III–V FET channel designs for high current densities and thin inversion layers

Abstract

III-V FETs are being developed for potential application in 0.3-3 THz systems and VLSI. To increase bandwidth, we must increase the drive current I d = qns v in j� per unit gate width Wg, requiring both high sheet carrier concentrations n , and high injection velocities vin. Present III-V NFETs restrict control region transport to the single isotropic r band minimum. As the gate dielectric is thinned, I d becomes limited by the effective mass m * , and is only increased by using materials with increased m * and hence increased transit times. 1 The deep wavefunction also makes r -valley transport in low-m * materials unsuitable for < 22-nm gate length ( Lg ) FETs.Yet, the L-valleys in many III-V materials2 have very low transverse m, and very high longitudinal mass mi' L valley bound state energies depend upon orientation, and the directions of confmement, growth, and transport can be chosen to selectively populate valleys having low mass in the transport direction3,4. The high perpendicular mass permits placement of multiple quantum wells spaced by a few nm, or population of multiple states of a thicker well spaced by -10-100 me V. Using combinations of r and L valleys, n , can be increased, m * kept low, and vertical confmement improved, key requirements for <20-nm Lg III-V FETs.Consider FET scaling. To increase bandwidth r :1, capacitances and transit delays must be reduced r:1 while maintaining constant voltages, currents, and resistances. In short-Lg FETs, the gate-source Cgs . ! oc sWg and gate drain Cgd oc s� fringing capacitances are a significant fraction of the total capacitance, and limit!,. Cgs . ! and Cgd are only weakly dependent on lateral geometry, hence the (Cg S ,f + Cgd)� V I Iddelay is reduced r:1 only if Id IWgis increased r:1. Id is determined by the sheet carrier concentration n , = ( Cg_ch I Lg � X � s -Vii.) I q, where the gate channel capacitance Cg_ ch = [1/ Cox + 1/ Cdep/h + 1/ C D ost is the series combination ot dielectric Cox = ss tO, Lg� ITeq' wavefunction depth Cdep/h = ssemiLg� I Tin v (T in V is the wavefunction mean depth) and density of states Cdos = q2 . dns I dEf capacitances. In the ballistic case, Cdos = q2 g(ml l molyl2 Lg� 127Tit 2 , where g is the # of populated valleys, and mi l and mol the effective masses parallel and perpendicular to transport; near equilibrium, Cdosis 2:1 larger.Given ballistic transporf andEf-EweJ1»kT, vin j = (4/37!X2(Ef-E...., )lml l Y'2 =( 4/37!X2qCg_ch ( � S -V/h)l m-,Pdoy2 . We scale by maintaining constant Vin j while reducing Cox I LgWg' Cdep/h I LgWg' and Cd . .! Lg� by r:1 This requires fixed transport mass mi l ' T eq and Tin V reduced r :1, and C dos increased r : 1 by increasing the # of valleys or the perpendicular mass.At a given dielectric thickness T eq ulv , there is an o p timum m * maximizing I d' We fmd I d I� =Jo ·K 1 ·«�s -V/h )11V)31 2 , where Jo = (41 371X2q I moyl2 V Vt\q2 mo I 27Tit 2 ) = 84 mA/J.m and K I = n . (mol Im . ) 1 1 2 (1 + (Cdos,o ICeqvulv) ' g ' (m�2 m:i2 I m.)r312is the normalized current density. Ceq U Iv = ...

Performance Comparison of GaSb, Strained-Si, and InGaAs Double-Gate Ultrathin-Body n-FETs

“…However, such devices suffer from a very low density of states (DOS) in the channel leading to small gate capacitances, particularly when a small equivalent oxide thickness (EOT) is used [3]. Therefore, only few electrons can travel at a high velocity, limiting the achievable ON-current. A material such as GaSb with a low energy separation between the bulk Γ-and L-valleys (26 meV at room temperature), a small transverse (m L,t = 0.1m 0 ), and a large longitudinal (m L,l = 1.3m 0 ) effective mass in the L-valley [4] could provide a viable solution to the DOS bottleneck encountered in InGaAs [5]. In effect, in an ultrathin-body (UTB) configuration with confinement along the (111) crystal axis, GaSb offers many conduction subbands, projected from the bulk L-valley, capable of carrying electrons at high velocities.…”

Simulation Study of Thin-Body Ballistic n-MOSFETs Involving Transport in Mixed $\Gamma$-L Valleys

“…This conclusion differs from that of [2] because of the thin 1.8-nm channel considered here. Given the strongly nonparabolic InAs valley, the E C edge mass increases from its bulk value of 0.023 to 0.08 m 0 for a 1.8-nm thin quantum well [3], [8]. Simultaneously, the proposed -L minima designs begin to outperform InAs (100) as EOT is scaled.…”

“…1). The idea behind (111) SG-ETB-MOSFETs is to quantize the anisotropic bulk L-valley, leading to the formation of multiple L[111] subbands with light in-plane transport mass [3]. This necessitates either a channel material which in bulk has its L-valley either below (e.g., Ge) or only slightly above that of the valley (e.g., GaSb), such that in the thin, quantized channel the 0741-3106 © 2013 IEEE Although we have considered and analyzed many channel designs, we here report in detail only those cases showing high performance [9].…”

“…A light effective mass also leads to low density of states (DOS), and consequently III-V channel materials provide diminishing benefit over Si as the effective oxide thickness (EOT) is scaled below 0.6 nm [2]. This loss of DOS can be compensated, under (111) confinement, using several eigenstates of the highly anisotropic L[111] band-edge states, or by aligning the and lowestenergy L[111] band-edge states (Table I) [3], [5]. Previous works contributing toward this idea assumed an idealized interface at the bottom surface of the channel [5].…”

Millimeter-Wave Series Power Combining Using Sub-Quarter-Wavelength Baluns

“…Millimeter-wave transistors have small carrier transit delays, hence mm-wave HBTs have thin collector depletion regions [21] and mm-wave FETs have short gates and small gate-drain spacings [22]. Breakdown voltages are therefore low.…”