Optimum Baritt Structure

The recently studied planar-doped triangular barrier structure is analyzed theoretically for a possible application as a barrier injection transit time device (BARMT). The small signal impedance as well as the large-signal power characteristics are calculated. Compared to the conventional Bm structures the new device offers a sign&ant improvement in its frequency range and generated power. The idea of two-terminal devices with a negative dynamical resistance arising from the transit-time effect in semiconductor structures was first considered by Shockley in 1954. A number of devices have been since proposed and implemented based on this principle (Ref. [l], Chap. IO), the most important of those being the IMPATT and BARITT diodes. All transit time devices contain a high-field region in which carriers travel at saturated velocity and develop a charge density wave which contributes to a phase delay between the ac current and voltage. For physical reasons this delay cannot exceed 90” and therefore one needs an additional delay to achieve a negative resistance. Such initial phase shift can be obtained from an input element which injects carriers into the high-field transit-time region. In IMPATT diodes, injection occurs due to avalanche breakdown in a reverse-biased p-n junction. In BARITT structures the injection mechanism is provided by thermionic emission over a barrier. The avalanche injection is equivalent to an LC input circuit without significant active resistance, which creates a 7r/2 phase delay. The thermionic injection mechanism corresponds to an RC circuit which contributes a positive (unwelcome) active resistance and gives rise to a delay which depends on the parameters of the structure. Because of this the BARITT diode operates at lower power and lower e5ciency than the IMPATT diode. On the other hand the noise associated with thermionic injection over a barrier is significantly lower than the avalanche noise in an IMPATT diode. In its original version the BARITT represents a reachthrough npn (or pnp) diode[ll. In the working state of the device the central p (or n) layer is depleted of carriers giving rise to a potential barrier for thermionic injecting. The downhill portion of this barrier works as a base and the uphill portion as an injector. The flat region near the top introduces parasitic positive resistance. Various multilayer structures have been proposed, e.g. nipun structure in Ref. [2], in order to reduce this resistance and thus improve the power and efliciency characteristics. The optimum limit of the BARITT corresponds to a structure in which the central layer is ultra-thin, i.e. represents a charge sheet. Such structures (planar-doped by MBE n-i-p+-i-n) have been recently studied experimentally[3,4] and theoretically[5]. Fortunately, it is precisely in the charge-sheet limit that the structure admits of a rigorous analytical theory. In Ref. [51 the planar-doped diode was analyzed in a static regime. In the present work we analyze its high-frequency characteristics and investigate its potential application as a BARITT device. The structure, shown in Fig. 1, includes a nearly intrinsic (i) semiconductor layer sandwiched between two n+ doped layers. In the process of growth of the structure by MBE a thin p+ layer is built in the i region. All acceptors in this layer are ionized forming a negatively charged sheet which gives rise to a potential barrier of triangular shape. For the transit time performance the triangular barrier (TB) must be strongly asymmetric, that is L, 4 L2. The dc voltage is applied with t to the longer side so that the downhill slope, L2, plays the role of a high-field transit-time region and the uphill side, L,, that of an injector. The smti-signal impedance of the base is calculated in the same way as in the conventional BARITT theory (Ref. [l], p. 619) and is given by Z(o) = R iX with RJzcosa ~ [cos a cos (a t e)] roe (la) .+,+E$E [sin a -sin ((I t O)]} (lb) where 8 = o&/u, is the transit time angle at frequency o, u, saturation velocity, e the dielectric permittivity, and Q the injection phase delay. The form of eqn (1) corresponds to the characteristic BARlTT boundary condition which relates the ac particle current density 83 to the ac field SE at the injection plane (which in our case is at the top of the barrier), viz. sJ(L,) = uSE(L*) (2) where 8 is related to a of eqn (1) by