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programNSF NASCENT ERCMicroelectronics Research CenterElectrical and Computer Engineerin

Banerjee, Sanjay K.

Hsu, William

Mantey, Jason

Register, Leonard F.

Applied Physics Letters

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William Hsu

Jason Mantey

Leonard F. Register

Sanjay K. Banerjee

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Comment on “Assessment of field-induced quantum confinement in heterogate germanium electron–hole bilayer tunnel field-effect transistor” [Appl. Phys. Lett. <b>105</b>, 082108 (2014)]

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Comment on “Assessment of field-induced quantum confinement in heterogategermanium electron–hole bilayer tunnel field-effect transistor” [Appl. Phys. Lett. 105,082108 (2014)]William Hsu, Jason Mantey, Leonard F. Register, and Sanjay K. Banerjee  Citation: Applied Physics Letters 106, 026102 (2015); doi: 10.1063/1.4905865 View online: http://dx.doi.org/10.1063/1.4905865 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/2?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Assessment of pseudo-bilayer structures in the heterogate germanium electron-hole bilayer tunnel field-effecttransistor Appl. Phys. Lett. 106, 262102 (2015); 10.1063/1.4923467  Response to “Comment on ‘Assessment of field-induced quantum confinement in heterogate germaniumelectron–hole bilayer tunnel field-effect transistor’” [Appl. Phys. Lett. 106, 026102 (2015)] Appl. Phys. Lett. 106, 026103 (2015); 10.1063/1.4905866  Assessment of field-induced quantum confinement in heterogate germanium electron–hole bilayer tunnel field-effect transistor Appl. Phys. Lett. 105, 082108 (2014); 10.1063/1.4894088  Tensile strained Ge tunnel field-effect transistors: k · p material modeling and numerical device simulation J. Appl. Phys. 115, 044505 (2014); 10.1063/1.4862806  A model determining optimal doping concentration and material’s band gap of tunnel field-effect transistors Appl. Phys. Lett. 100, 193509 (2012); 10.1063/1.4714544    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.83.205.53 On: Mon, 27 Jul 2015 18:03:00Comment on “Assessment of field-induced quantum confinement inheterogate germanium electron–hole bilayer tunnel field-effect transistor”[Appl. Phys. Lett. 105, 082108 (2014)]William Hsu,a) Jason Mantey, Leonard F. Register, and Sanjay K. BanerjeeDepartment of Electrical and Computer Engineering and Microelectronics Research Center,The University of Texas at Austin, Austin, Texas 78758, USA(Received 22 October 2014; accepted 31 December 2014; published online 15 January 2015)[http://dx.doi.org/10.1063/1.4905865]In a recent letter, Padilla et al.1 pointed out the impor-tance of the diagonal tunneling current in the Ge electron-holebilayer tunnel field-effect-transistor (EHBTFET), which waspreviously proposed with favorable ON-current and subthres-hold swing (SS) based on simulations.1 This parasitic diagonaltunneling current is exacerbated by the built-in horizontal fieldalong the channel due to different field-induced quantum con-finement (FIQC) between the gate overlap and underlapregions, resulting in SS degradation of a EHBTFET. Toaddress the issue, they proposed a heterogate design whichemploys substantially different metal gate work functions forthe gate overlap and underlap regions to reduce the horizontalfield and thus the diagonal tunneling (Fig. 1(a)).1 The workfunction difference, 0.5 eV or larger, was optimized for anOFF-state drain voltage, specifically VD¼ 0.5V.1 However,the use of such a large work function difference could havepotentially deleterious effects on the ON-state performancewith low-VD CMOS logic.To examine the performance of the heterogate GeEHBTFET under different drain bias, a full quantum me-chanical (QM) simulation2 is carried out to evaluate thepotential difference/barrier between the gate overlap andunderlap regions. The QM (2D subbands) study is performedby solving the Schr€odinger and Poisson equations self-consistently in all the 1D slices (y direction) along the chan-nel (x direction) without the incorporation of band-to-bandtunneling (BTBT). Here, we define the gate voltage, VG, atthe eigenstate alignment of the first electron and hole sub-bands in the gate overlap region as VG,align. It is assumed thatthe change of the carrier density in the channel due to BTBTis too low to introduce band modification at VG¼VG,align,particularly given the abrupt turn-on predicted by Padillaet al.1 The performance evaluation is