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Substrate Materials for High Power Transistors

RF Power transistors are responsible for amplifying RF signals to an output of 5 – 1000 watts, which are then directed out from antennas.  These power transistors are used for wireless communication (cell phone base stations), radar, radio, broadcast television, and industrial power supplies.  The frequency of operation constrains the size of the semiconductor device, so operating at high power inevitably results in high densities of waste heat which must be conducted away from the chip.  The thermal heat density of a typical RF transistor is typically 50 - 100W/cm2, which compares to 1 W/cm2 for a typical low frequency silicon power device.  Materion, through its many business units and facilities, serves the RF Power Transistor market by supplying several types of substrates. 

To remove heat, most Si power transistors are soldered to their substrate with gold/silicon solder.  Since AuSi is a “hard” solder, the thermal expansion of the substrate should be between 3 and 10 x 10-6 /°C, in order to prevent cracking of the Si upon temperature cycling.   The substrate can either be a dielectric or electrical conductor, depending on the type of transistor.  Si VMOS and bipolar transistors must be soldered onto an electrically isolated pad on a thermally conductive dielectric.   Materion’s BeO ceramic substrates, known as Thermalox™ BeO, offer a unique combination of high thermal conductivity, high electrical resistivity and low dielectric loss over a wide frequency range.   Materion provides hot pressed BeO ceramic substrates, manufactured at its Tucson, AZ facility, to meet this requirement.   Materion’s facility in Newburyport, MA then screen prints patterns of thick film MoMn paste that is fired onto the BeO, plated with Ni and then brazed into a package using braze alloys supplied by Materion’s facility in Buffalo, NY. 

In the mid-1990’s Motorola invented a new type of RF transistor called LDMOS. LDMOS transistors could be soldered directly to an electrically grounded metal flange, thereby eliminating the need for BeO.    The elimination of BeO reduced the cost of the package, as well as reduced the overall thermal resistance since most BeO packages contain a flange brazed beneath the BeO.  LDMOS transistors ramped up in popularity by the end of the 1990’s.   With the conversion from analog to digital cell phones in the early 2000’s LDMOS transistors dominated the market for amplifiers for cellular base stations.

Going forward, Materion is addressing other requirements of the RF transistor industry by manufacturing air cavity packages comprised of a ceramic frame brazed to a metal flange.   The industry is demanding flanges with higher thermal conductivity and lower cost.  Materion offers brazed ceramic packages with flanges made of CuW, CMC and CPC (Table 1).   Various Materion locations are working with key customers to develop flanges made of copper, which will require an entirely new method to solder Si onto the flange due to the severe thermal expansion mismatch between Si and Cu. 

The next generation of high power transistors are being made from a new semiconductor material called gallium nitride (GaN).   At the request of several of the GaN fabricators, Materion is also working to develop new flange materials that offer higher thermal conduction than conventional flange materials, high stiffness, and a good thermal expansion match to GaN.

Table 1:  Properties of Various Substrate Materials for High Power RF transistors

Material

Electrical Resistivity (Ω-cm)

Thermal Conductivity

(W/m-K)

Thermal Expansion Coefficient

(10-6 /°C)

Used with These of RF Transistors

Si

Variable

151

3

VMOS, bipolar, LDMOS

GaN

Variable

130

3.2

FETs

99.5%  BeO

1E13

285

7.6

Si VMOS, bipolar;

SiC  SIT

11:89 Cu:W

5.3E-8

200

6.5

Si LDMOS

1:1:1  Cu/Mo/Cu  (CMC)

N/A

230

8.9

Si LDMOS,  GaN

1: 4: 1 Cu/ CuMo/Cu  (CPC)

N/A

220

7.3x – 10y

Si LDMOS,  GaN

Cu

1.7E-8

390

17

Si LDMOS  (under development)