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PMOS transistors operate by creating an inversion layer in an n-type transistor body. This inversion layer, called the p-channel, can conduct holes between p-type "source" and "drain" terminals. The p-channel is created by applying a negative voltage (-25V was common [ 18 ] ) to the third terminal, called the gate.
It is of immediate concern in p-channel MOS devices (pMOS), since they almost always operate with negative gate-to-source voltage; however, the very same mechanism also affects nMOS transistors when biased in the accumulation regime, i.e. with a negative bias applied to the gate.
Alternatively, rather than static logic gates, dynamic logic such as four-phase logic was sometimes used in processes that did not have depletion-mode transistors available. For example, the 1971 Intel 4004 used enhancement-load silicon-gate PMOS logic, and the 1976 Zilog Z80 used depletion-load silicon-gate NMOS.
However, PMOS transistors are relatively easy to make and were therefore developed first — ionic contamination of the gate oxide from etching chemicals and other sources can very easily prevent (the electron based) NMOS transistors from switching off, while the effect in (the electron-hole based) PMOS transistors is much less severe ...
The PMOS transistor's channel is in a low resistance state, connecting Vdd to Q. Q, therefore, registers Vdd. On the other hand, when the voltage of A is high (i.e. close to Vdd), the PMOS transistor is in a high resistance state, disconnecting Vdd from Q. The NMOS transistor is in a low resistance state, connecting Vss to Q. Now, Q registers Vss.
PMOS: Mohamed M. Atalla, Dawon Kahng: Bell Telephone Laboratories [2] [3] NMOS: 10,000 nm: 100 nm: PMOS Mohamed M. Atalla, Dawon Kahng: Bell Telephone Laboratories [4] NMOS May 1965: 8,000 nm 150 nm: NMOS Chih-Tang Sah, Otto Leistiko, A.S. Grove Fairchild Semiconductor [5] 5,000 nm: 170 nm: PMOS December 1972: 1,000 nm? PMOS Robert H. Dennard ...
For many years, NMOS circuits were much faster than comparable PMOS and CMOS circuits, which had to use much slower p-channel transistors. It was also easier to manufacture NMOS than CMOS, as the latter has to implement p-channel transistors in special n-wells on the p-substrate, not prone to damage from bus conflicts, and not as vulnerable to ...
However, the nMOS devices were impractical, and only the pMOS type were practical working devices. [8] A more practical NMOS process was developed several years later. NMOS was initially faster than CMOS , thus NMOS was more widely used for computers in the 1970s. [ 10 ]