Nonlinear transient response of extrinsic Ge far-infrared photoconductors

Abstract

Physical mechanisms responsible for nonlinear phenomena and anomalous transient response of cooled extrinsic far-infrared photoconductors are discussed. A simple model describing carrier generation, trapping, and impact ionization is presented, which describes the transient response on fast time scales 10−3 to 10−4 sec, neglecting changes in space charge. Carrier heating by a dc electric field produces relatively fast, damped oscillatory response to external excitation. A small-signal analysis of these equations is a test of stability. An analysis of the role of ideal electrical contacts and space charge is also presented. The very slow (∼1 sec) overshoot and transient response commonly observed in cooled extrinsic photoconductors is explained by the dynamics of trapped space charge near the injecting electrical contact. A small-signal analysis determines the characteristic time constants for these processes, which are typically ∼1 sec. Calculated examples of the recombination and ionization coefficients, dc I-V curves, differential equation flow diagrams, and transient response are presented for parameters typical of p-type Ge photoconductors doped with shallow acceptor levels, and suggestions for the design of more stable photoconductors are presented.

Chaotic motion of space charge wave fronts in semiconductors under time-independent voltage bias

“…The transport coefficients V , K and R are, respectively, the average (drift) velocity of microscopic charge carriers (holes in the case of p-Ge), and coefficients describing the creation of free carriers via impurity impact ionization and the destruction of free carriers by capture onto an available impurity trapping site (i.e., neutral acceptor). All are nonlinear functions of electric field, and their forms and plots have been discussed extensively in the literature [5,13,14]. In this paper we use the same forms as in Ref.…”

“…Many of these phenomena have been successfully explained by means of a drift-diffusion model which includes impurity trapping of mobile holes and impact ionization of neutral acceptors [6,13]. Although much work has been done on this model problem (see [14] and references therein), important basic questions concerning its asymptotic description and chaos under time independent voltage bias are still open.…”

Transient and temperature-dependent phenomena in Ge:Be and Ge:Zn far infrared photoconductors

“…Recently, however, some attempts have been made to solve the differential equations describing steady state carrier transport, including the boundary condition for an ideal ohmic contact (52). The equations governing the physics of the near-contact region will be reviewed and a summary of the steady state solution will be given.…”

Germanium:gallium photoconductors for far infrared heterodyne detection

“…6 The photoconductive gain in turn is given by G =-'rr= Ir (Eb)X (lb) where T tr is the mean time for carriers to transit the interelectrode distance 1, A is the drift mobility, and Eb is the dc bias field on the detector. Thus there are three fundamental material parameters that completely characterize such a detector: mobility; recombination time; and quantum efficiency.…”