Graduate Students:
Y. Kwon* and
K. Hein*
Professor
D. Pavlidis
TRW, National Aeronautics and Space Administration NAGW-1334
U.S. Army Research Office DAAL-03-87-K-0007 and Honeywell Inc. BO4 931579
InAlAs/InGaAs HEMTs grown by three different growth techniques, namely Molecular Beam Epitaxy (MBE), Metalorganic Vapor Phase Epitaxy (MOVPE) and Chemical Beam Epitaxy (CBE) are being investigated experimentally. Careful optimization of the heterostructure designs and the use of submicron (0.1um) T-gate technology allowed state-of-the-art cut-off frequency (fT) performance.
Strained channel In(0.52)Al(0.48)As/In(0.65)Ga(0.35)As HEMTs grown by MBE were fabricated using 0.1um T-gate technology. A strained channel was used to enhance the fT performance due to its higher electron mobility, velocity, sheet carrier concentration and conduction band discontinuity (delta Ec). A fT value as high as 220GHz has been obtained for these devices.
SEM photograph of Quasi-1D HEMT gate using InAlAs/InGaAs heterostructures.
Features of this study include small parasitic regions of only 0.1 um
and variable channel widths from 0.05 um to 0.3 micron.
0.04 um long T-gate realized using the 5th lens mode which was recently
added to our E-beam. A bilayer process is used for this purpose and a special
mask set compatible with 5th lens mode has been employed to study sub-0.1 um
effects in HEMTs.
Double heterojunction (DH) HEMTs were studied to incorporate more carriers and better carrier confinement. Both single (SH) and double heterojunction devices were fabricated at the same time for comparison. Compared with SH HEMTs, DH devices showed a lower fT value (150GHz vs 180GHz) due to the inferior quality of the bottom heterointerface, but a higher fMAX value (310GHz vs 280GHz) due to the better carrier confinement. DC current level was also improved as much as 45% in DH structures and kink effect was also reduced.
In addition to exploring the different growth techniques, a novel submicron self- aligned offset-gate technology has been developed to enhance the gain and the maximum oscillation-frequency (fMAX). The self-aligned features allow the close proximity of the source and gate and thus permit minimization of the parasitic source resistances. Furthermore, the offset-gate feature allows a large gate-to-drain spacing and thus larger gm/gds and Cgs/Cgd ratios which favor again the fMAX performance. A record high fMAX/fT ratio of 2.6 has been achieved using this technology with 0.25 um long gates and pulse-doped InAlAs/InGaAs HEMTs. The fMAX values are in the order of 280-300GHz. Sub 0.1 um offset self-aligned DH-HEMTs showed record fMAX of 350GHz with Idmax=1.2A/mm.
InAlAs/InGaAs HEMT technology has been extended to quasi-1D HEMT structures. Quasi-1D HEMTs employ narrow stripes across the source and drain, which helps to confine the carrier transport laterally. Due to this lateral confinement, structures with smaller channel width showed higher normalized transconductance and higher fNAX characteristics. Various channel widths are realized to study the impact on DC and microwave performace. Due to the enhanced charge control in the quasi-1D HEMTs enhanced Gm/Ids and Gm/Gds were observed. Comparedwith conventional HEMTs the quasi -1D HEMTs showed degraded fT due to additional parasitic capacitances and improved fMAX due to better carrier confinement.
Work is in progress on the study of multichannel and inverted InAlAs/InGaAs HEMTs as well as the microwave characteristics of quasi-1D HEMTs including their noise properties. A sub 0.1um technology is under development using the 5th lens mode of operation which was recently added to our E-beam. Modeling based on delay time analysis and 2D ensemble Monte Carlo techniquescomplement these studies.
Calculated average velocities of DH- and SH- HEMTs under minimum delay
conditions Vgs=0V, Vds=1.2V(DH) and Vgs=0.2V, Vds=1.2V (SH). Dotted lines
represent the electron velocities averaged over the channel electrons,
including buffer electrons. DH structures showed ~15% lower peak channel
electron velocity due to more electrons traveling at or close to the bottom
interface and the interface roughness scattering associated with the later.
fT and fMAX for various pitch Quasi-1D HEMTs; Vgs for maximum Gm and Vds=1.5V
The results indicate that fT decreases for smaller pitch devices due to the
higher rate of increase for Cgs than Gm. fMAX however, shows a maximum around
a pitch of 0.41um due to enhancement in the conductance and capacitance
ratios. the fMAX degradation for smaller pitch values is related to the
dominant fT effect in that region.
