode, the diode’s avalanche breakdown voltage, and the produced harmonic distortion. The area of the diode junction primarily affects C and R.

The practice of electronics design is inviolably an exercise in making trade-offs. As the frequencies at which PIN diodes are used have increased, the required capacitance of the diodes must be smaller in order to achieve acceptable performance. This has primarily been achieved by reducing the area of the junction. This reduction in capacitance came at the price of a commensurate increase in series resistance, which resulted in increased insertion loss for a series-connected application or decreased isolation for a shunt-connected application. Short of increasing the I layer thickness, which also produces increased series resistance, there was nothing else the design engineer could do.

Series resistance can also be defined in terms of the semiconductor physics properties of the diode.

Where l is the thickness of the I layer, µamb is the ambipolar mobility of the charge carriers injected into the I layer and Q represents the amount of free charge carriers injected into the I layer.

As frequencies increased to the point that the µamb of Si produced series resistance was too large, materials with greater value µamb, such as gallium arsenide (GaAs), were adopted. For millimeterwave (mmW) applications, even the higher value of µamb of GaAs has shortcomings.

To solve this need for better resistance and lower capacitance at millimeterwave frequencies, MACOM has developed heterojunction PIN diode switches using a novel aluminum gallium arsenide (AlGaAs) structure to address the limitations of GaAs and Si PIN diodes. The AlGaAs PIN diodes are also three-layer diodes, but with a significant difference: aluminum (Al) is used as a p-type dopant in the anode layer of the diode. The I and N layers of the diode comprise GaAs. The addition of Al to the anode layer increases the band gap of the diode junction with respect to that of a GaAs PIN structure. This difference produces a greater barrier to diffusion of holes from the I layer back into the P layer when the diode is under forward bias, thus increasing Q, the amount of free charge carriers in the I layer. This increase in the forward-bias charge carrier population in the I layer reduces the series resistance of the AlGaAs PIN diode, without changing the diode’s reverse bias performance.

The net effect is that one formerly inviolable trade-off has been eased: for an AlGaAs PIN diode and a GaAs PIN diode with identical I layer lengths and identical resistance values, the AlGaAs PIN diode can have a smaller junction area with lower junction capacitance, enabling improved circuit performance.

Glossary:

Anode: The layer of a diode which has been doped with acceptor atoms.

Avalanche Breakdown Voltage / Breakdown Voltage: The reverse bias voltage at which a specified magnitude of reverse current (typically 10 microamperes) flows. The symbol for avalanche breakdown voltage is VBR or VB.

Cathode: The layer of a diode which has been doped with donor atoms.

Diode: A two-terminal, passive electronic device, which generally is capable of rectification.

Dopant: A foreign substance which is added to a semiconductor material to achieve a desired effect. For example, the acceptor-atom material which is added to a semiconductor to form an anode layer is a dopant.

Forward Bias: The condition in which the voltage applied to the anode of a rectifying semiconductor diode is negative with respect to its cathode.

Insertion Loss: The reduction in transmitted power, typically expressed in decibels, which results when a component or other structure is inserted into a transmission line. This term is used when the loss is intended to be small.

Isolation: The insertion loss, typically expressed in decibels, produced by a component. This term is used when the insertion loss is expected to be large.

Intrinsic Layer "I layer": The layer of a PIN diode which has the doping concentration typical of that which is considered to be the native state of the semiconductor. IN a PIN diode, the intrinsic layer typically has a concentration of donor atoms whose doping concentration is several orders of magnitude lower than that of the cathode layer.

PIN Diode: A semiconductor diode comprising three layers. The central layer is intrinsically doped (the l layer) and is between a layer which is heavily doped with acceptor atoms (the P layer) and a layer which is heavily doped with donor atoms (the N layer).

Series Resistance "RS": The opposition to current flow of a semiconductor junction where the junction is modeled as a parallel circuit. The symbol for series resistance is RS.

Switch: A device which allows or prevents a signal from propagating between points is a transmission medium.

Reverse Bias: The condition in which the voltage applied to the anode of a rectifying semiconductor diode is positive with respect to its cathode.

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