Base Transit Time in Abrupt GaN/InGaN/GaN and AlGaN/GaN/AlGaN HBTs
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Abstract Base transit time, Tb, in abrupt npn GaN/InGaN/GaN and AIGaN/GaN/AIGaN double heterojunction bipolar transistors (DHBTs) is reported. Base transit time strongly depends not only on the quasi-neutral base width, but also on the low field electron mobility, Pn, in the neutral base region and the effective electron velocity, Sc, at the edge of base-collector heterojunction. p,1 and S, are temperature-dependent parameters. A unity gain cut-off frequency of 10.6 GHz is obtained in AIGaN/GaN/AIGaN DHBTs and 19.1 GHz in GaN/InGaN/GaN DHBTs for a neutral base width of 0.05um. It is also shown that non-stationary transport is not required to study Tb for neutral base width in the range of 0.05um for GaN-based HBTs. Introduction Wide bandgap group III-nitride semiconductors are currently being pursued for possible high temperature and high power applications. Current gain as high as 105 is reported for GaN/SiC HBT [1]. In order to investigate high frequency performance the behavior of the base transit time, -b, needs to be investigated. Mohammad et al. [2] has reported the dependence of Tb on base doping concentration in a graded GaN/InGaN HBT. The double integral formulation of Tb by Kroemer [3] for HBTs is based upon the assumption that excess minority carrier concentration at the edge of base-collector depletion layer is negligible. Roulston [4] emphasized upon the use of a finite carrier velocity (saturation velocity) at the edge of the base-collector depletion region, thereby, introducing the component of base transit time due to velocity saturation. A more general formulation for carrier velocity at b-c heterojunction was used by Hafizi et al. [5]. Jahan et al. [6], based upon a self-consistent calculation of thermionic and tunneling components of the total current, proposed a tunneling factor for the determination of carrier velocity at the b-c junction. In this paper, the method formulated by Jahan et al. [7] is used in the determination of the effective electron velocity at base-collector junction which affects the electrons transport across base-collector junction. In this paper, Tb in abrupt GaN/InGaN/GaN and A1GaN/GaN/AIGaN HBT is reported. The computation of Tb includes the effects of bandgap narrowing, carrier saturation and partitioning of the total current into thermionic and tunneling components. Results obtained from an ensemble Monte Carlo simulation are used in the determination of low field mobility, p,n.
G 6.7
Mat. Res. Soc. Proc. Vol. 537 © 1999 Materials Research Society
Theory Base transit time, Tb, can be expressed as a sum of T-bl + T-b2, where NaB(Y) dydx I Zbl=WB nibZ2(x) WB INAB(X) xrB Dn(y) nib (y)
)
and
b2
I NAB(WB) cnib(WB)
WB nib2(x) AB(X)
where NAB (X) is the base doping concentration, nib2 (x) =nie2 exp(Ag /kT) is the effective
intrinsic carrier concentration in base region. The effective bandgap narrowing across the - x). Eg (GaN) x (1- x)Egb
emitter-base heterojunction, Ag (InxGa1 xN) =x. Eg (InN) + (I
(Egb=l.0 eV)[8] where x is the In-mole fraction in InxGalxN and
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