Investigation of Graded and Tensile-strained Channel Metamorphic HEMTs
| Autor: | Yin-Kai Liao, 廖英凱 |
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| Rok vydání: | 2006 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 94 In this thesis, in order to improve the small gate voltage swing of conventional high electron mobility transistors (HEMT), we adopt the HEMT with inversely-graded channel (IGC-HEMT) grown on InP substrate. The experimental results show that the IGC-HEMT has not only wider gate voltage swing but also good thermal stability. The reason for the wider gate voltage swing is that the balance between gate modulation capability and electron saturation velocity. The reason for good thermal stability is that the inversely-graded channel increases the conduction band discontinuity (ΔEC) at the channel/barrier interface and therefore improves the carrier confinement. The indium composition in the inversely-graded channel elevates with the increase of the depth and the electron saturation velocity also increases with the increase of the indium composition. The increase of electron saturation velocity can compensate for the increase of the distance from gate electrode to the effective 2-DEG position due to the decrease of gate voltage. This design makes the extrinsic transconductance unchanged even though the device is operated at wider gate voltage range. The indium composition in the inversely-graded channel elevates with the increase of the depth and therefore the bandgap decreases with the increase of the depth. Consequently, the maximum conduction band discontinuity is achieved at the channel/barrier interface. The maximum conduction band discontinuity provides good carrier confinement; hence, it has excellent thermal stability. The experimental results of IGC-HEMT are as follows: The drain-source saturation current density at VGS = 0 V (IDSS0), extrinsic transconductance (gm,max), fT, and fmax are 381 mA/mm, 314 mS/mm, 46 GHz and 55 GHz. The off-state breakdown voltage, maximum output power (Pout), associated gain (GA) and power added efficiency (P.A.E.) are 7.1 V, 12.8 dBm, 19.5 dB and 42.0 %. The IDSS0 at 420 K maintains at 90.7 % of the value at 300 K and the gm,max also maintains at 94.4 % of the value at 300 K. Therefore, the IGC-HEMT is suitable for high-temperature applications. In order to solve the problem that high mobility comes from In-rich channel which leads to small breakdown voltage, we design the “Tensile-strained metamorphic HEMT” (TS-MHEMT). First, we decrease the Indium composition in the channel to reduce impact ionization and increase breakdown voltage. High mobility and high carrier concentration are still maintained because of the tensile-strained channel and “V”-shaped graded channel structure. Additionally, we adopt the design which uses metamorphic HEMT grown on GaAs substrate to decrease the cost. The experimental results of TS-MHEMT are as follows: TS-MHEMT has high drain-source saturation current density (IDSS0 = 514 mA/mm), high extrinsic transconductance (gm,max = 404 mS/mm) and better microwave characteristics (fT = 58.5 GHz and fmax = 72.2 GHz). The breakdown voltage is elevated and therefore improve the power characteristics (Pout = 17.2 dBm, GA = 21.1 dB, P.A.E. = 49.4 %). Consequently, the tensile-strained metamorphic HEMT can fulfill the requirement of high-frequency and high-power at the same time. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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