High Electron Mobility Transistors for LowNoise Operation D

High Electron Mobility Transistors for Low-Noise Operation D. L. Pulfrey Department of Electrical and Computer Engineering University of British Columbia Vancouver, B. C. V 6 T 1 Z 4, Canada pulfrey@ece. ubc. ca http: //nano. ece. ubc. ca Day 3 B, May 29, 2008, Pisa

High electron-mobility Transistor • Note the Schottky barrier

Schottky barrier band diagram

Schottky barrier under bias • Negative potential on n-type semiconductor • discontinuity in EF

Forward bias in SB- and PN-diodes 2. What is the driving force here? ΦB -q. Va 1. What is the bottleneck here?

Two heterojunctions in a HEMT Metal/Al. Ga. As HJ Al. Ga. As/Ga. As HJ 2 -DEG in the potential "well" y Note the doping

Simplifying the quantum well • Triangular to finite square • Finite to infinite square • SWE becomes: • wavenumber is Ey a • bc’s: • solution: E is quantized

Energy is quantized E 1/m* 10 X Ga. As vs. Si Energy Wavefunction Probability density

For a finite well • Wavefunction not completely confined • Use undoped spacer

Employment of a spacer layer Provision of electrons from remote donors is called MODULATION DOPING

Formation of sub-bands and 2 DEG 2 m 2 m Empty Partially filled 2 nd sub-band • ns 0 1013 cm-2

2 DEG concentration ns Independent of E !!

Controlling ns by VGS Thick barrier layer Thick-enough barrier layer q. VGS Depleting the channel Threshold condition • How would you make an enhancement HEMT?

• Often modeled by SPICE LEVEL 1: IDsat=Wg Cg(VGS-VT)2 /2 Lg

HEMT attributes • Excellent lattice match no surface scattering ( ). • Electrons and donors separated no I I scattering, i. e. , • Undoped spacer also helps mobility. • Electrons confined to a well of width < e i. e. , about 15 nm for Ga. As at 300 K. • Size-quantization of energy levels - standing waves - only 2 -D scattering • and gm

Start with a high and preserve it!

High performance HEMT Why the funny gate? f. T= 270 GHz, fmax=490 GHz

NOISE Noisy DC signal d. B use RMS values What is a signal of -30 d. Bm ?

Thermal noise Brownian motion So, an equivalent circuit representation of thermal noise is v. R > vd, so present without current From Nyquist: This P can be transferred from a real resistor R to a noiseless resistor R. • What "colour" is this noise? • How much thermal noise in 50 Ohm R? -> 1 n. V over 1 Hz

Shot noise Forward-biased junction microscopically -> EC Transition over barrier is random event (probability of state occupancy) Important in HBTs, but not in FETs, except in sub-threshold operation.

Flicker noise Defects cause ''traps" Escape time: tends to be long Empirical expression: Colour of this noise? Prevalent in MOSFET channel. Keep L short. Use a HEMT.

Induced gate noise Gate • The induced gate noise is correlated with the channel (drain-current) noise. • Coupling is via capacitance • Impedance decreases with frequency • Important at high frequencies

Non-Quasi-Static operation Recall: QSA q(x, y, z, t' ) = f( VTerminals, t') q(x, y, z, t' ) f( VTerminals, t < t') At high frequencies, this breaks down. Consider example of charging a capacitor: R v(t') C q(t')=f(v(t')) v(t') Model capacitor by q(t')=f(v(t<=t'))

Non-Quasi-Static Equivalent Circuit including noise sources vns G RG + Rgd Cgd D + Cgs Zps + vn. Rg RL Rgs vsig ind + S vng

Noise Figure Important to have high fmax

• What is Associated Gain? • What is the "black" gain?