Digital Integrated Circuits A Design Perspective The Devices
The Diode The diodes are rarely explicitly used in modern integrated circuits However, a MOS transistor contains at least two reverse biased diodes that impact the behavior of the device. In particular, the voltage dependent capacitances contributed by these parasitic elements play an important role in the switching behavior of the MOS digital gate. Typically, diodes are also often used as Electro-Static Discharge (ESD) Protection devices to protect chip inputs and outputs. Therefore, a review of diode fundamentals is desirable (especially reverse bias conditions)
The Diode The pn-junction is the simplest of the semiconductor devices It consists of two regions of p- and n- type materials Both the regions are separated by a thin transition region such a device is called a step or abrupt junction device p-type material is doped with acceptor impurities (e.g., boron). Thus, holes are the dominant majority carriers. n-type material is doped with donor impurities (e.g., phosphorus). Thus, electrons are the dominant majority carriers. B Al Cross-section of pn-junction in an IC process p n A SiO 2 A p n B One-dimensional representation Al A B diode symbol
The Diode Bringing the p- and n-type materials together causes a large concentration gradient at the boundary. Electron (hole) concentration changes from a high value in the n-type (ptype) material to a very small value in the p-type (n-type) material This gradient causes: electrons (holes) to diffuse from n (p) type material to p (n) type material movement of mobile electrons (holes) leaving behind immobile donor (acceptor) ions in n (p) type material. The p (n) -type material is negatively (positively) charged in the vicinity of the pn-boundry. The region close to the junction from where all mobile carriers have left is called depletion (space charge) region. The remaining immobile ions create an electric field across the boundary causing drift current Under equilibrium condition: drift currents are equal and opposite to diffusion currents (zero net flow)
Depletion Region p hole diffusion electron diffusion hole drift electron drift n (a) Current flow. Charge Density - ρ + x Distance (b) Charge density. Electrical Field ξ x (c) Electric field. Potential V ψ 0 -W 1 W 2 x (d) Electrostatic potential. P is more heavily doped than n (N A >N D ), where N A and N D are the acceptor and donor concentrations
Depletion Region Under zero-bias condition, there exists a voltage across the junction, called built-in potential This potential (Φ 0 ) has the value N AN 0 = ΦT ln ni Φ 2 kt ΦT = = 26mV q D at 300K The quantity n i is the intrinsic carrier concentration in a pure sample of the semiconductor. It equals approximately 1.5 x 10 10 cm -3 at 300K for silicon
Diode: Static Behavior Assume forward biasing potential of p is raised w.r.t. n-region. This applied potential lowers the potential barrier. Thus, the flow of mobile carriers across the junction increases as the diffusion current dominates the drift current. These carriers traverse the depletion region and become minority carriers. Eventually, these minority carriers recombine with the majority carriers. The net result is a current flowing through the diode from the p- region to the n-region, and the diode is said to be forwardbiased.
Forward Bias Metal contact to p-region n p0 p n (W 2 ) L p p n0 Metal contact to n-region p-region -W 1 0 W 2 n-region x diffusion Typically avoided in Digital ICs
Reverse Conduction Potential of p is lowered w.r.t. n. This results in a reduction in diffusion current, and the drift current becomes dominant. Thus current flows from the n to p regions. Since the minority carriers is very small, this drift component is very small. Thus, it is fair to say that the diode operates as a nonconducting or blocking device when reverse-biased (i.e., one way conductor).
Reverse Bias Metal contact to p-region n p0 p n0 Metal contact to n-region p-region -W 1 0 W 2 n-region x diffusion The Dominant Operation Mode
Diode I-V I V Characteristics Deviation due to recombination
Diode: Static Behavior Ideal diode equation I D = I S ( e V D / Φ T 1) I s is the saturation current (function in diode area and doping levels) When V D <<0, I D =-I S