conducted by investi-gating the eigenenergy difference, DE, because we onlyfocus on the magnitude of the barrier between the gate over-lap and underlap regions induced by work function differen-ces. The magnitude of the barrier at VG¼VG,align isdetermined using jDEj ¼ jE1e,UL  E1e,OLj ¼ jE1e,UL E1h,OLj, where the E1e,UL and E1e,OL are the electron eigene-nergies of the first subbands in the gate underlap and overlapregions, respectively, and E1h,OL is the hole eigenenergy ofthe first subband in the gate overlap region. Note that themagnitude of the subband overlap, jE1e,UL  E1h,OLj, isrelated to the diagonal tunneling current. For simplicity, theeigenenergies are all extracted at the midpoints of their re-spective regions. The midpoints also serve as good referencepoints for estimating built-in electric fields from the simu-lated band diagrams (not shown).The basic heterogate Ge EHBTFET device structure fromRef. 1 is shown in Fig. 1(a), although parameters vary in thiswork. High-j gate oxides (0.6 nm equivalent oxide thickness)are used. The gate overlap length (Lg,OL), gate underlap length(Lg,UL), and the distance between the gate and source (Ls) areall set to 50 nm. A Ge (100) channel thickness of Tch¼ 10 nmis considered. The device consists of an intrinsic Ge regionFIG. 1. (a) Schematic cross section of the simulated heterogate Ge EHBTFET(Lg,OL¼Lg,UL¼Ls¼ 50 nm, equivalent oxide thickness¼ 0.6 nm), and (b) ei-gen energy difference, DE, as a function of VD with the work functions of topgate 2, /TG2¼ 4.1, 4.2, and 4.3 eV.a)Author to whom correspondence should be addressed. Electronic mail:william.hsu@utexas.edu0003-6951/2015/106(2)/026102/2/$30.00 VC 2015 AIP Publishing LLC106, 026102-1APPLIED PHYSICS LETTERS 106, 026102 (2015) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.83.205.53 On: Mon, 27 Jul 2015 18:03:00(modeled as ND¼ 1 1015cm3) with heavily doped sourceand drain (NA¼ND¼ 1 1019cm3). With the work functionof the portion of the top heterogate that covers the gate over-lap region (top gate 1) set to 3.6 eV, and the work function ofthe bottom gate set to 5.6 eV, three different values are consid-ered for the work function of the portion of the heterogateabove the gate underlap region (top gate 2) in our simulations,/TG2¼ 4.1, 4.2, and 4.3 eV.Figure 1(b) illustrates the DEVD relation with different/TG2 at VG¼VG,align. For /TG2¼ 4.1 eV, the DE variesquickly with increasing VD in the low-VD regime and nearlysaturates in the high-VD regime. In addition, the DE shows asmaller shift with increasing /TG2 in the low-VD regime ascompared to the high-VD regime. Both these behaviors canbe explained by drain-induced barrier modification (DIBM),3where the channel potential is more strongly controlled bythe drain than by the gate at lower drain bias. In the high-VDregime, where the gate dominates the drain with respect tothe channel potential, the change in DE with different/TG2 become very close to the change in /TG2. Ultimately,there exists an ideal value for /TG2 that results in a low andnear-constant jDEj as a function of VD. The optimum /TG2 isfound to be close to 4.2 eV in simulations (Fig. 1(b)). Theseresults suggest that the heterogate EHBTFET, which caneliminate parasitic tunneling utilizing work function differ-ence between the two top gates without degrading theON-state performance in the low-VD regime, also is a prom-ising TFET design. Note that although the evaluation resultsin this comment are performed with EHBTFETs, the hetero-gate design concept should be considered to be general forall the TFETs based on 2D-2D or 3D-2D tunneling.This work was supported in part by the NRI SWANprogram and NSF NASCENT ERC.1J. L. Padilla, C. Alper, F. Gamiz, and A. M. Ionescu, Appl. Phys. Lett. 105,082108 (2014).2Sentaurus Device, version G-2012.06 (Synopsys, Inc., Mountain View, CA,2012).3W. Hsu, J. Mantey, L. F. Register, and S. K. Banerjee, Appl. Phys. Lett.103, 093501 (2013).026102-2 Hsu et al. Appl. Phys. Lett. 106, 026102 (2015) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.83.205.53 On: Mon, 27 Jul 2015 18:03:00

Comment On >Assessment Of Field-Induced Quantum Confinement In Heterogate Germanium Electron-Hole Bilayer Tunnel Field-Effect Transistor> Appl. Phys. Lett. 105, 082108 (2014)

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Comment On >Assessment Of Field-Induced Quantum Confinement In Heterogate Germanium Electron-Hole Bilayer Tunnel Field-Effect Transistor> Appl. Phys. Lett. 105, 082108 (2014)

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