Hot electron noise in n-channel JFETs between 150 and 300°K

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HOT ELECTRON NOISE IN N-CHANNEL JFETs BETWEEN 150 AND 300°K S. K. KIM and A. VAN DER ZIELJ" Department of Electrical Englneenng, University of Minnesota, Minneapolis,MN 55455, U.S.A. and L. M. RUCKER~ Department of Electrical Engmeering,University of Florida, Gainsville,FL 32611, U S.A.

(Recaved 6 March 1978;in revisedform 22 May 1978) Abstract--The noise parameter ar = Sl(f)/(4kTg.) of n-channel JFETs was measured as a functnonof the voltage Vos-V, with the temperature T as a parameter between 150 and 300°K. It was found that aT could be approximated by the formula ar --"(300/T)a~. indicatingthe presence of hot electron effects. When the noise spectrum Sl(f) of the short-circuited drain current of a junction fluid-effect transistor (JFET) in saturation is expressed as

SRf)

=

ar4kTgm

(1)

where k is Boltzmann's constant, T the absolute temperature of the device, and g,. the device transconductance at saturation, the value of the dimensionless parameter aT is found to lie close to 2/3 for a longtSupposed by Army Research Office contract with the University of Minnesota. ¢Suppurted by N.S.F. conU'actwith the Umversity of Florida. 20

t

channel device with negligible series resistance between the channel and the source contact[l]. A value somewhat larger than 2/3, but always less than unity, is observed when the series resistance is signiticant[2]. For shortchannel devices the value of aT can be larger than unity because of hot electron effects, chiefly because the effective electron temperature is larger than the lattice temperature. We have measured a r for various saturated shortchannel JFETs (4.1~ meter channel length) as a function of V a s - V p with the temperature T as a parameter: here Vas is the gate-source potential and V, the pinchoff voltage of the device. Typical results are shown in Fig. 1.

"

Device'

FD

I10 #1

VDS = + 3 V

18

f= 4MHz

16

0.6

/

m 9 - K .,a

~ I 0.2

Fig. 1 Device FD 110 No. 1. aT

I 0.4

I 0.6

I 0.8

I 1.0

I 1.2

I 1.4 Iv¢~ - vpl

I 1.6 In Volt

1.8

Vp with the absolute temperature T as a parameter under saturated conditions a [ ffi4 MI-Izand Vm ffi3 V.

VS V ~ -

1259

1260

S K KIM and A VAN DER ZIEL I.I

_.....o

2ro*K

ID

0.9 °-'

0.7 0.6 0.5

0

O

V

J

I

I

I

0.2

0.4

0.6

0.8

I

l.O

I

1.2

I 1.4

I L.

i

1.8

I,. v.,, Fig. 2 Device FD 110 No 1, but (T/3OO)ar vs Vos - Vp with T as a parameter. Note that the curves are not qmte close together

We do not understand the rise m a r with decreasing V ~ s - Vv for low values of V 6 s - Vr, at lower temperatures, but for V o s - Vv larger than 0.4 V a r at a given temperature T always increases with increasing V o s Vv, whereas at a given value of V6s - Ve the parameter ~ r always increases with decreasing T and can be appreciably larger than unity. We believe that these phenomean can only be caused by hot electron effects. In Fig. 2 we have plotted (Tl3OO)ar as a function of V o s - Vv. The curves now lie very close together, indicating that approximately aT

= a3oo

(300/T).

(2)

Extrapolating back to 77°K would yield a77 4, and a noise temperature of g,,, of the order of room temperature. It could be, of course, that a r would increase somewhat faster below 150*K than eqn (2) would indicate, but it would be very surprising if a r were an order of magnitude larger. In a rocent paper Nougier et al.[3] reported noise measurements in homogeneous n-type silicon bars and in =

n-channel silicon JFETs at 77°K and found noise about 70 times larger than expected theoretically. They interpreted that noise as hot electron noise. We carried out low temperature measurements[4] of the noise resistance /L, of channel of JFETs and obtained results quite compatible with those published by Nou#er et al. We interpreted the noise as generationrecombination noise, and found corroboration in the fact that the noise resistance near 77°K showed an actwation energy of the order of the activation energy of donors, as expected from the g-r noise theory. We shall not discuss the two alternate interpretations of the noise generation at 770K in detail. The aim of this note is chiefly to report on hot electron effects in nchannel silicon JFETs above 150*K. III~¥_,I(I~CI~S 1. A. van der Ztel, Proc IRE, .~, 1808 (1962). 2 W. C Bruncke, Proc IEEE, 51,378 (1963) 3 J P. Nou#er, D Sodinl, M Rolland, D Gasquet and G Lecoy, Solid-St. Electron. 21,133 (1978).

4 S. K Kim, A. van der Zlel and L M Rucker, Sohd-St Electron. 21, 1099 (1978).