Advanced Semiconductor Devices Y-BRANCH SWITCH (YBS) Anubhav Khandelwal.

Advanced Semiconductor Devices

Y-BRANCH SWITCH

(YBS)

Anubhav Khandelwal

OUTLINE• INTRODUCTION –Need for efficient electronic switches• YBS

– Principle of operation – Ballistic Transport – Characteristics– Fabrication

• YBS as efficient switch• APPLICATIONS

– Theoretical Predictions– Demonstrated devices :

• Diodes • Transistors• Schmitt Trigger• Logic Gates: NAND

• SUMMARY

Need for efficient electronic switches

• Problem of switching bottleneck in modern communications network

Need: • Ultra-fast switching• High packing density• Low power dissipation

YBS be the solution?

YBS: Principle of OperationAssuming Ballistic transport

STEM

RIGHTLEFT

Fundamental limit for switching: i

Fs L

vVe

Li

For a YBS manufactured by etching through a GaAs/AlGaAs 2DEG, with ns=4×1011 cm−2 and Li ~ 200 nm, ΔVs ~1 mV

YBS: Ballistic Transport

• Ballistic Transport – Branch width < Electron free wavelength

42

2

1ooC VOVV

VL=VOVR=-VO

VC

Classical:

Ballistic:

0CV

(1) PHYSICAL REVIEW B, Vol. 62, No.24, 15 DECEMBER 2000-II

(1)

YBS: Characteristics1. For symmetric YBS, applying +V and –V to VL and VR will always result in

negative Vc

For asymmetric YBS, Vc is negative for lVl greater than certain threshold VL=VO

VR=-VO

VC

2. (Theoretically) Possible to achieve gain without external biasing due to self coupling between the branches.

(1) PHYSICAL REVIEW B, Vol. 62, No.24, 15 DECEMBER 2000-II

(1)

YBS: Fabrication

YBS as efficient switch

i

Fs L

vVe

(1) APL VOLUME 83, NUMBER 12 22 SEPTEMBER 2003

1. Speed • Small capacitance of central branch and

small contact resistance (few kΩs). Switching at 50GHz has been

demonstrated.

• Theoretically, self coupling in ‘gateless’ YBS result in switching at THz range

2. Size• YBS with sub-100nm thick branches

demonstrated. With branched nanowires, can go down further.

3. Switching energy • Fundamental limit for switching (single

mode coherent transport) is not Thermally limited in YBS

• Switching voltage in FET is Thermally limited

TkVe Bs 10log

ApplicationsTheoretical predictions

tVV oL cos tVV oR cos

tVV oC 2cos2

Rectifier

Second and higher harmonic generator

VVL VVR

CV

Logic AND

LV RV

CV

VC as a function of VL

• Diode if VR=0V

• Transistor if VR is varied

Reversible logic using YBS

• Minimum energy dissipation due to information erasure is

Currently, much more than kT being dissipated

2lnkT

IRREVERSIBLE LOGIC e.g. NAND

• Ideally, avoid information erasure by zero energy dissipation

Practically, always some energy dissipation but

2lnkT

REVERSIBLE LOGIC

Reversible logic using YBS

(a) A Fredkin or “Controlled Exchange” gate based on four YBSs (b) The corresponding truth table A is the control, exchanging the inputs B and C if it is set to high.Note: It is as universal as NAND/NOR

(a) (b)

Erik Forsberg, “INSTITUTE OF PHYSICS PUBLISHING, Nanotechnology 15 (2004) S298–S302”

YBS as Diode & Transistor

Diode: VR = 0V- VL<0V, VC follows VL linearly- VL>0V, VC saturates

Triode: VC as a function of VL for different values of VR

Note: Room temperatureoperation demonstrated onYBS etched on GaInAs/InP Heterostructure

H. Q. Xu, I. Shorubalko, D. Wallin, I. Maximov, P. Omling, L. Samuelson, and W. Seifert “IEEE ELECTRON DEVICE LETTERS, VOL. 25, NO. 4, APRIL 2004”

YBS as Schmitt-Trigger

(a) SEM image of a YBS together with a schematic view of the measurement setup. A bistable mode of operation was realized by coupling the left branch to the right sidegate, i.e., Vgr=Vbl . All voltages are related to ground

(b) Measurement setup in combination with the equivalent circuit of the YBS (shaded area)

Schmitt-Trigger characteristics

Demonstration of the bistable switching characteristic in feedback mode for Vbias=2.0 V. The hysteretic loop both for Vbl and Vbr is shown vs the voltage Vgl applied to the left sidegate.

Logic Gates using YBS: NAND

(a) SEM image of a NAND logic gate realized by integration of a TBJ with a point contact and the circuit setup for characterization.

H. Q. Xu, I. Shorubalko, D. Wallin, I. Maximov, P. Omling, L. Samuelson, and W. Seifert “IEEE ELECTRON DEVICE LETTERS, VOL. 25, NO. 4, APRIL 2004”

(b) Measured output voltage V and the corresponding input voltages V (dashed line) and V (solid line), for the NAND logic gate at room temperature. V = 10V and R=2.3M. The applied logic low and high inputs were set to 0 and 1.5 V, respectively, and the measured logic low and high outputs were set to 0.8 and 3.2 V.

(c) Experimental truth table for NAND logic gate

SUMMARY• Principle of operation, fabrication and

characteristics of YBS

• YBS as efficient electronic switch for high speed, low power operations like in communications networks

• YBS as diode, transistor, schmitt trigger, NAND

• Reversible logic possible through YBS