industry GaAs microelectronics
Most of time PAs operate at a fairly low power level, so the power added efficiency (PAE) can be relatively low. To combat this, in the last few years the makers of these amplifiers have unveiled two- state PAs, which increase efficiency when the device is operating at low and medium output powers. The technology to do this tends to be referred to as either a BiHEMT or BiFET process, and it involves the monolithic integration of HBTs and pHEMTs on a single die. The result is the combination of a PA and a switch.
With this technology it is possible to operate at high efficiencies far more often (see Figure 1). The input signal can be routed through the high-power path to deliver high efficiency at high power levels; and it can be directed through the low-power path to realize good efficiency at low powers.
At WIN Semiconductors Corporation of Taiwan we have been developing and refining our own BiHEMT process for uniting a HBT and a pHEMT on a single die. Specifically, our first-generation technology, which we unveiled at the CS Mantech conference held in 2007, involves the monolithic integration of InGaP/GaAs HBTs and E/D pHEMTs and uses a double-recess architecture and a bi-layer T-shape gate.
More recently, we have simplified our process, which in its latest guise incorporates just D-mode pHEMTs with HBTs. This makes good sense from a commercial perspective, because most BiHEMT applications for DC bias circuits and either RF or power-mode switches can be achieved with the pairing of D-mode pHEMTs and HBTs.
One of the strengths of our latest H2W process is that it trims the on-resistance of the pHEMT. In turn, this can lead to lower insertion loss and smaller layout area when the pHEMT performs as a power mode switch (as shown in Figure 1). The new generation H2W process is also very simple, requiring just 17 mask layers – five less than the previous H2W process, and just one more than we use for our stand-alone HBT technology. This means that our latest H2W process combines superior pHEMT characteristics for better RF switch performance with a simpler, better BiHEMT process for making products for producers of handsets and tablets.
Fabrication processes MOCVD growth is employed for forming our epiwafers. They feature an InGaP layer to
completely separate the pHEMTs from the HBTs, which form the upper section of the epitaxial stack. This isolation holds the key to independent optimization of both types of transistor. For example, by increasing the indium composition in the channel we have enhanced channel mobility, leading to a 40 percent reduction in on-resistance with the new generation H2W (PH50-20) process.
A cross-section of devices formed with this latest process is shown in Figure 2. Fabrication yields 2µm InGaP/GaAs HBTs and 0.5µm D-mode pHEMTs combined with epitaxial mesa resistors, thin film resistors, metal-insulator-metal (MIM) capacitors and through wafer vias. These BiHEMTs draw on our fourth generation HBT process, which includes two interconnection metal layers - Metal1 and Metal2 - and a thicker SiN dielectric sandwiched between them, which delivers improved mechanical and moisture protection.
The Metal 1 layer is a 1 µm-thick film of evaporated gold, and plating of this metal creates a 4 µm-thick layer for Metal 2. MMIC designs that can be
Figure
1.Amplification is more efficient over a wider range of conditions when two amplifiers are used for high power mode and just one for low and medium power modes
Figure
2.The new generation H2W process requires just 17 mask layers, five less than the previous generation process
October 2012
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