technology conference report
The engineer’s next step was to determine whether pHEMT degradation was caused by HBT overgrowth in particularly, or just the thermal history associated with this growth. To answer this question, pHEMT epiwafers were heated under arsine for between 10 and 90 minutes at the temperature employed for HBT growth. Measurements of carrier mobilities and doping profiles revealed a shocking result: Even though the HBT is grown at a lower temperature than the pHEMT, growth at this temperature can still impact the location of carriers, leading to a broadening of the carrier concentration profile and a reduction in carrier mobility in the channel.
To determine the origin of this carrier broadening, the engineers studied their samples by secondary ion mass spectroscopy (see figure 5). This revealed that indium, phosphor, and aluminum atoms were unaffected by heating. Silicon, however, an element renowned for its low thermal diffusion coefficient, spread out from the δ- doped layers and into the AlGaAs spacer layer and InGaAs channel, where it degraded carrier mobility.
Takeda and his co-workers offered two suggestions for improving δ-doped pHEMTs with a high silicon concentration : increase the spacer layer thickness between the channel and electron supply, which invokes the penalty of a cut in the transistor current; or turn to a uniformly doped electron supply layer.
Complex channels
More and more countries are developing III-V expertise, and this year a partnership between researchers from India Institute of Technology - Kharagpur and National Chiao Tung University, Taiwan, presented a paper on the development of an InAlAs/InGaAs HEMT with a novel composite channel. This transistor could be used to make a W-band power amplifier for radar applications.
The researchers aimed to improve HEMT performance by increasing indium content in the InGaAs channel, which
Figure 6. A partnership between researchers India Institute of Technology - Kharagpur and National Chiao Tung University, Taiwan has developed novel pHEMT architectures.
leads to several benefits. It deepens the quantum well, confining more carriers to the channel, it almost eliminates three-dimensional carrier movement, and it enhances mobility via a reduction in the electron’s effective mass.
To realize a high-indium-content channel that is free from phase separation, the researchers developed transistors with penta-composite channels that featured graded layers with differing indium content. These structures accommodated lattice mismatch between the layers, and through optimization of indium fraction in the InGaAs layer, δ-doping and spacer thickness, they promise to deliver high currents with good linearity at high frequencies.
The engineers best results were realized with a HEMT
Figure 5. Annealing a pHEMT wafer under arsine at temperatures associated with HBT growth causes
diffusion of the silicon dopant
32
www.compoundsemiconductor.net July 2010
featuring an In0.78Ga0.22As channel and a total channel thickness of 14 nm (see Figure 6). This transistor had a drain current of 1029 mA/mm and a transconductance of 648 mS/mm. The device’s cut-off frequency and maximum oscillation frequency were 125 GHz and 250 GHz, respectively, values that the team describes as “remarkable”, given the gate length of 0.25 mm. This is much larger than that used by today’s fastest transistors.
If the team goes down this scaling route and makes progress, then it may well be taking part at next year’s CS-Mantech. If this meeting is anything like the last few years, there’s bound to be a good number of GaAs related papers presented in Palm Springs, CA, in mid-May 2011.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83